rubber tecnology for seismic

stress curve rubber

1. BRIDGESTONE'S WORK ON SEISMIC ISOLATION

Bridgestone started devel opnlent of seismic isolation devices in 1981 and has spend 10 ye~rs since then perft)rming research. especially in the areas of design. materials. adhesion between steel and rubber components. and manuf'atcturing and quality control technology. The underlying goal held by Brid- gestone throughout this research has been the establishment of fundamental technologies which would lead to the development and installation or' satie and reliable base isolation systenls; in other words turning the promise potential.

1.1 Base Technology

(1) Establishment of Durable and Sate Design Method

Base isolation bearings must bear large Io~ids f~r long periods of- time while also being capable of large delSz>rm~ttion during an earthquake. Under these conditions rubber bearings must satisfy requirements Jbr spring characteristics and ability to deform. In order to ensure sal~ design. it is paramount to get a clear view of the conditions of stress and atrain that occur inside rubber bearings. ifler three years of research, Bridges tone was the first company in the world to successfully devel oped non-linear FEM analysis techniques lbr large deforina techs of rubber bearings. One property of rubber material is that it degrades depend- ing on the environment. Also. creep occurs when rubber is placed under a load for a long period of time. To research these phenomena. long-term degradation prediction methods for these materials based on theoretical and experimental studies was developed. The result of this works was Bridgestone's introduc- tion in 1986 of a general durability and safety design method called CANNA (kif~ Assurance with Large Deformation Analy sis) which combines degradation analysis with actual bearing performance. ~ - - .

(2) Development of H~gh Damping Rubber Bearangs

A damping function is indispensable to a seismic isolation system and this has. up to now. required a separate mechanism. If this function could be added to rubber. it would be a great advantage from the standpoints of building design and construc tion. However. trying to give rubber material a large attainability {damping ability). creep characteristics and temper- ature dependency become worse. Six years of research on this point has resulted in the achievement of a highly functional high-damping rubber bearing with low-creep and low tempera- ture dependency. A characteristics of this rubber bearing is not only its high-demping effect (heq O. I 5) for large earthquakes. but it demonstrates a superior damping effect lbr small and medium size displacements as well. This device has already been installed at a total of 4 sites including the computer canter of

(3) Establishment of Manufacturing Technology of High Reliability

Because a seismic isolation systetn is critical to the founda- tion of' a building and must have longer serviceability. a produc- tion method which consistently produces bearings within close tolerances and high quality is essential. A high level of quality control etTecting such areas as the production of rubber com- pounds. heat history at the time of vulcanization and long-term stability of adhesion, was needed. Research and development was conducted with special emphasis on the reliability of adhesion because rupture distortion of rubber bearings is a key criteria in their use in buildings.

1.2 Application Technology

Multi-stage rubber bearings were developed to increase the range olV applications for rubber bearings. For example. rubber bearings using high-damping rubber can be used in floor seismic isolation systemS. Also in a Tuned Mass Dampel- system 1or use in high-rise buildings. a combination of natural rubber and multi-stage rubber bearings can be applied to make a device which is free of friction and easy to maintain. A Super Plastic Rubber with even higher damping capacity (heq 0.4) than high-damping rubber has been developed. This material not only exhibits stress and strain behavior sirnllar to rubber but is also very suitable as a damping material for reducing the swaying of skyscrapers due to earthquakes and strong winds. Practical applications of this material are current ly being studied.

2. HIGH-DAMPING RUBBER MATERIALS 1,2)

2.1 High-damping Rubber

High-damping rubber bearings have two functions. flexibil- ity and damping as an intrinsic property of the high-damping rubber itself. which consequently eliminate the need for a separate bearing and damping system. As can be easily imagined. there are many antipodal sub- jects to be satisfied simultaneously for the realization of a high-damping rubber bearing. in particular high-damping with low creep and low temperature dependent properties in the rubber. It is unquestionable that high creep yields large local stress and strain inside rubber bearings and may in extreme be responsible for the tilting of n structure. High temperature and velocity dependent properties. on the otherhand. change the stiffness and damping of rubber bearings over the use tempera- ture and frequnency ranges. The high-damping rubber bearing has quite a high- damping capacity, an equivalent damping factor (h~,) of more than O. I 5 with a smooth hysteresis loop. shown on the next page. It also shows low creep and Ipw temperature dependent properties similar to those in a standard low damping rubber bearing. Creep and the temperature dependence of modulus are given in Fig. 2. I and Fig. 2.2.

2.2 Damping Generation Mechanism in High-damping Rub- ber aearingsm

We can understand the mechanism of damping generation in a high-damping rubber as follows. There are two types of cross-link real rubber vulcanizates. few chemical cross-link and a large number of physical cross links whick consist of the molecule-molecule interaction junctions weakly adhered to each other by molecular attraction forces. When an external fbrce is given to rubber vulcanizates. all molecules begin to move to the extension direction. Although chemical cross-links move as tightly f~xed cross-lin k points in the systenl. in the case of physical cross-links one molecule slides on the other one and another cross-link point is reproduced at different position on a molecule. This sliding or rubbing of molecules generates friction heat which is dissipated outside of the system. In untilled rubber vulcanizeted. since the molecular accretion fBrce in physical cross-links is very weak. then friction heat is negligibly small. This is the reason why a hysteresis energy , i.e. a damping capacity. is small in untilled rubbers. On the other hand. when we mix special Fillers in rubber, a tremendous number of' physical cross-links are newly produced in the system. in which new physical cross-links have much stronger interaction between fillers and rubber molecules compared with the molecid-molecule interaction thorca. If we deIbrm such a system, since a molecule adhering to the surface of filler must slide on the surface against its strong attraction force, ia vast amount of friction heat will be generated. This is the mechanism of high-damping rubbers. This can be easily understood if one considers the alltErence in consumed energy in pulling up a silk thread from its bundle and u sticky thread form a mixutre of coal tar and threads.

REFERENNES

1) Kojima. H. and Fukahori. Y., Rubber World, Vol 35. No 202 (1990)

2) gukahori. Y. and Seki. g., Polymer in Press

3. DYNAMIC PROPERTY OF HIGH-DAMPING RUBBER BEARING

In this section we describe important properties of high damping rubber bcarings for seismic isolation

3.1 Test Apparatus and Test Piece

The seismic test apparatus can impose a fixed force of vertical load to test piece with air or hydraulic actuater and in the shearing (horizontal) direction the test piece is fixed on the test table. By applying dynamic force to this test table with hydraulic actuators, the dynamic properties of horizontal dis- placement are studied. In the tests. the high-damping rubber bearing shown in Fig. 3.1 and a 1/4 scaled down model were used.

3.2 Horizontal Restoring Force Characteristics 1)

(1) Basic Characteristics

Fig. 3.2 shows the restoring force characteristics of the scale down model. In this figure (a) is the loop up to shear deforma- tion of rubber (horizontal displacement/rubber thickness) of 150% with O2Hz sine wave actuation under the condition of eompressive stress ff 50 ks/era". The result shows a loop with a generally smooth shape. Restoring force stiffness decreases as displacement (shear deformation) increases. High damping rubber bearings have such property that by ~a~ale of deformation it underwent. the siftheSS becomes lower in tlae range below the deformation. Fig. 3.2 (b) shows hystersis loop after the test of rubber shear deformation 7-- 150% shown in Fig 3.2 (a). We see the lower stiffness in a small deformation. ~r~d stabilization after several cycles. We defined equivalent stiffness K and equivalent damping ~m~tor (constant) h~q for hysteresis loop of optional amplitude X~ as follows. (refer Fig. 33)

Here Fm is the load at maximum displacement. z2XW. W is ~th¢ former area in loop. the latter area of triangle ABC A. B.C. D show the area of square contacting exterior of hysteresis loop. Fig. 3.4 shows change of equivalent stiffness and equivalent damping factor in the test piece shown in Fig. 3.1 We can see that the change of equivalent damping factor is small and keeps a fixed value. Observed on a long time basis, this stiffness recovers and returns to its initial value. Therefore, a typical of high-damping rubber bearings is the average value of initial stiffness as shown here and stabilized stiffneSS.

(2) Reiterating Deformation Property To evaluate change of equivalent stiffness and equivalent damping factor after repeated deformation. deformation with a shear strain of 75% was repeated 200 times continuously to the scale down model. The result showed that equivalent stillfines lowered by 20% after 200 cycles. This is because of a fall in elasticity caused by a rise of temperature brought about by conversion of energy absorbed by high-damping rubber to heat. Considering that in a normal large-scale earthquake the maxi- mum number of large deformations is usually around 10 time. we c~n Say that the change of stiffness in a given earthquake may be almost none

(3) Temperature Dependency Table 3. I shows temperature dependency of stiffness and damping factor relative to 20°C 1 .00. The stiffness and damping factors increase as temperature decreases If rubber material is left alone for a' long time at low temperatures, crystallization will occur (rigidity increases as it becomes brittle). It has been verified. however, that the charac- teristics of high-damping rubber are superior to those of natural rubber for low temperature crystallization.

3.3 Marginal Performance 2)

Fig. 3.5 shows shear deformation and marginal deforma tion of actual 150 ton rubber bearing comprising 31 rubber sheets laminated between steel plates of 4.4mm thickness and 778ram diameter for evaluation of an atomic power seismic isolation system. It shows a 1oad-deformation property with an initial soft spring which hardens from the point around 200% of shear strain until it breaks. The marginal deformation capability is around 450--500% and breakage stress at the time of failure is around 60 kg/cm2.

3.4 Vertical Property 1)

Regarding vertical properties, it was confirmed that stiff ness was as hard as natural rubber bearings. Test results under condition of vertical load 80+-20 tons (load fluctuation +-20% for design load) for the test piece shown in Fig. 3.1 shows that vertical stiffness is greater than 1,OOO times horizontal stiffness and the equivalent damping factor is around 5%.

REFER PYRES

1) Fujita. 72. Suzuki, S., and Fujita. S-. The 1989 ASME Pressure Vesseles and Piping Conference. Vol 181. p923-28 (1989)

2) Oka. Y. et hi.. Summaries of Thechnical Papers of Annual Meeting Architectural Institute of Japan B Structure I pp759. 760. 1990 (in Japa nes)

4.DURABILITY OF HIGH-DAMPING RUBBER BEARINGS 1)

Since long time durability is required for rubber bearings. inside rubber must be protected from ambient air and ultraviolet rays with a protective surface of rubber. Regarding the durabil- ity of rubber bearings. especially estimate of restoring properties (stiffness and damping) and failure (breaking) with the passage of time. evaluation of reliability and safety over long periods of time are important. For this study promotioned deterioration test based on chemical reaction velocity (treats conversion of- chemical reaction time and temperature) is applied.

4.1 Promotional Deterioration Test for Rubber Bearlag

Promotional deterioration testing sets the relation between chemical reaction and temperature from activated energy pos aessed by rubber material to estimate property changes in bearings under actual conditions. It uses this relationship to promote chemical reactions under high temperature. FOE this test is conducted by leaving rubber bearings for a specified numbers of hours under high temperature and under compres- ; sion to create accelerated deterioration which simulates actual ; conditions.

4.2 Change of Restoring Force Property

To examine the change of restoring force characteristics of high-damping rubber with the passage of time, we examined rubber bearings (Fig. 3. 1 ) deteriorated by the above method. Incidentally, activated energy applied by the rubber material is 20.3 kcal/mol, and under promotional temperature 100°C of this test, 24 hours under 25°C is equal to 2.5 years. Fig. 4. I shows the changes with the passage of time of horizontal equivalent stiff- ness and equivalent damping factor. This test showed that equivalent stiffness gradually decreases after an initial increased and equivalent damping factor remained almost unchanged. The change of stiffness in 60 years will be a maximum of around 20% compared to the initial value.

4.3 Change of Breaking Property

the same method as applied to the restoring force property test is applied to the change of breaking properties with the passage of time. According to shear breaking test (performed under a eompressive sterss 50 kg/cm2) of rubber bearings used for the tests shown in Fig.4.1, no lowering of breakage stress and strain were found under the condition of deterioration corresponded to that of 60 years. (If longer deterioration is applied, the lowering can he found.) Fig. 4.2 shows the broken section of the test sample. It confirms that adhesion between rubber and steel laminations is greater than the bonding of the rubber itself.

REFERENCES

1) Kojima, H. and Fukahori, Y., Rubber World, Vol 35, No202 (1990)

5. CHARACTERISTICS OF HIGH-DAMPING RUBBER BEARING

5.1 Seismic Isolation Performant 1,2)

To examine seismic isolation with high-damping rubber bearings in the event of large earthquakes. we performed a shaking table test using a model of a base-isolated building reduced to a scale of I : 0.263 (refer Fig. 5.1). In the test we examined seismic isolation performance for input of several earthquake waves (acceleration value targeted 3m/se as actual basis) and found response acceleration of the building was decreased to 1 /2-- l/ 10 at its top. The degree of seismic perfor- mance differed depending on the wave. If seismic isolation is applied to building housing impor taut equipment, evaluation of seismic isolation of such equip ment is also necessary. F-iS. 5.2 shows the result of floor response spectrum from response acceleration of a building for the E1 Centro and the Hachohe NS wave (damping ratio of equipment was estimated at 1%) and dashed line and solid line show seismic isolation and non-seismic isolation respectively. Seismic isolation decreases response acceleration of equipment in a very wide lange of feequencies. Regarding isolation performance for medium and small scale earthquakes which are encountered several times year, earthquake response observation results was recorded at a test seismic isolated wooden two stories residence built in Chiba prefecture, Japan Fig. 5.3 shows representative results with the maximum value of each response of horizontal direction of base-isolated residence recorded during earthquakes from July, 1988 to Dec. 1990. Since damping is secured for this degree of deformation. a comparatively satisfactory seismic isolation effect was gained. although this result con not be applied to learge scale earthquakes.

5.2 Shaking of Building Against Wind The stiffness of high-damping rubber bearings increases in small deformation, but equivalent damping factor keeps its fixed value. These characteristics are not favorable for sufficient seismic isolation effect, as earlier said, for medium and small scale earthquakes. but good in preventing building movement caused by wind.

5.3 Others Since high-damping rubber bearings do not require any special damping apparatus, they use little space in a structure's foundation and installation is relatively simple. By using rubber materials of suitable hardness (elasticity), the design of stable rubber bearing (i.e. diameter is much greater then total rubber height) becomes possible. g

EFERENCES

1) Suzuki, S. et el., Trans. of JISM, 1/O]. 57. NO. 536, Set. C. pp1129-1136, 1991 (in Japanese)

2) Fujita, T. et el., Procedings of The Eight Japanese Earth~ quake Enineering Symposium. VoL 2. pp1779-1784, · 990 (in japanese)

6. MODELING OF RESTORING FORCE CHARACTERISTICS OF HIGH-DAMPING RUBBER BEARING AND EARTHQUAKE RESPONSE ANALYSIS

6.1 Restoring Force Model of High-damping Rubber Bearing 1)

Regarding the modeling of restoring force properties of high-damping rubber bearings. employment of the hi-linear Model, I~amberg-osgood (R-O) Model and Rate Model may be considered To explain more precisely the restoring force prop- el-ties, we show these three models which are modified to a displacement dependent type. In each model, a restoring force property was indicated with one skelton curve (common to each model) and hysteresis loop which was established By each model. The skelton curve and hysteresis loop of each model were established so that actual restoring force properties could be reproduced as faith- fully as possible. Fig. 6. 1 shows the restoring force property gained by each analysis model The figure shows results for a regular input (sine wave of oscillation 0.5 Hz) and the analysis result shows exact reproduced of test result at each amplitude level using the model modified to displacement dependent type. Especially, in cases the g-o and the Rate Model were used, a smooth hyster- esis loop was gained, which was closer to the actual one.

6.2 Earthquake Response Analysis 2)

We performed an earthquake response analysis for a base- isolated building in which high-damping rubber bearings were imployed, using each model stated above. Fig. 6.2 shows analy- sis results along with test results. The results were gained by input of the E1 Centro US wave, 12.2 m/s: (equals to actual 3. 21 m/sJ) to base-isolated building (refer to Fig. 5. 1 , reduced scale model). The top chart in Fig. 6.2 is input acceleration, 2nd and 3rd chart is response acceleration at the ground and the top floor respectively, and bottom chart is response displacement of the rubber bearing. (Each value is a reduced scale value and to gain actual values conversions are necessary, such as accelera- tion to 0.263 times, displacement and time to 1/0.263 times.) Each analysis result shows comparatively similar test results. For evaluation of the seismic isolation performance of the building, no large difference was fonnd and values close test results were found. We also calculated floor response spectrum but the result from the hi-linear Model was a larger response value gained in an overall wide frequency area. However, the results gained by using the R-O Model and the Rate Model showed comparatively excellent reproduction. Therefore for the evaluation of not only seismic isolation performance of buildings but also floor response spectrum the loop shape of each model aflrkcted the results.

REFER EN CES

1) Fujita, r., SuzukL S., and Fujita. S-, The 1989 ASME Pressure Vessels and Piping Conference. VOl. 181. pp23-28, · 989

2) SuzukiS. et el., Trans of JSME, VOl. 57, No536, SeT C, pp1129-1136, 1991 (in Japanese)

7. DURABILITY FORECASTING SYSTEM (LALDA) 1),2)

One of the most important factors in realization of a base isolation system is the establishment of the longevity of rubber bearing effectiveness. Rubber bearings support vcritical com- pressive building loads under usual circumstances. but moreover in an earthquake they must follow the horizontal large move- ment of buildings, which produces the intense loca] sterss and strain inside the rubber or at the interface between rubber and steel plates. The maximun local strain sometimes amounts to several hundred percent during a major earthquake. In addition, rubber bearings are exposed to atmosphere over long periods of time, neroely oxygen, ozone and sometimes the sun, all of which produce enviromental degradation in rubber and steel plates and their interfaces which in turn effect the performance of bearings. Since buildings are generally designed for life of around 60 years or so, it is required that the bearings they rest on have minimum serviceability of equal length. As a result, the theoretical and quantitative establisheat of the long term dura- bility of rubber bearings is an essential and indispensable l~ctor to the widespread realization of base-isolation systems. Prior to the development of high-damping rubbber bear- ings, Bridestone developed a durability forcasting system called LALDA (Life Assurance with Large Deformation Analysis). Flow charts for LALDA are shownin Fig. 7.1. In LALDA, the local maximum stress and strain which are yielded on deformed rubber bearings are simulated through a large deformation FEM considering the following factors : - lnitial compressive deformation by building load ; - Increase ofcompressive deformation resulting from creep and rocking motions of the building and - Large horizontal deformation during earthquakes. In addition, LALDA estimates the degree of chemical degradation which occurs in rubber, iron plates and their inter- face over 60 years. Many types of ageing tests are carried out for rubber and the bonding between rubber and iron plats. The degradation rate at room temperature can be estimated by extrapolating results obtained at a higher temperature, base on the concept of the Archchins plot, ie H log(R) RT +C Where R : reaction rate. H : activation energy, a : gas constant, T : absolute temperature and C : constant. In an actual experi- ment, K is given by the reciprocal of the period (tc) and the special properties are degradeted to some critical level (=reten- tion rate). The Arrhenius plot is represented in Fig 7.2. Combining these groups of data we are now able to deter- mine the safety factor concering the durability of rubber bear- ings, thereby making possible the design of the base isolation systems with total safety.

REFER EN CES

1) Fukahori, Y, Nippon Comb Kyoukaisi, Vol. 60. No 397. 1987 (in Japanese) 2) Kojima, H- and fukahorL Y., Rubber World VoL 202. NO. 35 (1990)

8. A LARGE DEFORMATION FINITE ELEMENT ANALYSIS

8.1 A Large Deformation FEM

Bridgestone developed a new numerical method for the stress-strain and analysis of rubber belvtl~il~lgs under large defor mation. the first of its kind in the world. This method. called "Large Deformation ELM". supports the main function of LALDA and is capable of handling nonlinear elasticity and incompressibility of rubber-like materials under large deforma- tibn in a two or three dimensional condition. A few examples of the calculation are given in Fig. 8.1. Fig. 8.2 and Fig. 8.3.

8.2 Safety Design of Rubber Bearings

Typical dowel type rubber bearings shown in Fig. 8.4 common to the U.S. and New Zealand, are neither fixed to a foundation nor the building they support. Dowel-type bearings were originally used to avoid the large 1oc~xl strain which appears at the upper left and lower right of the bearing in Fig. 8.4. However, such bearings produce new problems because of their unstable structure. l~-irstly. quite a large local strain appears at the upper right and lower left of the bearing in Fig. 8.4 under simultaneous large compression and shear. In addition, the disconnected nature of dowel bearings induce rocking during earthquakes. In contrast Bridgestone's high-damping rubber bearins are bolted firmly to both a build- ing and its foundation. and designed to minimize the maximum local strain produce in the bearing even under credible deforma- tion over 60 years. (Fig. 8.5)

REFERENCES

1) Fukahori. Y. SekL W.. Composites '86 Proceedings of the Third Japan- U.S. Conference on Composite Materials. NO. 397, 1986

2) Seki, W. and Fukahori. Y. et al. Y-. Rubber Chem. Tech. Vol. 60. NO. 856. ·987

9. APPLICATION

9.1 Base Isolation for Buildings

The l~rst structure in Japan using bese isolation was con- structed in 198 I Since then about 50 such structures have been built. Most early base isolation devices consisted of natural rubber bearings and elastic plastic dampers. but in recent years structures using high-damping rubber bearings have increased because it is more economical. easier to construct. and more maintainable. Photo 9. 1 shows the installation condition of the rubber bearings in the largest building in Japan (the Second Computer Center of Tohoku Electtrie Power Co.. Ltd.: 10.032m~). which uses high-damping rubber bearingS. Table 9.1 lists some exam- ples of buildings using rubber bearings of Bridgestone Corpola- tion. which has so far delivered rubber bearings to more than 30 buildings. Of all base isolating rubber bearings. those for nuclear power plants have the severest performance and quality At present. Central Research Institute of Electric Pwer Indus try is conducting various basic evaluation tests of rubber bear ings as a national project. Those tests evaluate the performance of natural rubber bearings. and high-damping rubber rubber bearings with lead core in it. intended for a base isolation for nuclear power plants. Photo 9.2 shows Bridgestone's natural rubber FBR-type nuclear power plant. One of the largest rubber bearings in the world, it has a diameter of' a rated 160Cm. a rated mass of 500tons. a natural period of 2 seconds. and a maximum deformation of lOOcm. 9.2 Base Isolation of Bridges'~ Generally. bridges are composed of the superstructures (roads and beams) and piers which sLipport the superstructures. Seismic isolation of bridges is mainly aimed at protection of this pier structure. This purpose is different from that of seismic isolation of buildings which is mainly intended to protection of' the equipment and furnishings they contain. In the case of' bridges. shaking caused by an earthquake shakes the superstruc- ture. creating an inertia of horizontal movement at the crest of piers. This horizontal force generates an strong bending moment at the root of piers causing their breakage. Therefore. for anti-earthquake design. the most important task is to make the substructure (piers) of bridges sal~ t~z>r this bending moment The basic three ideas of seismic isolation of Bridge are as follows. (see Fig.9. I )

(1) Disperse force of inertia of superstructure.

In conventional bridge constructed with fixed bearings and movable bearings. horizontal forces concentrate on the fixed part at the time of an earthquake. The counter- measure is to disperse the three of inertia all over the piers. using bearings which have deforming capabilities and resto r i n g [b roe.

(2) Decrease force of inertia by prolonging inherent cycle

(3) Decrease shaking and displacement degree by absorption

Replacing conventional bearings by high-damping rubber bearing which supports the superstructure at the top of piers is considered the most suitable approach to seismic protection [br bridges. The properties of high-damping rubber bearings exclude detrimental shaking caused by wind and braking load and display seismic isolation effect. prolonging cycle. as earth- quake scale becomes larger.

9.3 Floor Seismic Isolation System Using Multi-stage Rubber Bearing 2)

Seismic isolation bearings for floors and equipment. carry a load off several tons per unit. This is very small load for rubber bearings. To achieve a natural period oF about 2 SeC. required for seismic isolation for such a small load. in a single rubber bearing it would be very bulky and unstable. The "Multi-stage rubber bearing" was developed to offset this shortcoming. (refer Fig. 9.2) A single isolation unit is composed of small stable rubber bearing fixed on four corners of a stabilizer. The structure prevents small rubber bearings from rotation under horizontal deformation and maintains stability against buckling. Such a floor seismic isolation system can also employ high-damping rubber bearing. Photo 9.3 shows a floor seismic isolation system on a shaking table. In the shaking table test input wave correspond to the response in the level of the building in which the base- isolated floor System was located for an earthquake and is calculated by modeling of the building. The results demonstrut ed that maximum acceleration on the isolated floor was reduced to 122 Gal. although maximum input acceleration was 450Gal.

9.4 Shaking Control Apparatus for Multistory Building

Flexible structures such as skyscrapers and towers are designed to be survive earthquakes by prolonging their inherent cycle. However. the comfort of inhabitants in the face off shaking caused by strong wind and after earthquakes. is a problem. To deal with this problem 2 kinds of shaking control apparatuses were developed and are now going to be put to practical USe. (Fig. 9.3)

(1) Tuned Mass Damper with Multi-stage Rubber Bearings 3)

A tuned mass damper (TMD) is an apparatus (installed at the top of a building) which is tuned to harmonize with the natural frequency of the building. It shakes sympathetically with the shaking of the building to lessen its motion. Design of/reD systems to date includes one which uses water sloshins in a water tank and another a steel mass roller which mechanically rolls. However. both methods have shortcomings. The water tank method is too bulky at the level necessary to be effective and the roller method, since it has friction. does not roll until shaking reaches a high degree. TMD using multi-stage rubber bearings supports heavy loads and since large deformation is generated by shearing of the rubber. an ideal apparatus which is compact and has almost no friction can be materialized. The mass of TUb is around I% of the effective mass of a building Into openings of each stage of the stabilizer oil dampers are inserted. Photo 9.4 shows a TMD model on a shaking table undergoing testing which conllrmed its effectiveness.

(2) Visco-plastic Rubber Dampern 4)

Visco-plastic rubber damper is increases the damping of a building. Fig. 9.5 shows a hysteresis curve made by shear deformation of this damper. The shape of the curve is almost a perfect oval and shows ideal visco-elastic property. Shear defor- mation dependences of stiffness and damping are little and equinelent damins factor is very large (h=,~ 0.4). Therefore effective shaking control can be expected even against small shakes. To measure the effect of a shaking control apparatus using this new material, we made a five story building model. In evey story a video-plastic rubber damper was installed. Measurements were taken of the response of the building at the time input cartquake wave was gradually increased The results showed that the response acceleration of the 5th story was reduced to 1/ 2-1/3 by daniplab apparatus. As the response acceleration was proportionate to the scale of the input earthquake wave, linear analysis and design of video-plastic rubber damping systems is possible. The video-plastic rubber damper is effective f~k~T even small deformations and makes very little noise. And this damper requires no container and has excellent durability. Furthemore, a compact apparatus is possible because energy absorption per unit section area is large.

REFERENCES

l) Kawashima, K et aL, Proceedings of the 43rd ~4nnual Con f- JSCE, 1988 (in Japanese)

2) Morikawa, S. et al., International Meeting an Earthquake Protection of Building, 1991

3) Fujita, [. Matsumoto. Y.. and MasakL N. TranS. of JSME. Vol- 56. NO. 523. SeT. C. pp108-113. 1989 (in Japanese)

4) Fujita, S et al. I l th SMiRT. 1991