Stressed log bridges

Abstract

The stressed log bridge was conceived as a means of recycling discarded utility timber poles, or logs, which are difficult to dispose because of having been treated with preservatives. To minimize wastage, the logs are trimmed to obtain two parallel flat surfaces, against which they are stacked and laterally stressed as in the familiar stress laminated wood decks. The log decks introduced in this paper are recommended to be stressed by means of aramid or glass fibre tendons which are inert and extremely flexible; because of which, the prestress losses can be virtually eliminated. The paper presents the details of the design, construction, and testing of a prototype stressed log bridge built to demonstrate the concept. The bridge was post-tensioned with aramid fibre cables known by the trade name of PARAFIL ropes. The tendons were first stressed to the maximum level corresponding to a high pressure that can be sustained by the wood and then, within half an hour, destressed to about 68% of the initial force. Observations over a period of about 10 months have confirmed that restressing of the deck is not required. Mainly because of economy, the use of glass fibre reinforced plastic rods is recommended instead of the aramid fibre cables. Key words: aramid fibre, bridge, glass fibre, log, prestress losses, stress laminated wood deck, timber bridge.