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?

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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

2 Answers

Anonymous 0 Comments

A suspension bridge have a single or pair of wires that hold up all of the bridge deck. It is under load even when the bridge deck i not installed and then the load changes during construction. When there is a load on one part like a heavy truck result in the wire slightly change position, So the bridge decks need to be relatively flexible

In cable stay bride there are multiple wires that work independently, which mean there is less motion. The force on the bridge segment is alos not vertical like in a suspension bride but has a horizontal part too. So the bridge segment needs to be stronger to handle horizontal compression, in a suspension bridge segment can hand independently from the vertical wires and there is no horizontal force in the bridge

The result is the cable stay bride can be stong enough so the bride deck can handle twisting forces without adding a lot of material. In a suspension bridge that can have a lighter deck its tickets and the lack of horizontal compression on the bridge result is that they would twist more if there only was a single wire support.

In general the same material can do more works the farther it is away from the center. So a truss bride likely use less material if you haver steel on both side compare to just in the middle. It alos make the road part simpler with a single open span.

Anonymous 0 Comments

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.