Journal of Bridge Engineering

Results of an experimental load test of a posted, three-span, steel-girder-and-slab bridge are presented. The bridge was designed to have three simple supported spans, each consisting of a cross section with nine noncomposite steel girders. Through diagnostic load testing, it was determined that the girders were acting compositely with the concrete deck and that significant restraint was being developed at the bearing supports. A finite-element model of the main span was generated using the measured response of the bridge. Results of analyses conducted using the numerical model were compared to the measured response and found to be quite similar. Using this model, along with models that account for the loss of composite action and/or support restraint, a variety of load ratings for the bridge was determined. By comparing the various load ratings, and by considering the nature of the unintended composite action and support restraint, decisions regarding the determination of a safe bridge rating are discussed. Final conclusions suggest that the current posting levels may be unnecessary.

A simple, approximate method for determining the live-load girder distribution factors of irregular slab on I-beam bridge layouts is presented. The method can also be used for the live-load analysis of bridges subjected to one truck, as in the case of fatigue, or two (or more) different truck configurations, as in the case of permit loading. The method takes into account the longitudinal as well as transverse effects of the truck loads. It is based on isolating strips of the deck slab in the transverse direction directly under the wheel loads and treating them as beams on elastic supports. The stiffness of the supports at the strip locations is computed based on the geometric properties of the beams and location of the trucks in the longitudinal direction. The accuracy of the method for shear and flexure of I-beams in simple and continuous span bridges is verified using a detailed three-dimensional finite-element analysis.

A field test of a complicated statically indeterminate floor system on a historic steel truss bridge has been conducted to obtain a more accurate load rating of the structure than possible using calculated predictions of wheel load distributions. The empirically determined wheel load distributions resulted in the identification of the floor system, particularly the transverse floor beams, as the element governing the load rating of the structure. Favorable lateral distribution of wheel loads among the stringers by a glued laminated treated timber deck was noted. The experimental determination of wheel load distributions has provided an inexpensive means of intelligently allocating limited county resources for the maintenance and rehabilitation of an inventory of aging bridges.

Footings that are subject to axial load and bending can have a portion of their footing area that does not act in bearing. To design for such a loading condition, the engineer must compute the maximum bearing pressure, the bearing pressure at the remaining corners, the location of the line of zero pressure, and the percentage of the spread footing area acting in bearing. For footings subject to bending about both principal axes, the solution of this problem can be quite cumbersome and little design information is available to assist the engineer in solving this complex problem. This paper presents a solution to this complex problem, including the general principles and assumptions upon which the solution is based, the derivation of the equations used to solve this problem for any number of nonbearing corners, a design example using the derived equations, and a practical applications section, showing a sample computer application of the equations. This paper emphasizes practical design information for the practicing bridge engineer.