The curve redirects the push. If the dam was straight or if the bridge did not have the curved support underneath it, the force would be pushing directly through them. With the curve, much of the force is redirected along the curve. You can see this with a paper cup. Press on the side of the paper cup. You have to press fairly hard to get it to dent. Then cut the cup apart, and flatten the paper across the space between two books. Press downwards. You should find that it is much easier to bend the paper now.