Seismic fragility analysis of skewed bridges in the central southeastern United States

• Seismic fragility analysis of skewed bridges in the CSEUS is performed.
• The bridge system becomes vulnerable as skew angle increases.
• Expansion bearings and NSD columns are the most vulnerable components.
• Seismic design and retrofit for columns effectively reduce bridge system damage.
• Retrofits with transverse shear keys increase demands in columns and abutments.

Skewed bridges are often encountered in the highway bridge system when the geometry cannot accommodate straight (unskewed) bridges. The objective of this study is to investigate the influence of skew angle on the seismic response of bridges using nonlinear time-history analysis and probabilistic seismic assessments. Six types of skewed and straight bridges (including multi-span simply supported, multi-span continuous, and single-span skewed bridges with steel or concrete girders together with non-integral abutments) commonly used in the central and southeastern United States (CSEUS) are considered for establishing three-dimensional numerical bridge models. The six bridge types are further categorized as: (1) non-seismically designed (NSD) bridges, (2) bridges with seismically designed (SD) columns, (3) bridges retrofitted by (i) column jackets, (ii) isolator bearings (IBs) and keeper plates (KPs), (iii) restrainer cables (RCs) and shear keys (SKs), or (iv) seat extenders (SEs) and shear keys (SKs). Probabilistic seismic demand models incorporating geometric and material uncertainty parameters for the bridges under a suite of ground motions are established to develop corresponding sets of fragility curves in terms of vulnerable bridge components. System fragility curves are further developed through a combination of the component fragility curves in the bridges. Comparisons of the fragility curves between the straight and skewed bridges indicate that the larger the skew angle, the more vulnerable the bridges, regardless of NSD bridges, bridges with SD columns, and retrofitted bridges. Formulas that consider effect of skew on the values of fragility parameters in the fragility curves are derived for each bridge class, component type, and limit state. Finally, the retrofit of columns and seismically designed columns can reduce column damage probabilities without significantly increasing demands to the other bridge component types, leading to a lower bridge system risk than that for the NSD bridges. However, although the other three retrofits (IB&KP, RC&SK, and SE&SK) can reduce transverse and/or longitudinal demands on the bearings, the column demands remain a similar or worse damage level than that for the NSD bridges, resulting in a similar or higher risk for the three retrofitted bridge systems.