Analogies are a great way of discussing problems that you don't quite understand in terms of something that you do. Also sometimes analogies can be poorly choosen. I think we've got a bit of both here.

I think a bridge is a poor analogy because a bridge is a whole system, a hash function is a component of a whole cryptosystem. So if for hash function we substitute steel rod and consider this rod in the system of a bridge I think we've got a better analogy.

This immediately throws up how ignorant we are about whole cryptosystems. We understand how a steel rod in a bridge behaves - finite element analysis has become so good that the other day I was looking at 3D renderings of how an element from a car's crash absorber would collapse under certain loads alongside actual photographs of the same part experimentally tested and the crushed and twisted metal matched the prediction fold for fold. I defy anyone to make equally good predictions about cryptosystems of practical levels of complexity.

Back to the actual analogy. When choosing a steel rod as part of a whole bridge system there are elements beyond the simple mechanical strength of that part versus the applied load. Is the load multiplied by stress raisers such as bolt holes or welds? Is the applied load below the fatigue limit for this material? What is the strength of the material after it has been corroding for twenty years?