This RM Bridge FAQ: Influence surfaces example has a .pdf document and .tcl files to show how users can:

• use the influence surfaces for transversal analysis;
• use influence surfaces for the traffic load evaluation of wheel loads on grillages;
• create a mix of Macro2 and Macro3 within the generation of lanes for one surface;
• define a uniform load to be used at the influence surfaces;
• proceed if there are many load positions to be defined.

More basic information about Influence surfaces can be found at the link below:
communities.bentley.com/.../influence-surfaces