To Build a Bridge Part VIII: Spanning Over it All

The main function of a bridge is to provide a navigable path over some dip in the terrain.  Be it canyons, water, traffic or anything else: the key component to the bridge is the "superstructure", the part that takes you over the open air.  As has been mentioned previously, there are many different systems that do this, but for this bridge, prestressed girders are used.

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.

These are concrete beams that have very high strength, steel strands running through them.  Reinforcing steel in concrete is normally "60ksi" steel.  That means it yields when 60,000 pounds ('k' represents "kips", which means 1,000 pounds) of force are applied to one square inch of it (yielding doesn't actually mean "failure", but we very rarely design above yield strength of rebar, and only for extreme events when we do).  The steel strands used in prestressed concrete have an ultimate strength of 270ksi.

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.

Though prestressed girders can theoretically be of any shape and size, the designer is normally limited to just a few selections.  The DoT normally designs a few general shapes that they think are well suited to specific span lengths (the longer the span, the bigger the beam shape is).  The designer than chooses where or how much steel to put in that shape to make his or her bridge work.  By standardizing just a few shapes, local fabricators can build just a few forms of the appropriate shape, and then crank out beams quickly and cheaply.  For the Greenfield bridge, I choose "45W" girders.  This is WisDoT's specification for a girder that is 45 inches (just under four feet) tall and "wide flange".  These are pushing their limits at the ~107 foot span I had, but with enough steel they were up to the task.  The bigger the shape, the more expensive the girder.  More importantly, though, the bigger the shape the higher the road needs to be above a underlying highway, since the bigger shapes will impinge upon the clearance of the traffic below.  Thus, the smaller the girder you can reasonable use the better.

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.