COMPARATIVE SEISMIC RISK ANALYSIS OF RC BRIDGES DESIGNED FOR DIFFERENT CODES

Abstract

Most of the country codes, still, utilize Force Based Design philosophy for designing of structures for earthquake actions. In this methodology, the effect of inelastic energy dissipation is taken into account indirectly using a ‘Response Reduction Factor’ or ‘Behavior Factor’. Contrary to buildings, the energy dissipation in bridges takes place in substructure, i.e. mainly in the piers. The capacity of a pier to dissipate energy depends on reinforcement detailing in the plastic hinge region. Different national codes differ in the value of response reduction factor and also in guidelines for reinforcement detailing of piers, and hence are expected to have different performances during an earthquake of a given intensity. The problem with force based design philosophy is, that the damage is evaluated in terms of force to strength ratio. But it is now generally known that displacement or strain are better parameters to evaluate structural performance against seismic action. It has led to evolution of Performance-Based Design (PBD) philosophy, where structures are designed for predefined performance levels usually expressed in terms of inelastic rotation or drift ratio. Priestley (2000) have proposed a non-iterative performance-based design procedure known as Direct Displacement Based Design (DDBD) method. Recently, a rigorous method based on incremental nonlinear dynamic analysis of structures has been proposed for estimating collapse probability. In this procedure, the structure is analyzed using a set of ground motion records with gradually increasing seismic intensity, till it collapses. Considering the various variabilities and uncertainties associated with the process, the collapse probability is estimated. The main objective of the present study is to compare the seismic performance of bridge piers designed for different codes and using the DDBD, in terms of their collapse probability. Two existing/proposed bridges with different spans and pier heights are considered for this study. Both the bridges are designed for a hazard corresponding to Indian seismic zone V on rock outcrop. The design forces are estimated and reinforcement detailing is provided using different national codes namely, Standard Specifications and Code of Practice for Road Bridges Section: II (IRC: 6-2014),Standard Specifications and Code of Practice for Road Bridges Section: II (IRC: 6-2017), Code of Practice for Concrete Road Bridges (IRC: 112-2011), RDSO guidelines on seismic design of railway bridges vi (RDSO, 2015), Eurocode 8: Design of Structures for Earthquake Resistance Part 2: Bridges (CEN, EN 1998-2:200X,2003), Caltrans, Seismic Design Criteria and Bridge Memo to Designers, Version 1.4 (Caltrans,2006), and the AASHTO LRFD bridge design specifications (AASHTO LRFD,2012). The bridge piers are also designed using DDBD procedure for varying identical drift ratios. The bridges designed for different codes and drift ratios are analyzed using non-linear static and non-linear dynamic methods. The modelling parameters such as moment capacity and moment curvature are obtained from the pier sections and reinforcement detailing. Degrading energy and Pivot type hysteretic models are used to simulate the pinching effect in RC piers during earthquake. For non-linear Incremental Dynamic Analysis, guidelines of ASCE-41 (2013) and FEMA P695 (2009) are used with far field ground motion suit given in FEMA P695 (2009). Both Sa,avg (0.2T-3T) and Sa(T1) have been considered as intensity measure for fragility analysis. Comparative discussion in terms of ductility capacity, collapse margin ratio, and collapse probability of bridges is presented for the bridge piers designed using both FBD and DDBD procedures. Results of study conducted on bridge piers clearly shows that not only response reduction factor, but also, load combination factor, MCE to DBE conversion factor and selection of importance factor affects the performance of bridge piers designed using FBD method. For bridge piers of a given diameter designed using the DDBD method, performance of piers degrades as design drift ratio increases. It is interesting to note that, the collapse probability of flexible bridge piers (having larger slenderness ratio) is comparatively less than rigid piers.