Vehicle Model for Highway Bridge Impact

Abstract

This paper suggests a simple vehicle model that may be used to estimate impact in highway bridges. In this model each axle load consists of a constant load together with a sinusoidally varying dynamic component with a frequency equal to the natural frequency of the bridge. For maximum impact the dynamic load components must be in phase with respect to time. Maximum bending moments in a simple girder are determined by the span, the static axle loads, the relative magnitude of the dynamic load component, and a bridge factor defined as the number of dynamic load maxima applied by an axle during its passage over the bridge. It is shown that for many bridges, the first natural frequency is simply related to the span and that the bridge factor is about 6. Impact values are predicted for a range of spans with a dynamic load component equal to 10% of the static load. A companion paper uses field studies to review this percentage.