Location: Outside U.S.
Country: Great Britain
Date: November 2007
Why does a bridge have rollers under it?
How do the rollers of a bridge work?
There are many, many types of different bridge structures, and I do
not know which type you are specifically referring to, but here is a
general answer. As temperature rises, a bridge expands in length.
There must be some way to accommodate this. One way is to allow the
end (or ends) of the bridge to move by mounting one or both ends on a
roller. On a hot day, a very long bridge can increase its length by
several feet. If some means was not used to accommodate this, the
bridge could buckle. The "roller" method is usually only used on
smaller, rigidly-constructed bridges. Long suspension bridges, on the
other hand, are designed so they are higher in the center. they are
usually not set on the rollers. Increased length caused by rising
temperature, simply causes the center part to rise higher.
A bridge of any substantial length is going to be affected by changes in
temperature. As the bridge heats up on a hot summer day with lots of sunshine
on the pavement, the different sections will try to expand. On a cold winter
night, it will contract. While this expansion and contraction may not make
much difference in relatively short distances, for much longer bridges even a
little change can become significant. (a few inches may not seem like much on
a mile long bridge, unless you are trying to pour the concrete into those few
inches!) Rollers or other forms of expansion joints exist to allow multiple
sections of a bridge to mesh together, while still adapting to changing
conditions. They act much like a very sturdy set of wheels, allowing the
upper section to roll back and forth.
Rollers are one type of mechanism that allows expansion and rotation of the bridge.
As the temperature changes a bridge expands or contracts due to thermal expansion
of the metal and concrete that the bridge is made of. The ground stays at a fairly
constant temperature. Other causes of bridge motion are: a bridge support may
settle, the earth may move, heavy loads cause the bridge to deflect, wind might
make the bridge deflect, and the bridge may shrink or creep as the concrete cures.
Rollers are one type of mechanism. Other kinds are rocker bearings, knuckle pin
bearings, plain sliding bearings (flat plates) and elastomeric bearings (slabs
of rubber-like material that shear). All of these support the bridge and allow
the bridge to move relative to the bridge supports.
A book called "Bridge Bearings and Expansion Joints" is useful.
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Update: June 2012