|Prestressed girders stacked together. At the ends are the prestressing strands. Peeking out on top are "shear connectors", they help the beams integrate into the concrete deck that is poured on top of them.|
A form is prepared for the correct shape and length of the beam. Steel is put in: both the prestressing strands and standard reinforcing that will help with various forces and to make sure the beam doesn't crack or spall through temperature or chemical demands. Then the strands are pulled on: adding tension to them. Once the strands are at the correct stress concrete is poured into the forms around the steel. Once the concrete has set, the strands are released and that tension transfers into the concrete, compressing it.
|The forms surround the beams are strong, steel forms since they are compressed by the pretensioning of the steel strands.|
Prestressed girders have become the dominate bridge type. Because of the ability of precasting plants to turn so many out so fast: they are very cheap. By using fast-setting concrete, every form is expected to turn out one girder per day. And do so for very little money once the initial investment in equipment has been made. Almost every bridge from 40 or 50 feet to 150 feet is made out of prestressed concrete girders. Bridges that aren't straight, or have some other, odd feature may still use another system. But most bridges in that range (and most bridges are in that range) come from prestressed concrete.
Concrete is very good in compression, and very bad in tension. That is the fundamental principle that underlies prestressed concrete. It forces the concrete into compression before any load is applied, and allows for concrete to do what it does best.
Technically, prestressing doesn't actually make the beams any stronger than if you simply put all the steel in without prestessing it. The theoretical, final failure would occur at the same load. However, by prestressing it you're making it so that the girders don't bend, deform and crack under normal loading. They are much stiffer, and much more resilient and elastic with prestressing.
So how much are the girders prestressed? Well each bridge has to be designed, but for my very modest, Greenfield bridge, each girder had about 1.5 million pounds of force applied to it through prestressing. That means the strands were pulled on with the weight/force of 750 tons, or over 400 cars. As a point of reference, Greenfield had 16 girder lines and sports a surface area large enough to hold around 75 cars parked on top of it.