Study On Seismic Performance Of Small Hollow Concrete Block Buildings

A study on the seismic performance of multistory small hollow concrete block (CB) buildings with core-and tie-columns which are widely used in China is presented in the thesis. In the thesis two major parts are included. In the first part several fundamental aspects concerning the performance-based design of structures and confined CB buildings are described. Chapter 1 gives an overview of the performance-based design and the seismic investigations and application of the CB buildings at home and abroad. Based on the analysis of data collected from the experimental studies performed in China a general expression for estimating the shear strength of hollow CB walls is suggested in chapter 2. Values of shift angle at cracking and ultimate stages and the ductility for different kind of CB walls are provided; the CB walls are classified into five categories according to their damage level. A four-linear skeleton curve corresponding to the damage category is developed. Meanwhile, the values of each characteristic point on the skeleton curve are also suggested. In Chapter 4 based on the damage feature observed from the shacking table tests of CB building models,with reference to the general standard of classification relevant to performance level of structures, a five-performance level classification for CB structures is established, and correspondingly a description of damage and the damage index in terms of interstory drift are given. Chapters 4 and 5 are devoted to the pushover analysis and the estimation of the target displacement, which represent the two basic components of the nonlinear static analysis (NSA) in implementing the structural performed-based design. Chapter 4 introduces fundamentals and methodologies of the pushover analysis focusing on lateral load pattern. A polynomial distribution is formulated from the curve-fitting of the test data collected. The approaches for approximate estimating the maximum nonlinear displacement of SDOF system proposed by different researchers are discussed and compared. It is found they all can be expressed in the form of the displacement modification factor. Numerical comparison shows all approaches yield results with minor difference and approximating to those obtained from the nonlinear time history analysis (NTHA) for elastic period over 1sec, while relatively large errors for the period within 1sec. In the second part principals,methods, basic relationship and parameters developed/provided in the first part are applied to numerical examples. A comparison of results of shacking table tests and the NTHA of a seven-story CB residential building model the availability of the analysis model, hysteretic model and relevant parameters adopted, and the computation program developed are verified in Chapter 6. Furthermore, NTHA for three typical CB buildings, one six-, one seven-and one eight-story, were conducted. Factors considered in the analyses include: site condition, peck acceleration level and direction of the input ground motions. The results of these analyses provide basis for comparison for the next chapter. From the NTHA it is shown that for the six-story building considered the aseismic requirement for the transversal direction can be basically satisfied, while that for the longitudinal direction under earthquake that occurs rarely can not be satisfied in most cases. For seven-and eight-story CB buildings the requirement of the seismic fortification intensity VII can be met. It seems an appropriate increase in the maximum story number and the maximum total height of CB buildings with core-and tie-columns is permissible. Chapter 7 illustrates the NSA for the same three typical CB buildings as used in the above chapter. In the analyses three lateral load patterns were adopted, and the results by using NSA is compared with those by NTHA. It is shown that: (1) The capacity curves obtained from NSA consist of three linear segments with two remarkable turning points, which represent the slight and the moderate damage characteristic points, respectively. (2) The capacity curve obtained from the inverse triangular pattern is in general more close to that of the NTHA than other two patterns. (3) The target displacement obtained from the pushover analysis by using the inverse triangular pattern combining with the improved capacity spectra method is relatively close to that from NTHA in most cases for site conditions I, II and III categories with error less than 25%;and higher error for site condition IV. (3) Due to the inherent approximation the NSA underestimates both the maximum interstory drift and the maximum inert-story shear forces. Therefore, it could overestimate the seismic reliability of structures. Finally, seismic deformation evaluation for two CB buildings is illustrated in Chapter 8. The methods used in the evaluation include: the NTHA, NSA and themethods specified in the China《Code for seismic design of buildings》(GB50011-2001) and the 《General rule for performance-based design of buildings》(CECS 160:2004). The results indicate that there are some differences in the aseismic performance levels predicted by different methods. Moreover, it is found the requirement for a structure, which is satisfied under minor and fortification earthquake, could not be met under major earthquake event. It is also shown that through details of seismic design made according to the《Code for seismic design of buildings》requirement for masonry buildings no collapse occurring under major earthquake could not be ensured. Therefore to evaluate deformation of CB buildings under major earthquake is necessary. Finally, the limitations both of the《Code for seismic design of buildings》and the《General rule for performance-based design of buildings》in estimating the aseismic performance were identified.