Control of footstep vertical vibration for Vierendeel truss - supported steel footbridges

In this paper three dimensional static and dynamic analyses for steel pedestrian bridges have been carried out. The footbridges considered are of a total length of (20, 30, 40 and 50m). The bridge is considered to act as continuous of two equal spans or as a two simply supported spans disconnected at the intermediate support. The latter design philosophy is advantageous in case of vehicle accidents so that only one half of the bridge will need maintenance. The bridge skeleton consists of two Vierendeel trusses acting as the main edge girders and the deck consists of parallel transverse sections covered with a steel plate. The BS5400 has been adopted in the present study for the footstep vibration serviceability. The bridge fundamental frequency and deck acceleration have been obtained and compared with the tolerable limits to avoid human discomfort. The analysis parameters were the bridge span, the height of the Vierendeel truss and whether the bridge is simply supported or continuous. Results have shown that for the continuous bridges of spans less than or equal to 15m, the frequency is greater than the minimum limit and the vibration serviceability requirement is satisfied for the height range of the Vierendeel truss. For spans equal to or greater than 20m the frequency is less than the limit and the deck acceleration has been checked. For the simply supported bridges of spans 10m to 25m the frequency is less than the limit and the deck acceleration has been checked. For the same section properties the maximum acceleration has been increased by (10% to 50%) as compared with the continuous cases. Depending on the mass and stiffness for different analysis cases, a regression analysis has been done to develop equations for estimating the fundamental frequency.