Bridge Expansion Joint for Bridges with Timber Decks

Timber deck bridges require expansion joints that accommodate both thermal movement and the moisture-related swelling and shrinkage of the wood. The joint must be compatible with the timber and the timber preservative treatment. Timber movement is more complex than concrete or steel movement because wood swells and shrinks with changes in moisture content in addition to thermal expansion. A timber deck may expand by 1-2% in the transverse direction when the moisture content increases from 15% to 25%. This moisture-related movement must be added to the thermal movement when calculating the required joint capacity. Moisture content variation in timber decks depends on the climate and the deck exposure. An exposed timber deck in a temperate climate may have a moisture content ranging from 12% in summer to 20% in winter. The corresponding movement in the transverse direction is approximately 0.5-1.0% of the deck width. For a 6-meter wide deck, this corresponds to 30-60 mm of transverse movement. Joint orientation on timber deck bridges must accommodate both longitudinal and transverse movement. The longitudinal movement is driven by thermal expansion of the deck length. The transverse movement is driven by moisture-related swelling and shrinkage of the deck width. The joint must be designed for the vector sum of these movements. Compatibility between the joint materials and the timber preservative treatment is critical. Creosote-treated timber is incompatible with many rubber compounds, causing swelling and deterioration of the rubber. EPDM rubber has better compatibility with wood preservatives than neoprene. The pad manufacturer should be consulted to verify compatibility with the specific preservative treatment. Fastening of the joint to the timber deck requires stainless steel or hot-dip galvanized bolts to prevent corrosion in the wet environment. The bolt holes must be sealed with a waterproof sealant to prevent water ingress into the timber. The bolt pattern must provide adequate restraint against lateral displacement without splitting the timber.