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TR.32.12.1 Generation of Moving Loads

This command is used to generate Moving Loads using previously specified load definitions.

Predefined moving load system types may be used to generate the desired number of primary load cases, each representing a particular position of the moving load system on the structure. This procedure will simulate the movement of a vehicle in a specified direction on a specified plane on the structure.

General Format

TYPE j x1 y1 z1 *{ XINC f1YINC f2ZINC f3 } ( { YRANGE | ZRANGE } r)


n total number of primary load cases to be generated
i load case number for the previously defined load case to be added to the generated loads
j type number of previously defined load system
x1, y1, z1 x, y and z coordinates (global) of the initial position of the reference load
f1, f2, f3 x, y, or z (global) increments of position of load system to be used for generation of subsequent load cases.

Use only XINC & ZINC if Y up; Use only XINC & YINC if Z up.

r (Optional) defines section of the structure along global vertical direction to carry moving load. This r value is added and subtracted to the reference vertical coordinate (y1 or z1)in the global vertical direction to form a range. The moving load will be externally distributed among all members within the vertical range thus generated. r always should be a positive number. In other words, the program always looks for members lying in the range y1 and y1+ABS(r) or z1 and z1+ABS(r). The default r value is very small, so entering r is recommended.

The ADD LOAD specification  may be used to add a previously defined load case to all the load cases generated by the LOAD GENERATION command. In the example below, the SELFWEIGHT specified in load case 1 is added to all the generated load cases.

Sequential load case numbers will be assigned to the series of generated primary load cases. Numbering will begin at one plus the immediate previous load case number. Allow for these when specifying load cases after load case generation.


  1. Primary load cases can be generated from Moving Load systems for frame members only. This feature does not work on finite elements.    
  2. This facility works best when the roadway, as well as the movement of the vehicle are along one of the global horizontal (X or Z) or (X or Y) directions.

    For bridge decks which are skewed with respect to the global axes, the load generation may not yield the most satisfactory results. In such cases, the M. Bridge Deck workflow is recommended is recommended. The Bridge Deck workflow works on the influence line/influence surface method, and is considerably superior to the moving load generator. It also has the advantage of being able to calculate the critical load positions on decks modeled using plate elements.

  3. The x1, y1, z1 values of the starting position of the reference wheel must be provided bearing in mind that the reference wheel has to be at the elevation of the deck. An improper set of values of these parameters may result in the wheels being positioned incorrectly, and consequently, no load may be generated at all.


LOAD 1 DL Only
TYPE 1 0. 5. 10. Xi 10.
TYPE 2 0. 10. 10. Zi 15.
LOAD 22 Live Load on Pavement
10 TO 20 30 TO 40 UNI GY -5.0
LOAD COMBINATION 31 10 0.75 22 0.75