Converting Bridge Object to Unlinked Model
The "best practice" approach to use the Bridge Modeler is to develop the model by using the parametric definition of the Bridge Object via commands in the "BrIM" menu. If additional flexibility is needed, you can always use "BrIM > Update Linked Bridge Model" menu command to "Convert to Unlinked Model" and make additional changes manually. By doing so, you are however loosing the advantage of parametric definition for modifying the model and for reviewing the results. Therefore, it is recommended to use this approach only when the model is fully finalized. In addition, it is also recommended to create a copy of the file before converting to unlinked model, such that you can always go back if changes to the parametric bridge definition are needed.
Verifying Results of Moving Load Analysis by Static Load Case
Results of step-by-step and influence-based moving load analyses my be verified by running a simple static load case with the static loads representing the truck placed to the location causing maximum effect for a desired response quantity.
Although the values will be different, as the result of a moving load analysis is envelope of internal forces for multiple truck locations, while the static load will produce internal forces only for one truck location (depending on where you apply the static load representing the truck), this analysis may serve as a simple check. The static load case should always produce results smaller or equal to the the moving load analysis.
For illustration purposes, let's say you are checking midspan moment on a simply supported beam due to a unit moving load. The midspan moment obtained from moving load analysis will match the midpan moment from the static load case only if the unit static load is placed at the midspan. But for real structures, the truck location causing the extreme forces may not be that obvious.
Moving Load Envelope for Nonlinear Model
You can obtain moving load envelope for nonlinear model by applying BRIDGE LIVE load pattern in a NONLINEAR TIME HISTORY load case (analysis case in V11). The key steps for setting up such model include the following:
- Define load pattern (load case in V11) of type BRIDGE LIVE. Use small vehicle speed if you wish to disregard the dynamic effects.
- This will automatically create a Multi-step static load case (analysis case in v11). Change this to a Time-History load case and set analysis type to nonlinear, direct-integration. Set the damping and other parameters as necessary.
- When plotting the results, choose Envelope option.
Multiple Vehicles Acting Simultaneously in a Single Lane
Multiple vehicles acting simultaneously on one lane can be modeled in SAP2000 by specifying a special vehicle on the General Vehicle Data Form. This special vehicle would be defined such that it contains the two vehicles separated by a fixed or variable distance between them. For example, definition of two HL93 trucks would consist of six axles: axles 1 to 3 would represent the first vehicle and axles 4 to 6 would represent the second vehicle.
Pinned condition for both abutment and bearings
- The bridge abutment foundation spring property was defined as pinned. At the same time the bearings at the abutment location were defined as pinned. This causes local instability since the common joint to which both of these links are connected does not have any rotational stiffness and nothing prevents this joint from spinning about its own axis. The solution would be to re-define one of the links (either abutment or bearing) as fully fixed.
Mixed Models - Very Basic Introduction
For typical bridges, the entire bridge model can be developed within the bride modeler, i.e. using menu commands available in the Bridge menu. When you however need to model aspects not available in the current version of the bridge modeler (such as the superimposed loads or parapet loads), these need to be modeled "outside" of the bridge modeler, using other menu commands. In this case, you would be developing what we call a "mixed model" in which the bridge object created by the bridge modeler co-exists with regular modeling.
Miscellaneous Tips
- In general, anything you change that has been created by the bridge modeler will be overwritten when the linked model is updated.