Design Criteria for Portable Timber Bridge Systems: Static versus Dynamic Loads

Interest in portable bridge systems has increased in the U.S. due to heightened awareness of the need to reduce environmental impacts and costs associated with road stream crossings. However, design criteria are needed for portable bridges to insure that they are safe and cost effective. This paper discusses different portable bridge categories and their general design criteria. Specific emphasis is given to quantifying the effects of dynamic live loads on portable bridge design. Results from static and dynamic load tests of two portable timber bridges demonstrated that dynamic loads can be significantly greater than static loads. Under smooth bridge entrance conditions, the mean dynamic bridge deflections were 1.13 times greater than static bridge deflections. Under rough bridge entrance conditions, mean dynamic bridge deflections were 1.44 times greater than static bridge deflections. INTRODUCTION There is considerable interest in the U.S. for portable bridge systems designed for forestry and related natural resources industries as well as other more traditional applications, such as for military or construction uses. In typical civilian construction applications, portable bridges are used when a permanent highway bridge is being replaced and a temporary bypass is needed during the construction period. Also, portable bridges are needed to serve as temporary structures during disaster situations, e.g. when a flood washes out a highway bridge. In addition, there are many situations where temporary access is needed across streams in remote areas for the construction or maintenance of utility structures. Access to our forests and other natural resources requires an extensive roadway network over a wide range of geographical conditions. Environmental concerns are the primary reason for the current interest in portable bridges used in forestry and natural resource applications. Forest roads typically require a large number of structures to cross streams and other topographical features. Rothwell (1983) and Swift (1985), in separate studies on forest roads, found that forest road stream crossings were the most frequent sources of erosion and sediment introduction into streams. Taylor et al. (1999) reviewed several studies that documented significant increases in sediment levels downstream from stream crossings. Bridges for forest roads can be permanent or temporary. Permanent bridges, which are typically designed for service lives of 40 to 50 years, are not economically feasible for short use periods frequently encountered in forest operations. Also, permanent bridges for low-volume forest roads are commonly designed to lower standards than most public access facilities and can be a potential liability to bridge owners if public access is possible. One solution to short-term bridge needs is the concept of portable bridges. Blinn et al. (1998) and Mason (1990) summarized portable bridges available in the US. If properly designed and constructed, portable bridges can be easily transported, installed, and removed for 1 The authors are, respectively, Graduate Research Assistant and Associate Professor, Biosystems Engineering Department, Auburn University, AL, USA; Engineer, Trus Joist MacMillan, Charlotte, NC, USA; and Project Leader, USDA Forest Service Forest Products Laboratory, Madison, WI, USA.