Why the Bridge beam support looks different from the building beam at support 2

[johndeng]

Hi I only deal with buildings. When I pass by the bridges, I can see all the beams are sitting high above the abutments. It looks like some details to lift it up at the support bearing. What's the considerations? For building beams, it only sits on a thin bearing plate.
Thanks

 

[EngDM]

At bridge supports the reason you might see a gap from a distance is that they have a special bearing pad or hinge connection to allow for expansion and contraction of the bridge deck, which is typically a much thicker assembly than just your typical 3/4" steel bearing plate that you might see in a steel structure. These connections are closer to actually being true pinned connections, as opposed to conventional steel frame where you just kind of assume pinned, but in reality it is semi-fixed/rigid and will offer some moment resistance.

The plates/assemblies that these bridges sit on are $$$ from what I recall.

 

[Lomarandil]

Accommodating the thermal movements is the real driver behind most bridge bearings (at least for non-seismic regions). Getting true pin behaviour is incidental in a lot of cases.

 

[BridgeSmith]

Many older bridges used a pin and rocker bearing system, which typically requires an 8-10 inch radius on the rocker to keep the contact stresses in an acceptable range. The height was often matched for the fixed bearings to keep the abutments similar.

Also, bridge girders need to have a significant clearance from the ground (earth berm), so tall bearings minimize the amount of concrete needed for the abutment.

The shift from relatively cheap labor and expensive materials 50-60 years ago to today, where that balance is reversed (relatively cheaper materials, and labor cost, especially skilled labor, through the roof), has changed many things in bridge design. For instance, you'll rarely see plate girders with transverse (vertical) stiffeners anymore. The labor to fabricate them, fit them up and weld them in is much higher than the the cost of the steel for a thicker web that doesn't need stiffeners.

 

[johndeng]

@BridgeSmith,

You are the expert! But I am a bit more confused. Is that the tall bearing is not for the thermal movements? Nowadays new bridges no longer use the tall bearings any more?

 

[BridgeSmith]

The tall bearings were used to accommodate thermal movements. There are many different configurations; most of ours are like this.

Most of our new bridges, and as far as I know, those in most other states as well, use steel reinforced elastomeric bearings. Those are typically a few inches high.

Our most common abutment configuration is a fully-integral abutment, where the girders are cast into a concrete diaphragm, which is rigidly connected to a cap on steel piles. The translation from thermal expansion and contraction of the superstructure, as well as rotations from loading, are accommodated through flexure of the piles. For those, the 'bearing pad' is a 1/2" thick plain elastomeric pad.