Loads - Moving Loads

The standard AASHTO loads are built into the moving loads database, however you can add and save custom moving loads as well.  The moving loads can be applied in any direction, so they can be used to model crane loadings (which are typically applied in 2 or 3 directions at the same location).  You can have up to 5,000 moving loads in each model.

To Apply a Moving Load

1. From the Spreadsheets menu, select the Loads - Moving Loads spreadsheet.
2. Specify a pattern in the Pattern field by selecting it from the drop down list.
3. In the Increment field specify the distance for the load pattern to be stepped through the path.
4. Specify the path by defining the Joints in the remaining fields and indicate if you wish the pattern to be moved Both Ways through the path.

Note

• You may skip joints when specifying the path.  The moving load feature is “smart” in the sense that it will try to find a way to get from one joint to the next joint in the load path sequence.  The load path taken will usually be the most direct route between the joints and may be verified by animating the moving load.
• For example, in the image below, if you want the moving load to move left to right along the beam member (M4), there if no need to specify the intermediate joint (N4) in the moving load path.

To Animate a Moving Load

1. If there is not a model view already open, click    on the RISA Toolbar to open a new view and make any adjustments you wish to appear in the animation.
2. On the Window Toolbar click the Model Display Options    button.
3. Select the moving load or load combination from the drop-down list at the bottom of the options and then click on the Animate button.

To Include a Moving Load in a Load Combination

1. To include a moving load in your analysis, specify it in the Load Combinations spreadsheet in one of the BLC fields and enter a corresponding BLC Factor. 
2. You may either type in the moving load tag (i.e. M1, M2, etc.) directly or you may select it by clicking the red arrow  to open the Set BLC Entry dialog. Choose the appropriate moving load from the drop down list.

Moving Loads Spreadsheet

The Moving Loads Spreadsheet records the moving loads for the member elements and may be accessed by selecting Loads  Moving Loads on the Spreadsheets menu.

Each moving load definition is automatically assigned a Tag on the left side of the spreadsheet.  These labels may not be edited.  Moving loads are included in your analysis by referencing this label on the Load Combinations spreadsheet.

The Pattern field is the name of the moving load pattern used for that particular moving load definition.  You can access the drop-down list of valid pattern names by clicking the down    arrow in this cell.  You may access the Moving Load Patterns and add or edit your own patterns by clicking on the Moving Load Patterns   button on the RISA Toolbar.

The Increment field is the distance that the moving load will be moved for each step in the moving load analysis.

The Both Ways field is a check box that indicates whether the moving load pattern is to be applied in both directions of the load path or just one way along the load path.  If the box is checked the load is first run from the start joint all the way to the last joint of the load path.  The load is then turned around and the last joint is now treated as the first joint in the load path.  The load is then run back to the first joint in the load path.

The last 10 fields are the joint numbers that are used to define the load path for the moving load.  The moving load feature is smart in the sense that it will try to always find a way to get from one joint to the next joint in the load path sequence.  The load path taken will usually be the most direct route between the joints.  If you have a long series of co-linear members, or if there is only one valid path between your start and end joints, you usually will only need to specify the first joint and the last joint in the series.  If there are several members that branch from a joint that are all part of valid paths to the next joint in the sequence, the member with the lowest member number will be the one chosen.  To control exactly which route is taken in this situation, use joints at each intersection point.  See the figure below:

In the example moving load path shown, you would need to specify joints A, B, C, and D as the load path joints.  You would not have to specify the joints that were in between the points where the load path changed direction;  I.e., the moving load would automatically go in a straight line from joint A to B, etc.

Moving Load Patterns

You may access the Moving Load Patterns and add or edit your own patterns by clicking on the Moving Load Patterns   button on the RISA Toolbar and then clicking on Add Pattern or Edit Pattern.

The file that the moving load pattern database is stored in is ML_LIB32.FIL.  The path to this file is specified in the Application Settings on the Tools menu.  You can add up to 500 different moving load patterns in the pattern database.

When you add a new pattern, the new pattern must have a unique name and can consist of up to 50 different loads.  The sign of the load Magnitude will control which way the load is pointing in the direction specified in the Direction field.  The direction field can be any of the 3 global directions or the 3 local directions for the members that the load will travel over.  Note that if your load travels over multiple members, a local direction load will be applied based on the local axes of each member it crosses over.  There is also a special code, “V”, which causes the load to be applied in the direction of the current vertical axis, whatever it is (X, Y, or Z).  The Distance is the distance between the loads.

Note: If you have two loads at the same location but in different directions, you may input them one after the other with a Distance between them of 0 ft/m, as shown in the image below:

Moving Loads Solution Procedure

Moving loads are handled internally by applying the loads at discrete locations that are then moved through the model.  A static solution is performed for the model at each load location.  Typically, once the first solution is solved, the remaining loads are just solved against the existing stiffness matrix, so the stiffness matrix would not be rebuilt for each load position.

Note:

• Models that contain tension/compression only items will have their stiffness matrix rebuilt at least once at each load position.  This can make the model solution take much longer than usual.
• Multiple moving loads may be assigned to a single load combination.  However, they cannot be assigned different "start times".  If a delay between moving loads is required, it must be accomplished by adjusting the moving load pattern.
• The load increment may be adjusted to "slow down" a moving load when multiple moving loads are applied.

Moving Loads Results

Load combinations that contain a moving load, will step the moving load through the load path and perform a solution for each position.  The results are enveloped, giving maximum and minimum results of these solutions.

For these result spreadsheets, the maximum and minimum values are shown for each section location, for each active member. The governing load combination and step location is also shown for each result value under the "LC" column. The first number is the load combination, the second is the step number: (load combination - step number).

Step Location

The step location tells you where the moving load was located along the moving load path when the maximum or minimum result value was obtained. This can help you recreate the static model to verify results.

The total number of steps is calculated as follows:

Note

• The results for every load position are not stored; just the maximums and minimums. Therefore the program is unable at this time to give a detail report solution for the moving load results. However this is an enhancement that we do plan on adding in the future.

Moving Load Step Point Load Generation

Because solving moving loads produces so much output (full model design at each moving load step increment), the output is limited to Enveloped results. This means that the moving load results only show the maximum and minimum values for each output result value.

However there are times when you may want to investigate the full model design (all member results, detail reports, deflected shape, etc.) at a specific step. You can easily create a static load case of the moving loads at a specific load step using the Point Loads from a Moving Load feature.

Instructions to use this feature:

1. Determine which moving load step you want to investigate.

2. Access this feature from the Drawing toolbar button , or from the Insert - Point Load from a Moving Load menu.
   1. Select which Basic Load Case you want the static point loads to be added to.
   2. Select which Moving Load or Load Combination with a moving load you want the point loads generated from.
   3. Select the Moving Load Step you want the static point loads generated from.

    

   

3. Click Apply.

4. Create a Load Combination that includes the Basic Load Case selected in step #2a above and solve to see the full results.