Alternative Truss Design for Footbridges

Abstract

Footbridges are constructions that are needed where a pathway has to be provided for people to cross some physical obstacle like a river or to cross traffic flows. They can be of various types e.g. beam, arch, suspension, truss and cable-stayed. From the various types of bridges, the focus was on steel truss bridges. The research sought to address the problem of over-design in classical trusses which can be solved through, though still far from ideal, varying the cross section area for different truss intervals. Instead of using single elements to connect truss nodes, multiple members of uniform cross sections were used in parallel so that variation in area is achieved through use of a number of members that make up the area. The objective of the research was to make a catalogue of trusses for footbridges of three spans and two widths. The trusses were intended to approach a fully stressed design as much as possible by varying the number of members in parallel from two to five. For the diagonal elements, only two members were used. The members were connected by horizontal beams. Distances between members on either side of the beam were determined such that these members had equal forces and the joint was in equilibrium. The bridge deck was a frame consisting of C-sections with an overall dimension of 2.44 m by 0.61 m and 3 mm tear plate surfacing welded on it. Two deck widths of 1.22 m and 1.83 m were considered. A number of trusses were analysed from which optimal ones were selected. The design for all trusses and connections were done according to the Eurocodes. The result obtained was a catalogue of trusses for different lengths of footbridges, connection details for the truss members, drawings and details. Three dimensional drawings were also produced to show the completely assembled footbridge. To conclude, a truss consisting of members in parallel is advantageous for slender trusses where it can save up to 21.3% and 26.4% by mass of steel for 1.83 m wide and 1.22 m wide footbridges, respectively when compared to the classical design. It can achieve 75% fully stressed design with most members being utilized to greater than 70% capacity. In addition, it is aesthetic and ease of assembly and disassembly is ensured by use of bolted connections. Also, elements can be replaced while the footbridge is still in place. Use of cross sections of different wall thickness is recommended in order to save more weight. Another recommendation is to galvanise the components of the footbridge to protect them against rusting. Finally, further research should be done on the design of a truss consisting of plates with different thicknesses for the elements to achieve as close as possible a fully stressed design. In this case compression elements will have stiffener ribs.