Physics - Structures and Materials
Structures and Materials
Beams are often used for the task of spanning spaces. When a beam is only
supported at the ends, it tends to bend under its own weight. When it
bends, there is a region of it in tension, a region in compression and a
This is a problem if the beam is made of stone or concrete, because they
are both strong under compression but weak under tension - the bottom of
the beam will fracture.
* Use steel reinforcement in underside - steel is strong in tension.
* Pre-stress the whole beam with steel wires/rods so that it is always in
compression. This is done by setting the concrete around tensioned rods.
Once the concrete is set, the rods attempt to contract and thus put the
concrete in compression.
* Build an arch instead of using a beam - in arches and domes all parts
are under compression. To see this, think about the fact that each stone
has to 'push up' to support the stones above it.
Beams will bend in other ways in other situations. For instance a
cantilevered beam will experience tension along the top and compression
along the bottom. This is the opposite of a beam supported at each end.
In this case reinforcement would need to be placed in the top of the beam.
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