DETAIL LRFD BASED DESIGN OF BOTTOMLESS CULVERTS

摘要

Bottomless culverts are three sided culverts that are founded on either spread footings or pile supported footings. The absence of a bottom slab connecting the walls makes these culverts different (more flexible) than their four-sided box counterparts. These structures are increasingly used in highway facilities such as waterway crossings, fish passages, and pedestrian undercrossing. They are considered environmentally friendly because they allow natural stream beds to remain unchanged, thus they continue to provide habitat support for native plants and animals. For this reason alone, it is anticipated their use will continue to grow in the future. Traditional methods of analysis and design rely on simplified soil pressure "envelopes" to obtain the forces in the culvert walls and top slab. These traditional methods, by virtue of their historical record of success with minimal failures, enjoy a high level of credibility among practicing engineers. In most cases, this credibility is well justified and should not be ignored. However, this approach appears to be too conservative for bottomless culverts, since the "semi-rigid" nature of these structures affects the soil pressure distribution from the surrounding fill, and hence, the forces within the culvert walls and top slab. It follows that analysis methods that are better able to capture a more accurate soil-structure interaction provide better means of estimating the soil loading and structural response for these culverts. The computer program CANDE (Culvert ANalysis and DEsign) by NCHRP provides this capability through detailed soil modeling and finite element analysis. As the effort towards implementing the LRFD code continues, it is important for engineers to look for opportunities - not only to migrate to the new code - but to also upgrade their analysis methods and design assumptions to take advantage of the more modern and refined analysis tools. This is especially true where modern analysis tools also allow for the incorporation of more detailed material properties based on laboratory testing. By upgrading their analysis tools while transitioning to LRFD, engineers stand to benefit from the new technology by achieving better designs, and realizing what could be significant cost savings in both design and construction. The focus of this paper is to identify and preserve the salient features of traditional methods of designing bottomless culverts while they are transformed into a new and improved LRFD based approach.