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Using the Support Movement Functionality
Introduction
LimitState:RING is most commonly used as a tool to rapidly assess bridge ultimate load carrying capacity, but the inbuilt ‘support movement wizard’ can provide valuable insights on pre-collapse bridge behaviour. Using this mode, users can explore the likely causes of crack patterns seen in bridges in the field, and also examine potential load paths in a given bridge. This article shows the user how to set up and solve a simple single-span bridge problem using support movement mode. Once familiar with the concept, significantly more complex scenarios can also be considered using support movement mode, e.g. involving multi-ring and / or multi-span arch bridges.
Support Movement Wizard
When the centering supporting the arch barrel of a single-span arch is lowered, the thrust from the arch is transmitted to the supporting abutments. In practice these are liable to move apart slightly and this example explores this case.
Try this:
- First set up the software so that problems are solved immediately after a vehicle is dragged with the mouse. This is achieved by selecting Tools>Preferences>General>Solve after dragging vehicle
- Begin a new project by selecting File>New
- Click Finish to complete the New Bridge Wizard using the default parameters.
- Add a heavy vehicle to the project. Select Tools>Loads>Vehicle Database>European Union Vehicle>EU Triple Axle (2.60m) and click Import.... Click OK then select EU Triple Axle (2.60m) from the Vehicle dropdown list. Click OK.
- Drag the vehicle off the left-hand side of the bridge using the mouse. You will find the load factor is reported as ‘locked’ as, even if the load factor is increased to infinity, the arch will not collapse with the load positioned here.
- Now move the right-hand support outwards by 10mm. Select Tools>Support Movement Wizard then click Pick support block to move. Select the right-hand support block (green) and in the x (mm) column, type 10mm. Finally, click OK.
- Now solve the problem. Click the green solve button, press F5 or select Analysis>Solve.
You will obtain the result shown in Figure. 1. LimitState:RING has identified a suitable load path, together with implied hinge / crack positions, caused by the movement of the support. If a real bridge under consideration has cracks that are different to those predicted by the software, this is likely to indicate a different underlying cause (e.g. differential vertical settlements of the supports). By exploring different support movements, the potential underlying causes of cracks can be investigated.
Figure 1: Line of thrust and hinges in unloaded arch bridge subject to outward support movement
Try this:
- Magnify the displacement using the slider (highlighted in Figure 1) to show the mode of response more clearly. Note that the reported ‘Total work done by moving supports’ is in this case likely to be of much less interest than the observed mode of response (the ‘Total work done by moving supports’ is the horizontal component of the thrust exerted on the support multiplied by the specified displacement).
- Now drag the vehicle with the mouse to various locations across the bridge, as shown in Fig. 2:
Figure 2: Lines of thrust and hinges in loaded arch bridge subject to outward support movement
It is evident that the presence of the vehicle loads leads to migration of the hinges from their original locations. When the applied load is greater the degree of migration becomes more marked, and there is even the potential for complete reversal of the intrados / extrados location of the hinges at a given cross section (the axle loads can be modified by setting the axle load factor via Tools>Partial factors). Clearly the implied repeated opening and closing of hinges under the action of a moving vehicle is likely to be detrimental, potentially leading to rapid deterioration of the structure.
Finally it should be borne in mind that since the original, undeformed, geometry is used in all calculations the support displacements are instantaneous, and it is not currently possible to e.g. determine the amount of support movement required to cause collapse.