आरसी संरचनाओं का प्रदर्शन-आधारित भूकंपीय डिजाइन
Overview
The
occurrence of recent earthquakes in countries across the world raises the need
for a fundamental change in the present seismic design procedure appearing in
Indian Seismic Codes. Present seismic design codes are force-based, that is,
forces and displacements within elastic limits are calculated, and their
combination is used to design the structural and non-structural components.
Serviceability checks are applied using displacement limits and ductile
detailing. Inelastic responses are calculated by applying the response
reduction factor, which relates to force or displacement amplification;
however, such an indirect approach causes misjudgement in the actual building
response [Zameeruddin and Sangle, 2016].
Figure 1: Past
earthquakes tremors across the world
With an aim to communicate the safety-related decisions, the design practice focuses on the predictive method of assessing potential seismic performance, known as performance-based seismic design (PBSD).
PBSD is a generalized design philosophy in which design criteria are expressed in terms of achieving stated performance objectives when the structure is subjected to the stated levels of seismic hazard [Ahemad Ghobarah,2001]. PBSD permits the design and construction of buildings with a realistic and reliable understanding of the risk to life, occupancy, and economic loss that may occur because of future seismic events [Ronald Hamburger, 2006]. PBSD is an iterative process, which begins with the selection of performance objectives (that are defined by the owners, designers, and building officials), followed by the development of a preliminary design (considering stated set of performance objectives), an assessment of whether the design meets the performance objectives, and finally redesign and reassessment, if required, until the desired performance level is achieved [ATC 58, 2013]. Fig. 1 displays the flowchart representing key steps in the PBSD procedure.
Fig. 1. PBSD flow diagram [FEMA 445, 2006]
State-of-art-of-development
Performance-based
methods have the potential to significantly magnify many aspects of building
design, including seismic protection. This potential was used in practice and
further modification was done to get it integrated with the present building
codes. As
it addresses the seismic design this approach has been termed as “Performance
-based Seismic Design” [Freeman, 2000].
PBSD has been in practice since late 1900’s. This appeared firstly in the documentation published by the Structural Engineering Association of California (SEAOC, 1960) and Portland Cement Association (PCA). The SEAOC publication introduced a lateral force equation which accounts seismic hazard (Z), the performance of the structural system (K), and the dynamics of structures (C, a function of period T). Later, some additional coefficients related to ductility and elastic excursions were added to this equation.
From last Five decades and more, the Applied Technological Council has spearheaded the development of a framework for the performance- based earthquake engineering practice under the funding of Federal Emergency Management Agency (FEMA), Natural Science Foundation (NSF) and the department of defense (DOD). [Moehie J. P, 1992; Boquan LIU, 2004]. Three fundamental publications have laid the foundation for the development of performance-based earthquake engineering.
ATC-13(1985)
The
report, funded by FEMA provided a methodology to estimate the probable repair
costs for California buildings damaged by earthquakes. Based on the expert
opinions of knowledgeable engineers and intended for application this has been
used to estimate of Probable Maximum Loses (PML) of broad classes of buildings
rather than individual structure [Freeman, 2004].
