Applied material: tin, tungsten, gold, silver, lead, zinc, tantalum, niobium, titanium, manganese, iron ore, coal, etc. Advantages: high concentration ratio of dressing, convenient adjustment and easy to get obvious separation.
The shaking table beneficiation can not only be used as an independent beneficiation method, but also is often combined with methods such as jigging, flotation, magnetic separation by centrifugal concentrator, spiral classifier, spiral chute and other beneficiation equipment.
The shaking table is mainly used for the separation of copper, tungsten, tin, tantalum, niobium, chromium, gold and other rare metal and precious metal ores. In addition, it is widely used in the separation of iron, manganese ore and coal. Before flotation, it was also used in the dressing of nonferrous ores.
It can be used for different operations such as roughing, concentration, sweeping, etc., to separate coarse sand (2-0.5 mm), fine sand (0.5-0.074 mm), sludge (-0.074 mm) and sand with other different particle sizes. It is very effective equipment for selecting fine-grained materials below 1 mm, especially below 0.1 mm.
The bed surface can be made of wood, FRP (glass fiber reinforced plastic), metals (such as aluminum, cast iron) and other materials. Common shapes of the bed surface are rectangle, trapezoid and diamond.
There is a feeding chute on the upper right of the bed surface, the length of which is about 1/3~1/4 of the total length. There are many small holes on one side of the feeding chute, so that the slurry can be evenly distributed on the bed surface.
Connected to the feeding chute is the flushing tank, which accounts for 2/3~3/4 of the total length of the bed surface. Many small holes are made on the side of the tank so that the flushing water can be evenly fed along the longitudinal direction of the bed.
The light mineral particles in the upper layer are subject to great impact force, and most of them move downwardly along the bed surface to become tailings. Accordingly, this side of the bed surface is called the tailings side.
The heavy mineral particles at the bottom of the bed move longitudinally by differential movement of the bed surface, and are discharged from the opposite of the transmission end to become concentrate. The corresponding position of the bed surface is called the concentrate end.
The horizontal and longitudinal effects of mineral particles of different densities and particle sizes on the bed surface are different. The materials finally spread out in a fan shape on the bed surface, and a variety of products of different quality can be obtained.
The amount of feeding ore is related to the granularity of the feed. If the ore grains are relatively coarse, the required amount of feeding ore is large. However, if it is too large, it will cause zoning problems. In this case, it is necessary to move the concentrate intercepting plate to increase the flushing water and the horizontal slope of shaking table surface.
The Cr2O3 content in a certain lean chromite ore in Zimbabwe is only 8.19%. Fote has conducted research on the beneficiation technology and equipment of the lean chromite ore, finally decided to adopt the beneficiation method: tail discharging by the strong magnetic separationfull-grain separation by shaking table. The indicators are relatively good.
Step 2 Then use Fote magnetic separator for strong magnetic separation to remove qualified tailings with a yield of 50.21%, and the tailing grade is only 2.19%. As a result, the amount of ore entering the shaking table is reduced by half, and the number of shaking tables is greatly reduced. At the same time, after throwing the tail, it creates favorable conditions for the sorting of the shaking table and further improves the sorting index.
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It is widely used in separating gold, tungsten, tin, tantalum, niobium and other rare metals and precious metal ore. Can be used for roughing, concentration, scavenging different operations, separating coarse sand (2-0.5mm), fine sand (0.5-0.074mm), slime (-0.074mm) of different grain grade. It can also be used to separate iron, manganese ore and coal. When processing tungsten, tin ore, the table effective recycles particle size range for the 2-0.22 mm.
The working principle of shaking table is to use the combined action of the specific gravity difference of sorted minerals, alternating movement of bed surface, and transverse oblique water flow and riffle, to allow loose layering of ores on the bed surface and fan-shaped zoning, then different minerals can be separated.
911Metallurgist is a recognized supplier of high-quality shaker tables that are precision-made to produce the best gravity separation. Our team of experienced engineers manufactures and assembles our tables at the suppliers factory site where the machines are built to very high standards under strict quality control conditions. The tables are constructed of the highest quality materials on the market and have been tried and tested in the field over many decades. Shaking tables provide the most efficient gravity separation of sub 2mm materials. With over a century of use concentrating minerals, 911Metallurgist units have proved themselves as the market leaders. 911Metallurgist customers are currently using tables to produce concentrates of gold (alluvial and milled ore), tin, tungsten, tantalum, and chromite, where the tables are usually used as the final stage in gravity circuits.
The most generally accepted explanation of the action of a concentrating shaker table is that as the material to be treated is fanned out over the shaker table deck by the differential motion and gravitational flow, the particles become stratified in layers behind the riffles. This stratificaton is followed by the removal of successive layers from the top downward by cross-flowing water as the stratified bed travels toward the outer end of the table. The cross-flowing water is made up partly of water introduced with the feed and partly of wash water fed separately through troughs along the upper side of the table. The progressive removal of material from the top toward the bottom of the bed is the result of the taper of the shaker table riffles toward their outer end, which allows successively deeper layer of material to be carried away by the cross-flowing water as the outer end of the shaker table is approached. By the time the end of the shaker table is reached only a thin layer, probably not thicker than one or two particles, remains on the surface of the deck, this being finally discharged over the end of the table.
The physical and mechanical principles involved in the concentrating action of a shaker table are somewhat more complicated than this explanation implies. Mathematical calculations and experimental data are extremely usefulin studying these principles, but they tell only a part of the story and do not explain the highly efficient separations that tables are known to be capable of making.
Unless the shaker table feed contains a considerable percentage of bone gold and other material of specific gravities intermediate between that of rock and gold, extremely high tabling efficiencies may be expected. If a shaker table could be operated on feed consisting of nothing but a mixture of individual gold and slate particles with a size range of approximately -in. to 48 mesh, an almost perfect separation would be obtainable even on an unclassified feed. With such a feed a well-operated shaker table would probably recover not less than 98 per cent of the gold while eliminating not less than 95 per cent of the slate. This implies almost perfect stratification according to specific gravity without regard to particle size, and it is improbable that it could be attained entirely as a result of the motion of the deck and the flow of water in a plane parallel to the deck surface.The question then arises as to what the other forces or factors are that might contribute significantly to the efficiency of the separation on a table.
As far as is known, no exhaustive studies have ever been made of the principles involved in shaker table concentration by either ore-dressing or gold-preparation engineers. Bird and Davis probably have given more attention to the subject than anyone else, but their experimental work was of a preliminary nature. It was done on minus 4-mesh raw gold and on synthetic mixtures of various products derived from this raw gold by screen sizing and sink-and-float fractionations. They used an apparatus which they called a stratifier. This was a channel-shaped box 12 ft. long, 5 in. deep and 1 in. wide, inside measurements. It was suitably mounted with one end attached to an eccentric and pitman. Stratification experiments were made by filling the box with gold and water and running it at a speed of 360 strokes per minute with the eccentric set to give -in. stroke. The amount of water used was sufficient to permit complete mobility in the bed during the operation of the stratifier. At the end of each run, after the water had been allowed to drain off, one side wall of the stratifier was removed and cross-section samples were taken of the bed to determine by screen-sizing and sink-and-float tests to what extent stratification had been accomplished. Bird and Davis say that their aim is to bring out the fact that stratification, contrary to the common brief, will not account for the separation effected by the gold-washing table, and that cross-flowing water, in addition to removing the top strata found on the table, must also have an important selective action in completing the separation according to specific gravity, both in the upper and in the lower strata found between riffles.
The theory of Bird and Davis as to the selective action of the crossflowing water is that only a part of the water flows over the top of the bed between riffles; the remainder flows through interstices in the bed. These interstices are comparatively large near the top of the bed but become progressively smaller toward the bottom, thus forming in effect V-shaped troughs. In this way the water currents would be relatively swift near the top of the bed and become progressively slower toward the bottom. According to Bird and Davis, With paths for the water such that the top strata are subjected to relatively swift currents and the lower strata are subjected to progressively slower currents, the separation actually occurring on the shaker table can be explained. As the coarse particles at the top receive swift currents and each successively finer size at the lower levels receives slower currents, the velocity of the water matches the size of materials comprising the different strata. Under these conditions a separation occurs in the lower strata similar to that in the top strata, only it takes place more slowly. The slow currents of water within the bed carry the fine gold particles along from riffle to riffle, at a more rapid rate than they do the fine bone and shale particles.
Although stratification due to the nearly horizontal action of the shaker table deck and the flow of water in a plane parallel to it is probably not sufficient to account entirely for the separation made by a table, it is, nevertheless, the fundamental principle of the shaker table just as hindered settling is the fundamental principle of a jig. Although these processes are of diametrically opposite characteristics, there is some possibility that a shaker table may utilize to a minor extent the hindered-settling principle. For convenience in this discussion, the stratification due to the more or less horizontal action of the shaker table deck and flow of water will be referred to as shaker table stratification. This type of stratification is illustrated by the separation that takes place when a box of large and small marbles is shaken and agitated in a horizontal plane in such a way that the large and small marbles collect into separate layers. It is a familiar phenomenon that the small marbles will collect in a layer on the bottom while the large marbles collect in a top layer. The principle of hindered settling can be illustrated by placing a mixture of large and small marbles in an upright cylinder of suitable size with a perforated-plate bottom. If water of sufficient volume and pressure is forced upward through the perforated plate so as to keep the marbles in teeter for a short interval, the marbles will separate into layers, with all the large marbles in the bottom layer and all the small ones on top. The separation is the reverse of that obtained by shaker table stratification. In these illustrations of stratification and hindered settling it is assumed that the marbles are all of the same specific gravity regardless of size. If some marbles have higher specific gravities than others the effect will be to increase their tendency to settle toward the bottom, regardless of whether this tendency favors or opposes the stratification or hindered-settling action. The heavier the small marbles, the easier the separation by shaker table stratification and the more difficult by hindered settling. Conversely, the heavier the large marbles, the more difficult the separation by shaker table stratification and the easier by hindered settling.
In line with principles referred to above, complete separation according to specific gravity could hardly occur on a shaker table or in any other concentrating device as a result of either shaker table stratification by itself or hindered settling by itself when the material to be separated consists of particles varying a great deal in both size and specific gravity. In gold washing the aim is to separate gold particles from particles of refuse according to specific gravity without reference to size of particles, as the ash content of a particle is almost directly proportional to its specific gravity. This separation can be accomplished more effectively by utilizing a combination of shaker table stratification and hindered settling than by relying on either of these two alone, and it is quite conceivable that both processes actually do play a part in the operation of a concentrating table.
To explain how a certain degree of hindered settling might occur on a table, we must assume, as Bird and Davis did, that although a part of the water flows across the top of the bed the remainder of it flows through interstices in the bed itself between adjacent riffles. This seems to be a reasonable assumption and it is one that is also made by Taggart in his discussion of the theory of shaker table concentration. The cross flow of water from one riffle to the next might be somewhat as illustrated in Fig. 8, in which a-b is a line along the surface of the deck perpendicular to the riffles, and C and D are two successive riffles. If the bed is kept in a mobile condition between riffles by the motion of the table, and if the water flows from riffle to riffle approximately as indicated in Fig. 8, it is quite probable that to a certain degree a hindered-settling effect is attained along the upper side of each riffle in a zone indicated by the arrows in Fig. 8. Although the effect of hindered-settling along any individual riffle might be relatively slight, the cumulative effect along the entire series of riffles across the width of the deck might be of sufficient magnitude to influence materially the character of the shaker table separation.
We should expect a hindered-settling effect to be very beneficial as an ally to stratification on a table. The weak point about shaker table stratification is that it tends to deposit all fines at the bottom of the bed, even fine gold of low specific gravity. This fine gold, after penetrating to the surface of the deck, would be guided toward the refuse end by the riffles and would tend to go into the refuse if it were not brought to the top of the bed by some means or other and then carried over the riffles by the cross flow of water and subsequently discharged with the washed gold. Bringing the fine gold to the surface is a function that hindered settling would accomplish very effectively, as one of the fundamentals of hindered settling is that it brings the light, fine particles to the top of the bed. As far as the coarse particles of gold are concerned, evidently they are brought to the surface by stratification and started on their way to the washed-gold side of the shaker table by the cross flow almost instantly after the feed strikes the deck. Anyone who has operated a gold-washing shaker table is familiar with the rapidity of this separation and the way in which it causes all light, reasonably coarse gold particles to be discharged from a rather narrow zone at the head-motion end.
If the suppositions in the foregoing paragraph are correct, the process of separation of gold and refuse on a shaker table may be summarized as follows: Almost immediately after the feed strikes the table, sufficient stratification takes place to bring all coarse, light particles of gold and possibly some coarse particles of refuse to the top of the bed. The cross flow of water carries the coarse gold particles across to the gold-discharge side very rapidly, whereas any coarse particles of refuse at the top of the bed are carried toward the refuse end much more rapidly by the differential motion of the shaker table than they can be transported transversely by the cross flow of water. After removal of the coarse gold, and as the bed progresses diagonally across the table, the shaker table stratification action brings medium-sized gold particles to the surface, and these are removed across the tapering riffles by the wash water. The tapering riffles and continuous removal of material by the cross flow causes the bed to become thinner and thinner toward the refuse end. When the point is reached where the thickness of the bed is less than that of the coarse refuse particles, these particles stick up through the surface of the bed and the transverse pressure exerted on them by the cross flow is diminished, as their surfaces are only partly exposed to this flow. This helps to keep them on their course toward the end of the shaker table and prevents them from being transported by the water in the same direction as the medium-sized gold. Toward the outer end of the riffles the extremely fine gold is being brought to the surface by a hindered-settling action immediately behind each successive riffle. Since the material subjected to this action consists of light, fine particles of gold and heavy refuse of a much larger average particle size, the action should be particularly effective in bringing the fine gold to the surface and allowing it to be carried off into the washed gold by the wash water.
This explanation presumes that to some extent there is a greater opportunity for hindered-settling conditions toward the outer end of each riffle than near the head-motion end. Although this presumption may be questionable, it is possible that, as the bed becomes thinner, a greater proportion of the water follows a coarse along the surface of the deck and contributes to the upward current required for hindered-settling conditions as each riffle is encountered.
In this discussion of shaker table principles shape of particle has been disregarded because it is believed that, as a rule, this is not an important factor in the gold-tabling process. Almost invariably the gold particles are somewhat more cubicle and less platy or flaky than refuse particles, but there is little evidence to show that refuse particles of one particular shape are more difficult to separate on a shaker table than those of some other shape. As for the gold, the shape of particles in sizes suitable for tabling are pretty much alike in all golds. Yancey made a study of the effect of shape of particle. He decided that, for the gold he used in his study, shape of particle is a factor of minor importance in tabling this unsized gold, in so far as the over-all efficiency of the process is concerned. Size and, of course, specific-gravity difference are the major factors.
Of considerably more importance than shape of particle is the particle-size factor. It is evident from the nature of stratification and hindered settling that the separation of gold from refuse becomes more difficult as the range of sizes to be treated in one operation increases. The increasing difficulty as the size range increases is apparent from the following considerations: Assume that we are dealing with two minerals, one of high and one of low specific gravity, and that a mixture of 10-mesh particles of the two minerals will separate readily into two layers by either shaker table stratification or hindered settling, one layer containing all the light particles and the other layer all the heavy particles. Now, if we add two more sizes of heavy particles to the mixture, say 8-mesh and 14-mesh particles, obviously, according to the principles of stratification and hindered settling, the separation by either process into two layers according to the specific gravities of the two minerals will be somewhat more difficult than with the original mixture of nothing but 10-mesh particles. The greater the number of sizes of heavy mineral added to the mixture, the more difficult will be the separation. This reasoning applies likewise to the particles of the light mineral, and it all sums up to the fact that if a shaker table feed contains too wide a range of sizes some of the sizes will be cleaned inefficiently.
In actual practice there is no objection to a considerable variety of sizes in the feed; in fact, if all particles were of the same size there might be some disadvantages, because the bed would be less mobile and less fluid and conditions within the bed would be less favorable for efficient separation than when there is some variety of sizes. For efficient shaker table operation, however, it is important to guard against having too wide a range of sizes in the feed.
In the use of tables in gold preparation, the importance of correct operating conditions can hardly be overemphasized. It is a peculiarity of tables that they give excellent results when correct operating conditions are maintained, but with conditions upset and unbalanced the results are likely to be as far on the bad side as they were on the good side under favorable conditions. This is especially true if the washing problem is somewhat difficult. Naturally, when there is an almost complete absence of bony material in the shaker table feed and the problem is mainly one of separating low-ash gold from slate and other rock, fair results may be obtained even under haphazard operating conditions; but if the washing problem is at all difficult the results are likely to be either extremely good or extremely bad, depending on whether or not correct operating conditions are adhered to. Some of the factors on which operating conditions are dependent will be discussed briefly.
It is a comparatively simple matter to build foundations substantial enough so that they will not have a tendency to shake or vibrate as a result of the motion of the tables. A reinforced-concrete slab need not be more than 6 or 7 in. thick to provide a perfectly rigid foundation, even at a considerable height above the ground, if properly supported on reinforced concrete pillars. It is important to provide tables with substantial, rigid foundations that will not deteriorate after a few years of service. Even a slight shaking or vibrating motion in the foundations is likely to interfere with the action of the tables and lead to serious loss of shaker table efficiency.
One of the first essentials for successful shaker table operation is uniform flow of gold and water to the table. The significance of a steady, uniform feed is apparent from a consideration of the mechanical process involved in the shaker table separation of gold from refuse. The material fed to a shaker table spreads out in a fan-shaped bed. This bed covers virtually the entire shaker table deck. Along the outer edges of the bed at the points of discharge the refuse has separated from the gold and discharges over the end of the shaker table while the gold discharges over the side, assuming that the corner of the shaker table is the dividing point between gold and refuse. However, the amount of material discharging over the side of the shaker table in proportion to that discharged over the end will vary if the rate of feed varies and other conditions remain constant. For instance, if a shaker table is set to give highly efficient results with a feed of 7 tons per hour of a given gold, it will discharge approximately the correct percentage by weight over the refuse end as refuse. If the feed is decreased by several tons per hour, however, without any compensating adjustments being made, a larger percentage of the total material is likely to discharge over the refuse end. This means an unnecessary loss of gold and a low shaker table efficiency. If the feed should be increased by several tons per hour the reverse of this probably would happen, with a certain amount of refuse going into the washed gold and raising its ash content.
Variations in feed rate also affect adversely the conditions for separation of gold from refuse within the bed itself. For instance, for any particular setting of the shaker table when a given gold is treated there is an optimum thickness of bed and an optimum ratio of water to solids in the feed that should be observed when high shaker table efficiency is important. The process of separating particles of refuse from particles of gold cannot be highly efficient except under these optimum conditions, and it is quite obvious that if the feed rate decreases it will tend to decrease the thickness of the bed in certain areas on the table, and the ratio of water to solids will change, as the amount of feed water and wash water are usually more or less independent of the tonnage of solids in the feed. Such interference with the actual separating function of the shaker table is likely to cause an incomplete separation.
With further reference to optimum separating conditions within the bed itself, it is important to maintain always the right kind of distributionthe term distribution in this connection referring to the shaker table distribution of the material with which the constantly moving bed on the shaker table is maintained. The shaker table distribution should be such that the quantity of solids discharged per unit length along the side of the shaker table decreases gradually from the head-motion end toward the refuse end. It should be observed in qualification of this statement, however, that it is usually advantageous to have the washed-gold discharge start at a point a foot or so away from the cornerthat is, the corner directly across from the feed box. Usually there is a large volume of water discharging from this corner zone, but ordinarily it is preferable to have almost no solids discharging with it. Beginning at the end of this corner zone, however, there should be a very heavy discharge of washed gold in the first 3 or 4 ft., and the amount discharged from each successive zone from there to the corner at the refuse end should decrease gradually. There should be some discharge of solids virtually all the way to the corner, but as the corner is reached the discharge should be almost zero. Under these conditions there will always be some refuse material discharging immediately around the corner, but the amount of refuse from the first 6 or 8 in. next to the corner on the refuse end should be negligible in quantity. The bulk of the refuse should discharge over a zone of considerable width, starting not less than 1 or 2 ft. up from the corner.
Although this more or less ideal distribution is fairly easy to attain with an average raw-gold feed, it may be more difficult of attainment with a type of feed in which there is an abnormally high percentage of refuse, especially if the refuse consists mostly of high-ash bone gold. This condition often is encountered in the re-treatment of middlings from primary stages of washing.
However, regardless of the character of the feed, the nearer this ideal distribution is approached, the better the results will be. Once the correct balance between shaker table adjustments and the volume of feed gold, feed water, and wash water has been found, good distribution will maintain itself automatically as long as none of the operating factors are allowed to change. It is self-evident, however, that an increase or decrease in the amount of water going to the tableeither feed water or wash waterwill upset this distribution just as quickly as a change in the feed tonnage unless other compensating adjustments are made.
It is of paramount importance, therefore, to have a feed system that will eliminate as far as possible fluctuations or variations in the rate at which gold and water are fed to the table. With regard to the gold, not only the quantity but also the quality and physical characteristics should be kept constant. This is true particularly with reference to the size distribution of the feed. Any change in size distribution, such as may result from segregation in an improperly designed bin ahead of the tables, can upset the distribution of the material on the tables. The only sure way to get a steady feed is to feed the gold to the shaker table by means of a positive-type feeder, such as a belt, screw conveyor, apron feeder, or rotary star or paddle feeder. A sliding gate device instead of mechanical feeders is almost certain to be unsatisfactory, even when a water line can be placed inside the gate to keep the material moving. The mechanical feeders should be provided with variable-speed drive for adjusting the feed to the desired tonnage. This adjustment cannot be made satisfactorily by varying the size of the opening through which the gold discharges onto the feeder. The feed bin should be of such size and design as to eliminate segregation as far as possible. Any attempt to dispense with feed bins is likely to result in unsatisfactory operating conditions, although it is being done at many plants. A customary practice, for instance, is to draw a middling product from a set of jigs and after dewatering run it through a crusher directly to the tables. Such procedure nearly always provides a variable feed for the tables whereas a constant feed could be obtained by dropping the discharge from the crusher into a bin and having mechanical feeders between the bin and the tables.
Changes in the size distribution of a feed are sometimes caused by difficulties in the dry screening of run-of-mine gold. If dry screening is used and the amount of surface moisture in the run-of-mine gold varies, a finer shaker table feed will be produced when the gold is excessively moist than when it is dry. Naturally, particles near the upper size limit will go through the screen readily if the gold is dry whereas if the gold is wet these particles are likely to go into the oversize. The resultant variation in the size character of the feed can interfere with shaker table efficiency as readily as segregation in the bin. Wet screening eliminates this difficulty.
In connection with the problem of segregation and variations in the size-consist of shaker table feed, a comparatively recent development at a shaker table plant in Alabama is worth noting. This plant went into operation at the Praco mine of the Alabama By-Products Corporation in 1944. Incorporated in this plant is a newly-designed system for reducing to a minimum the problem of segregation. The 16 tables in this plant are provided with small individual feed hoppers of about 1500 lb. capacity. Transfer of the 7/16 in- to 0 shaker table feed gold to these hoppers from 100-ton storage bin is accomplished by means of a horizontally operated bucket conveyor, tradenamed Side-Kar Karrier by its manufacturer. After passing under the 100-ton storage bin where the buckets are filled up with gold through multiple openings in the bottom of the bin, this conveyor moves on a track laid in a horizontal plane across the tops of the 16 feed hoppers. Each individual hopper is spring-suspended and as gold is withdrawn out of the bottom by the shaker table feeder, the hopper rises due to decrease in weight. As it rises it automatically engages a tripping mechanism in the conveyor buckets overhead, causing the buckets to discharge their load into the hopper. Thus a few buckets at a time are dumped into each hopper and the effect of small increments dumped at frequent intervals is obtained, giving a flow of gold to each shaker table of more average and uniform size-consist than when gold is run in a continuous stream into a large feed bin until the bin is filled.
As a further deterrent to segregation, the gold is fed from the bottom of the hopper to the shaker table by means of a tapered auger so as to draw continuously from the entire width of the hopper and avoid segregation within the hopper. For further details of this plant, the reader is referred to an article published in 1944.
With regard to the water supply for a table, it is just as important to have a steady, uniform flow of water as of gold. The water pipes and valves should be so arranged in a shaker table plant that each shaker table gets its flow of water quite independently of the others. If a common water header is used it should be big enough so that, regardless of how the water adjustments are changed for one shaker table or group of tables, the volume of flow to the others will not be changed. The source of the water supply, of course, should be maintained with a fairly constant pressure or head. This can be accomplished more effectively by using a gravity tank at a considerable height above the level of the tables than by drawing water directly from a pumping circuit. Clean water is to be recommended strongly in preference to dirty water from the washer circuit. Wash water sometimes carries enough solids in suspension to interfere with the flow through pipes and valves, and accumulation of solids sometimes may stop a valve entirely. Under these conditions the flow of water varies almost continuously and there will be too much one minute and not enough the next. The solids in the water are likely also to be sufficiently abrasive so that frequent replacements of the valves and fittings will be necessary. All of these troubles can be avoided entirely by using a supply of clean water for the tables.
The riffling, shaker table speed, length of stroke, and other adjustments, such as shaker table slope, longitudinal, and cross slope, must in each case be balanced by the various other operating factors, so as to get the desired results. The speed that the shaker table manufacturer provides for when he supplies each shaker table with its individual motor drive is usually quite satisfactory. This speed is usually between 250 and 300 r.p.m. All shaker table head motions are designed so that the length of stroke is adjustable within a certain range. This range usually is from to 1 in., or slightly over. The coarsest shaker table feed requires the longest stroke. For a raw-gold feed of average size, say 5/16-in. to 0, a stroke of 7/8 to 1 in. usually is satisfactory. A slightly longer stroke on such a feed usually will give about the same shaker table efficiency with slightly higher capacity. A report giving experimental data as to the effect of speed, stroke, and other variables on shaker table efficiency has been published by the Bureau of Mines. More recent work published by the Illinois Geological Survey emphasizes the importance of the longitudinal slope and the speed of reciprocation, two factors which are not readily adjustable on ordinary commercial tables. A slower speed is found to improve the performance, in opposition to the results reported by the Bureau of Mines. The discrepancy is noted by the author, and has not been explained.
As to type of riffling, it seems to be generally agreed now that high riffles are advantageous in the tabling of bituminous gold, and it is customary to have the main riffles start with a height of not less than in. at the feed end and taper to a feather edge at the outer end. The -in. height probably represents a minimum; riffles 2 in. high are now used on the Deister Plat-O tables; and these tables are recommended by the manufacturer for the cleaning of shaker table feeds as fine as 3/8-in. to 0. There is a great deal of variation in the spacing of high riffles. In some designs there is only one shallow riffle between two higher riffles. Another design, intended to emphasize the importance of the pool effect, provides four or more shallow riffles between successive high riffles. About the only suggestion that can be made with regard to riffling is that the coarser the feed, the more advantageous are high riffles. Unless the shaker table feed is extremely fine, with maximum particles size less than in., there seems to be no good argument for the main riffles to be less than or 1 in. high. On such gold, riffles lower than this would tend to reduce capacity. With coarser feeds higher riffles can be used advantageously.
As to the comparative merits of wooden riffles and rubber riffles, one can be substituted for the other without changing the shaker table results appreciably. It seems evident, however, that the efficiency, as far as ash reduction and gold recovery are concerned, is slightly less with rubber covering and riffles than with linoleum covering and wooden riffles. The difference would be only a few tenths of one per cent less ash at the same recovery, using the linoleum and wooden riffles. Usually this is more than offset by the greater operating economy of the rubber covering and riffles. Although the rubber combination costs about twice as much as linoleum and wood, it is supposed to last 10 or 12 times as long.
In summarizing, the principal adjustments and factors to be considered in putting a shaker table into operation on a certain feed, are: feed rate, as to volume of both gold and water; slope of the shaker table (longitudinal and cross slope); riffling system, shaker table speed, and length of stroke. A shaker table installation should be so designed that any or all of these adjustments and factors can be changed easily to meet requirements during the procedure of placing the tables in operation. In starting a shaker table plant, the main objective should be to find the combination of shaker table adjustments and operating factors that will give the correct shaker table distribution described previously in the discussion of feed uniformity. The quantity of water to be used is from two to three times as much by weight as the feed of gold, but it should be adjusted as nearly as possible to the minimum amount that will keep the products discharging uniformly from all zones around the edge of the table. To most nearly attain the ideal distribution on the table, it is usually necessary to have the supporting channels under the shaker table deck several inches higher at the refuse end than at the feed end. As to the cross slope, it should be the minimum at which it is possible to attain good distribution. In other words, the flatter the shaker table is in the crosswise direction, the better, provided the distribution is good. The length of stroke and shaker table speed should be adjusted so that the bed will be kept in a state of uniform flow and mobility all over the deck. On the raw-gold feed, these operating conditions can be attained fairly easily, but it may be more difficult in the treatment of middling products. Difficulties sometimes can be overcome by making slight changes in the riffling and by use of auxiliary water sprays directed at certain areas in the bed. Anything that is done should be directed toward getting and maintaining a distribution on the shaker table as nearly ideal as possible.
The launder system in a shaker table plant should be so designed that a splitter can be used for dividing the washed gold from the refuse at some point along the washed-gold side instead of at the corner, if desired. The correct shaker table distribution will sometimes give too high an ash content in the washed gold if the split between washed gold and refuse is made at the corner, and in such instances the best solution is an adjustable divider or splitter that can be set at any desired point along the washed-gold side.
The tonnage a shaker table will handle effectively depends to a great extent on the washability and size of the gold. In treating an ordinary 5/16-in. to 0 raw-gold feed, high efficiency with respect to both cleaning and recovery usually can be obtained with a feed of as much as 10 tons per hour. High efficiency in this case means an efficiency that could not be improved appreciably by lowering the tonnage. If the gold is extremely easy to wash, higher tonnages can be cleaned with equally good efficiency. The claim sometimes is made by shaker table manufacturers that their tables will handle efficiently as much as 15 to 20 tons per hour of 5/16-in. to 0 gold. On an average feed of this size, however, feed-tonnages of more than 10 tons per hour are likely to cause a decrease in efficiency. With feeds as coarse as -in. or 1-in. to 0, it is not unusual to treat from 12 to 15 tons an hour per table. Modern tables will handle minus 1/8-in. feed at the rate of 7.5 tons per hour.
One of the important considerations frequently overlooked in the design of a shaker table plant is that the making of necessary shaker table adjustments is extremely difficult unless representative samples can be taken easily. Often the more or less permanent washed-gold and refuse launders around the tables are laid out in such a way that it is next to impossible to get dependable samples of the products from individual tables. Either the launders should be so designed that they can be partly removed during sampling, or they should be built with enough spacing between the edge of the shaker table and the launder so that the necessary sampling pans for taking zone samples can be inserted at any place around the table. Provisions should also be made for conveniently sampling the composite washed gold and composite refuse from each table, in addition to the feed to individual tables. Without dependable samples it is sometimes difficult to tell whether or not an individual shaker table is operating correctly; and, owing to the segregation of products into various discharge zones, haphazard sampling is sometimes worse than no sampling at all.
The laboratory shaking table is widely used for the gravity separation of sands too fine to treat by jigging. The physical principles utilised in tabling must be understood if preparation of feed and application of control are to be efficient.
Consider a number of spheres rolling down a slightly tilted plane under the urging influence of a flowing film of water. Some of the spheres (shaded) in Fig. 170 represent heavy mineral and others (white) light gangue. The largest sphere travels fastest and the smallest one slowest, under the combined influence of streaming action and gravitational pull. Of two spheres having the same density, the larger moves faster. Of two having the samediameter, if the slope is relatively gentle and the hydraulic urge relatively strong, the lighter sphere travels faster. If during the otherwise free downward travel of these spheres the whole plane is moved sideways, then the horizontal displacement of the spheres varies in accordance with the lengthof time they take to roll down. This is represented here on the right, which shows that the largest light sphere has undergone the least horizontal displacement because it travelled fastest, whilst the smallest heavy one has been carried furthest to one side. From this it is seen that if a suitable displacing movement can be applied to a plane, the feed can be spread into bands according to the size and density of its constituent particles. If these bands are collected into separate vessels as they leave this deck, the feed will have been segregated into three main products:
A particle light enough to respond mainly to the hydraulic influence of the flowing film of water moves down-plane with little horizontal displacement. A typical particle, unlike a sphere. will either slide or skip downward, rather than roll, provided it is reasonably free to move. Apart from the limited use of the automatic strake in concentrating metallic gold, continuous lateral displacement across the sorting plane cannot handle an adequate tonnage and is not used in the mill.
With the Laboratory shaking table a reciprocating side motion is applied to the sloping surface or deck down which the pulp is streaming. If this shaking action was applied symmetrically in both directions across the stream, each particle would move an equal distance in each direction, and separation into bands would not occur. The displacing stroke must be applied gently, so as not tobreak the grip between particle and deck. The deck accelerates, and in doing so imparts kinetic energy to the material on it. Then the deck motion is abruptly reversed so that it is snatched away from under the particles resting immediately above it. These continue to skid sideways (across the flow) until their kinetic energy has been exhausted. It is therefore essential to provide a differential side-shake which builds up gently and then breaks contact between deck and load.
This is provided by the shaking mechanism or head motion of the shaker table. The slower the particle travels downstream, the further it slides sideways under the influence of the shaking motion. Thus far discussion has been limited to a series of individual particles fed to the deck from one starting-point. If, instead, a layer several particles deep is fed from a starting-line, it becomes possible to handle a greatly increased load on the deck. The operating conditions have now changed. In the cross-section through such a layer, as seen normal to the direction of shake, the mixed feed first stratifies itself under the disturbing influence of the shaking action. The smallest and heaviest particles reach the deck, the largest and lightest stay uppermost, with a mixture of large heavy and small light grains between. This arrangement exposes the large, light particles to the maximum sluicing force of the film of water as it streams down the laboratory table. a force that can be controlled in intensity by varying the volume of water used and the slope of the deck. It is thus possible to exert some degree of skimming action to accelerate the downward movement of the uppermost layer without disturbing those below. The particles next to the deck are pressed to it by the material above, and therefore can grip it with greater firmness than would be given by their own unaided weight. They thus are able to cling during fast sideways acceleration, and are only freed and set skidding by the sudden reverse action.
The overlying particles have only a precarious hold. This aids the discriminating action of each stroke. The bottom particle travels furthest, breaks free at stroke reversal and is the first to skid. Those above it sway backward and forward and consequently receive less lateral movement. This accentuates the separating action by giving the bottom (heavy mineral) particles the maximum horizontal displacement per stroke and the upper (light gangue) grains the least. This aids the sorting discrimination. If the feed has been properly prepared by hydraulic classification, ensuring that all the grains have similar settling characteristics through vertical currents, film sizing can now take advantage of the variation in cross-section between the heavy andlight particles in each stratum, sweeping down the lighter and leaving the heavier untouched. The particles thus segregated are then removed in separately discharged fractions, called bands, at the far end of the tables deck. It would not be possible to form and maintain an evenly distributed thick bed of the kind called for by the foregoing considerations if a smooth plane deck were used. Riffles are therefore employed to provide protected pockets in which stratification can take place. They are usually straight and parallel with the direction of shake, but may be curved or slanted. The deck, instead of being plane, may be formed to provide pools in which the feed can stratify. The riffles must:
Thus (a) rules out as bad practice the use of stopping riffles set high above the rest, sometimes used to arrest and spread entering feed. If all riffles are not of similar initial height the stratifying action and transfer between them is upset. Smooth delivery is best achieved with a feed box integral with the moving deck, and aligned with the vibrator. It should let the feed down gently to the head riffles. Items (b), (c), and (d) are arguments against the use of curved riffles, which increase wall friction and upset stratifying action. A badly maintained mechanical action and deck coupling may mislead the engineer into redesigning his riffle plan, just as an incorrect stance may cause the unwary golfer to modify his swing instead of standing correctly. In the standard Wilfley table the riffles run parallel with the long axis, and are tapered from a maximum height on the feed side (nearest the shaking mechanism) till they die out near the opposite side, part of whichis left smooth. Where the riffles stand high, a certain amount of eddying movement occurs, aiding the stratification and jigging action in the riffle troughs.
As the load of material is jerked across the Laboratory Shaker Table, the uppermostlayer ceases to be protected from the down-coursing film of water, owing to the taper of the riffle. It is therefore swept or rolled over into the next riffle below. In this way the uppermost layer of sand is repeatedly sluiced with the full force of the current of wash water, riffle after riffle, until it leaves the deck. This water-film is thinnest and swiftest while climbing over the solid riffle, and the slight check and down pull it receives while passing over the trough between two riffles helps to drop any suspended solids into that trough.
At the bottom of the riffle-trough, then, the particles in contact with the deck are moving crosswise as the result of the mechanical shaking movement. At the top they are exposed to the hydraulic pressure of a controllable film of water sweeping downwards. In the trough of the riffle the combined forces-stratification, eddy action, and jigging-are arranging them according to density and volume.
Provided the entering particles have been suitably sorted and liberated, good separation can be achieved on sands in any appropriate size range from an upper limit of about i to a lower one of some 300 mesh. The difference in density and mass between particles of concentrate and gangue determines the efficient size range which must be maintained by hydraulic classification or free-fall sorting of the feed. A further separating influence is applied hydraulically along each riffle as the water in it gathers energy from the decks movement. As it gathers speed in the forward half of its cycle, the water flowing along the trough parallel to the axis of vibration is accelerated. When the decks direction is abruptly reversed this flow is only gently checked relatively to the more positive braking force exerted on the skidding particles in the riffle. There is thus a mildly pulsed sluicing action across the Laboratory Shaker Table, in addition to the steady stream at right angles to it, down-slope. This cross-stream helps the particles to travel along the riffles.Since separation depends to a large degree on the hydraulic displacement of the particle, its shape influences its reaction. Flakes of mica, though light, work down and cling to the deck, and may be seen moving nearly straight across, even at the unriffled end where they meet the full force of the stream. Where there is no marked influence in density between the constituent minerals of a pulp, the shape factor aids a flat particle to move along the deck to the concentrates zone, and under like conditions helps an equi-dimensional one to move down-slope toward the tailings discharge. Shape factor can therefore help tabling in some cases, and be disadvantageous in others, depending on whether it reinforces or opposes differences in size between the classified particles of value and tailing.
Small scale table concentration tests have many critics. Many metallurgists consider that such tests are of problematical value because of the difficulties involved in conducting and interpreting them.Many kinds of small-scale ore dressing tests are difficult to conduct, and there is, perhaps, good reason for thinking that table concentration tests are amongst the most difficult.Interpretation of results from small-scale tests is the responsibility of the metallurgists and engineers in charge, and it is often held that small-scale table concentration tests are particularly difficult to interpret.
Firstly, there are difficulties due inherently to the small-scale nature of the operations; for example the smaller width of all mineral bands on the table and the less complete separation due to the shorter length of travel between the feed and discharge points.
Secondly, there are the effects of batch operation owing to the fact that the mineral particles behave differently during the initial period when the sample is just beginning to spread over the table, the middle period when feed and discharge are even and continuous, and the final stage, when the last of the sample has been added and the table is beginning to empty itself.
If the test must be conducted as a small-scale batch test, difficulties due to the first two causes are inevitable, but by proper attention to the equipment and technique used for laboratory table concentration tests, difficulties due to inevitable causes may be minimized.
Unfortunately, it is common to find that insufficient attention has been given to the careful design of laboratory concentrating tables, and it is believed that difficulties arising from this cause, combined with crude testing techniques, are largely responsible for difficulties in interpreting results. If proper attention is given to the points mentioned, there seems no reason why the results obtained should not be a reliable guide to the optimum performance of a commercial plant.
The present paper describes the development of the concentrating table used in the laboratory operated jointly by the Mining Department of the University of Melbourne and the Ore Dressing Section of the Commonwealth Scientific and Industrial Research Organization. Although the paper contains some discussion of the technique of table concentration testing, the bulk of it is devoted to describing the steps taken to improve the mechanical rigidity of the table and the convenience of its adjustments and controls.
In order to comprehend the reason for the modifications made, it is helpful to consider, first, how a mixed feed of dense and light particles, say galena and quartz, behaves in an ordinary batch table concentration test.
It is supposed that the feed rate is uniform throughout the test and that the side slope and cross water are adjusted so that when stable conditions have been established on the table, the line of demarcation between galena and quartz will be on the concentrate end of the table 2 in. from the corner.
Galena is scarce because the quartz moves more quickly; quartz appears well up the slope of the table because the forces tending to wash it across the table are not fully operative. There is little galena on the riffled portion of the deck, so that more quartz particles remain in the riffles where they have little opportunity to be forced by the galena to the top of the bed in the riffles, from where they would be washed down by the cross water.
As the feed continues to flow, more galena appears on the table, and when stable conditions have been established, the line of demarcation between galena and quartz moves down to a point 2 in. from the corner. This condition continues until feeding ceases. Shortly it will be noted that there is scarcely any quartz on the table and that the line of demarcation between the galena and the remaining quartz moves down the concentrate end of the table towards the corner.
The first effect occurs because the quartz moves across the table more quickly than the galena. The second effect occurs because the cross water washes the galena further down the unriffled part of the deck since there is practically no quartz to stop it.
It will be found, then, that if in a batch test a table is fed- uniformly and neither the cross, water nor the side slope is altered, the line of demarcation between concentrate and tailing will start at a point well up the concentrate end of this table, move gradually to a stable point and, at the end of the test, move rather quickly to a point much closer to the corner of the table.
If a clean separation is to be obtained, it will be necessary to move a splitter to follow this line of demarcation. However, it is common to find the movement of the separation point so great that moving a splitter is not alone sufficient to cope with the large changes which occur. In this case it is necessary to alter the side slope of the table.
However, the head motion used on the laboratory table had been in service for a number of years, and had become badly worn. As alternative plans for a replacement were being considered, Mount Isa Mines Ltd. offered to donate to the laboratory a commercial Deister Plat-O head motion in excellent mechanical condition. This offer was gratefully accepted. For compactness, a frame was built to accommodate the table deck directly above the case containing the head motion, the movement being transferred through a lever arm pinned to the frame. The arrangement is illustrated in Figs. 1 and 3.
Lever arm lengths can be adjusted readily to give a stroke length ranging from 5/16 in- to 1 in. The sharpness of the kick can also be adjusted. To date no experiments on the effect of either of these variables have been conducted. The speed is constant at about 300 strokes per min. and adjustment can only be effected by changing the driving pulley.
The frame is of welded construction. The base is made of 5 in. channels, and the rest of the frame of 3 in. channels and 2 in. and in. angles. The ample sections combined with the cross-bracing give a rigid frame.
A deck of this kind has only one major defect for test workthe difficulty of avoiding contamination of successive runs owing to solids lodging between the riffles and the linoleum surface. This trouble has been minimized by using a waterproof adhesive as well as the nails to attach the riffles. Another source of contamination in the old model table was a flat-bottomed feed box which was difficult to clean. The feed box now used was made from a short length of 1 in. dia. pipe and may be seen in Fig. 1. This type of feed box is very easy to clean.
The deck is supported on four slipper rods which slide in seats arranged in independent pairs at each end of the table. Each pair of seats can move freely about a pivot, the pivots being aligned accurately. This arrangement provides a very rigid support, which accommodates itself easily to change of slope. A clear view of the rods may be seen at A, in Fig. 2, while the seats may be seen at A in Fig. 3.
The deck is connected to the head motion through a shackle and pin, (A and B, Fig. 5), while a spring attached at an angle beneath the deck keeps the slipper rods seated. A crank operated by hand-lever (A in Fig. 4) applies tension to the spring. Either one of two decks with slightly different riffling may be used. To remove the deck, spring tension is released by turning the hand-lever, and detaching the spring. The pin A (Fig. 5) is removed from the shackle B and the deck lifted off. To fit the other deck, these operations are repeated in reverse order. The changing of decks can be effected in about two minutes.
The table is provided with two adjustable splitters, a concentrate-middling splitter on the concentrate end of the table, and a middling-tailing splitter on the tailing side of the table. An external view of the splitters is shown in Fig. 6.
The concentrate end of the table is faced with a 1 in. wide strip of 16 gauge brass sheet, its edge being flush with the edge of the linoleum deck surface. The splitter itself is a vertical sheet of brass, the top edge of which is about 3/8 in. below the deck surface. The splitter and its small attached launder are mounted on a split block which slides along two brass rods mounted on brackets underneath the table. The halves of the block are held against the rods by crossed leaf springs tensioned by a small knurled nut. The method of attachment is shown in Fig. 2. The cutter moves readily when slight pressure is applied, and maintains any set position.
The cross slope of the table is adjusted by a lever arm attached to the pair of slipper rod seats at the concentrate end. A second lever operates a locking nut at the back of the pivot. The two lever arms are shown in Fig. 4. When using this simple two lever arrangement, it has been found that when the locknut is released the cross slope of the table may change suddenly and jerkily. To improve this feature, a vertical screw type of adjustment is being attached to the lever arm B.
When the cross slope of the table is changed, a couple is applied to the bridge bar (D, Fig. 2) connecting the two slipper rods at the head motion end. To avoid applying a twist to the shackle E, the nut F tightens onto a shoulder on the pin G and not onto the bridge bar. The clearance is so small (0.001 in.) that there is no perceptible slackness although the shackle can twist quite freely.
The top edge of the table deck is not parallel to the axis about which the deck is tilted. Consequently, if the launder distributing cross water were attached to the deck, the water distribution would change when the cross slope was changed. To avoid this, the launder has been attached to the main frame by two pieces of 1 in. x 3/16 in. flat steel bent appropriately. The launder is attached by hinges and may be folded up out of the way to facilitate changing of decks. The method of attaching the water launder is made clear in Fig. 4.
A common method of feeding a table for batch test work is by scoop. The discussion given of the behaviour of dense and light minerals in a batch test in which the feed is quite regular enables conditions to be foreseen when the table is fed by scoop. Suppose a somewhat extreme example in which a scoopful is fed onto the table in five seconds, and successive scoopsful added every 30 seconds subsequently. In the period immediately after adding each scoopful, the quartz added will move more rapidly than the galena, and so will push the line of demarcationbetween concentrate and tailing up. Subsequently, the corresponding amount of galena will arrive at the table edge, and so will push the line of demarcation down. This cycle will be repeated for each scoopful added. The result will be that the line of demarcation between concentrate and tailing will fluctuate. The extent of the movement will depend on the irregularity of the feed, and although with care the fluctuation may be minimized, the operation will inevitably be tedious and time-consuming, and even the best result will leave much to be desired.
Experiments with a launder feeding method have shown that it has decided advantages. The V-bottom launder used is shown in Fig. 1. The feed is spread fairly uniformly along the bottom of the launder, and the rate of feed regulated by the rate of feeding water to the head of the launder. About 90% of the feed will flow without further alteration, but some additional wash water isnecessary near the end of a run to clean down the sides of the launder.
More elaborate launder feeding methods with progressing water jets, etc., have been proposed, but although these would appear to have further advantages, the simple method described has proved satisfactory. It does not give absolutely regular feed, but the changes occur, gradually and are easy to cope with.
Experiments with continuous circulation have also been conducted. The arrangement is shown, in Fig. 7. Concentrate, middling and tailing separate on the table and are deflected into a common pump, which discharges the, mixed feed into the dewatering cone shown. The overflow runs to waste and the discharge returns to the table. This system gives far more regular feed than any other method tried. It works very well for demonstration purposes, but quantitative tests have not yet been undertaken. The method proposed is to establish equilibrium conditions, and then take timed samples.
Three product hoppers are used, two small hoppers which are fixed, to the table framework being provided for concentrate and middling, while the tailing is collected in a large hopper fitted into a framework mounted onwheels. The large mobile hoppers of 30 gal. capacity are extremely useful in the laboratory for many purposes, such as the collection and settlement of slime, collection of jig and table tailings, and in fact any large quantities of ore pulp.
Both the fixed and mobile hoppers are closed with rubber bungs from inside, the bungs being fixed to long brass rods with T-handles. The clearance below the hopper outlets is sufficient for a 3 gal. bucket.
A laboratory concentration table was modified by incorporating a sturdier head motion, main frame and supports, and altering the controls so as to make them positive, convenient and independent of each other.
The advantages from the modifications to the table construction cannot readily be expressed in quantitative results. The important effect is that every operation, such as feeding the table, adjusting the side slope or product splitters, and handling the products, is easier, and the table itself is much less prone to erratic disturbances due to lack of rigidity in the framework, supports and adjustments. It is felt that these substantial mechanical improvements are bound to express themselves in improved metallurgical performance.
Shujin Li, Cai Wu, Fan Kong, "Shaking Table Model Test and Seismic Performance Analysis of a High-Rise RC Shear Wall Structure", Shock and Vibration, vol. 2019, Article ID 6189873, 17 pages, 2019. https://doi.org/10.1155/2019/6189873
A building developed by Wuhan Shimao Group in Wuhan, China, is a high-rise residence with 56 stories near the Yangtze River. The building is a reinforced concrete structure, featuring with a nonregular T-type plane and a height 179.6m, which is out of the restrictions specified by the China Technical Specification for Concrete Structures of Tall Building (JGJ3-2010). To investigate its seismic performance, a shaking table test with a 1/30 scale model is carried out in Structural Laboratory in Wuhan University of Technology. The dynamic characteristics and the responses of the model subject to different seismic intensities are investigated via the analyzing of shaking table test data and the observed cracking pattern of the scaled model. Finite element analysis of the shaking table model is also established, and the results are coincident well with the test. An autoregressive method is also presented to identify the damage of the structure after suffering from different waves, and the results coincide well with the test and numerical simulation. The shaking table model test, numerical analysis, and damage identification prove that this building is well designed and can be safely put into use. Suggestions and measures to improve the seismic performance of structures are also presented.
With the fast urbanization in China, the population growth in cities has led to ever-increasing demand for high-rise buildings to accommodate commercial and residential needs. High-rise buildings are very common in the densely populated cities all over the world, such as New York and London [1, 2]. Accordingly, pertinent regulations have been developed to ensure the safety and reliability requirements of high-rise buildings. High-rise buildings that are designed and constructed according to codes and standards are deemed as complied with all regulatory requirements for the buildings. With the innovation and advancement of technology and materials, some high-rise buildings are planned and designed beyond the specifications of codes and standards, particularly for fast-developing countries such as China. To ensure the safe and reliable function of these buildings, it is imperative to investigate the behaviors of these buildings, in particular, the behavior under horizontal loadings such as wind and earthquake. It is also essential to identify and quantify if possible the characteristics of these buildings with a view to provide guidance for the future design of similar buildings.
The shaking table test is one of the most widely used techniques to assess the seismic performance of structures made of various materials. Commonly, it is widely used for assessing linear/nonlinear and elastic/inelastic dynamic response of structures. Martinelli et al.  presented the nonlinear dynamic response of a shaking table test for a full-scale seven-story reinforced concrete shear wall building, where four simulated earthquake records with increasing intensity were used as excitation. Saranik et al.  conducted a shaking table test to investigate the inelastic behavior of a two-story steel portal frame with bolted connections. Furthermore, it is not only used for structural dynamic tests but also for geotechnical behavior. Chen et al.  conducted a series of shaking table tests on a plaster model of a three-story and three-span subway station to investigate the seismic failure characteristics of the structure on the liquefiable ground. Lin et al.  undertook shaking table tests on three embankment slope models to study the seismic response of the embankment slope with different reinforcing schemes. The effectiveness of shaking table tests has also been studied. For example, Srilatha et al.  investigated the effect of frequency of base shaking on the dynamic response of unreinforced and reinforced soil slopes through a series of shaking table tests.
In shaking table tests, most researchers used scaled models as specimens. For example, Liu et al.  carried out shaking table tests on a 1/30 scaled model with and without base isolation bearings to assess the seismic performance of an isolated museum structure in high earthquake intensity regions. Lu et al.  tested a 1/50 scale high-rise building model on a shaking table for Shanghai World Financial Center Tower. The dynamic characteristics, seismic responses, and failure mechanism of the structure were investigated, and weak positions under seldom-occurred earthquakes were identified. Some researchers use prototype structures. For example, Lignos et al.  conducted a shaking table test on a full-scale high-rise building to demonstrate the effectiveness of the numerical models used. Graziotti et al.  performed a shaking table test on a two-story full-scale unreinforced masonry building to study its response, characteristics, damage mechanism, and evolution during the experimental phases.
The above literature review suggests that the shaking table test is an essential tool to assess and verify the dynamic behavior of structures. It is particularly imperative for those structures that exceed the limits of the specification of design codes and standards. It is with this regard that the present paper is in order. The Shimao Building (numbered 13 in the A2 block) is an iconic building in the central business district area of Wuhan, China, located on the bank of Yangtze River. It is a combined commercial and residential building with 56 stories. The lateral-force resisting system of the structure is the reinforced concrete (RC) shear wall with a height of 179.6m, exceeding the limit for high-rise buildings specified by Chinese Regulation Technical Specification for Concrete Structures of Tall Building (JGJ3-2010) . Therefore, it is required to verify the structural seismic performance after the regular structural designing according to the codes. Besides, the irregular building shape in both plane and elevation complicates the analysis and determination of seismic resistance of the structure. To ensure the safety of the building, it is necessary to examine the behavior of the building under seismic loading. For this purpose, a shaking table test and its numerical analysis as well as structural damage identification were also conducted.
This paper focuses on the investigation of the seismic behavior of Shimao Building. Firstly, the results of a shaking table test on the 1/30 scale building model will be presented. The structural dynamic characteristics and the responses under different levels of earthquake loading will be investigated, and the failure mechanism and cracking pattern of the tested model will also be obtained. Then, the corresponding finite element model will be established to analyze its seismic performance. At last, a damage identification method based on the autoregressive (AR) model will also be presented to identify the damage of the test building. According to the analysis of experiment, finite element model, and theoretical identification of the test building, seismic performance of the prototype building will be obtained, and this study can make sure that the prototype building is designed reasonably and can be put into use safely. Finally, suggestions and measures to improve the prototype building seismic performance will also be presented.
A 1/30 model of Shimao Building was designed and built for the shaking table test to represent the main characteristics of the prototype building. The experiment was undertaken in the laboratory of School of Civil Engineering and Architecture at Wuhan University of Technology, China. Figure 1 shows the representing plan and elevation of the prototype building. It can be seen from Figure 1(b) that the plan of the building is shaped as T (maximum 32.85m in length and maximum 19.5m in width), and this shape of building may be susceptible to earthquake excitation. The vertical configuration of the building consists of a core tube accommodating staircase, lift well, and pipe shaft and shear walls located at the outer walls and some inner walls. The floors and roof of the building are reinforced concrete beam-slab structures. In addition, the upper (vertical) structure is fixed on the reinforced concrete base.
According to the Technical Points (No. 65, 2015) of Special Inspection for Seismic Fortification of Out-of-Code High-rise Buildings , issued by the Chinese Construction Ministry, the total height of the building 188.6m (including the roof truss) with 56 stories exceeds the limit height of 170m specified by the Class B shear wall structures according the Chinese code (JGJ3-2010) . Furthermore, the building has a set back at the height of 128.35m (the 41st story), as shown in Figure 1, leading to a disagreement between the centroid and stiffness center. As a consequence, the eccentricity violates the requirements of seismic conceptual design that building structures should be symmetric in both plan and elevation. Overall, a shaking table test of this building can be quite necessary.
Dynamic characteristics of the prototype building were calculated through Chinese structural design software PKPM. Although the maximum seismic responses of the structure under Frequent 6 (defined at Section 3), shown in Table 1, satisfy the requirements of the code (specified by the Chinese Seismic Design Code of Building Structures), considering larger earthquakes happen and making extra certain, the shaking table test is still needed for further verification.
In this paper, the similitude law is determined by the dimensional analysis method . Firstly, similar conditions are obtained, and then other similarity constants are obtained based on . Inertia force, restoring force, and gravity are required to be simulated in the test, and thus, elastic modulus and density of the model material are strictly controlled. The essential requirement is , where the subscripts and represent the scaled model and the prototype building, respectively. That is,where , , , and denote the similitude ratio of elastic modulus, equivalent density, acceleration, and geometry, respectively. In the present case, three controllable similitude ratios should be determined in advance to obtain others. Specifically, , , and are chosen in this paper, and thus, can be calculated using Equation (1).
Considering the shaking table size and the height requirement of the laboratory, the dimension scaling parameter is chosen as 1/30. Based on the tested characteristics of materials in the test, the similitude law of elastic modulus is determined as 1/3.05. The total weight of the model (including self-weight and artificial mass) and prototype building are 3.06ton and 40400ton (including live load), respectively. Thus, the mass ratio is 1/13202.6, and the similitude ratio of equivalent density can be obtained as 2.0245; all the main model similitude relationships and calculation formulas are shown in Table 2.
Since the aim of the shaking table test is to investigate the seismic behavior of the original structure subjected to different intensity of earthquakes, including failure mode and mechanisms, it is necessary to use the same materials as the prototype building. The materials used for model construction (specimen) are microconcrete (mix proportion is shown in Table 3), galvanized steel wires, and meshes which are similar to the materials in the prototype building. Ordinary Portland Cement P.O. 32.5 is chosen to construct concrete. A batch of specimens in the form of cubic and prism type were cast to measure the strength and elastic modulus of microconcrete. The testing method of the specimens strictly followed the requirements of the Standard for Test Methods of Concrete Structures (GB50152-2012) . The microconcrete mix proportions are shown in Table 3. The elastic modulus of materials is shown in Table 4. The height of the model is 6.437m, with 6.287m for the model itself, and 0.15m for the base. The photo of the completed model is shown in Figure 2.
The test variables include two types: different fortification intensity and different types of earthquake waves. According to related researches on the statistics of the peak acceleration of ground motions in China, the seismic intensity of a specific site exhibits the extreme distribution of the III type (Weibull distribution). The fortification intensity is defined as the intensity with 10% exceedance probability, which is also called as the moderate or basic intensity for simplicity. Similarly, the rare and frequent intensity is defined as the intensity with 23% and 65% exceedance probability, respectively. Furthermore, for the moderate intensity of a specific site, the frequent and rare intensity is about 1.55 lower and 1 higher than the moderate intensity, respectively. In this paper, the scaled building under investigation is located in Wuhan with Degree 6 as the fortification/basic intensity . This intensity is associated with medium occurrence (10%). Therefore, Moderate 6 means ground motion with Intensity 6 (the peak ground acceleration (PGA) is 0.05g) and Frequency 6 means ground motion with Intensity 4.45 (PGA 0.018g), whereas Rare 6 means ground motion with Intensity 7 (PGA 0.1g). Ultralarge earthquakes are not specified in GB50011-2010 , and Rare 7 (actually Intensity 8) is introduced here for the purpose of studying the nonlinear or even collapse performance of the scaled building. For a chosen recorded seismic excitation, the similitude law (shown in Table 1) was used to scale the acceleration and time.
According to the dynamic characteristics and site condition of the prototype structure, three seismic runs are chosen for simulating the shaking table test input wave: (1) El Centro wave, with a peak acceleration of 3.41m/s2; (2) Taft wave, with a peak acceleration of 1.53m/s2; and (3) artificial seismic wave (USER1), supplied by the construction designers, with a peak acceleration of 0.18m/s2. The time-history curves are shown in Figure 3. The main specifications of the shaking table used in the present experiment are shown in Table 5.
The testing procedure is shown in Table 6. It can be seen that the EI Centro wave is the first wave in each test condition, followed by the Taft wave and artificial seismic wave. Before and after inputting different fortification intensity seismic waves, low peak white noise excitation is conducted to measure the dynamic characteristics parameters such as natural frequency, mode, and damping ratio.
The main measurement of structural response is acceleration, displacement, strain, etc. Several acceleration sensors, displacement sensors, and strain gauges are arranged at the different heights of the model to measure the responses of the model structure under different seismic fortification intensities. Accelerations and displacements were measured by the large dynamic signal acquisition and analysis system DASP2003, developed by Orient Institute of Noise and Vibration. 14 acceleration sensors were used for different purposes, namely, 2 for measuring vertical accelerations, 10 for horizontal accelerations, and 2 for torsion of the building. Dynamic strain was obtained by the dynamic and static testing instrument DH3817. Five displacement sensors were used to measure the deformation along the direction of shaking. The positions of acceleration sensors and displacement sensors are shown in Figure 4.
When subjected to Frequent 6, there were no noticeable shaking and visible damages, it can be predicted that the test model can remain in a serviceable condition after Frequent 6, and there was no damage. In the case of Moderate 6, the model responded with little vibration, but no cracks and structural damages, which may indicate that the model is still in serviceable conditions, and there was no need to strengthen. No visible cracks and significant damages occurred after Rare 6. However, the model responded with more vibrations and little crack, which indicated that the model was minor damaged, even though the test building was still in the serviceable condition. Some part of it might need to be repaired. When subjected to Rare 7, it is observed that the model vibrates significantly, together with a large number of cracks in the upper part of the model and spalling of concrete. It can be concluded that the test building is not collapsed even when subject to Rare 7 but lost much of its lateral load resisting capacity. Since the prototype building is represented as the model, the damage pattern of the prototype building can be obtained. The damage of different floors after the test is shown in Figure 5.
Low peak white noise excitation was used before and after seismic excitation for capturing the dynamic characteristic of the model. Results are shown in Table 7. It can be seen that the natural frequencies of the test model maintain the same under Frequent 6, indicating linear behaviors of the structure in this stage. Furthermore, the second- and the third-order frequencies of the test model decreased slightly after Moderate 6, reflecting slight decrease of the structures stiffness. Next, after Rare 6, the natural fundamental frequency decreased by 3.9%, indicating that damages may occur at a certain lateral-force resisting component of the model structure. Finally, after Rare 7, the natural frequencies of the test model decreased significantly. It can be inferred that the 1st mode of the prototype structure is the Y direction, the 2nd mode is the X direction, and the 3rd mode is torsion. The ratio of the 1st mode periods between torsion to the Y and X direction is 0.33 and 0.49, respectively, which is far smaller than the limit value 0.85 given by the Chinese code (JGJ3-2010) . Furthermore, after analyzing the structure stiffness degradation curves in accordance with the 1st order natural frequencies of the model, it can be obtained that the stiffness of the structure declines with the increasing magnitude of earthquake excitation, with a minimum stiffness to 81.9%.
Acceleration amplification factor is the ratio of the maximum absolute value of acceleration response of each story to the maximum input acceleration at the bottom of the model. This factor is of great significance to analyze the seismic performance of structures, describing how many times the accelerations at each story are amplified compared to the base seismic excitation. Hence, the acceleration amplification factor can be obtained through dividing the peak accelerations of the testing stories by the peak accelerations of the shaking table in this test. Then, the envelope diagram of the building in different test conditions can be drawn. Figure 6 shows the envelope of acceleration amplification factors in the main vibration direction (Y direction) with different seismic intensities, and the peak acceleration of some floors in a specific condition and acceleration amplification factor are listed in Table 8.
As can be seen, the acceleration amplification factors along the floors of the structure are nearly invariable except for the top floor, reflecting the lateral stiffness at different floors (except for the top floor) is uniformly distributed. Furthermore, the acceleration amplification factor was almost unchanged after suffering from Frequent and Moderate 6, which indicated that the lateral-force resisting components of the model are seldom damaged. However, the acceleration amplification factor increases sharply on the top floor and roofing layer, indicating that the whiplash effect cannot be ignored in this case. Usually when damages are increasing, the stiffness of structures is reducing, leading to the elastic-plastic phrase, which can result in a smaller acceleration amplification. It can be seen in Figure 6 that the acceleration amplification factor of the same floor continued to decrease with an increasing excitation intensity, reflecting a decreasing structural lateral stiffness and an increasing degree of damage as the seismic intensity increases. However, the decline of the acceleration amplification factor was not obvious after suffering from Rare 6, which indicated that some lateral-force resisting components of the model have already be damaged. Thus, the experimental phenomenon coincided well with the theory.
The displacement response of the model was converted to the displacement response of the prototype by a similar law. The formula to translate the maximum displacement response from the test model to the prototype building should be as follows:Di is the maximum displacement of the prototype on the ith floor, Dmi is the maximum displacement of the model at ith floor, mg is the maximum acceleration of the shaking table determined by the similitude law, ig is the maximum acceleration of the shaking table measured during the test, and Sd is the displacement similarity coefficient.
The maximum displacement and corresponding displacement angle of the prototype structures roof under different seismic levels are listed in Table 9. It can be seen that as the seismic wave intensity increases, both the maximum displacement and displacement angle of the roof increase. Both the maximum displacement and displacement angle of the prototype structure can meet the requirements of the Chinese code (JGJ3-2010) . The prototype building will not collapse and even have a relatively good integrity after severe earthquake action.
Figure 7 shows the envelope diagrams of maximum displacement in the Y direction of the prototype structure along the floors. It can be seen that the displacements of the prototype structure increase as the stories increase. Furthermore, the effect of the El Centro wave was significantly larger than that of the other two waves. Owning to the whiplash effect, the displacement response of the top floor and roofing layer is much larger than that of other floors. The lateral displacement curves under Frequent and Moderate 6 were not flat, which was small and had obvious bending shear deformation characteristics. So the structure had not been damaged yet. The lateral displacement curves under Rare 6 and 7 were relatively flat and obvious, which means that some components have already been damaged, and the stiffness of the structure has declined.
The story drift of representative floors under different seismic waves is listed in Table 10. It can be seen that all the maximum story drift of the structure occurred in the top of the structure, especially on the 56th floor, which means that the upper part of the structure is relatively weaker than others. The stiffness is reduced as the structure becomes smaller above 41st floors, which leads to the increase of story drift. All story drifts of the structure under the testing earthquakes are smaller than the value specified in the Chinese code (JGJ3-2010) , which indicates that the structure can meet the seismic resistance requirements of the code.
There are symmetrical accelerometers arranged at the 41st and the top floor. The displacements under different seismic intensities of these two stories can be obtained by integrating the accelerations. Hence, the torsion can be obtained by the ratios of the displacements to the sensors distances. Torsion angle of the model under different seismic levels is shown in Figure 8. It can be seen that the torsion deformation is small before the inputting of Rare 6, reflecting a good torsional stiffness. However, the torsion deformation became larger under Rare 7, which indicates that some part of the structure has been damaged.
According to transformation formula, the hysteresis curve of the prototype structure under different earthquake levels can be obtained by the displacement historical response and shear historical responses. The shear responses can be calculated by quality distribution of floors and corresponding acceleration responses. Taking Rare 6 as an example, considering the limited pages of this paper, the hysteresis curve under different waves is shown in Figure 9. Actually, the hysteresis curve of the structure under Frequent, Moderate, and Rare 6 change with the external excitation while the change of stiffness is however not obvious, which indicates that the building is basically in the elastic working stage. However, it can be seen that the hysteresis curve becomes irregular under Rare 7, which indicates that some parts of the structure have already been damaged and the structure has gone into the elastic-plastic phase.
In order to verify the experimental results, a finite element model of the test model was established by ANSYS. Elastic-plastic analysis of the test model was conducted. Three-dimensional BEAM4 element was used to simulate the beams and embedded columns, and SHELL63 was used to simulate the floors and shear walls. The material properties were obtained from the measured tests, and the nonlinear performance of materials had been considered. The input seismic waves used in the finite element model were the same as the shaking table test. Real properties of the materials of the model had been taken into account. The finite element mode contained 78899 nodes, beam elements 4599, and shell elements 72414 totally. The height is 179.4m, which is the same as the prototype building.
The results of the finite element analysis indicate that first three order vibration modes of the model include the translation mode in Y direction, X direction, and torsion mode. The first three order vibration modes are shown in Figure 10. All the three vibration modes reflect the coupling between translation and torsion.
Table 11 shows the free vibration characteristics of the model in experimental results and finite element simulation results. It can be seen that the finite element simulation result of the first periods and second periods is similar to those of experiment results, and divergences between the two are 0.07% and 2.41%, respectively. However, the divergences of the third periods became much more significant, which is still within an acceptable level. The ratio of the first mode periods between torsion and translation in the Y direction is 0.38 in the finite element simulation, while the test result is 0.33. Both the two results are far less than the limited value of the Chinese code (JGJ3-2010) . Moreover, the influence of higher vibration modes to the structure can be quite large because of the high aspect ratio for high-rise buildings. It is usually difficult to capture the higher vibration modes of the building by an experiment, and the computational analysis thus shows its advantage and is an important supplement. The first 30 vibration modes and periods were analyzed through the finite element method. It can be concluded that the vibration modes became localized after the 15th order, and the vibration of the top model is much more obvious than others, which indicates that the whiplash effect is quite remarkable. Based on mass participation ratio and vibration maps, it can be concluded that the vibration mode of the structure is coupled translation and torsion, and the torsion has great influence on the seismic response of the structure.
It can be seen in Table 12 that both the acceleration amplification factor of the finite element model and experimental model continued to decrease after suffering from Frequent 6, Moderate 6, and Rare 6, reflecting that the lateral stiffness of the structure has decreased and the damage of the structure increased. The acceleration response of finite element simulation is similar to the shaking table test.
In order to compare the experimental results with the calculated results, the maximum displacement of the test floors under different earthquake levels is listed in Table 13. The envelope of interstory drift under different earthquake waves is shown in Figure 11.
It can be calculated that both the story drift angle of the finite element model and test model under Frequent and Moderate 6 can meet the seismic resistance requirements in the code specification (1/800). The maximum story drift angle of the finite element model under Rare 6 is 1/350, which is larger than the limited elastic value; however, it still can meet the requirements of plastic story drift angle in the Chinese code (JGJ3-2010) . As can be seen in Figure 11, all the peak story drift occurs in the upper part of the structure, especially near the 50th floor, which is relatively weaker than the other parts of the structure. Furthermore, story drift has increased above 41st floors, reflecting a decline of the stiffness, which coincides well with the experimental analysis. Hence, we can reach the conclusion that all the results of finite element simulation coincides well with the results of the experiment, which indicates that both the finite element simulation, and the shaking table model test are reasonable.
In this section, an identification method based on the AR model is presented to identify the damage location and degree of the test model after suffering from simulated earthquakes. Firstly, the AR model is briefly introduced and established by the acceleration response of the test model. Secondly, the plain version of the least squares (LS) method is used to solve the unknown parameters of the established AR model. Then, a judging factor based on the residual variance of the AR model is proposed to estimate the degree of structural damage. Finally, the proposed damage factor of the model building after different earthquake intensities is calculated by MATLAB. The damage location and degree identified by this method are compared with the testing results as well as the numerical results.
The AR model is widely used in the field of structural damage identification , and it is attempt to account for the correlations of the current time parameter with its predecessors in time series, in which the output variable depends linearly on its own previous values and on a stochastic term. It can be implemented to represent the dynamic response of structures . The AR model does not need any specific structural characteristics but the output response data; hence, it is widespread for complex structures [20, 21]. In this research, the AR time-series model is used to describe the acceleration time histories of the shaking table. A noisy AR model of order is described by equation :where is the output of the AR model, it is the discrete-time signal, and in this paper, the acceleration responses are used. is the random noise. is the unknown order of this model at prior and varies from 0 to . denotes the AR coefficients, which need to be estimated. This model can be simplified as follows :where , , and .
However, finding out the optimal order of the AR model is not trivial. The order is not as larger as better. When the order of the AR model increases, the residual sum of squares theoretically decreases, while the calculating errors rise. Therefore, these two aspects should be both considered in the modeling. In literature, there are some criterions achieved this goal , such as Akaikes Information Criterion (AIC) or Bayesian information criterion (BIC), proposed by Akaike and Schwarz, respectively. The AIC will be used in this paper, and it is presented as follows:where is the estimated variance of residual errors when the order of the AR model is .
After the unknown parameter of the AR model is obtained, a factor needs to be proposed to judge the damage of the structure. The step of the method can be clarified as follows:(1)Dividing the obtained response acceleration data before damage into two parts, part and part , serves as benchmark data, from which of the undamaged situation will be estimated. While serves as the unknown inspection data to be estimated in the healthy state of structure.(2)Estimating by equation (5) and the residential of based on by equation (6).(3)Dividing all the observed data into part and . Estimating the residential of and of based on the obtained .(4)Calculating the average of and to obtain . represents the final residential of observed data to be estimated after damage.(5)The damage identification factor is calculated as the ratio between the residential variance of to , shown as
It is clear that if the data to be estimated is coming from the undamaged structure, IF will be close to one. Otherwise, will be larger than ; that is, the IF will increase as the damages of the structure rise.
In this part, the IF of different stories and seismic intensities will be presented. It can be seen in Table 6 that before and after all the testing waves, the white noise is used to test the model; hence, the identification of white noise will be conducted here. Figure 12 lists the IF after different earthquake intensities of some representing floors based on the white noise excitation. It can be concluded that the IF becomes larger as the intensity of earthquake increases, indicating that the damage of the test building rises while intensity increases. Furthermore, the IF of the top story is larger than that of other stories, reflecting the whiplash effect too.
When comparing the damages of all stories after the same seismic intensity, the damage variation along stories can be studied. For the sake of simplicity, Figure 13 shows the IF along some stories, taking the white noise response after suffering from Frequent 6 and Rare 7 as examples here.
It can be concluded that after Frequent 6, all the IF ranges from 1.0 to 1.25, indicating very little damages occurred in the model building. Even though the IF of the 1st floor and top floor is the smallest and largest respectively, there is only a little difference. However, after suffering from Rare 7, the damage increases obviously, the damage degree of 50th, 52nd, and top floors is larger than that of other floors, and the damage of 14th, 28th, and 8th stories is quite significant as well, while the damage of the first story is the smallest. This variation can also be found in Table 8 of the peak acceleration and acceleration amplification factors. The IF of 41st floor is not quite large but increased rapidly above 41st floor, indicating that the 41st floor is not in a serious damage condition as the floors above. This is not limited to the earthquake intensities in Figure 13, and the same conclusion can be drawn after analyzing all the white noise response data of the model building.
Moreover, after studying the IF of the three types of waves used in the test, the variation of IF is nearly the same with that of white noise, and the results will not be detailed here. However, the comparison of the effectiveness between different types of waves cannot be obtained, probably due to no relative data to be used to calculate the healthy residential of benchmark data ().
To summarize, we can reach the conclusion that the identification results are reasonable and coincide well with the results of the experiment and numerical simulation, which indicates that the identification method presented here is effective, and not only the location but also the degree of the damage can be identified by the new identification factor.
The prototype building is represented as the testing model in this paper. Based on all the analysis, it can be concluded that after Frequent 6, almost no changes occur in the structure which is still in the elastic stage. After Moderate 6, no visible damages occur, and natural frequency decreased slightly, which indicates that the stiffness of the prototype building was changed slightly in this condition. However, under Rare 6, the 1st natural frequency decreased by 3.9% and other parameters had little of changes, which suggests that some part of the prototype building will be damaged in this condition. Under Rare 7, visible cracks and spalling of concrete occur, and the natural frequency of the model decreased significantly, which means that the prototype building has been damaged significantly in this condition.
Acceleration response of the top part of the structure is relatively large, which indicates that the whiplash effect of the building is significant. The torsional deformation is not apparent when an earthquake is small, but it became more substantial when the level of input earthquake increased, which indicates that the effect of torsion on seismic response of the structure is increased. Furthermore, the effect of torsion is large above the 41st floors, especially on the 52nd floor, showing that these floors may be weaker than other parts relatively. However, as for the same level of earthquake intensity, the maximum displacement, displacement angle, story drift, and torsional angle of the model caused by the El Centro wave are the largest among the three types of input waves, followed by the Taft wave and artificial seismic wave. Thus the El Centro wave may be the most dangerous wave to the prototype building.
Finite element simulation results coincide well with the experimental results. Higher vibration modes of the building show that vibration modes have become localized after 15th order, and the vibration mode of the structure is translation-torsion coupled; the whiplash effect at the top of the structure is quite remarkable.
The damage degree and location identified by the proposed factor in this paper also show that the upper part of the building has more damage than the lower part, but the damage of 8th28th floor is also quite significant. With the increase of the earthquake acceleration, the damage of the building increases apparently. The identification results indicate that the identification method is effective and can be used in other similar cases.
The results of the test, the numerical analysis, and the identification prove that the building in the A2 block developed by Wuhan Shimao Group was designed reasonably, which can entirely meet the requirement in the Chinese Code and can be safely put into use. Even though the design of this building can meet the seismic design requirements, some measures should be taken to improve the seismic performances. Firstly, the connection between the shear wall of the bottom floor and the base can be strengthened to avoid horizontal joined-up cracks under big earthquakes. Then, the effect of torsion is large above the 41st floor of the building, but the damage of the 8th28th floor cannot be neglected either. More structural reinforcements may be necessary for these floors. The top of the structure also needs to be strengthened since the whiplash effect is obvious.
Copyright 2019 Shujin Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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