Dear Jorge,
Structural dynamics is just that, no matter what structure, be it a bridge or something altogether different. As I remember, the first time I had to resolve the problem of how to include a gap in a system was in 1975. It is coming back, now and then, like a 5-year tide.
Your total restraint at each end of the bridge is made up of soil, abutment, rubber pad and a gap.
- Both the pad and the gap are N/L elements. That means that if you insist on the spectrum response method, you have to develop an equivalent spring, representing all four elements.
- That linear spring is forcing-dependent, valid for a specific forcing or a specific response spectrum.
- The method of finding the equivalent k is a bit round-about, but quite rational.
- The reason for concern is when a method used makes the seismic effects smaller, than they really should be. With a rubber pad it can go either way, i.e. you can artificially increase or decrease the response.
- There is less doubt with a gap. If you choose to ignore it, for computational convenience, you are artificially increasing the natural frequency. This usually results in decreasing of the computed response, which is unconservative and unsafe.
- Unless the people who did those analyses know how to handle the two nonlinearities, the inclusion of only the linear stiffness component is unsafe.
- One of the ways to avoid creating equivalent springs is to conduct a time-dependent N/L simulation, but that is luxury for most engineering offices.
- If you wanted to see many examples of practical handling of nonlinearities, you could borrow from a library my first book, entitled "Dynamics of structures and machinery. Problems and solutions", Wiley 1982.
- I am not in a position to comment on political and legal aspects of the situation.
Still, I believe that I confirmed your suspicion with regard to the quality of the results.
Sincerely,