PERFORMANCE-BASED SEISMIC DESIGN OF HOSPITALS

Abstract

Hospital buildings have immense post-earthquake importance. The functionality of the hospitals following a major earthquake is critical for the well being of the community. For post-earthquake operability of a hospital, the hospital building structure should not suffer any significant damage and the hospital nonstructural components including equipment and services (also known as Operational and Functional Components, OFCs) should remain functional. Further, the cost of the OFCs in a hospital is about 85% to 90% of the total cost of the hospital. Thus, ensuring the seismic safety of the OFCs in a hospital is of utmost importance. Currently the design codes rely on Force-Based method of design. In course of time, the limitations of Force-Based method of design have become apparent to engineering community. In the last two decades, a new design philosophy, namely, Performance-Based Design (PBD) methodology has evolved. Indian design code specifies an Importance factor of 1.5 for the design of hospital buildings. It is necessary to evaluate, in the light of PBD, if such an Importance factor can lead to a hospital that retains its post-earthquake functionality. It is also worthwhile to investigate, if there is any correspondence between Force-Based method of design and the PBD. For avoiding structural damage in a major earthquake, the member performance objective has to be stringent. Many of the OFCs are drift sensitive, and, for this reason, the interstory drift is a critical design parameter for hospitals. The structural elements of the hospital building should meet Immediate Occupancy (10) level of performance and the interstory drift of the building has to be limited to 1%. In totality, a hospital building should exhibit an Operational level (OL) of performance under ground motions corresponding to a level of Maximum Considered Earthquake (MCE). The present study considers frame buildings and frame-wall buildings of 4-story, 10-story and 20-story heights. The plans of buildings have been taken as per the existing Block 3 of All India Institute of Medical Sciences (AIIMS), New Delhi, and Ward Block of Guru Teg Bahadur Hospital (GTBH), New Delhi. To investigate if Force-Based method of design may lead to OL buildings, buildings have been designed with increasing Importance factor (or increasing force level) and the performance of the buildings has been studied using Nonlinear Static Analysis (NSA) and Nonlinear Time History ii Analysis (NLTHA) under Spectrum Compatible Ground Motions (SCGM) scaled to MCE level. From the study, it has been found that the pushover analysis fails to reveal column sway mechanism in some of the buildings, but this gets manifested in the NLTHA. The frame and frame-wall buildings designed using Force-Based method of design with suitable increase in design force level, may satisfy 10 performance level, but generally fail to satisfy 1% interstory drift limit. Further, it has been found that there is no unique Factor to increase the design force level which can render 10 level of performance for all building heights. For frame-wall buildings, it is difficult to set the wall sizes from a theoretical basis. Thus, Force-Based method of design is not suitable for the design of OL hospital buildings. A design method to be applicable to OL hospital buildings should incorporate both member performance level and interstory drift. A parametric study on the stiffness, strength and ductility ofOL buildings has been performed by designing OL buildings using an iterative procedure based on a combination of Displacement- Based design method and Force-Based design method. It has been observed that the number of iterations is quite large to satisfy member performance level and interstory drift limit. Relationships between various design parameters, with height of buildings, have been developed to reduce the number of iterations. A Unified Performance-Based Design (UPBD) method has been proposed that incorporates both member performance level and interstory drift limit. The initial beam sizes and wall sizes are obtained to meet the ductility demand arising from the member performance level and interstory drift limit. The base shear is obtained as per Pettinga and Priestley, for frame buildings, and as per Sullivan et al., for frame-wall buildings. The proposed method has been validated by applying to the design of buildings of various heights. It has been observed that the proposed method can successfully render OL hospital buildings with limited number of iterations. It has been observed from the study that up to 4-story height, frame is an economical system for OL buildings. Above 4-story height, frame-wall system has been found to be increasingly more cost effective than the frame system. To study the seismic safety of OFCs, floor response has been investigated in terms of: (i) Floor Amplification Factor (FAF), (ii) Component Amplification Factor (CAF) and (iii) Total Amplification Factor (TAF). For hospital buildings considered, it has been found that FEMA provisions give a fair estimate of FAF but underestimate the CAF. It has also been found that iii FEMA provisions underestimate TAF for OL buildings, which may result in unsafe design of anchorages for OFCs in hospitals.