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INTRODUCTION OF STRUCTURAL ASEISMIC

 RESEARCH IN CHINA IN RECENT FOUR YEARS

LUO Qifeng, SHI Chunxiang, CAO Binzhen, YAN Shulong and XIONG Hua

Research Institute of Structural Engineering and Disaster Reduction, Tongji University, Shanghai 200092, China

In recent four years, aseismic theory and practice in China, which mainly include structural aseismic theory, aseismic analysis and design of reinforced concrete structure, special structure, general structure, lifeline system, structure foundation, structure vibration reducing and isolation, have made much progress.

 

I.  STRUCTURAL ASEISMIC THEORY

1. The development of Earthquake Engineering

In a general frame of the development of Earthquake Engineering in the word, Hu (1999) reviews the 50 years' development of the field in China. According to author's point of view, the development in China may be reviewed in 3 stages, namely the beginning stage in 1950s-1960s, the developing stage in 1970s-1980s, and the spreading stage in 1990s. Finally, a few problems of current common interest and the possible trend of development in the early new century are raised. Hu and Zhou (1999) brief the experiences and lessons learning from the destructive earthquakes in China and abroad in recent years. It is pointed out that the damage action of the large velocity impulse in ground motion in near field should be considered in seismic design. Numerous earthquake examples show that there are many weaknesses in aspects of earthquake protection of high rise building, overpass bridges, and other infrastructures and industry facilities. Damaging of these structures and facilities could result in tremendous economic losses and social impact. It is stressed that earthquake disaster mitigation being as a scientific and engineering branch should be studied by systematic engineering methodology in order to reduce earthquake disaster as possible. In addition, this paper stresses that in order to acquire new breakthrough in earthquake engineering field in the new century, it is very important to explore high performance low-cost earthquake protection measures and to produce close cooperation among seismologist, civil engineer, industry and material expert, social scientist, economist and management expert.

2.  Seismic Resistance of Engineering Structures

Wang (1999) makes some comments on the present Seismic Design Code for Buildings (GBJ11-89). The latest achievements and focuses in the seismic design code development in the world are concluded. It is suggested that the following subjects be required to carry out for the model code of early new century for the seismic design of buildings: the earthquake risk assessment realized by replacing the seismic intensity by the seismic zoning parameters, and by determining the risk levels by return periods or exceeding probabilities; the earthquake design level and the earthquake performance level for varied structures; the improvement of the current seismic design spectra involving the long period components and the near field effect of strong motion; the practical use of energy spectrum, displacement spectrum as well as the accumulated damage spectrum; the methodology of seismic analysis of structures; the displacement-based criteria of structural damage and collapse, etc. The model code of early new century for buildings would be a guideline of seismic design codes for varied engineering objects. Regulation of Building Seismic Strengthening Technique (JGJ116-98) is a mandatory standard approved by the ministry of construction. It comes into effect on March 1st in 1999 year. Li (1999) briefly introduces the compile basis, principle, aim, scope of application and main contents of regulations. The Code for seismic Design of buildings (GBJ11-89) has been revised. Dai (2001) briefly introduces some major improvement in this revised edition. They are the new zoning map of design parameters of earthquake motion, the modification of design seismic response spectra, the torsion analysis and deformation checking for building structures, the requirement for regular building, the control axial pressure of RC columns, the detail measure of end in RC structural wall. The design method has proposed for some new structural system in this revised edition, which are seismic steel structures of tall buildings, seismic RC tube system and isolation earthquake of multi-story structures, and so on. Hong and Xie (1999) presents the decision-making method of earthquake-resistant fortification standards of engineering structures, which is based on the initial cost analysis and the earthquake loss analysis of structures. The relation between the structural initial cost and the design intensity is established, and the method of estimating the earthquake loss is presented. The analytical expressions of the optimal design intensity and return period are deduced by using the decision-making method, and therefore, the conclusions of great significance to earthquake-resistant design are drawn. Gao et al. (1999) discusses the anti-seismic levels for buildings of various importances by using analytical method of engineering policy-making. The engineering costs and earthquake damage and to type B and type D buildings are analyzed, and optimal anti-seismic levels for buildings of various importances put forward. It is an important and essential point in the field of earthquake engineering to assess seismic damage of building structures. The study on seismic damage assessment becomes more and more urgent after many buildings suffered severe damage in 1995 Kobe earthquake. Wang and Liu (2001) aim at to review and discuss systematically accomplishment of seismic damage assessment of building structures in three levels of material, element and structure. Ye and Zhou (2000) deal with the simplified methods for seismic response analysis of building structures. Some approximate procedures are studied, including the pushover analysis, the capacity spectrum method and so on. Some important problems are discussed, and an improvement is made on the capacity spectrum based on the research. With analysis of a typical structure, it is shown that the method suggested by the authors may be in performance-based design as a simple and effective tool. Zhou et al. (2002) discuss the probabilistic meanings of three levels of earthquake protection in seismic design code for buildings, it is pointed out that except basic design earthquake intensity the probabilistic definition of the two levels of earthquake protection is ambiguous that can only be understood in average meaning. On basis of mutual relations among three level probabilistic earthquake protection intensities, which are considered in seismic design code, a simplified method for estimating design basic intensity of structures with different service period and acceptable hazard level is presented. The estimated design intensity corresponding to different return period in this way is unable to reflex the real situation of the local seismic of the considered site, but gives a general assessment of the basic design intensity. Hence it can be used as reference for determining design intensity of normal structures. In addition, the basic design accelerations and amplification factors in accordance with three level design intensities are also discussed. By reviewing the code provisions about the checking calculation method of structural elastic-plastic deformation response to major earthquake, Guo et al. (1999) give some suggestions with the aim to improve on it. The main reason for the difference between the calculated nonlinear seismic response and actual seismic response is analyzed. Based on a large number of test data, a series of elastic-plastic interstory drift index for R/C frame-wall structures, R/C wall structures and frame-tube structures is suggested.

3.  Structural Aseismic Design Theory

Cheng et al. (2000) discusses some key problems on performance-based seismic design. It is concluded that the future performance-based seismic design should contain the following characteristics. (1) The design process should consider two types of target performance levels, which reflect the “generality” and “individuality” of seismic design respectively. (2) The reliability-based format is used directly, and the application of reliability in member level should be improved to that in structural system level considering different seismic performance requirements. (3) Reliability-based structural optimization procedure under cost-benefit criterion should be employed. The theory of performance-based seismic design is a new concept in the world in 1990s. It is a revolution of the current seismic design procedures. Li et al. (2001) introduce the development and main properties of performance-based design theory. Its importance to improving the seismic design theory is also pointed out. Zhang (2001) introduces the recent development situation on the certainty seismic design theory and the reliability seismic design theory based on the structural performance. The existing questions in these theories that are based on the deformation failure criterion have been reviewed. The damage of buildings from the 1995 Hyogo-ken Nanbu (Kobe) earthquake indicated the necessity of structural design to meet the performance requirements of an individual building. Xiao (2000) introduces the activities associated with the Integrated Research Development Project (1995-1998) organized by Ministry of Construction, Japanese Government, and current efforts to revise the Building Standard Law toward the performance-based design format in Japan. Luo and Zhang (2002) put forward the temporal-frequency response spectrum method for analysis of strong ground motions. Temporal-frequency response spectrum is the response history of a series of single freedom quantity point system, which have same ramping and different natural period, under the effect of strong ground motions. It is amplitude-time-frequency 3-dimmensions spectra. Cao et al. (2001) put forward a new thought which integrates wavelet theory and random vibration theory to solve the problems in earthquake engineering, by reviewing the history and characters of wavelet theory, and pointing out the problems which exist in structural earthquake-resistant theory. Shi and Luo (2000) introduce a spectra analysis method, which is suitable for analyzing nonstationary and nonlinear process, and suggest a method to solve the end-swing problem during realizing HHT transform. Li (2000) discusses the problems in seismic design of structures based on reliability theory. These problems include the aim and objective of seismic design, principles and standards of seismic design, statistics of seismic effects, and the seismic design approach of structures based on reliability theory. The author suggests the solutions to some of the problems though some of them still remain to unsolve for further study. The ideas proposed in this paper may be useful for revision of the national code for seismic design of buildings. Lü and Wang (1999) give the material model of minimum-cost design of seismic structures based on damage performance, and its characteristics and difficulties are analyzed. Then the material model of optimization is translated into an artificial evolutionary model, and the intelligent optimum design of seismic structure is realized via genetic algorithms. The results show that genetic algorithm is an effective way to attain the goal of structural intelligent design, and its application prospect appears to be highly promising.

 

II.  REINFORCED CONCRETE STRUCTURAL ASEISMIC PROBLEMS

1.  Whole Structure

Xiong et al. (2002) point out that the subsection curve and the initial damage value should be adopted in a damage model by combining the theory of damage mechanics with the experiment results of tensile stress-strain curve of plain concrete, the existing tensile damage models of concrete are discussed. A new and revising subsection curve damage model of concrete is given out. Cao and Luo (2002) briefly discuss the seismic performance of unbounded prestressed concrete structures, and make a conclusion that if the design and construction of the unbounded prestressed concrete structures are right and desirable, they have similar seismic performance as bounded prestressed concrete structures. Lü and Lu (1998) develop the nonlinear time history analysis method for RC frame structure in which ‘a shear type model' is used for the whole structure and ‘a semi degrading trilinear model with a descending branch' is selected as restoring force model for the interstory columns. Jiang et al. (1998) give elasto-plastic seismic response analysis of frame-wall structures by door-like elements for frame and four-spring elements for shear wall. Hu and Huang (2000) propose a nonlinear finite element model, which is suitable to analysis of seismic performance of prestressed concrete. In this model, a concrete structure would be divided into bar units. Du et al. (1999) study the inelastic response of structure having plane eccentricities by non-coincidence of centers of mass and stiffness, and of reinforced concrete frame under bi-directional ground motion. Jiao and Tao (2000) find a damping modification factor formula of response spectrum. Xü et al. (2002) recommend a steel bar index, a ratio of partial prestress and a limit coefficient of tensile stress in a prestressed concrete frame. Wang et al. (1998a; 1998b) design two new energy dissipation devices. Xü et al. (2000) present the detrimental effect on aseismic behavior of shear wall structure with some supporting frames when the level of transfer story is arranged at higher level. Hu et al. (2000) consider the frame with special-shaped reinforced concrete column of large bays. Cao et al. (2000) consider high-rise bracing frame with special-shaped columns of large bays and rigidity variation along the height. Wang et al. (1999) consider a mid-high rise big-bay RC frame with special-shaped columns. Huang et al. (2002) consider an infilled frame with special-shaped columns, all have done experimental studies on aseismic behavior of respective structures. Yuan (1998) presents an analytic method for earthquake-resistant capability of a damaged concrete frame structure .In the method, a damage index D is served as a measure of a member's residual energy dissipation capacity. He and Ou (2000) introduce a design of structural seismic damage performance based on improved capacity spectrum method. Zheng and Huang's (2001) testing results indicate that there is not obvious difference in aseismic behavior between self-compacting concrete frame and RC frame. Zhang and Sun (2001) adopt a simplified limit analysis approach to determine the limit state and its limit bearing capacity of RC structure, the nonlinear bearing capacity is equivalently linearized to some linear capacity, which makes the random seismic response calculated linear. Zhuang (2002) deems that the shell elements to simulate the flexible floor slab are more accurate in analyzing atrium structure systems. Li et al. (2002) present an analysis method of frame concerning its felted slippage under repeated load, and get M-φ)relation of nonlinear reinforced concrete structures by using numerical value integration method. 

2.  Partial Member

Cao et al. (2000) perform the aseismic property of the RC shear wall with rim and two-storied concealed bracing. Zhang et al. (2000) carry out the experimental research on the aseismic property of the RC shear wall with two-storied concealed bracing and slits. Dong et al. (2001) think about the influence of the aseismic effect on the RC shear wall with concealed bracings when the reinforcement proportion of the concealed bracings is changed while the amount of the overall steels remain the same. Jiang and Lü (1998) provide a multi-component-in-parallel model of shear wall structure. Li et al. (2000) give  test results, which show that the horizontal bearing capacity of a frame-supported continuous masonry wall is less 20%30% than the ordinary masonry wall resting on the ground. Zhang et al. (2001) solve nonlinear seismic response of RC shear wall structures based on pattern of self-equilibrating stresses. Cao et al. (1999a; 1999b; 2000) considering T-shaped columns, 十-shaped columns and L-shaped columns of smaller shear span ratio ( =2.375), Hu et al. (1999) make experiment to analyze the aseismic behavior of L-shaped columns of shear span ratio ( =3.833), and then they suggest to set concealed column for rasing seismic capacity of column. Zhao et al. (2000) analyze the influence of wrapping area and wrapping manners of carbon fiber sheet on strengthening the RC columns for their ductility. Li et al. (1998) suggest a simplified calculating method for the ultimate loading capacity of laminated columns with high strength concrete containing steel tube.

 

III. SPECIAL STRUCTURAL ASEISMIC PROBLEMS

1.  Study of Vertical Fluid-Storage Tank

Sun et al. (1999a; 1999b) establish analytical model of base rubber isolation for flexible cylindrical steel fluid-storage tank, present the vibration equation for the system and the analytical method for solving the shear force and overturning moment. They also establish a test model of the base rubber isolation and make the test of earthquake simulation. Sun et al. (2000) present a simple analytical model and put forward equation for the rolling auto-recovering isolation of the fluid storage tank, they establish a test model and make the earthquake dynamic test on it. The results show that isolation system for mitigating short period earthquake excitation is effective. Wang et al. (2000) make a research on engineering method of isolation design for stand steel storage tank, they give an idea that use response spectrum method in normal design to perform seismic isolation design. Yang and Luo (2002) introduce the capacity spectrum method into aseismic analysis of petrochemical equipment, and give one demand spectrum curve for transforming ground spectrum into capacity spectrum.

2.  Study of Dynamic Characteristics for Tall Structures

Cheng finds the law of the influence of P-Δ effect to the structural dynamic responses by through investigation of five types of structures. On this basis it is suggested that the influence to elastic dynamic responses of structures is not always unfavorable, such influence is not significant. For those tall and slender structures (such as TV towers, chimneys and tower shaped tall buildings) with drastic decreasing of mass and stiffness at the top of portion, the elasto-plastic responses should not be disregarded. Hu and Wang (2000) discuss a steel space-truss structure of LuoYang city. The difference of the response of the structure between taking account of the influence of geometrical nonlinear and linear is studied when it is acted by the wind loading, and then the dynamic character of the structure is analyzed. By using step-by-step integration procedure, they calculate the seismic response of the structure subjected to the three-dimension and single-dimension seismic actions considering geometrical nonlinear and linear. Li and Xiao (1998) give the mechanical model of soil-pile-structure interaction of the transmission tower system and the equation of motion, compile the program for computer and use the program to calculate the earthquake responses of an actual transmission tower. Zhang and Yu (1999) measure the acceleration response of CCTV tower, Shanxi TV tower and Tianjin TV tower in environment vibration. The mode frequencies and the damping rations of all three towers are obtained by using the method of identification.

3.  Lifeline System Aseismic Problems

Guo and Fen (1999) present a simple method for aseismic analysis of pipe crossing fault. Some current earthquake countermeasures for pipe are proved by numerical calculation. Fen and Zhao (2001) consider the interaction of pipes and soil when they conclude the buckling responses of buried pipes to fault movement, using the Finite Displacement Analysis Method with thin shell elements. Du and Chen (2000) consider that it is difficult for fuel assembly in nuclear power station to avoid nonlinear seismic response induced by fiction, slip deformation and impact under strong earthquake, they proposed a seismic response analysis method in which the factors mentioned above are considered based on the nonlinear dynamic behaviors of fuel assembly given by the experiment on shaking table. Liu and Sun (1999) discuss the several problems on asismic performance appraisal of the using electric equipments of nuclear power station and put forward some methods to resolve them. Li et al. (1999) discuss the characteristics of damage of electric power system in previous earthquakes. According to the results of analysis, the seismic reliability of high –voltage electrical equipments, high voltage substations and the high–voltage power system, several suggestions on improving aseismatic capability are presented. Zhang et al. (1999) present a reliability analysis method of water supply pipe net system, and they edit reliable software for analysis. Liu and Wang (2001) use the method of extremum of product function to analyze upper bound seismic base shear force of existing over pass and the calculation formulas are gotten. Wu et al. (2001) proposed a new method based on Newmark-βmethod and modified Newton-Rapshon method to solve geometrically non-linear seismic response equation. The linear and the nonlinear seismic response analyses of long-span cable-stayed pipeline bridge are discussed with this method. Sun et al. (2000) investigate the dynamic characteristic and response of the pipeline systems conveying fluid and supported above the ground considering the interaction between the media and the structure. With respect to certain media and structural parameters, some preliminary conclusions are obtained about the effect of the fluid flowing on natural frequency and the dynamic response.

4.  Others

Wang et al. (2000) make experimental study on aseismatic properties of high strength concrete adqueduct bentframe. Based on experiments of three specimens of C50 high strength concrete bentframe models suffering from horizontal cyclic load they deal with aseismatic behavior of the high concrete aqueduct bentframe from several aspects, which are experimental phenomena, failure characteristic, hysteretic curate and ductility coefficient. The results show that aqueduct bentframe made of high strength concrete has a good aseismatic performance and the structure ductility meets the aseismatic requirements of engineering on the given condition of this experiment. Liu et al. (1998) make research on the soil-structure –liquid interaction in order to identify the failure mechanism of the caisson quay wall. Wang and Xie (2001) have made strong ground motion observation on some concrete dams, such as Danjiang dam, Gezhouba dam, Xinfengjiang dam and Tanling dam, they introduce the instruments, measuring method used in vibration observation of a prototype concrete dam, and the obtained preliminary results. Du et al. (2000) combine a contact force model simulating the contact condition between the contact interfaces with an explicit finite element method in which the artificial boundary condition is a transmitting boundary is used to solve the difficult problem that the nonlinear behaviors of geological structure are unavoidably existing in the near-field foundation for every dam site and the effects of the energy dispersion in the infinite foundation are also important for the seismic response analysis of arch dams.

IV. GENERAL STRUCTURAL ASEISMIC PROBLEMS

1.  Aseismic Problems of Brick Adobe Houses

Zhao (2000) analyzes the reasons of failure occurred in rural buildings during two recent earthquakes. The importance of earthquake fortification for rural buildings in region of intensity VI is indicated. Some necessary fortification measures are introduced, which can be used for reference in the design and construction of new residential buildings in villages. Dou et al. (2000) analyze the reason of earthquake damage through field investigation of earthquake disaster in the Zhangbei area of Hebei province. Aiming to the situation of local earthquake activity, the seismic protection measures of rural buildings were proposed. Based on the kinds of common houses collapsed easily in earthquake zones. Ding et al. (2000) conduct the seismic simulation shaking table tests for single story brick masonry and adobe house models with and without simple collapse resistant measurements. From the test results, the effective of their suggested simple measurements are obvious. The findings provide a scientific basis for mitigating seismic damage of rural houses in seismic zones.

2.  Seismic of Brick-Concrete Composite and Masonry Buildings

Zhang (2000) discusses some problems existed in the seismic design of brick-concrete composite buildings according to the basic principles of conceptual design. Hu (1999) explains the diversified reasons that cause light damage to multi-story brick buildings showing similar phenomena. This will bring difficulty to the investigation and appraisal of engineering projects due to earthquake damage. Several types of destroyed multi-story brick buildings and several special features of earthquake damages are introduced as local points. It is also mentioned that three factors should be paid attention in distinguishing earthquake damages. This can be used as a reference for the investigation of post-earthquake damages to buildings. Huang et al. (1999) analyze the horizontal seismic effect on unit floor area and seismic bearing capacity for brick wall on unit length for multistory masonry buildings. Designers can obtain the results by consulting the tables and simple calculation. Zhang et al. (1998) analyze the examples of the masonry buildings with R.C. framework bottom floor by following three methods respectively, bottom shear force method, response spectrum technique and elastic step-by-step integration. The bottom floor seismic shear force obtained by the three methods was compared, and then the calculating formula for the amplifying coefficients of the bottom floor seismic shear force was proposed. Shi et al. (2000) carry out experiments of longitudinal wall piece between windows using DS type of porous masonry units. The window openings of wall have several different sizes. Reinforced concrete tie columns are located in walls between windows, and there are horizontal steel reinforcements in some wall specimens. The experiments results show that the shear strength and deformable capacity of walls can be increased and the seismic behavior of the walls can be improved due to putting the reinforced concrete tie columns and horizontal steel reinforcements in the wall. Based on the quasi-dynamic shearing tests of stonewall, the practice experiences and seismic disaster, Yu et al. (2000) give aseismic design method of stone structure. The reliability and practicality of their suggested methods are verified. In order to study the aseismic behavior of six-story masonry buildings with small-size concrete block in intensity area, Zhou et al. (2000) carry out the test of a 1st/4 scale model circle static load and shaking table tests. Some results, such as earthquake-resistant capacity and collapse mode, are obtained. A nonlinear resistant model, based on the test results, is able to analyses the model building and the analytical results are in good agreement with the tests. The anti-seismic performance of prototype building is evaluated by the nonlinear analysis and similarity theory. In order to study the possibility of constructing nine-story masonry building with concrete block in intensity , Miao et al. (2000) propose a new bearing wall system of small-size hollow concrete blocks, with installed tie columns and grouted reinforced cores, and a 1/4 scale model was built. The circle static load test and shaking table test were carried out. Some results, such as earthquake-resistant capacity and collapse mode of the test model, are obtained. The test results show that the concrete block masonry buildings with installed tie columns and grouted reinforced cores have good seismic performance and the construction of nine-story concrete block masonry buildings in the regions of intensity is quite feasible. Yang et al. (2000) make research on the hollow concrete block wall with construction-core column system. The aseismic behaviors of these structures have been studied by contrast testing. The test results show that the earthquake-resistance of concrete block walls was improved obviously by the use of construction-core column system.

3.  Others

Cai (1999) presents the revised contents related to the steel structure buildings in Code for Seismic Design of Buildings (GBJ11-89). Based on structures with a typical configuration but different stories as well as 80 suitable seismic records, the statistical amplifying factors for estimating the elastic-plastic displacement of braced and un-braced steel building frames subjected to severe earthquake have been obtained through a great number of numerical calculations. Li et al. (2000) propose a table of factors with reasonable reliability for elastic-plastic displacement check to implement aseismic design of multistory and tall steel building in practice. Yu et al. (1998) introduce aseismic analysis methods and mechanical models of tall buildings based on program DASTAB. The newest developments of structural analysis of tall buildings are also introduced. Cao et al. 1999) make research on the T-shaped column with concealed column. Based on the tests of seismic behavior of 4 T-shaped columns with small shear span ration(λ=2.375), the influence of axial pressure ratio on bearing capacity, rigidity, ductility and the function of setting up concealed column in raising seismic capacity of T-shaped column are analyzed. And finally, theoretical calculation is completed. The calculating results agree with the measuring ones. In connection with the characteristic of eccentric buildings and elastic-plastic analysis, Cai et al. (1999) give a simple computational space model. Based on this model, a direct time history elastic-plastic seismic analysis method is presented. At the same time, a special computer program ZZC is coded, which can solve the calculation problems of different kinds of eccentric buildings by considering the effects of floor deformation. Using the orthogonal lateral resistant dynamic model, Cai et al. (1999) analyze the elastic seismic response of asymmetric multistory buildings with different eccentric conditions such as first story, middle story, top story, or all stories are asymmetric, respectively. The effects of eccentric ratio, fundamental lateral frequency, the frequency ratio of lateral to torsion, and lateral frequency of orthogonal direction on the structure nominal base and maximum shear coefficient in eccentric story are researched. Lin (2001) designs a multistory structure model and several secondary structure models to study the seismic response of secondary structure and the interaction between main building and secondary structure set on the main building. The experiment is carried out on the shaking table and the results show that the secondary structure may affect the vibration in different ways. Depending on the parameters of secondary structure, the response peak value of main building with secondary structure may become lager or smaller than that of main building without secondary structure.

 

V. FOUNDTION ASEISMIC PROBLEMS

1.  Foundation Aseismic Design

Fan (2000) introduces the basic requirement for subsoil and foundation design in the aspects of building location, choice of foundation type, seismic calculation of natural subsoil and pile foundation, and earthquake resistant measures for poor subsoil. Under the seismic action, flat ground base, which is composed of soil and weathered rock and one-way surface slope of underground weathered rock is great, mostly results in the soil sliding on the surface of the weathered rock and the damage is always disastrous. Because of soil nonlinear elastic-plastic property, if general slip surface method were still used in analyzing the stability, the effect of the passive earth pressure of soil would be overestimated, Zhang and Wang (1999) analyze the stability of this kind base and give treatment method when nature base cannot satisfy the stability we need. Zhou et al. (1999) reviews soil sites classification and rating of the corresponding characteristic periods of design spectra that have been offered in the course of implementation of the current seismic design code for buildings. An improved scheme of seismic site classification is proposed which covers the ranges of the equivalent shear wave velocity and overburden thickness for 4 types sites and the continuous rating of corresponding characteristic periods. In addition, the necessity and feasibility of continuous changes of the characteristic periods in design spectra are discussed and the interpolation of the characteristic period across boundary lines of the site classification are given in the paper. Based on the relation between rotation (torsion and rocking) and translation components of ground motion, the artificial waves of rotation components are synthesized. In terms of 228 translation acceleration records recorded at 76 array during 1971 earthquake of San Fernando, America, Sun et al. (1999) analyze the average response spectrum curves of rotation component of ground motion for two types of site soil (hard and soft). The design rotational response spectrum curves proposed in this paper can be of practical use to earthquake engineering design.

2.  Saturated Soil Liquefaction

Based on the two-phase theory, the saturated soil is simplified as one-dimension model, Lu et al. (1999) present theoretical discussion on the expansion of liquefaction volume of the saturated soil. It is shown that the expansion of liquefaction volume retards with time and will slow down with the increase of density. The liquefaction volume will also develop faster with increase of dp/dn. Miao et al. (2001) study Rayleigh wave method to appraise the improvement effects of the gravel piles for liquefaction foundation. The measuring results of Rayleigh wave method are agreement with SPT's results. The relative formula of the shear wave speeds with N63.5 is presented based on the regressive analysis for measuring results of Rayleigh wave method and SPT. The research results are conveniently applied for Rayleigh wave method in the engineering. On the basis of virtual work principle, Lin et al. (2000) deduce the formulate for the floating response of buried pipeline, considering the effects of nonlinear soil constraint and the initial axial force acting on the nonlinear increment element method. Some results are given. Zhang (2001) makes investigation on seismic disasters and conclude that large horizontal residual post-liquefaction deformation occurred in level ground of structural sandy soil during the past strong earthquake, such as the 1964 Niigata earthquake and the 1995 Hogoken-Nambu earthquake in Japan. Liquefaction-induced ground deformation induced by seismic liquefaction can be divided into three types: (1) smaller pre-liquefaction deformation in shaking, (2) larger post-liquefaction deformation in shaking and (3) residual post-earthquake deformation. All three types of ground deformation should be properly considered in seismic design of a pile foundation in liquefiable level ground.

3.  Soil-Structure Earthquake Disaster

Chen et al. (2001) make shaking table tests of structure suppressing vibration control and including soil-structure interaction (SSI) effects. By the testing of four model structure on the rigid base and soft soil, the effects of soil-structure interaction on both the earthquake response of structure and the efficiency of suppressing vibration control for the top of super structure simulating tuned mass damping (TMD) are analyzed and discussed. Comparing the earthquake response of the main structure which is in the condition with different bases and the same earthquake ground motion input, the results of the tests show that SSI effects has the double action of reducing and increasing the efficiency of structure suppressing vibration control, its comprehensive effect is related to the frequency spectrum characteristic and peak acceleration value of input seismic waves. Because of the double action of SSI effect, the suppressing vibration control effects by TMD designed based on the condition of rigid base are not satisfactory in soft soil condition. Some factors affecting the efficiency of suppressing vibration control under the condition of soft soil ground are also discussed. Guo et al. (2001) study technique for earthquake rupture test of soil site. Two soil site model tests are conducted with seismic simulation shaking table. Some meaningful conclusions are drawn. Meng et al. (2001) present the state of art of the research on the residual strain and pore pressure of soil under earthquake loads. The existing problems in calculating the residual strain and pore pressure of soil are also pointed out and the key points of future study are summarized as well according to the current lab condition and bases. Dou et al. (1999) review the effects of soil-structure interaction and analysis methods. Some problems on soil-structure dynamic interaction are discussed. Ding et al. (1999) point out the limitation of program FLUSH for soil-structure interaction, and a method of transmitting boundary replacing consistent boundary is presented. That 2nd order transmitting boundary has the same accuracy as consistent boundary, and is better than 1st order transmitting boundary is shown by a typical engineering example. The reformed program WMNS-F1 is more practical than FLUSH.

4.  Seismic Damage of Pile Foundation

Liu (1999) gives a summary on seismic damage of pile foundations since 1960s, especially after Hyogoken-Nanbu earthquake in Japan (1995). The typical pattern and causes of pile foundation damage under non-liquefiable and liquefiable soil conditions are discussed. Some commentaries on seismic design of pile foundation and earthquake protection countermeasures are given in this paper. In order to investigate the seismic response of pile-nonlinear frame wall interaction system, there is need to compute the impedance functions of pile groups. Song et al. (1999) calculate the dynamic impedance functions of pile groups by using the impedance functions of single pile and dynamic interaction factors, and the characteristic of impedance functions of pile groups with different configurations in soft, medium and hard soil are discussed. These researches show that the impedance functions of pile groups are strongly frequency dependent and with the increment of soil shear wave velocity they increase greatly, but the soil shear wave velocity has a very little influence on normalized stiffness and damping of pile groups. Song et al. (2000) investigate the seismic response of pile-nonlinear frame wall interaction systems, in which door-like elements and four-spring wall elements and the impedance functions of pile groups are computed by using the impedance function of single pile and dynamic interaction factors respectively deal the frame and wall. By way of hybrid-frequency-time-domain procedure, the authors investigate the effect of frequency dependence of impedance functions of pile groups on the seismic response of structure. The authors also investigate the effects of soil-pile-structure interaction on the seismic responses of structures based on the shear wave velocity of soil and intensity of seismic motion.

 

VI. PESEARCH ON VIBRATION REDUCING AND ISOLATION FOR STRUTURAL ENGINEERING

1.  Research on Vibration Reducing

(1)  Inactive control 

Zhou et al. introduce performance of viscoelastic structure, analysis method of viscoelastic structure and its application in engineering structure (Zhou, 1998). They invent lead viscoelastic damper (Zhou, 2000), mixed steel yielding-friction energy dissipators (Zhou, 1999), the property experiment and comparative experiment are conducted. Hytseresis ring is also presented. Sun et al. (1999) design a new added stiffness circular ring energy dissipator (ASCRED); the property experiment of ASCRED under cyclic reversed loading is conducted. The working behavior and energy-dissipating behavior are investigated and the restoring model of ASCRED is set up. Wu et al. (2000) study the distribution problem of the maximum interstory shear force under mild earthquakes among structural members and viscous dampers. The relationships the maximum interstory shear force, the maximum interstory additional force and the maximum interstory shear force are proposed for structures with viscous dampers. A simple method of calculating the elastoplastic interstory drifts of structures with viscous dampers under strong earthquakes is suggested. Fan and Shen (2000) apply the vibration reducing system of viscous damper to reticulated shells. Numerical calculations of vibration reducing and properties experiment for reticulated shells are carried out.

(2)  Semiactive control 

On the basis of explaining the applied principle of semiactive control of ER controllable dampers for wind vibration and earthquake responses of engineering structures, Qu and Xiang present the suitable range of ER variable dampers in civil engineering structures (Qu and Xiang, 1998). Liu and Li (1999) propose a new method using variable stiffness for a semiactive structural control system; a shaking table test of semiactive structural control system and method using variable stiffness has been completed. Ou et al. (2000) design a smart piezoelectric-friction damper, and analyze the influence of damper's figure parameter. Two kinds of piezoelectric-friction dampers with the adjustable frictional force and imitated viscous characteristic are designed concretely. Qu et al. (2000a, 2000b) present a smart friction damper. Based on the classical LQR theory, they put forward a quasi-optimal control strategy for wind-induced vibration control of high-rise steel-truss tower using semi-active friction dampers. Zhou et al. (2000) present the properties and the mechanical model of magnetorheological (MR) damper. The parameters of MR damper are designed and a semi-active control algorithm based on the theory of modern optimal control is established. Hong et al. (2003) present the control approach to irregular building excited by multi-dimensional earthquake ground motion by using semi-active tuned liquid column damper (TLCD). Guo and Zhang (1998) design a passive control device - the pendulum oil damper (POD) system, the experimental study and theoretical analysis are carried out.

(3) Active control 

Sun and Liu (1999) present differential equations of the excited structure that equipped with ACWPS setup, and the control theory is also established. Simulation results demonstrate that the ACWPS is a promising structural control method. Li et al. (2000) present piezoelectric intelligent control device. An active control algorithm based on the theory of modern optimal control about the piezoelectric intelligent control device (an active control device for the response of frame structure induced by earthquake) is established. Jia et al. (2000) present an actual computing example for a tall structure using Tuned Liquid Damper (TLD) to reduce vibration. The results show that it is better to control the muti-mode structure responses simultaneous that only to control the first-mode response. Li et al. (2000) utilize multi-TLD for controlling the multi-mode responses of tall buildings and high-rise structures to reduce the structural responses to earthquake ground motion. The mechanical models and the equations of motion for the systems of tall buildings with differently installed ways of TLDs are established based on the method of the volume of fluid to solve the dynamic liquid pressure and the state equation. Wang and Gu (1998) put forward a simple method for the design of an active tuned mass damper for vibration control in tall buildings. Hu and Hu provide a new idea to determine the optimal parameters of tuned mass damper for reducing seismic response, design of dampers for multi-degree-of-freedom systems and effect of detuning of damper parameters are investigated.

(4)  Hybrid control 

Yan et al. (1998) propose the concept of multi-structural hybrid control system and the corresponding hybrid controller; the hybrid controller is composed of a passive damper and an active variable stiffness controller (DAVS controller). The state equation of a two-structural hybrid control system is established. Based on the instantaneous optimal control, the control law of a multi-structural hybrid control system is suggested. Zhu and Tang (1998) study the relative advantages of several passive control strategies including the separate or combined use of base isolation and TMD system to reduce the response of tall buildings through the use of traveling wave and power flow approach.

(5) Research on isolation

Rubber bearing 

Li et al. (1999) design a smart damper using electro-rheological fluid. It is combined with other isolation devices in isolation structure; a smart isolation system would be created. Zhao et al. (1999) present two kinds of close-loop feedback controllers for building structures equipped with base-isolation hybrid protective system by using the reaching law method of the variable structure system of sliding model control theory. The compound equation of motion of the above-mentioned hybrid control structure system, which is suitable for numerical analysis, has been constructed. Liu et al. (1999) provide the fundamental mechanic characteristics of Chinese lead rubber bearings, which include vertical, horizontal and extreme properties of rubber bearings. Han and Zhou (Han M. and Zhou X. Y., 1999) provide a type of the protective device, which consists of buffers installed in isolation layer as the second defence measure. A structural analytic model is set up in consideration of soil-structure interaction. The motion equations of non-collision are deduced. The program is established. Zhou et al. (1999) set up an analytical model for the rubber bearing combined with two different sections, formula of lateral stiffness of the combined rubber bearing is deduced. Authors also analyze the influences of the typical parameters for the combined rubber bearing. Jiang and Tang (1999) present the seismic response of a reinforced concrete frame building model with laminated-rubber bearings base isolation and with no base isolation through shaking table test. Liu et al. (2001) propose a method for analyzing the vertical stiffness of rubber bearings by a new concept of stiffness factor, and a simple formula is derived on the basis of stiffness factor method.

 Steel tubular bearing 

Sun et al. (1999), Zhang et al. (2000), Wang et al. (2001) give results of nonlinear analysis and test of the changeable stiffness base isolator with concrete filled steel tubular (CFST) short columns. The isolation function and energy-dissipation are discussed as well. The results show that the base isolator can dissipate energy and reduce the responses to the earthquake effectively. Xiong et al. (2001) propose a base-isolation device with concrete-filled steel tubular short column. Through pseudo-static test of the device the restoring force hysteretic curve is measured and its characteristic parameters is given. The inelastic analysis of earthquake response of a 7-story brick building is performed. Based on the results of shaking table test, the isolation effects of the device are examined.

Others 

On the basis of the equivalent process in pendulum seismic isolation structure, Wang and Su (1999) present an effective method of dynamic analyzing by using general program. Analysis results indicate that it is capable of reducing the seismic response of the structure obviously. Xiong et al. (2000) analyze the test results on full-scale brick masonry structure models with and without sliding base isolation. They deduce that the sliding base isolation structure is better than the fixed base structure in aseismic behavior. Li et al. (2000) present the differential vibration equations of shear-typed multistory buildings with FPS bearings, the isolation effects of FPS isolator and some horizontal and vertical vibration responses laws of the structures to horizontal ground motions are expounded.

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