Question

An old existing timber bridge with a concrete deck is currently in service in the south west of Western Australia. It has been proposed to increase the strength of the bridge by adding steel plates to the timber beams to enable larger trucks to use the road on which the bridge is located. The existing bridge has 460* 100mm timber beams at 1.8 metre centres. The concrete deck (which is fixed to the timber beams to enable composite action) is 140mm thick. A proposed improvement option, which involves strengthening the bridge by adding a 20mm thick plate to the bottom of the timber beam, needs to be examined to determine whether it would be worthwhile. It will be necessary to evaluate the percentage improvement of the maximum elastic moment. Assume: for steel: E 200 GPa σ -300 MPa for concrete: E-25 GPa σ compression -32MPa Ơ tension -2.4 MPa Ơ compression -15 MPa Ơ tension -20 MPa for timber: E 10 GPa The following steps need to be carried out to complete this question: A. For the existing bridge and the proposed improvement: Transform the bridge section into an equivalent concrete section. Locate the position of the centroid (to avoid confusion, take it from the top of the bridge, as the bottom is modified in the improvement option) Determine the I value of the concrete transformed section Draw transformed and composite stress diagrams to determine limiting materials and maximum stresses in all materials (the diagrams on the next page may assist you). Determine the stresses at the top and bottom of each material » Calculate the maximum elastic moment. B. Determine the percentage improvement of the proposed option capacity over the capacity of the existing bridge.

Answer 1

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