Assignment Questions Question 6 (Required knowledge: Stresses 2,4 & 5 lecture material) 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 400*120mm timber beams at 2.0 metre centres. The concrete deck (which is fixed to the timber beams to enable composite action) is 150mm 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 0 = 300 MPa for concrete: E = 25 GPa = 32 MPa A Ocompression A Otension = 2.4 MPa for timber: E = 10 GPa A compression = 15 MPa Otension = 20 MPa The following steps need to be carried out to complete this question: A. For the existing bridge: Transform the bridge section into an equivalent concrete section. Locate the position of the centroid from the top of the bridge. 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 (use the diagrams on the next page to assist you). Determine the stresses at the top and bottom of each material. Calculate the maximum elastic moment. B. Repeat the above procedure for the proposed improved bridge. C. Determine the percentage improvement of the proposed option capacity (max. elastic moment capacity) over the capacity of the existing bridge. Existing bridge: Proposed improvement: 150 mm 400 mm 20mm *120mm steel plate fixed to base of timber beams 120 mm p6 of 8 Existing Bridge: transformed composite Proposed Improvement Option: transformed composite Scan page with Student ID card placed here. p7 of 8