The null point is the position of zero movement in the bridge deck. When the bridge deck is pinned at a single pier, it provides the location of null point with no deck movement. However, when the bridge deck is pinned to more piers, the position of null point has to be calculated. The determination of null point is important because it serves to estimate the forces on the piers by deck length changes and to calculate the sliding movement of sliding and free bearings.
For symmetrical deck founded on two identical fixed piers, the null point should be midway between the two fixed piers. However, if one pier is taller than the other, the null point would be shifted to the shorter stiffer pier.
This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.
If you have a query, you can ask a question here.