PERFORMANCE BASED SEISMIC DESIGN OF LOW TO MEDIUM RISE RC FRAMED BUILDINGS FOR INDIA

Abstract

Seismic Hazard in India is due to the earthquakes originating from movement of plates in Himalayan region. Since, forces due to the resultant collision have not been fully neutralized, Himalayan mountain range is still rising and providing potential source of earthquakes in Northern India. Seismic activity in Southern India is believed to be due to intraplate fault movements. Structural Engineering contributes in mitigating this hazard on fronts such as optimum estimation of expected earthquake levels, formulating the engineering design criteria for structures and other facilities, formulation and implementation ofconstruction practices which result in to safe and economical structures. Due to limitations on resources, the aspect of safety and economy ofstructures is very important for developing countries like India. Development ofseismic design practices can be seen in its evolution from a simple specification of designing buildings for lateral load of 10 %of weight of the building to the recent form of Performance Based Seismic Design (PBSD). Lessons learnt from the past earthquakes have greatly contributed in this development. US trend of development ofseismic design specifications is reflecting in Codes ofPractices of many countries including India. However, there is major obstacle in implementing the design methodology in ATC and FEMA documents outside US, due to inherent differences in seismicity, types of construction materials, types of structural systems in use, and design and construction practices in vogue in an individual country. Present work deals with exploring the potential ofPBSD and elements needed for its use in India. Most buildings in India are four storied, buildings with five to ten floors u can be grouped as few, while those having more than ten stories are rare. Among these multistoried buildings, use of steel frames and concrete shear walls is still rare. Therefore, majority of framed buildings in India can be classified as low to medium rise reinforced concrete (RC) moment resisting framed (MRF) buildings. Earthquake Damage Survey Reports show that the seismic damage in Indian urban areas is mainly due to the RC, MRF buildings under residential occupancy. Therefore, among the various types ofstructural systems used in India, RC, MRF type is chosen for present studies. Research work aiming at evaluating and evolving a design methodology attain wide parametric scope. Present work is divided in four major sections which includes studies related to: (i) Relevance of available PBSD guidelines for use in India. (ii) Consideration of modeling guidelines in PBSD procedures and Indian Code based procedure. (iii) Investigations for adequacy ofcurrent Indian Code design provisions. (iv) Exploring possibility of improving current design provisions using available PBSD methodology. Accordingly, attempts are made in this work to understand the differences in seismicity of India and that of US. Regarding guidelines for modeling of structural systems, specifications for modeling of RC elements given by ATC, FEMA and available literature are studied. Major finding of this study is that modeling guidelines for linear analysis and design are not very different in these documents and practice in India but for nonlinear analysis which forms the core of PBSD, there in is absence of experimental data from Indian side. Fortunately literature is available for experimental investigation of brick masonry infilled RC frames in India. It is found that the analytical behaviour of brick masonry infills in India is much different than the behaviour reported in FEMA and other documents based on research in western countries. On account of this difference, a detailed study carried out in present work for infilled RC frames has resulted in arriving at material model and component behaviour model for brick masonry in India. Proposed models are validated using the experimental data from India. It is found that investigations carried out are able to explain the field performance of infilled frames in India. Studies for alternatives to obtain candidate structures for nonlinear analysis in PBSD indicate the importance of preliminary design. Literature review in this aspect shows that structure designed on the basis of codal provisions provide most appropriate candidate structures. This helped to evaluate the performance of Indian code compliant buildings. On the basis of comparative studies for codal provisions, inconsistencies in Indian codes could be identified. These are addressed through comparison with similar provisions from codes of other countries as well as through design exercise carried out in present study. Preliminary studies for implementation of PBSD methodology for code designed buildings pointed out that result for evaluation using nonlinear analysis are dependent on codal procedures of force based analysis and design for assumed values of Response Reduction Factor, R. This being inconsistent with principles of PBSD, necessitated in depth evaluation for response reduction factor applicable for Indian buildings. It is found that the constituents of response reduction factor R, viz., iv Case studies for performance evaluation in accordance with ATC and FEMA specifications for buildings designed using seismic design criteria in Indian Code show that methodology used in this research is useful to provide the elements necessary for implementation ofPBSD in India. Using the proposed energy based ductility it is proved that any general forcedisplacement curve can be represented in elasto-perfectly-plastic (EPP) form that can be considered as an equivalent representation of the original form for the purpose of computing the ductility factor. This equivalence is then used to propose two methods called as, Energy-Ductility Based Response Reduction Methods (EDBRRM) as replacements for the Capacity Spectrum Method of ATC 40. These methods utilize the well-established equal-energy principle in the constant acceleration region of the Elastic Design Response Spectra (EDRS) and the equal-displacement principle in the constant velocity region of the EDRS. These principles have been used traditionally to compute the Inelastic Design Response Spectra for elasto-perfectly plastic behavior of single degree of freedom systems. It is expected that the proposed Energy-Ductility Based Response Reduction Methods will be used for computing the inelastic demand in Nonlinear Pushover Analyses Methods ofPBSD of future dates.