Capacity Limit States for Nonductile Bridge Columns

Abstract

The ShakeCast software platform, used by the California Department of Transportation (Caltrans), utilizes near real-time ground shaking maps generated by the US Geological Survey in conjunction with demand and capacity models, to evaluate the likely damage to all bridges in the vicinity of an earthquake event. The ability to estimate with reasonable accuracy the likelihood and extent of damage to bridges following an earthquake is crucial to post-earthquake activities such as the mobilization of emergency response. While the development of seismic demand models has seen considerable progress, there is a significant gap in our current ability to correlate demands with capacity limit states, particularly for older California bridges. Whereas modern bridges designed after 1990 are expected to perform well, older bridges, particularly those built before 1971 (and referred to as Era-1 bridges in this report), are vulnerable to damage. It is the goal of this research to address this gap by developing a range of component capacity limit states (CCLS), from minor damage up to collapse, for pre-1971 Caltrans bridge columns through modeling and comprehensive simulations. A simulation model is developed for typical non-ductile bridge columns considering flexure, shear and mixed shear-flexure failure modes and incorporating critical effects at the material level (such as confinement in concrete and bar buckling in reinforcing steel) and the sectional level (such as bond-slip due to strain penetration). Given the prevalence of drift-based measures in seismic design and assessment, the first choice considered in the development of the CCLS models was ductility. A strain-based approach was used to correlate damage with capacity limit states for both circular and wide rectangular sections that typify Era-1 bridge columns. Findings from this phase of work exposed a major drawback in using ductility-based measures to characterize capacity limit states under random earthquake-induced loading. Hence, a major effort was dedicated to developing a damage-index based approach to classifying limit states. The proposed damage-based approach to developing CCLS models was validated against experimental data and then applied to single, two and three-column bents. Fragility functions were developed wherein exceedance probabilities of damage states were examined as a function of a seismic intensity measure. The new damage-based methodology was successful in predicting a range of capacity limit states associated with visual damage such as cracking of the cover concrete, spalling of concrete, buckling of longitudinal reinforcement, crushing of the core concrete and multi-bar rupture. Findings from the study will not only assist in post-earthquake emergency response efforts but also in prioritizing strengthening of older, nonductile bridges.