Question
I am analysing a multi-span bridge where the springing level of the spans changes across the piers. How can I model this?
Solution
It is possible to achieve this by modelling part of the pier (the step up or down) within one of the spans. This is done by adding two extra points to the end of a user-defined profile e.g. see Spans 1 and 3 in the attached file. The process can be a little tricky to perfect. Remember to assign additional blocks to the span so that there are sufficient number in both the arch and the vertical section.
Once you have achieved a geometry that you are happy with, you should check to ensure it matches the real bridge as closely as possible. It is also prudent to fix the blocks in the vertical portion of the span against sliding and hinging, as these are part of the pier. Similarly, where the contacts are rotated beyond the horizontal (e.g. at the right springing of Span 3 in the attached file) you should fix these against sliding and hinging also in order to suppress non-credible failure mechanisms.
Note that it is seldom necessary to model all the spans in a multi-span bridge in the same model - typically a loaded and adjacent unloaded span is sufficient.
