On a trip to Omaha, Nebraska, I was able to visit a pedestrian bridge. This bridge linked Nebraska and Iowa, crossing the Missouri River. The modern marvel that made this bridge possible is high strength steel.
Typically, structural steel is specified at 36 or 50 KSI, although you can get up to 70 KSI or higher. One KSI is equal to one thousand pounds per square inch; that is a lot of stress! The steel wires you see in the bridge have an ultimate tensile strength of around 290 KSI, that’s two hundred and ninety thousand pounds per square inch of stress that the wires can withstand in tension. Ultimate strength means that the wires can withstand the stress in tensile, not compression.
Steel this strong is not like typical structural steel, in that there is no yield plateau or strain hardening. This material makes long spanning bridges possible. It is also used in pre-stressed concrete. Until recently, the materials available did not have the properties necessary to make pre-stressed concrete. The materials did not have enough strength and had to much strain to be effective in the use of pre-stressing elements.
This super high strength steel is made of cables gathered into strands. In pre-stressed concrete, a seven-wire strand is usually used. In bridges, especially highway bridges the strands can be several inches thick. This new material is a great achievement and has opened up many new possibilities for architects and engineers. Just think what the possibilities for the future are.
No comments:
Post a Comment