Seismic retrofit of rectangular bridge columns using CFRP wrapping

Mesay Abebaw Endeshaw

This study investigated retrofitting measures for improving the seismic performance of rectangular columns in existing bridges. Experimental tests were conducted on 0.4-scale column specimens which incorporated details that were selected to represent deficiencies present in older bridges in Washington State. Two unretrofitted specimens were tested to examine the performance of the as-built columns incorporating lap splices at the base of the columns and deficient transverse reinforcement. Five columns were retrofitted with carbon fiber reinforced polymer (CFRP) composite wrapping and one specimen was retrofitted with a steel jacket. The specimens were subjected to increasing levels of cycled lateral displacements under constant axial load. Specimen performance was evaluated based on failure mode, displacement ductility capacity and hysteretic behavior. Failure in the as-built specimens was caused by either spalling followed by longitudinal reinforcement buckling and eventual low cycle fatigue fracture or lap splice failure. Reasonable energy dissipation and ductility were achieved in the as-built specimens. While results from this study and from past research indicate satisfactory column performance for displacement ductility levels of 4 or more, these results should be applied carefully due to possible scaling effects, and it is anticipated that full-scale columns may perform worse than the scaled specimens. Hence, it is conservatively recommended that all columns be retrofitted to ensure a ductile performance for displacement ductility demands of 2 or more. For retrofitting of rectangular columns, it is recommended that oval-shaped jackets be used whenever possible. Column specimens with oval-shaped jackets of steel and CFRP composite material performed similarly, both producing ductile column performance. Failure in these specimens was due to flexural hinging in the gap region between the footing and retrofit jacket, leading to eventual low-cycle fatigue fracture of the longitudinal reinforcement. Details and procedures for the design of oval-shaped steel jackets are provided in FHWA Seismic Retrofitting Manual for Highway Bridges (2006). Design guidelines for oval-shaped CFRP jackets are given in ACTT-95/08 (Seible et al., 1995). Oval-shaped jackets designed according to these recommendations can be expected to prevent slippage of lapped bars within the retrofitted region. Columns retrofitted with rectangular-shaped CFRP jackets all demonstrated ductile column performance. Failure in these specimens was due to flexural hinging in the gap region followed by low-cycle fatigue fracture of the reinforcement. The CFRP jacket designed based on ACTT-95/08 recommendations for rectangular-shaped retrofits resulted in satisfactory performance, but bulging of the CFRP jacket was observed towards the end of testing. Increased thickness of CFRP jackets resulted in reduced bulging of the CFRP jacket and, in the case of the specimen retrofitted with a CFRP jacket designed based on 150% of the ACTT-95/08 recommendations, improved performance. Design guidelines for rectangular-shaped retrofitting using CFRP composite materials are proposed for application to columns with cross-section aspect ratios of 2 or less. While no slippage of the lap splice was observed, it is conservatively recommended that rectangular-shaped CFRP wrapping be used only for the situation where controlled debonding of the lap splice is acceptable.

Seismic retrofit of rectangular bridge columns using CFRP wrapping

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