A Study on Live Load Deflection Criteria of Long-Span Steel Bridges

Under-clearance of bridges is one of the major considerations when planning and designing bridges. Especially for sea-crossing bridges, under-clearance is more important for safe passage of ships. Long span cable bridges with steel orthotropic deck usually show relatively large deflections and excessive deflections may be not only disadvantageous to the clearance plan, but also a problem in bridge serviceability. The clearance of sea-crossing bridge is designed with air draft from the waterline to top of mast, ship’s trim, psychological free space, sea level, height of wave and bridge deflection by live loads. At design stage, bridge engineers need a live load deflection limit to ensure adequate clearance. The limitation of live load deflection is also considered for ensuring the serviceability of vibration of bridges. For vibrational serviceability, the live load deflection limit is expressed as a ratio to the span or the formula by natural frequency. On the other hand, some standards specify vibration serviceability limit by peak or root-mean-square (RMS) acceleration. The purpose of this study is to propose new deflection limit criteria for long span steel cable bridges based on analysis and long-term measurement data. Structural analysis was performed on two steel suspension and three steel cable-stayed bridges with more than 200 m of main span under design live load model of DL24 and new live load model KL510 in limit state design code. Also, the deflection limit criterion was evaluated by probabilistic method using the deflection data measured for 1 year. The results of this study are expected to be useful data for bridge engineers designing long span steel cable bridges.