STAAD.Pro Help

TR.31.1 Definition of Moving Load System

This set of commands may be used to define the moving load system. Enter the DEFINE command only once with up to 100 TYPE commands.

The MOVING LOAD system may be defined in two possible ways: directly within the input file or using an external file.

General Format

TYPE j { LOAD f1 f2fn ( DISTANCE d1 d2dn-1 (WIDTH w) ) | load-name (f) }
( DISTANCE d1 d2dn-1 (WIDTH w) ) optionally as 2nd set 

The FILE option should be used only in the second case when the data is to be read from an external file. The file name should be limited to 24 characters.

Note: Moving Loads can be generated for frame members only. They will not be generated for finite elements.
Note: All loads and distances are in current unit system.

Define Moving Load Within the Input File

Use the first TYPE specification. Input Data must be all in one line (as shown above) or two sets of lines. If two sets, then the second set must begin with DIS as shown above. If two sets, then Load and Dist lines may end each line but last of each set with a hyphen (See example).

TYPE j  LOAD f1 f2fn
DISTANCE d1 d2dn-1 (WIDTH w)


TYPE j moving load system type number (integer limit of 200 types)
n number of loads (eg, axles), 2 to 200.
LOAD fi value of concentrated ith load
DISTANCE di distance between the (i+1)th load and the ith load in the direction of movement
WIDTH w spacing between loads perpendicular to the direction of movement (e.g., the width of vehicle). If left out, one dimensional loading is assumed. This parameter will double the total load since the fi is applied to each wheel.
Note: For a single moving load use: TYPE j LOAD f1 DIST 0

Define Moving Load Using an External File

Use the second TYPE specification.

TYPE j load-name (f)


TYPE j moving load system type no. (integer).
load-name the name of the moving load system (maximum of 24 characters).
f Optional multiplying factor to scale up or down the value of the loads. (default = 1.0)

Following is a typical file containing the data.

50. 80. 90. 100.
7. 7. 9.

Graphical representation of the previous load system

Several load systems may be repeated within the same file.

The STAAD moving load generator assumes:

  1. All positive loads are acting in the negative global vertical (Y or Z) direction. The user is advised to set up the structure model accordingly.

  2. Resultant direction of movement is determined from the X and Z (or Y if Z is up)increments of movements as provided by the user.

Reference Load

The first specified concentrated load in the moving load system is designated as the reference load. While generating subsequent primary load cases, the initial position of the load system and the direction of movement are defined with respect to the reference load location. Also, when selecting the reference load location with a positive value of Width specified, then the following two views define the reference load location.

Movement parallel to global X axis; Movement parallel to global Z axis

Notice that in the left view, the reference point is on the positive Z wheel track side; whereas in the right view, the reference point is on the least positive X wheel track side.

Specifying Standard AASHTO Loadings

Truck loads specified in AASHTO specifications are also built in to STAAD.

TYPE j { HS20 | HS15 | H20 | H15 } (f) ( vs )


TYPE j moving load system type no. (integer).
f Optional multiplying factor to scale up or down the value of the loads. (default = 1.0)
vs variable spacing as defined by AASHTO, for HS series trucks (default = 14 ft.)

Example 1

TYPE 1 LOAD 10.0 20.0 –
15.0 10.0
DISTANCE 5.0 7.5 –
6.5 WIDTH 6.0 
TYPE 2 HS20 0.80 22.0

Example 2

When data is provided through an external file called MOVLOAD

Data in STAAD input file


Data in external file MOVLOAD

10 20 15
5.0 7.5
20 20