Question about how steel beams fail

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Doug Wedel - 30 Jul 2008 22:27 GMT

Suppose you were pounding a steel beam into the ground with a pile driver.
After you had driven the
beam say five feet into the ground, you then arranged to hit the top of the
beam with a weight far heavier
than normal, heavy enough to cause the beam to fail.

Would the beam fail by bending or by snapping? Would the answer depend on
how much force was applied?

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dlzc - 30 Jul 2008 23:09 GMT

Dear Doug Wedel:

> Suppose you were pounding a steel beam into the
> ground with a pile driver. After you had driven the
[quoted text clipped - 4 lines]
>
> Would the beam fail by bending or by snapping?

The answer is "bending or crushing, depending on length".

> Would the answer depend on how much force
> was applied?

No, it usually has more to do with the unrestrained length (above
ground) and the speed of the impact hammer. Most steel is relatively
low carbon and low sulfur (what got the Titanic rivets and surrounding
metal had brittle failure when the iceberg hit), so it usually able to
deform gracefully.

http://en.wikipedia.org/wiki/Buckling
http://www.rocw.raifoundation.org/mechanical/BTec-Mec/som_2/lecture-notes/Lessio
n-01.pdf

If the driver moves too fast (say approaching the speed of sound), you
have essentially an explosive failure, as localized energy is too high
for the beam to stay in one piece. Think kinetic or explosive
welding...

David A. Smith

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Natural Science Forum / Physics / General Physics / July 2008

Question about how steel beams fail