SEISMIC BEHAVIOR OF SKEWED AND CURVED BRIDGES

Abstract

Skewed and horizontally curved bridges facilitate the crossing of roadways or waterways when it is not convenient or permissible to span perpendicular to an obstruction and to align straight along the traffic flow direction. Bridges, with such plan configuration, are common at Highway interchanges, flyovers and to some extent at river crossings. Bridges with such configurations have been reported to suffer severe seismic damage owing to rotation of the superstructure or displacement towards• the outside of the curve line due to complex• vibrations occurring during strong earthquake ground motions. Hence, it is a paramount to study the seismic behavior of skewed and curved bridges for enhancing the state-of-the-art of the seismic design and retrofitting practices. Two prototype bridges, which are typical, representative of medium-span,-- reinforced concrete, skewed and curved bridges, are taken from a series of seven design examples presented by the Federal Highway Administration (FHWA, 1997) to illustrate the AASHTO requirements for seismic design of bridges. Full three-dimensional bridge models which include bridge deck, bent caps, and ductile columns, spread and pile foundations, diaphragm-type abutments, seat-type abutments with abutment-backfill and abutment shear keys, have been developed using SAP 2000. The up-to-date linear and non-linear analytical models of materials and structural components are used to carry out the seismic response analysis by varying the skew and curvature of the bridge models. As a part of identification of the inherent dynamic characteristics of skewed and curved bridges, parametric studies on the modal behavior of the bridges have been conducted. The results of the analysis indicate the coupling of horizontal vibration modes of skewed bridges to excite the lateral translating mass in each of the principal directions of the bridge, andcoupling of the out-of-plane bending and longitudinal torsion modes of curved bridges to excite vertical translating mass. It has been also found that the principal directions of skewed bridges, along which the maximum base shear can be excited, are oriented neither along the longitudinal-transverse nor along the skewed longitudinal-transverse directions.