Bridge analysis methods

For bridge analysis the live load requirements are very different to those for ordinary structural analysis. A number of tools are available in GSA to simplify the generation of these load cases. Three different ways of using the tools are available for different design codes and different situations:

1. The most powerful tools are available for specific design codes and constant width carriageways.
2. For other design codes or more complex carriageway geometry, lane by lane optimisation is possible.
3. When an influence analysis is not appropriate, loads can be placed directly on a structure.

The steps to be carried out are described below and should normally be carried out in sequence:

Method A

For UK or HK design loading (HA and HB), assessment loading (HA and SV), EC1-UK loading (LM1 and LM3), generic EC1 loading, Australian (AS), or US (AASHTO or LRFD) only on constant width carriageways

1. Use the Bridge specification dialog to select the design code, limit state(s) to be considered and loading type.
2. Define a User axis – as method B
3. Define a Grid plane – as method B
4. Define an Alignment – as method B
5. Define a series of Paths – carriageways/footways only (otherwise as method B)
6. Define influence effects – as method B
7. Carry out a bridge load optimistion analysis which will produce analysed grid loading load cases and combination cases.

The bridge analysis generates data for a number of input modules. These are generally deleted automatically when the influence analysis is deleted.

Note: For large files with many influence effects the bridge analysis may take a long time.

The process may be split into two stages by carrying out an influence analysis (which includes optimisation in this method) and reviewing the influence lines and generated static bridge loads before expanding bridge loading and carrying out the final static analysis. Note that the user will then need to create combination cases manually, so this is only recommended for large models where the user wants to make manual changes to static bridge loading (for example to delete cases which will not govern design by inspection, such as sagging at supports). If the user wants to make minor alterations to the generated loading, without deleting cases, the recommended approach is to carry out a full bridge analysis first, then delete the static analysis cases (using the Analysis wizard, not Delete all results which will delete the carriageway paths and static bridge loads as well) and grid loading (using the Delete grid loads tool), then modify the static bridge loading, and follow steps 11 and 12 of method B.

Method B

To optimise on a lane by lane basis for any design code

1. Use the Bridge specification dialog to set the design code to Undefined.
2. Define a User axis with origin at the start of the alignment, chainage 0 and the x axis in direction of increasing chainage (or arrange the model so that the alignment starts at global (0,0,0) in the Global X direction).
3. Define a Grid surface on which the load is to be applied. Note that all bridge loading is applied in the Grid surface, and that the elements selected in this plane need to either all be 2D elements or all form a simple grillage with 1D elements only that meet at nodes. Further details of these restrictions are given under Grid point loads.
4. Define an Alignment, defined by a series of curvatures at chainages and the Grid plane.
5. Define a series of Paths – lanes/footways/tracks/vehicle path (defined by offsets from the alignment, note that positive offsets are to the right of the alignment, looking in the direction of increasing chainage, not in the direction of positive local y) along which bridge loading is to be moved. Paths need to be within closed panels of the structure (or within the 2D element mesh), except at the ends of the structure.
6. Define a series of Node influence effects or Beam influence effects that are to be maximised/minimised by the optimiser. To optimise for stresses in 2D elements, add small beams joining adjacent nodes. For axial stress a single beam is sufficient, for bending stresses a straight line of beams is needed.
7. Define a series of Path loads that are to be applied to paths.
8. Carry out an Influence analysis using the Bridge option in the Analysis wizard.

Tip: It is often useful to save the results at this stage to allow a quick return to this point.

9. Use the Optimise path loading tool to derive static bridge loads for each path load and each influence effect.

Tip: Increase the default group number by one to allow the group number and load case number to be made identical to the analysis case number, see step 12.

10. Use the Expand bridge loading tool to convert static bridge loads (or moving bridge loads) to Grid loads. Note that only the grid loads form part of the input data to a static analysis, and changes to the other data described above will not have any effect on subsequent analysis unless the old grid point loads are deleted and new ones generated using this tool.
11. Carry out a static analysis of the grid load cases using the Analysis wizard.

Tip: Create default analysis cases then delete the dummy load case created at step 10 to make analysis case numbers identical to load case numbers.

12. Using the Total effect information in the Static vehicle loads module as a guide, set up Combination cases to give the worst effect due to simultaneous loading on several paths.

Having gone through these steps once for a model it is sometimes useful to use the GSBridge dialog box to carry out steps 9 to 12 in one operation. Expanding bridge loads generates grid loading and 2D polylines. The Delete grid loading tool allows these to be deleted quickly before carrying out another analysis.

Note: Generated static bridge loads are not deleted automatically with the influence analysis Results in this method, and should normally be deleted before carrying out another influence analysis.

Method C

Placing load directly on the structure

1. Enable bridge analysis – as method B.
2. Use the Bridge specification dialog to set the design code to Other.
3. Define a User axis – as method B
4. Define a Grid plane – as method B
5. The Generate static vehicle loading tool can be used to place a vehicle directly on the Grid plane, otherwise:
6. Define an Alignment – as method B
7. Define a series of Paths – as method B
8. The Generate static vehicle loading tool can also be used to place a vehicle directly on a path, or offset from an alignment, otherwise:
9. Vehicles can be moved along paths at regular intervals as Moving bridge loads and Static bridge loads can be placed directly on paths.
10. Not used
11. Expand bridge loading – as method B.