how suspension bridges are able to cover so much with relatively few supports.


I assume it works kinda lika keystone. Thanks in advance.

In: 5

A suspension bridge hangs off of the big cables. The big cables hang off of the towers.

That’s really all there is to it. Sure, there are a lot of engineering details, but that’s the basic principle.

It’s the complete opposite of a keystone, actually. A keystone is there to turn the entire arch into a compressive load; the weight pushes down on the keystone, which transmits the force to both “legs” of the arch, which compresses them. So long as the materials for the arch are good in compression, like stone and brick, and you have a stable structure. The force goes from the road to the keystone to the arch to the ground.

A suspension bridge operates using tension, the opposite of compression. What holds the roadway up is the cables that connect the roadway to the huge, heavy suspension cables which are “draped” between the very tall supports. Each of these elements is in tension; the suspension cable is in tension between the two support towers, like a laundry line. The smaller cables are in tension between the suspension cable and the roadway, like a string holding up a plumb-bob. The roadway pulls down on the cable, which pulls down on the suspension cable, which pulls sideways on the support towers. The support towers are either balanced by having a suspension cable on each side or having a support cable to the ground at the end of the bridge. That transmits the load from the roadway to the cables to the suspension cables to the towers to the support cables to the ground.

What you don’t see in most suspension bridges is where these relatively small cables end up.

The cables transmit most of the forces of the bridge weight and vehicle weight along their lengths and these cables end up buried in tons and tons and more tons of concrete which is connected to the bedrock with very deep steel rods.

So the bits you can see are small, but the bits you can’t see are **massive**.