Dynamic amplification responses of short span bridges considering scour and debris impacts

Abstract

During extreme flooding events, bridges could experience large lateral displacements or even collapse due to either strong hydraulic forces or the effects of large waterborne debris, such as tree branches. The effects of waterborne debris on the structural behaviors of bridges include temporary accumulated dams and direct debris impacts. The additional lateral hydraulic forces and the acceleration of bridge scouring process due to temporary accumulated dams were investigated by researchers. Several debris impact force models were proposed to determine the loading effects applied on bridges. However, very few studies were conducted on the dynamic responses of bridges during flooding events considering the effects of large wood debris (LWD) as a combination of additional lateral hydraulic forces, foundation scour, and direct debris impacts. Therefore, the present study performed numerical analyses for bridges subjected to foundation scour and LWD impacts corresponding to flooding events. Besides, the effects of foundation scour and LWD impacts on the dynamic responses were simplified for bridge design purposes. Firstly, various short-span bridges with possible foundation scour depths due to debris accumulation were numerically simulated to obtain dynamic characteristics. Then, the hydraulic forces applied on the bridges were updated considering an extra blocking area due to debris accumulation. The collision forces due to floating woody debris were also introduced into the vehicle-bridge coupling system. Finally, the dynamic amplification factors (DAFs) with and without considering the effects of debris impacts were calculated and compared. It was found that the DAFs increase 20% at the maximum due to the effects of debris impacts. A modified factor was proposed to consider the effects. The reaction forces with and without the effects were also calculated to investigate the effects on the risk of deck unseating. Introducing possible debris impacts during flooding events into the dynamic analyses of bridges brings a more realistic estimation for the safety of bridges. The DAFs of bridges can be further taken as a guide for the supplements of design codes. The updated reaction forces would help to guide the risk mitigation of deck unseating.