Generalized Approach for Determining the Maximum Moment under HL-93 Loading on Simple Spans

With the institution of AASHTO HL-93 loading, calculating the maximum live-load moment on simple spans became more complex than it had been previously. In the past, to find the maximum live-load moment from the HS-20 design truck, the middle axle was offset from midspan by 0.711 m (2 ft 4 in.), and the moment under that axle was the maximum moment on the beam. However, with the combination of the design vehicle and the distributed design lane load, this general approach became obsolete. The purpose of this study was to develop a new generalized approach to determine the location and value of the maximum moment under HL-93 loading on simple spans. This was accomplished by creating moment envelopes and studying their extremum. Subsequently, equations, charts, and tables were developed that describe the location and value of the maximum moment on a beam under HL-93 loading, consisting of design truck or design tandem loading in combination with the design lane load for a simple span of any length.