Requires: Advanced Level
- Applying Moving Loads
- Creating New Trucks
- Analysis Options
- Load Influence
- Reporting Case Results
Moving Loads are an analysis feature to model truck loads on bridges or crane loads on girders. These loads can move along the length of the members in either the positive x-direction or when marked as "reversible" in both directions. This tool eliminates the need to create multiple load cases to simulate moving loads and provides much more intelligent reporting capabilities. The tool also manages the application of lane loads for worst possible effects, applying partial lane loads over members according to AASHTO requirements.
The tool comes with two pre-defined truck loads, the HS 20 Short and HS 20 Long, as defined by AASHTO. You can also create your own custom trucks and lane loads using the "<Add New Truck>" option.
VisualAnalysis does not provide full AASHTO support, or sophisticated bridge analysis tools, there are some significant limitations. Moving Loads can only be applied to a contiguous chain of members. This means that if there are two chains of members in structure that you want to load with moving loads, you must create two Moving Load Cases. Only one truck is allowed per moving load case. Moving loads can only be applied to member elements in the Global Y-direction. There is no way to distribute the moving loads laterally to multiple member chains. Moving Loads cannot be applied to plate elements, but they can be applied to members in a structure that contains plates.
Due to the method used to calculate moving load results, superposition is used, moving loads are not allowed on nonlinear structures. This means that cable elements, semi-rigid connections, or tension-only (or compression-only) elements will prevent the application of moving loads on a structure. In these situations you will need to manually handle moving loads with multiple load cases.
Results for Moving Load Cases are presented in Extreme Min. and Extreme Max. envelope form.
When reporting results for moving load cases in a Report View, you can include the "When?" report column to indicate the moving load position associated with the result in the column on its left. Include the "When?" column immediately following a column with results, like Mz, or Vy.
The Moving Load results also offer Influence Diagrams for nodes or members at points you specify.
Moving Load Cases can also be combined with results from any other type of load case using Result Superposition Combinations. This is the approach to take to get design checks for your moving load results.
Applying Moving Loads
Creating Moving Loads
Moving loads are applied by creating a Moving Load Case. Moving Load Cases are created using the New Moving Load Case... command under the Advanced tab in the Load Case Manager. Typically you will want to select the members to load before you choose this command, as it is usually easier to select them graphically. If you do not select the members first, you will have an opportunity to do so in the Wizard.
If you wish to create multiple moving loads, or to load multiple distinct sections of the model, you will need to create multiple Moving Load Cases.
Editing Moving Loads
Once you have created a Moving Load Case, you can modify the load that was generated by selecting it graphically in the Model View, or through the Find Tool. The properties will appear in the Project Manager's Modify tab.
Creating New Trucks
Adding New Trucks
Custom trucks are created using the <Add New Truck> option in the Select Truck drop-down list. Selecting this option takes you to the "New Truck" dialog box. A truck consists of one or more point (or wheel or axle) loads and an optional lane load.
You provide a descriptive name that must be unique in the TruckLoad.txt file (see below). Then you may define the axle (or wheel) loads one at a time. For each axle load, you specify the magnitude (usually negative for the gravity direction) and a distance. By convention, the first axle is at offset 0.0, and each additional axle is measured relative to that. Once you add a truck, it will be available for all future Moving Load Cases and future projects.
New trucks you define are saved in the text file TruckLoad.txt normally located in the Data Files folder.
You may open this file directly using Notepad or a similar text editor and manually delete any or all of the custom trucks you have made. You may wish to make a backup copy of this file before editing it, in case the file become unreadable by the software after you make your changes.
The Moving Load feature works by creating and analyzing a series of "behind the scenes" load cases that place the truck loads at incremental distances starting at one end of the member and moving across to the other end. The number of points where the load is placed along each member is determined by the number of Load Stepping Points per Member. This number is specified by through the command. The option is not available unless you have created a Moving Load Case in your project.
Increasing this number may yield more precise results, but will also increase the size of your project file, slow the performance of the analysis, and increase the output in reports.
Note: A nonlinear model will preclude the use of moving loads because the analysis relies on superposition, which is invalid for nonlinear models.
How To Use
Once a Moving Load Case has been analyzed, Influence Diagrams can be plotted for members. They are plotted by selecting a member in a Moving Load Case Result view and either right clicking on it, and selecting Influence Diagram from the context menu, or by selecting from the main menu. Influence diagrams are available for any node or member in the structure. For members, you specify the result of interest, and the offset along the member.
The influence diagrams (by default) are plotted for dy, Mz, and Vy at the center of the selected member. The items to plot, as well as the member offset are changed using the Filter tab of the Project Manager.
Influence lines represent the effect of the moving load on a particular point in the model as the load moves. Influence diagrams are constructed for a positive (upward Global Y) 1 kip point load. The vertical axis shows the magnitude of the result in question when the 1 kip load is applied, while the horizontal axis shows the position of the 1 kip load along the member chain. The typical use for these influence diagrams is to determine where to place other (perhaps non-moving) loads to achieve a maximum affect at some point in the structure. The moving load analysis does this automatically for the moving truck load and lane loads, but you might have other loads that need to be positioned in another load case.
For a more complete discussion of influence diagrams you might consult any one of a dozen standard textbooks, such as chapter 10 in "Structural Analysis" by Jeffrey P. Laible (ISBN 0-03-063382-6).
Below is screen shot of an Influence Diagram for 37.5 ft from the start of the left member of a two-span, pin supported, continuous beam.
Reporting Moving Load Case Results
Moving Loads are reported in much the same way as any other member results. Report tables, such as Member Internal Forces and Member Internal Stresses, work with moving load cases as well. There is also a table for reporting the Moving Load Cases you have defined.
Member Result Sections
One big difference you will notice when reporting moving load case results is that the "Number of member results sections" value has no effect on the number of such sections reported for moving load case results. This difference arises as a result of the way in which moving load case results are calculated and stored. The results presented for moving load cases are "envelope" results. The Moving Load Tool works by creating and analyzing a series of load cases that place the truck loads at incremental distances starting at one end of the member and moving across to the other end.
The number of points reported for each member is determined by the number of Load Stepping Points per Member. Once the load has been placed at every point necessary, and the analysis has been run, VisualAnalysis searches to find the maximum and minimum values for moment, shear, etc at each point. Then, it stores these values along with the location of the moving load when they occurred. Thus report tables cannot be shortened (interpolated) without losing the location information.
Determining Load Location for a Result
To find where the truck was when a given moment or stress occurred in a member you will need to manually include a "When?" report column. When this column is included in a report, it displays the location of the moving load that produced the result in the column to it's left. To include this column, right-click on the report table, and choose Table Properties from the context menu. Then you can insert a "When?" column into the table.
Then the Member Forces table will look something like this:
Member Case Offset Vy When? Mz When?
ft K K-ft
M1 HS 20 Short +Ex 0.00 -6.47 (On M2 @ 22.990 ft. ) 0.00 (On M1 @ 24.475 ft(Rev.) )
M1 HS 20 Short +Ex 0.75 -6.47 (On M2 @ 22.990 ft. ) 44.33 (On M1 @ 1.5743 ft. )
M1 HS 20 Short +Ex 1.50 -6.47 (On M2 @ 22.990 ft. ) 88.66 (On M1 @ 1.5743 ft. )
M1 HS 20 Short +Ex
2.25 -6.47 (On M2 @ 22.990 ft. ) 130.3 (On M1 @ 2.5941 ft. )
It should be noted that the "When?" report column does not return anything if you are using a truck that consists of only a lane load. This occurs because the lane load, which is basically just a uniform load, is not applied in an easily specified position, but instead could be applied over various parts of the whole member chain to produce the worst case results.