The following example, which demonstrates the implementation of influence-based moving-load analysis, considers a bridge system which will be subjected to a moving load such that a maximum response quantity may be resolved. Influence-based analysis proceeds as follows:

Step 1: Obtain deflected shape

CSI Software first generates lane-load points along and across each lane according to lane discretization. Deflected shapes are then obtained by placing a unit load at each lane-load point along the gravity direction. These unit loads are then transferred from the lane surface to joints within the structural system according to their tributary distribution. The lane-to-object connections which govern this transfer of load path may be reviewed through Display > Show Lanes > Show Structural Connection For Selected Point. If, for example, the model contains 1000 lane-load points, 1000 deflected shapes will be calculated, from which response quantities are determined.

Step 2: Obtain influence surfaces for response quantities of interest

For each response quantity (reaction, member axial force, etc.), the influence surface is then derived from the deflected shapes previously calculated. Values are exact at lane-load point locations, and linearly interpolated between points.

Step 3: Find minimum and maximum response quantities

Once the influence surface is generated, a software algorithm locates vehicular loading such that minimum and maximum effect is induced. Response is enveloped to obtain the maximum absolute value.


In summary, deflected shapes are first resolved for each unit influence load, then the influence surface is calculated for each response quantity as vehicle loads move along the structure. Since the first step provides for analysis, it must be done in its entirety, then the second and third steps may be done selectively for the response quantities desired.

Computational time for each phase of analysis may vary depending on model parameters, including the number of degrees of freedom, lane-load points, vehicles and axles, response measures, etc. Additional information is available in the CSI Analysis Reference Manual (Computation Considerations, page 467).

See Also