Seismic Design Criteria for Circular Lap-Spliced Reinforced Concrete Bridge Columns Retrofitted with Fiber-Reinforced Polymer Jackets

ACI Structural Journal, May/Jun 2005 by Elsanadedy, Hussein M, Haroun, Medhat A

In this study, an object-oriented computer code was developed for predicting the behavior of circular lap-spliced bridge columns retrofitted with advanced composite-material jackets. The numerical model is based on a moment-curvature analysis of the column section with the inclusion of bond-slip mechanism and fiber-reinforced polymer-confined concrete models. The developed software was calibrated through a parametric study comparing the experimental and predicted results for different test data available in the literature. Accordingly, an optimum computational tool was developed to accurately predict the performance of all columns. The emphasis of this paper is on the establishment of a seismic design procedure for circular lap-splice reinforced concrete bridge columns upgraded with fiber-reinforced polymer jackets.

Keywords: bridges; columns; lap splice; reinforced concrete; seismic design.

(ProQuest Information and Learning: ... denotes formulae omitted.)

INTRODUCTION

In many of the tall bridges built in California and designed using pre-1971 guidelines, the longitudinal reinforcement of the bridge columns was spliced with starter bars extending from the column footing with a lap length of 20 bar diameters. In addition, the transverse column reinforcement was typically Grade 40 No. 4 (12.7 mm-diameter) bars spaced at 305 mm (12 in.) on center, independent of column size, strength, or deformation demands. As a result, the transverse reinforcement provides inadequate confinement for the core concrete under compression and insufficient clamping action to the lap splices to prevent debonding. The cover concrete may therefore start to spall prematurely and anchorage of the lapped bars may degrade rapidly due to the splitting action under fully reversed cyclic loads.

To upgrade and retrofit circular bridge columns with poor lap splice details (short lap splices with insufficient hoop reinforcement), various retrofitting means have been developed by researchers and practicing engineers. Steel jacketing has been proven by Chai, Priestley, and Seible1 to be an effective method to retrofit columns with insufficient lap-splice lengths. Even though steel jacketing has been widely used in practice in California and elsewhere, relatively higher costs are expected due to the complexity during installation. Moreover, the corrosion of the steel jacket could be a potential problem in the future.

Advanced composite materials have been recently recognized and applied to bridge retrofit. The advantages of composite retrofit systems include: light weight, high strength or stiffness-to-weight ratios, corrosion resistance, and, in particular, the ease of installation. All of these advantages make these materials more suitable for retrofitting bridge columns. Moreover, contrary to other retrofit techniques, fiber-reinforced polymer (FRP) jackets will not affect the lateral stiffness of the columns and hence will not alter the bridge dynamic characteristics.

The cyclic performance of circular composite-jacketed bridge columns with poor lap splice details was studied through an experimental program conducted at the University of Southern California.2 The retrofit system used a series of prefabricated E-glass fiber-reinforced composite cylindrical shells with slits. This system was employed to retrofit circular half-scale bridge columns with insufficient lap splice length at the base, and it proved to be very efficient in terms of clamping on the lap splice region and enhancing the column ductility. Most recently, a comprehensive testing program was carried out at the University of California, Irvine3,4 to study the seismic performance of circular lapsplice bridge columns retrofitted with advanced compositematerial jackets. Eight half-scale circular columns were constructed with a 381 mm (15 in.) lap splice at their base and tested in flexure: two as-built columns and six samples retrofitted by six different composite-material jackets. A brittle failure was observed in the as-built columns due to bond deterioration of the lap-spliced longitudinal reinforcement. The FRP-jacketed circular columns demonstrated a significant improvement in their cyclic performance by reaching ductility greater than 6.0.

For engineers to use FRP for the purpose of seismic upgrade of substandard structural elements, design procedures should be developed. One of the fundamental objectives of this research is to establish practical design criteria for column retrofit by composite-material jackets. The retrofit design methodology of circular bridge columns with insufficient lap-splice length is addressed in this paper.

RESEARCH SIGNIFICANCE

Poor performance of reinforced concrete (RC) columns, primarily due to inadequate lateral reinforcement and insufficient lap-splice length of the starter bars, caused many bridge failures during recent earthquakes. As an effective and economical means for seismic upgrade of lap-splicedeficient columns, this research addresses the use of advanced composite-material jackets. This will provide an insight into promising materials such as fiber composites for various applications in civil engineering.