Performance and sensitivity analysis of UHPFRC-strengthened bridge columns subjected to vehicle collisions

Bridge columns made of normal concrete are evidenced to be susceptible to vehicle collisions. Particularly in the United States, vehicle collision has become one of the primary causes of bridge failures. This is largely due to the low crashworthiness of a conventional reinforced concrete (RC) column. Ultra-high-performance fiber-reinforced concrete (UHPFRC) as one of advanced concrete materials has been experimentally demonstrated to possess excellent strength, durability, impact resistance and energy-absorbing capacity. Accordingly, one type of UHPFRC-strengthened columns was proposed in this study as an alternative to RC columns that may be at risk for vehicle collision incidents. High-resolution finite element (FE) models were developed to investigate the performance of UHPFRC-strengthened columns subjected to vehicle collisions. In the high-resolution FE model, a three-span simply-supported girder bridge (including girder, pier column, column cap, bearing, etc.) was adopted and modelled. Material models regarding normal concrete and UHPFRC as well as the vehicle model were carefully calibrated by experimental data. The influence of initial gravity loads on impact responses was found to be pronounced, and a damping-based method was proposed to efficiently exert permanent loads on pier columns prior to a collision. Three different simplified models, as published in current studies, were investigated to replace the whole bridge model. Two single-column models with different boundaries were shown to have low accuracy. The pier-bent model considering the superstructure gravity was demonstrated as capable of predicting collision-induced responses that are in good agreement with the high-resolution FE model. The impact resistances of both RC and UHPFRC-strengthened columns were extensively investigated using the appropriate simplified model. The crashworthiness of UHPFRC-strengthened column was found to be considerably superior to that of RC column. An extensive parametric study was performed using response surface methodology to explore the influences of reinforcement ratios, thickness of UHPFRC jacket, UHPFRC strength and initial impact speed. The impact-resistant performance is mostly sensitive to changes in the thickness of UHPFRC jacket when the impact speed is not very high. On the contrary, the residual capacity of the bridge column is hardly increased by thickening UHPFRC jacket. In addition, the developed response surface models provided easy estimation of impact-induced responses of an UHPFRC-strengthened column, which have potential use as the surrogates of time-consuming FE simulations to efficiently examine the reliability and optimization of bridge columns under impact loadings.