Wearing Surfaces for Orthotropic Steel Bridge Decks

In the state of Alaska, a six-span 884.5-m orthotropic steel bridge crosses the Yukon River at a 6 percent grade. The bridge is on a gravel road, it has a roadway width of 9.2 m, and the orthotropic steel deck is supported by two steel box girders 154.9 cm wide and 414.0 cm deep. The structure is expected to support the oil pipeline, a future gas line, and a low volume of heavy loaded trucks and to respond to harsh winter temperatures. This structure was designed in the early 1970s with a 127-mm two-layer timber deck wearing surface. Since then, the wearing surface has been replaced twice with timber decks (1981, 1992) and is again scheduled for timber deck replacement in 1999. Alternative wearing surfaces for other orthotropic bridge decks are reviewed. Orthotropic steel deck flexibility is examined and design charts for selecting the thickness for a given wearing surface are presented. The design charts account for truck loads and temperature change; no provision is made for traction or abrasion. The use of these charts is illustrated. Strains caused by prepositioned truck loads were measured in the orthotropic steel deck. The deck had a two-layer 127-mm timber wearing surface. These strains were compared by analysis. The results showed that the timber-deck system was stiff. Live load tensile strains and the range of strain were low; thermal stresses are expected to be high. Cold temperature thermal cracking, abrasion, adhesion to the steel deck, and traction are important parameters for selecting a future wearing surface. Live load fatigue in the wearing surface should not be a problem for this orthotropic bridge.