Using the Support Movement Functionality
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.
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
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.