Impact Factor for Live Load

The impact factor for live load is applied slightly differently for AASHTO LFD and LRFD. For AASHTO LFD and LRFD, the impact factor is not applied to the value of individual live load. In addition, the analysis results of moments and forces due to the individual live load shown on the Analysis screen are those without the impact factor. The effect of the impact factor is only applied to the analysis results of moments and forces for the load combinations and envelopes.

For AASHTO LFD, only one impact factor, for both the truck and lane, is used for both manually input and program generated live loads. However, for AASHTO LRFD, Substructure applies, by default, 33% and 0% as the truck and the lane impact factors, respectively. But, you can change the default impact factors by clicking A/D Parameters on the Analysis screen. Furthermore, Substructure distinguishes manually input live load and program generated live load as follows:

Setting	Description
Manually Input Live Load (LRFD):	Substructure applies only the impact factor for the truck. The impact factor for the lane is not considered.
Program Generated Live Load (LRFD):	Substructure knows, in this case, that a live case is due to either truck or lane load or both. Thus, the impact factor for truck will be applied to the truckload and impact factor for the lane will be applied to the lane load accordingly. Same as above-mentioned, the impact factor will be applied to the analysis results of the load combinations and envelopes. The impact factor is not applied to the analysis results (moments and forces) for the individual live load, or the value of the individual live load.

Setting

Description

Manually Input Live Load (LRFD):

Substructure applies only the impact factor for the truck. The impact factor for the lane is not considered.

Program Generated Live Load (LRFD):

Substructure knows, in this case, that a live case is due to either truck or lane load or both. Thus, the impact factor for truck will be applied to the truckload and impact factor for the lane will be applied to the lane load accordingly. Same as above-mentioned, the impact factor will be applied to the analysis results of the load combinations and envelopes. The impact factor is not applied to the analysis results (moments and forces) for the individual live load, or the value of the individual live load.