One of the most difficult requirements for timber bridges has been meeting the live-load deflection limitation of l/500 (span lengthdivided by 500) at competitive costs. An 18- x 18-ft all-pennsylvania oak a-frame bridge has been designed in accordance with the 1991 aashto standard specifications for highway bridges, analyzed by a finite element method, and built and tested under an equivalent hs-20 truck loading at bucknell university to satisfy this live-load deflection requirement at a low cost. An 18-ft-long timber a-frame bridgeconsisting of two timber a-frames, six timber stringers, two steel hanger rods suspended from the apex of the frames, one steel transverse beam at midspan supported by the hanger rods, and panelized timber decking met the l/500 deflection requirement. The total cost of the bridge was $1, 500 for material and $2, 500 for student labor (estimated). This same a-frame-type bridge can be built economically and also satisfy the l/500 live-load requirement for spans up to 50 ft in length. This paper appears in transportation research record no. 1319, Bridge and hydrology research 1991.
Abstract