Lateral Load Distribution in Multi-girder Bridges

This study of the distribution of internal forces in bridge structures is motivated in part by the sense that simple formulations may be perhaps inaccurate, restrictive and uneconomical in the design process. In the age of computerized structural analysis, the differences between the results of so-called exact, and now less costly, analyses and simplified load distribution rules are apparent. The availability of a variety of structural analysis systems programs greatly expands the options available to the bridge engineer. For slab and longitudinal girder bridges, a structural solver which will handle a three-dimensional frame combined with plate and shell finite elements can produce an "exact" analysis. SAP, NASTRAN, FINITE, ANSYS and SAP80 are examples of this type of program. These systems are implemented on machines ranging from supercomputers to desktop microcomputers. Programs such as SAP80 are accessible to the individual engineer on a personal, not corporate basis. However, the size of the problem and the range of analytical refinement available to the PC is restricted at present. Some level of simplification is needed, but even so considerable refinement over the present distribution rules can be achieved. Application of the analysis system does not involve the development of new computer programs—rather an application of an input "language" to the task of describing the loads and structure for analysis. Important also is a clear understanding of the idealization and specific structural theory underlying the system to be used. Once a paradigm input is written, the solution for a family of bridge types is usually simple. This paper provides an overview of insights gained on idealization and simplification for analysis and selected illustrative results developed on AISI Project 332, "Lateral Load Distribution for Multigirder Bridges." Detailed information will become available in the project final report. Discussed are structural idealizations of the bridge and deck systems accomplished by use of (1) the "exact" idealization of a shell bending element (RFSHELL) with axial membrane forces placed eccentric to the girders and diaphragm elements, (2) the grid with plate bending elements (RPB12) and (3) the simple grid for which both the transverse effect of the deck and composite action are taken into account (GRID). Selected results will be used representing simple and continuous bridges with a variety of span lengths, widths, number of girders and slab thicknesses.