Let's consider the following example: we have a bridge with many members and we want to calculate maximum axial force in a member due to moving loads. The procedure for influence-based analysis follows the following steps:
Step 1: Obtain Deflected Shape
Based on the lane discretization, the program creates lane loading points along the width and length of each lane in the model. Then it places unit load in gravity direction at each lane loading point to determine the deflected shape. The unit load applied to the lane loading point is proportionally distributed to the tributary joints in the model. Using "Display > Show Lanes > Show Structural Connection For Selected Point" you can display the connections between the lane loading point and the joints in the model. So if our model contains 1000 lane loading points, the program would calculate 1000 deflected shapes, each corresponding to a unit load at each lane loading point. These deflected shapes are then used to determine all response quantities.
Step 2: Obtain Influence Surfaces for Response Quantities of Interest
Based on the deflected shapes calculated in the previous step, the program determines influence surface for each desired response quantity (such as reaction or axial force in a member). The value of the influence surface is exact at the lane loading points and linear interpolation is used to obtain values on the influence surface between the lane loading points.
Step 3: Find minimum and maximum response quantities
Once the influence surface is known, the program runs an algorithm that places the vehicular loading to a location in which causes maximum effect.
Summary
So there are two parts to the analysis – solving for the response (deflected shape) to the unit influence loads, then for each response quantity calculating the influence surface and moving the vehicles along it. The first part must always be done once and completely. The second part needs only be done for the requested response quantities.
Depending on the parameters of the model – number of degrees of freedom, number of lane load points, number of vehicles and axles, and number of response quantities requested – the relative time taken by these two parts may differ. Please read the section "Computation Considerations" at the end of Chapter "Bridge Analysis" of the
CSI