Behavior of FRP-Concrete Beam-Columns under Cyclic Loading

Abstract

Use of concrete-filled fiber-reinforced polymers (FRP) tubes (CFFT) for columns and piles has been studied extensively over the last decade. The focus, however, has been exclusively on the monotonic behavior of CFFT. An issue that has received little attention is the implications of using CFFT in seismic regions. Survey of damaged structures in recent earthquakes indicates that catastrophic failure of an entire structure may result from failure of few columns in a chain action. Since it may not be economical to design columns to respond to earthquake loads in their elastic range, dissipation of energy by post-elastic deformation is desired. Although, FRP materials are known for their linear elastic behavior, some FRP systems may exhibit non-linearity due to their laminate architecture and inter-laminar shear. Also, confinement of concrete core in CFFT improves its ductility. This study was carried out to evaluate the cyclic behavior of CFFT beam-columns, and determine whether non-linearity of FRP or confinement of concrete can provide seismic performance comparable to reinforced concrete (RC) columns or concrete-filled steel tubes (CFST).

The experimental work consisted of cyclic loading and unloading of FRP-wrapped concrete cylinders and FRP coupons, and reverse cyclic loading of CFFT beam-columns under constant axial load. Some measures of hysteretic performance, including cumulative energy dissipation, ductility and pinching effect were used to evaluate the cyclic response of tested CFFT beam-columns. The study resulted in a cyclic model for FRP-confined concrete in compression, and cyclic models for linear and non-linear FRP materials in tension and compression. A fiber element model was employed to predict the cyclic behavior of CFFT beam-columns. A parametric study was carried out on the cyclic behavior of CFFT beam-columns, and to compare the hysteretic response of CFFT beam-columns with those of RC and CFST members.

The two types of CFFT beam-columns tested under this study represented two different failure modes; a brittle compression failure for the over-reinforced white tube specimens with thick FRP tube and with majority of the fibers in the longitudinal direction, and a ductile tension failure for the under-reinforced yellow tube specimens with thin FRP tubes and off-axis fibers. The study showed feasibility of designing ductile CFFT members for seismic applications comparable to RC members. Significant ductility can result from the inter-laminar shear in the FRP tube. Moderate amounts of internal steel reinforcement can further improve the performance of CFFT members. Adding internal steel can be ineffective and may lead to premature failure. Slender CFFT members have less capacity than their short stocky counterparts. However, they are less susceptible to pinching effect and premature shear failure.