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.
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 Ⅷ 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 Ⅷ, 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 Ⅷ 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.
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 (