I'm an engineer. The cantilever design of the 35W bridge is very flimsy compared to the concrete arch bridge right next to the one that collapsed. Look at the concrete footings of the 35W bridge, they are very small. If those concrete footings or the support steel were to give way, the whole bridge could go down, like a deck of cards. Typical engineering rules of thumb designs for at least twice the worse case loading, and double fault protection.