Recommended Design Specifications for Live Load Distribution to Buried Structures, National Cooperative Highway Research Program, (NCHRP Report 647)

摘要

This report provides recommendations to revise the AASHTO LRFD Bridge Design Specifications relating to the distribution of live load to buried structures. The report details the development of simplified design equations (SDEs) for structural response based on threedimensional (3D) analysis of 830 buried culverts. In addition, the report provides guidelines for conducting 2D and 3D modeling for design situations with conditions not covered by the SDEs. The material in this report will be of immediate interest to roadway and bridge designers. Much analytical and experimental work has been conducted to investigate the distribution of surface loads through earth fills. The classic Boussinesq (1885) solution, and simple assumptions such as spreading a surface load over an area that is a linear function of depth, remain perhaps the most widely used calculation procedures. The approximate method for estimating the distribution of vehicular live loads through earth fill in the AASHTO Standard Specifications has, over many years, produced designs that have performed acceptably. The AASHTO LRFD Bridge Design Specifications use a different approximation method that significantly increases live load pressures on buried structures compared with the Standard Specifications. When combined with the increased dynamic load allowance prescribed in the LRFD Specifications, this increase is even greater. In addition, the basis of the approximation methods in both codes is neither well documented nor understood. The objective of this project was to develop (1) recommended revisions to the AASHTO LRFD Bridge Design Specifications relating to the distribution of live load to buried structures and (2) refined methods for analyzing the distribution of live load to buried structures, including selection of an appropriate soil models. This research was managed by Charles R. Nelson and Lee Petersen, CNA Consulting Engineers, in Minneapolis, Minnesota, with the assistance of Timothy J. McGrath, Simpson Gumpertz & Heger, Inc., in Waltham, Massachusetts. The report fully documents the research leading to the recommended design specifications for live load distribution to buried structures.