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Bridge loading and analysis

Getting started

To set up a bridge analysis in GSA you will need the following data:

AxisCan be global, no need to define.
Grid plane/grid surfaceGrid plane defines the geometry of a surface, while grid surface defines how loads will be applied to elements within a plane.
AlignmentThe setting out feature for bridges, defined in chainage, straights or radii.
PathsA track on which a vehicle can travel. A carriageway carries out full automatic analysis. You can also do lane-by-lane analysis for more complex cases.
VehicleDefine if using non-standard vehicle.
Bridge VUDLDefine if using non-standard VUDL.
Influence effectsA displacement or force at a particular point. A separate influence line is drawn for each one, and gives the effects of moving load at different locations along a path.
Path loadingOnly needed for lane-by-lane analysis.

Tip: It can be easy to generate 100s of load cases, so be selective when choosing influence effects, to speed up processing time.

Creating your bridge model

1. Define your axes

Bridge analysis - Step 1: Define your axes

  1. Open GSA and click New blank model.

  2. Select Layer from the dropdown menu above the graphic window, and choose Analysis.

  3. In the Explorer pane, select Geometry > Axes, enter your axes vectors.

  4. Click on Labels and display methods icon

  5. In the Axes tab, tick User axes.

2. Define grid plane and grid surface

Bridge analysis - Step 2: Define grid plane and surface

For this step, the grid plane sets up the basic geometry while the grid surface defines how the loads will be applied to elements within that plane.

  1. Select Explorer pane > Geometry > Grid planes, and enter your values in the grid plane table.

  2. Under the Axis dropdown menu, select the name of the axis label you have just created. Finally, add your elevation to the table.

  3. Return to Explorer pane > Geometry and select Grid surfaces.

  4. Enter your values in the grid surfaces table.

  5. In the Grid plane dropdown, select the grid plane you have just set up.

3. Define alignment

Bridge analysis - Step 3: Define alignment

  1. From the Data tab of the Explorer pane, select Bridge. In the video above, this is shown on the bottom left of the interface. Choose Alignments and select a name.

  2. This brings up the alignment table. Under Grid surface, select the grid surface you have just set up from the dropdown list.

  3. Continue adding values to your table until you have a curve.

  4. To view your curve select the Diagram settings tool: icon In the video above this is shown on the right of the graphic view.

  5. In the dialogue box, you will see an option to click Alignments and paths.

  6. Your curve will now be visible on the graphic interface.

4. Adding nodes and elements along an alignment

Bridge analysis - Step 4: Adding nodes

  1. In the Data tab of the Explorer pane select Nodes to bring up the nodes properties table.

  2. Add a node at the desired location by typing in its global coordinates.

We will now use the extrude function to create additional nodes.

  1. Click on the Select nodes icon: icon In the video, this is shown on the left.

Tip: Click and hold Shift to select multiple nodes at once.

  1. In the top menu select top menu select Sculpt > Extrude selection. In this example, we will extrude along the Y direction first.

  2. This will open the Extrude dialogue box. Tick Axes then select the axes button.

  3. Tick User and wait for your label to appear in the dropdown, then hit Ok. You can now tick the Y axis.

  4. Assign increments and length and make sure you have ticked to include beam elements along the extrusion. Hit Ok.

Assign element properties

  1. To change the elements shown on the interface, click on the Select elements tool icon to the left of the graphic display.
  1. Then enter a new property number in the right-hand properties panel.

Extrude elements along alignment

  1. Click and drag to select your elements. Then from the top menu, go to: Sculpt > Extrude selection to extrude the elements.

  2. Specify the direction of extrusion as your desired alignment by ticking Aligment and specifying which alignment number to extrude along. Be sure to also click Include transverse beam elements and Use existing nodes then hit Ok.

Adding supports

  1. Click the Select nodes icon, icon then click and drag on your grid to add supports.

5. Adding section and material properties, loading and running static analysis

Bridge analysis - Steps 5 & 6: Model behaviour

This section shows how to add the remaining information needed to create a model that can be analysed.
  1. In the Data tab of the Explorer pane go to Properties > Section library. Select the Wizard icon, icon or double click a cell to bring up the Section component definition dialogue box.

  2. Select your material from the dropdown in the table, then click Properties to choose a shape.

  3. Return to the Explorer pane and choose Specification then, Design.

  4. Choose a default concrete grade and rebar from the dropdown.

  5. To define the grade, select the Wizard tool, icon then select your grade from the dropdown.

  6. To add gravity loading, go to Explorer pane > Data > Loading > Gravity loading, and enter gravity loads in the table.

6. Run analysis to check model behaviour

  1. When complete, click the Analyse button in the top toolbar icon

  2. To view your model in the Analysis layer, select Window from the top menu, and choose your model from the dropdown list.

  3. Click the Isometric icon icon to change the graphical output.

  4. Select your analysis case from the Cases dropdown menu, above the graphic display.

  5. Click the Bending moment icon icon to bring up the bending moment diagram.

Bridge loading

1. Specify the bridge loading you will use

Bridge loading - Step 1: Specify bridge loading

  1. Explorer pane > Data > Specification > Bridge loading.

  2. After selecting a bridge design loading code and accepting the number of influence points, choose your vehicle type (if applicable) and, for generic EC1 loading, the Nationally Determined Paramters (NDP).

  3. Return to the Explorer pane and select Bridge > Paths. Enter the type of path(s) you are using, and their properties.

Tip: Highway engineers always measures things looking in the direction of increasing chainage and positive to the right.

  1. Exit the table to return to the graphic interface. Lanes, carriageways and footpaths will be visible as green dotted lines.

Note: Remember to keep your left values more negative than the right values, otherwise your dotted lines will show as red.

2. Set up influence effects

Bridge loading - Step 2: Influence effects

  1. Explorer pane > Bridge > Influence effects
    • Set up node effects
    • Set up beam effects

Tip: When filling out multiple cells in a table, use the double equals symbol (==) to autofill the rest of the line to be identical to the one above.

  1. In Node effects, you can apply properties to the four nodes across the pier.

  2. Once complete, this table will maximise for the sum of the vertical reactions on these four nodes.

  3. For individual bearing reactions use a different effect number for each node.

  4. Click the Diagram icon icon to draw the influence effects.

  5. In the dialogue box go to Bridge options to adjust your node/beam effect options.

Note: A moment is displayed on your graphical interface as two orange arrows.


3. Run bridge analysis

Bridge loading - Step 3: Run bridge analysis

  1. From the top menu select Analysis > New analysis task.

  2. In the dialogue box tick Bridge load optimisation, then click Next.

  3. Select the steps you want to run. Or use the default steps suggested.

  4. Click Analyse now to view your results.

Tip: Clicking 'Retain results at influence points only' will reduce the volume of data when processing.

4. Review results

Bridge loading - Step 4: Review results

  1. After running analysis, click the Isometric tool icon (to open a new graphic view).

  2. Select Layer from the Display dropdown menu and choose Analysis from the other dropdown.

  3. View your load and analysis cases in the Cases dropdown.

Note: BIE is beam influence effect, NIE is node influence effect.

  1. Influence lines can be viewed graphically by selecting the influence option in the Cases dropdown menu, then clicking the Diagram tool icon followed by Global results > Influence lines:

  2. To view output tables, select the Output tab at the top of the Explorer pane.

  3. Click on Bridge and select from the list below the output you'd like to see.

5. Envelope results

Bridge loading - Step 5: Envelopes

  1. Return to the Data tab of the Explorer pane and go to Tasks and cases > Combination cases.

  2. Select a cell, then click the Wizard tool icon

  3. Highlight your selected field, then select Envelope. Name your envelope, then hit OK.

  4. Your envelope will now be visible in the cases dropdown menu.