AASHTO LRFD Main Design Loads and Combinations – Live Loads
Multiple moving live loads over a bridge with a certain speed require a complex dynamic analysis. In practice, engineers have simplified this complex problem to an equivalent static solution to determine the forces and deflections due to live load.
HL-93 live model is determined from a probabilistic prediction that also represents the future live loads that can pass over the bridge during its economic life. The model has a uniform load surrounding a single truck.
Over the width of the bridge having multiple trucks at the same time at the same cross-section at the same speed is a very rare event. Therefore, the specifications have addressed this case by use of a simple factor for multiple presence of vehicles at the same time at the same location.
The static deflection of a stopping truck and total deflection of a passing truck over a bridge are not the same. Pass of a truck also amplifies the forces on the bridge. Engineers apply an amplification factor to static loads of truck to represent the dynamıic effects.
In multi-girder systems, not only one girder carries the entire load but a couple of girders share most of the loads. Over the years, engineers were able to compute the live load distribution factors to be used in computation proper share of the load on girders.
Interior girders are the ones close to the longitudinal axis of the bridge. Since each interior girder has a neighbouring girder on right and left, they have an advantage in load sharing with other adjacent girders.
Exterior girders have only one adjacent girder next to them. The live load on top of an exterior girder have only one-direction of distribution.
Since most of the maximum effects develop at the mid-span, it is important to compute effects at this location per lane. For uniform live load, classical equations can be used to determine moments without moving the load.
Moving load analysis is usually required to determine the maximum effects at the mid-span due to truck loading part of HL-93. Over the years, engineers have developed simple ways of computing maximum force effects without moving much the loads per lane.
Partial effect of computed lane moments for uniform load and truck load needs to be determined by use of girder distribution factors determined for interior and exterior girders.
See also; Live Load Calculator