Why are many cable-stayed bridges anchored to only the center of the roadway, while basically all other bridge designs connected to the outside of the roadway?

Question

I’m sure if other bridge designs (suspension, truss, etc.) could safely have just a central support, they would, as it would cut down material costs by a lot, but, it seems basically only cable-stayed bridges sometimes have this design. Googling various versions of the question, I couldn’t even find some sort of advanced engineering explanation of the forces, nonetheless something I could understand. Hopefully one of you can?

In: Engineering

Answer 1

You could have a suspension bridge with just one cable. The problem is how would you stop a bridge hanging from one cable from wobbling. The deck has little stiffness as each bit is hanging directly from the cable. Cable stayed bridges have each piece cantilevered from the tower, not hanging from the cable, and the bridge deck itself transfers force along the bridge.

As bridge spans get longer, cable-stayed bridges need higher and higher towers to keep the forces on the cables and on the deck sections manageable. Eventually it becomes more practical to make smaller towers, string cables over them and hang a suspension bridge from them. By the time you have got here, it makes a lot more sense to have two cables so that each deck span is suspended from both sides (making them stable), than to have one cable and find some other way to damp any swinging motion.