Question
I have a bridge model that contains backing over the supports. When I increase the height of the backing I would expect to see an increase in the capacity of the bridge / forces in the piers; however, this does not happen - why might this be?
Solution
There are two reasons why increasing the amount of backing in the model may not result in an increase in capacity:
1. The mode of response is such that backing is not utilized.
In order to have an influence on the solution, the backing needs to be in compression. If the mode of response of the bridge is such that the backing is in tension, or very low compression, then increasing the height of it will not have much effect. For example, this can occur in a multi-span bridge where the abutments are spreading apart.
2. The backing height has not actually been changed in the model.
LimitState:RING models backing as a bar element (rigid in compression but with no tensile capacity) attached to the centroid of a voussoir block. Entering the geometry dialog and increasing the height of backing above an abutment or pier by a small amount may not be sufficient to allow the backing to reach the level of the next voussoir centroid. To remedy this, either increase the height of the backing further, or increase the amount of blocks in the span such that the distance between centroids is lower.