Report No.: CCEER-97-2

Title: Effects of Confinement and Flares on the Seismic Performance of Reinforced Concrete Bridge Columns

Authors: Nadim I. Wehbe, M. Saiid Saiidi, and David H. Sanders

Date: September 1997

Performing Organization:
Department of Civil Engineering/258
University of Nevada, Reno
Reno, NV 89557


This study examines the ductility capacity of rectangular reinforced concrete bridge columns with moderate confinement and the effect of structural flares on the seismic performance of reinforced concrete bridge columns.

Four half-scaled rectangular bridge columns were built and tested. The transverse reinforcement ratios provided in the strong direction of the column specimens corresponded to 46 percent and 60 percent of the minimum lateral reinforcement required by AASHTO for seismic detailing. Each specimen was tested under constant axial load while subjected to quasi-static cyclic lateral loading in the column strong direction. The axial load indexes were 10 percent and 25 percent. The specimens exhibited displacement ductilities ranging between 5 and 7. A new equation relating the amount of lateral steel to the attainable displacement ductility was developed.

Push-over analysis was performed on four existing bridges supported by columns with structural parabolic flares. In two bridges, the column longitudinal steel was placed in the core and along the flares whereas in the other two bridges the longitudinal steel was placed along the flares only. The results indicated that plastic hinges in columns with structural flares do not necessarily form at points of maximum bending moments. The location of the plastic hinges depended on the longitudinal steel amount placed in the core and along the flare. The analysis also showed that flared columns with main reinforcement placed along the flares only are more vulnerable to seismic loads than flared columns with most of the longitudinal steel placed in the core.

Experimental and analytical studies were performed on two flared column specimens. The specimens represented 40 percent scaled models of prototype columns with main reinforcement placed along the flares only. The specimens were tested under constant axial loads while subjected to lateral drift reversals along the column strong direction. The results indicated that it is possible to predict with very good accuracy the plastic hinge location and the ductility of columns with structural flares.

In this study, new models pertaining to shear capacity of reinforced concrete columns and lateral deflections due to bond slip were also proposed (Abstract by authors).