What is a Suspension Cable?
A suspension cable is used to support large bridge spans. Created from thousands of small steel cables wound into one large suspension cable, the roadway of the bridge hangs securely from the cables. Using huge concrete blocks buried at each end of the bridge, the suspension cable's strength is displaced along the entire length of the bridge. In long bridge designs, the suspension cable allows the bridge to sway slightly in the wind instead of fight the shear forces that would eventually destroy a rigid span.
While the bridge benefits from the strength of the suspension cable, it does not actually connect to it. The bridge roadway is suspended far below the suspension cable by smaller drop cables. These drop cables are connected to both the bridge and the suspension cable by large clamps and bolts. By using many drop cables to support the weight of the bridge roadway, each drop cable is responsible for suspending only a short section of roadway.
The maintenance on a suspension cable bridge is never-ending. The large cables are inspected regularly for frayed cables, rust and corrosion. Special paint intended to fight the corrosion and rust is used to cover the cables. On many large suspension bridges, the painting of the cables is an ongoing task which is done 365 days a year. Workers who are not afraid of heights are tasked with actually walking the giant cables from one end of the bridge to the other. These cable walkers spend their days checking for any areas that might require attention.
Suspension bridges spanning saltwater are especially prone to cable rust and corrosion. These bridges utilize a rubberized coating instead of paint to protect the steel cables from the harsh saltwater elements. Bridges, such as the United State's Golden Gate Bridge in San Francisco, California, take their distinctive red coloring from the special coating protecting them from the elements. This coating is applied not only to the cables, but to the steel towers and roadway structure as well.
Another US structure, the George Washington Bridge in New York City, is said to have enough steel cable wound together to form the suspension cables supporting the roadway that it would circle the Earth at the equator. It took workers more than a year to wind the cables used in that bridge alone. Beginning as a single strand of small diameter steel cable, the cables were wound into the large components that are in use today.
@nony - I haven’t heard of a single suspension bridge that has collapsed. Perhaps there have been some, but I haven’t heard of any. However, I have heard of bridge collapses with regular piering supports, the kind that you think are safer than the suspension system.
At least to me that suggests that the suspension bridge cables are strong enough to secure the areas of the bridges they’re secured to. Yes, it takes constant maintenance but maybe that’s why we haven’t had any accidents. I think with regular bridges there is more of a tendency to set it and forget it, because you think the pier support is so rock solid.
@Charred - I think your concerns are valid. That probably explains why such a suspension system is rarely used for bridges. I can understand why it’s used in the Golden Gate bridge given its large span, but you don’t see these systems used in too many other places.
Engineering and safety concerns aside, there’s the issue of finding people who want to work on those bridges. How many people do you know who would like to climb the heights of a suspension bridge? I certainly wouldn’t.
Suspension cable hanging systems for bridges have never inspired that much confidence for me. I am relieved to know that the cables are not strictly responsible for supporting the bridge.
Still, the very fact that the cables are providing some support is a little disconcerting to me. It’s like the feeling of riding an extremely tall roller coaster with clanking wooden rails that rumble as the roller coaster makes its ascent.
Perhaps from an engineering standpoint it has structural integrity, but it can make your stomach turn. To me a bridge should be one hundred percent supported by beams from beneath. I don’t want to cross a bridge and watch cables swaying in the wind.
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