Computer-Aided Graphical Analysis of Bridges under Vehicular Loading

A computer-based, graphical approach is presented to construct the Influence Profile for determinate bridge structures that are subjected to multiple axle loads. An Influence Profile represents the variation of an internal or external force, or moment, at a specific location of a structure, under loading by an actual vehicle (e.g., AASHTO HS-20 truck, American Railway Engineering and Maintenance-of-Way Association (AREMA) Cooper E-80 train). The technique works by scaling the influence line ordinates by the individual axle weights of the vehicle, and by applying distance delays to account for the horizontal separation between axles. The scaled and shifted influence lines for individual axle loads are then superimposed graphically to obtain the effect of the entire vehicle. Beam and truss examples are given to illustrate the technique. A simple method is also presented for constructing influence profiles for compound systems, where the load is transferred through the deck and floor beams to the superstructure. Because of its graphical concept, the technique provides a practical approach for evaluating the force effects caused by an actual vehicular load; it is also well suited for teaching purposes.