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This page is devoted to frequently asked questions (FAQ) related to moving-load analysis. |
On this page:
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General
General frequently asked questions are listed as follows:
How
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are transverse moving
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loads
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applied?
Answer: Transverse (horizontal) moving loads are available since CSiBridge V17.2.0. Braking, acceleration and centrifugal horizontal loads can be defined.
How is the speed of a moving truck changed?
Answer: For
If absolutely necessary, you could try evaluating lateral moving loads in a separate model by the following manipulation:
- Create a copy of your model and rotate the entire model by 90 deg about global x axis. This rotation will align the lateral direction of the actual structure with the gravity direction in the SAP2000 model.
- Apply and evaluate moving loads. I would recommend using zero width lanes, as this would enable you to reuse lane definition from the original model.
- Manually (outside of SAP2000) superimpose the response from the lateral moving load analysis with the results obtained from the original model.
How can I change speed of the moving truck?
You can define the speed of a moving truck for step-by-step moving load analysis by entering the desired value of speed on the the "Multi Step Bridge Live Load Pattern Generation" form. Please note that you would first need to define -load analysis, once a load pattern of type BRIDGE LIVE and then click modify bridge load button Bridge Live type is defined, select Modify Bridge Load to access the "Multi-Step Bridge Live Load Pattern Generation" form form in which the speed of a moving truck may be specified.
When
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a vehicle
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is defined, how are two directions specified for its path of motion?
Answer: Depending upon analysis type, direction may be defined as follows:
- For influence-based moving-load analysis
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- , in which Moving Load is specified for load-case type, the vehicle
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- automatically
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- moves in two directions.
- For step-by-step moving-load analysis
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- , in which Multi-step Static is specified for load-case type, Forward or Backward vehicle direction may be specified on the Multi-Step Bridge Live Load Pattern Generation
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- form.
- To obtain
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- Multi-step
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- Static response for a vehicle which
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- may travel in either direction, results from Forward and Backward patterns may be enveloped.
Why are my bridge objects returning zero force during moving-load analysis?
Answer: The request for saving section forces should be made such that bridge objects return the proper response during moving-load analysis.
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Influence-based
Influence-based frequently asked questions are listed as follows:
How is the load from each truck axle distributed over shell objects?
Answer: During influence-based moving-load analysis, an interpolated influence surface determines the effect of loading from each truck axle. For additional details, please see the Influence-based moving-load analysis implementation article.
How are unit loads distributed from lane-load points to model joints?
Answer: For response, please see the Influence-based moving-load analysis implementation and Lane-load point connection articles.
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Why does deformed configuration from moving-load analysis not look correct?
See Deformed shape for moving load analysis.
Why are zero results generated for a bridge object which is updated as a solid model?
Answer: Analysis > Bridge Response > Moving Load Case Results Saved > Section Cuts must be selected for solid models, while it is not necessary for frame and shell models.
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Related Incidents:
Question 1: How does the program place the vehicle to calculate maximum effect? What algorithm is used? Answer 1: OPEN QUESTION |
Step-by-step
Step-by-step frequently asked questions are listed as follows:
Why does step-by-step moving-load analysis not run when the vehicle is defined using a uniform load?
Answer: Vehicles defined using a uniform load may only be used during influence-based moving-load analysis. To use such a vehicle in step-by-step moving-load analysis, the uniform load would need to be approximated and modeled using a series of equivalent point loads.
Influence-line
Influence-line frequently asked questions are listed as follows:
Why is the influence line not displayed as a smooth line?
Answer: Influence lines will appear piecewise linear when discretization is more coarse. A similar effect occurs when output-station spacing is fairly large for moment-diagram plots. The effect of refining lane discretization (Figure 1) is apparent in the influence lines which follow (Figure 2):
Figure 1 - Lane data
Figure 2 - Influence line
Step-by-step
Influence-based
How does the program place the vehicle to calculate maximum effect? What algorithm is used?
Open Question
How exactly are the unit load distributed from from lane loading points to the model joints?
See Influence-based moving load analysis implementation
What are good numbers for Max. Lane Load Discretization Lengths Along Lane/Across lane? I mean the optimum condition for getting accurate results and being efficient.
Influence analysis can be quite time-consuming, so using the largest discretization size possible is recommended, especially for preliminary analyses. For overall superstructure and substructure behavior, discretization on the order of 1/10 of the span length and 1/2 of the lane width is usually sufficient. For more local effects, a smaller discretization may be needed. Plot the influence lines for a few representative quantities to help see what you should be using. Again, start with larger values so that you can run many early analyses, then refine the discretization for your final results.
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