Civil Engineering Study Portal (UG): Performance-based Seismic Design of RC Structures

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].