Probably the question I get asked the most is “What bridge design holds the most weight?” or “What bridge design is the strongest?”
The answer to that question is not simple and probably is not going to be what you expect. The truth is, I cannot answer the question. There are too many variables that are not being defined.
Is the the Pratt truss or the Warren truss stronger? Actually, the answer is up to you. You can make the Warren truss stronger. Or you can make the Pratt stronger. It depends simply on the strength of the wood you use. You can use 2×4′s to build the Warren, and toothpicks to build the Pratt. Obviously the Warren is going to be stronger in this case.
A Better Question
I think though, many people are trying to ask whether or not one of these trusses has an inherent advantage over the others. In my mind, none of them do. If you look at my truss design page, you will see that each truss spreads a load out differently, but none with an apparent advantage. Conclusion: no bridge design has an inherent strength over another.
However, this conclusion only applies to this general setting. It may be that in a specific situation one bridge design would be better suited than another. It is up to you to examine how each truss works and decide which one to use.