A Study On Inelastic Response Of Multi-Storey Buildings To Near-field Ground Motions

Abstract

With the advancement in knowledge of inelastic response of structures, the design and construction practices of reinforced concrete buildings have been changing worldwide. Most of the codes are incorporating the near-fault factors and performance based designs in the seismic codes. However, further investigation is needed to identify the physical behaviour of multi-storey buildings to near-fault ground motions. At present, quantitative evaluation of response and its mitigation to near field ground motions is a popular topic in earthquake engineering field. The present study discusses the inelastic response of symmetric and asymmetric multi-storey buildings to near-fault ground motions. The possibility of design approach is based on ‘expendable top storey’ for the multi-storey RC- buildings to near field records. If such behaviour is feasible one can conceive of a structure whose top storey is permitted and designed to undergo large inelastic deformations while reducing damage in the lower storey. The concept was first proposed in an earlier research (RaghuPrasad, 1977). Such a concept juxtaposes the often-mentioned ‘soft first storey’ concept. Further in this report, the performance evaluation of multi-storey buildings near Chiplun fault in Mumbai, India is also discussed. The thesis is organized in the following chapters: Introduction in Chapter-1 contains detailed literature review on inelastic response of symmetric and asymmetric buildings, response of buildings to near-fault records, elastic and inelastic vibration absorber concepts and performance based designs. The literature reveals that considerable amount of research has been carried out on the elastic, inelastic response of structures and vibration absorber concepts to ordinary ground motions. Recently, the effect of near field ground motions on the response of multi-storey buildings is gaining much importance. Most of the research publications are available on response of symmetric buildings subjected to near field ground motions. But many problems are yet to be investigated. They are, identification of important ground motion parameters in near fault records, vibration absorber concepts and torsional response of structures subjected to pulse type ground motions. These problems are clearly mentioned in the recently published state-of-the-art review by Shuang and Li-Li (2007). In this report an attempt has been made to solve these problems. Effect of near-fault ground motions on symmetric multi-storey buildings in Chapter-2, describes simplified non-dimensionalized equations of motion to study the response behaviour of multi-storey buildings to near fault records. The non-dimensionalized equations of motion are expressed in terms of near fault ground motion parameters. The objective is to find a relation between ductility demand and near field ground motion parameters through neural network approach. For this a neural network modeling was done to predict the ductility demand in terms of peak ground acceleration, peak ground velocity, epicentral distance and pulse period of the near field ground motion. A thorough sensitive analysis is carried out, to ascertain which parameters are having maximum influence on ductility demand. In this chapter further, a comparative study is made on the inelastic seismic response of multi-storey buildings to pulse type and non pulse type ground motions. The study shows that, it is necessary to consider the effect of near fault ground motions separately and make provisions for the design in the codes of practice accordingly. Vibration absorber effect in multi-storey buildings in chapter-3, discusses the behaviour of top storey as a vibration absorber during near field ground motions. For this purpose, a five storey symmetric building model is considered as an example problem to demonstrate the effectiveness of the proposed concept. Response of the structure is obtained for the various combinations of absorber storey parameters such as mass ratio, frequency ratio and yield displacement ratio. Here mass ratio means mass of the absorber storey to that of the bottom storey and similarly for the frequency and yield displacement ratios. Observing the storey-wise variation of these responses, we can say that for a range of mass ratios, frequency ratios and yield displacement ratios, the inelastic response of top storey is large compared to the lower storeys. This range is termed as ‘effective range’. Further, in this range the top storey absorbs the vibration energy of the structure by keeping the lower storeys in elastic state i.e. acts as a vibration absorber. The top storey can also be termed as ‘expendable top storey’. Effect of near-fault ground motions on asymmetric multi-storey buildings in Chapter-4, discusses the inelastic response of asymmetric buildings to single horizontal component and two horizontal components of near fault ground motions viz., fault normal and fault parallel components. For numerical investigations eight building models are considered. Eccentricity has been created by keeping the stiffness and mass centre separately. The building models are subjected to strong motion records of Imperial Valley Array-5 (1979) and Northridge-Sylmar (1994). A detailed study on the effect of base shear strength, eccentricity and pulse period of near fault ground motions on the response is investigated. Performance of multi-Storey buildings in Chapter-5, reported a detailed procedure for the performance evaluation of structures. The procedure is applied to find the performance evaluation of multi-storeyed buildings located in near fault region. Chiplun fault in Mumbai, India has been chosen for the study because several features of that fault are clearly published (RaghuKanth and Iyengar, 2006). Results of performance evaluation of five and ten storeyed symmetric buildings with and without infill panels are studied. Ground motion records consistent with the hazard spectrum for the design are considered. The performance of the building near the Chiplun fault in Mumbai, India shows operational under UHS-500 (uniform hazard spectrum) event and it collapses when the building is exposed to UHS-2500 record. The thesis is concluded in Chapter-6 with an overall summary of the report and suggestions for further scope of the work.