Elastomeric Pad Expansion Joint for Timber Bridges

Timber bridges on low-volume rural roads are increasingly using elastomeric pad expansion joints as a simple and cost-effective solution for accommodating deck movement. The pad must be compatible with the timber deck and must accommodate both thermal movement and the moisture-related swelling and shrinkage of the wood. Timber movement is more complex than concrete 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 pad capacity. Compatibility between the elastomeric pad and the timber deck requires that the pad material does not attack the wood preservative treatment. 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. Bearing pressure on timber is limited by the compressive strength of the wood perpendicular to the grain. For Douglas fir, the allowable compressive stress perpendicular to the grain is approximately 2 MPa. The pad must be sized to keep the bearing pressure below this limit. A larger pad area is typically required for timber bridges than for concrete bridges of the same span. Fastening of the pad 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. Maintenance of elastomeric pads on timber bridges should include inspection of the timber at the pad location for decay or insect damage. Timber decay at the bearing location can cause the pad to sink, creating a step at the joint. Any decay must be treated or the affected timber replaced before the pad is reinstalled.