Seismic Resistance of Square Concrete Columns Retrofitted with Glass Fiber-Reinforced Polymer

ACI Structural Journal, Sep/Oct 2005 by Memon, Muhammad S, Sheikh, Shamim A

The capacity of key structural members, particularly columns, to absorb and dissipate energy without severe strength degradation dictates the survival of structures during a major earthquake. Reinforced concrete columns with inadequate confinement do not possess the necessary ductility to dissipate sufficient seismic energy. This research evaluates the effectiveness of glass fiber-reinforced polymer (GFRP) wraps in strengthening deficient and repairing damaged square concrete columns. Each of the eight specimens tested, representing columns of buildings and bridges constructed before 1971, consisted of a 305 × 305 × 1473 mm column connected to a 508 × 762 × 813 mm stub. Specimens were tested under constant axial compression and cyclic lateral displacement excursions simulating earthquake loads. Test results reveal that retrofitting with GFRP wraps significantly enhanced ductility, energy dissipation ability, and shear and moment capacities of deficient columns. Cyclic behavior progressively improved as the number of GFRP layers increased, causing both stiffness degradation and strength reduction rates to decrease. Improvements observed following GFRP repair of damaged columns depended mainly on the extent of damage sustained. GFRP-confined columns exceeded the performance of similar columns that contained transverse steel reinforcement in accordance with the seismic provisions of the current North American codes.

Keywords: column; concrete; confinement; ductility; repair; seismic.

INTRODUCTION

The ability of structures to withstand severe earthquake vibrations and to perform in a satisfactory manner in postelastic states depends mainly on the formation of plastic hinges and their capacity to absorb and dissipate seismic energy. Most building codes1 provide guidelines to ensure that the seismic energy is dissipated in beams and girders rather than columns. Despite this strong column-weak beam concept, plastic hinging in columns during severe earthquakes is still unavoidable. Hinging of columns at the base of the structure is, in fact, depended upon to develop the mechanism to dissipate energy. Therefore, column performance in the inelastic mode is of utmost importance for the safety of a structure during an earthquake. It is well known that appropriate confinement of the potential plastic hinge regions ensures the ability of columns to sustain inelastic displacement without significant strength and stiffness degradation during severe earthquakes.

An investigation of the damages to buildings and highway structures in recent earthquakes in California and Japan2,3 has demonstrated the vulnerability of concrete columns, particularly in the structures constructed prior to 1971. The amount of lateral reinforcement used in such columns was as low as 10 to 15% of that specified by the current seismic design codes, thereby putting these structures at risk of rapid failure in a severe earthquake. As a result, major efforts were directed toward developing and applying retrofitting strategies to upgrade such columns. A retrofit technique using glass and carbon fiber-reinforced polymer materials (FRPs) is such an innovation. The advantages of glass FRP (GFRP) over the conventional external confinement techniques (reinforced concrete jacketing and steel plate jacketing) include higher strength-to-weight ratio, greater contact area, increased resistance to corrosion, ease of installation, lower labor and construction costs, and maintenance of the original member stiffness.

In recent years, despite a limited amount of experimental data on the seismic behavior of FRP-confined concrete columns, external FRP systems have become widespread in field column applications. The main objective of this research is to study the effectiveness of strengthening deficiently built columns as well as repairing damaged square columns with GFRP sheets for seismic resistance. The work presented herein is part of a comprehensive research program4-6 that aims to study the use of FRP to improve the behavior of concrete structures under a variety of extreme loads. Results from a recent test series on GFRP-confined square concrete columns are presented in this paper.

RESEARCH SIGNIFICANCE

A very limited amount of experimental data exists on the seismic behavior of realistically sized square concrete columns confined with GFRP wraps. This research investigates the seismic performance of near full-scale GFRP-retrofitted columns typical of existing buildings and highways. The reported study addresses the repair of damaged columns with GFRP-a topic of significant importance for the construction industry. The seismic responses of GFRP-confined columns are compared with those of similar columns reinforced only with transverse steel in accordance with the ACI code provisions. Results can be used to design retrofitting schemes for deficient columns and also to develop design guidelines for retrofitting with GFRP.

EXPERIMENTAL PROGRAM

Eight large-scale reinforced concrete columns were constructed using typical lateral steel detailing from the pre1971 design codes. Seven of these columns were strengthened or repaired with GFRP wraps. One unwrapped column from this program and one from an earlier study6 were used as control specimens to evaluate the benefits of FRP retrofitting. All the specimens were tested under constant axial load and cyclic lateral excursions simulating seismic loading conditions. The main variables of the study were the number of GFRP layers in the potential plastic hinge region, the level of applied axial load, and the presence of column damage.