Residual Seismic Performance of Reinforced Concrete Bridge Piers After Moderate Earthquakes

An experimental investigation was conducted to evaluate the seismic ductility of previously damaged concrete columns. Eight circular concrete columns 600 mm (23.6 in.) in diameter and 1500 mm (59.0 in.) in height were constructed with three test parameters: confinement ratio, lap-splice of longitudinal steel, and retrofitting fiber-reinforced polymer (FRP) materials. The objective of this research was to subject reinforced concrete (RC) bridge piers to artificial earthquake motions using a pseudo-dynamic test (PDT), and then to examine their seismic performance in a quasistatic test (QST). The seismic enhancement of FRP wraps was also investigated. Six specimens were loaded to induce damage by a series of four artificial earthquakes, which were developed by the Korea Highway Corporation (KHC), to be representative of earthquakes in the Korean peninsula. Following the PDT, the six predamaged specimens were subjected to inelastic cyclic loading while under a constant axial load of 10% of the column axial capacity. Two reference specimens without predamage were subjected to similar quasi-static loads.

Test results showed that all specimens behaved almost linearly under moderate artificial earthquakes (PDT). Except for the ordinary specimens with lap-spliced longitudinal bars, most specimens predamaged during the PDT generally demonstrated good residual seismic performance. The predamage introduced during the PDT in ordinary specimens lowered their seismic performance. RC bridge specimens retrofitted with fiber composite wraps in the potential plastic hinge region exhibited enhanced flexural ductility.

Keywords: fiber-reinforced concrete; lap splice; pier; reinforced concrete; seismic; transverse confinement.

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INTRODUCTION

Until recently, Korea was considered to be immune from earthquake hazards because it is located relatively far away from active faults. It has been observed in the Korean peninsula, however, that the number of low and moderate earthquakes has increased year by year. Recent earthquakes in Turkey (1999), Taiwan (1999), India (2001), Sumatra (2004), and Fukuoka (2005) have caused considerable loss of life and extensive damage to structures. It is particularly noteworthy that the Turkey (1999) and Taiwan (1999) earthquakes are similar in scale, but the damage in Turkey was typically more severe because of the lower seismic preparedness. Hence, to protect human life and property from seismic hazard even in countries of moderate seismicity, assuring seismic safety of various infrastructures is very important. The collapse or near collapse of bridge structures during the 1994 Northridge earthquake and the 1995 Hyogoken Nambu earthquake were noted with concern in Korea and prompted safety evaluations of various structures that were or were not designed to resist earthquakes.

Reinforced concrete (RC) bridges are potentially among the most seismically vulnerable structures. The damage of RC columns in regions that experience inelastic action depends on the characteristics of the earthquakes as well as column details. The extent of this damage affects the bridge performance during the design-level earthquake and the feasibility of restoring the columns to their preearthquake conditions. The research reported herein has focused on the repair and strengthening of shear dominated RC columns that were damaged by minor or moderate earthquakes. Moreover, for practical reasons, lap splices of longitudinal bars were used in the plastic hinge region of most RC bridge columns that were constructed before the seismic design code of the Korea highway bridge design specification (KHBDS) was implemented in 1992. Therefore, the purpose of the research presented herein was to investigate the effect of lap splices of longitudinal reinforcement on the seismic performance of RC bridge piers and to evaluate the residual seismic performance of such piers with prior earthquake damage. The last objective of this investigation was to develop an efficient strengthening method for RC piers that were built with lap-spliced longitudinal bars.

It is known that closely spaced transverse reinforcement in the potential plastic hinge zone of bridge columns increases the ultimate strength and strain capacity of the concrete core. Chai et al.1 reported that the use of steel jacketing to retrofit columns was effective in restoring the flexural strength and the ductility of a damaged column that had suffered total bond failure of reinforcement spliced in the plastic hinge region. Saadatmanesh et al.2 experimentally studied the seismic behavior of RC columns strengthened with fiber reinforced plastic composite straps and showed that such straps were very effective in confining the core concrete and preventing the longitudinal bars from buckling under cyclic loading. They also conducted an experimental investigation into the flexural behavior of four earthquake-damaged RC columns repaired with prefabricated fiber-reinforced plastic wraps and verified the effectiveness of the proposed repair technique by showing that the flexural strength and displacement ductility of the repaired columns were higher than those of the original column. Aboutaha et al.3 have also reported the effects of confinement on the compressive strength and ductility of RC bridge piers. They investigated the effect of lap splice lengths of the longitudinal bars and the repair of lap splice failures in damaged concrete columns. A total of six specimens with a 457.2 x 914.4 mm (18.0 x 36.0 in.) cross section were fabricated and tested under axial load and cyclic lateral displacement to examine the effect of two confinement steel types and different repair methods. Results from the cyclic lateral load test showed that the retrofitted columns reached their design strength and had good shear strength and ductility. Lehman et al.4 experimentally investigated the performance of earthquake-damaged RC columns repaired by different techniques, depending on the damage level and details of the original columns: headed reinforcement, mechanical couplers, and cover concrete patching with epoxy injection. The effectiveness of each repair technique was determined by comparing responses of the repaired columns and original column with the design requirements.