18,000psi is a rather low value. Typical building codes allow
22kpsi for standard beams or 24kpsi for certain 'compact' shapes
in A36 steel. These numbers have a reasonable safety allowance
built in. If you actually put them in a tensile tester, you will
get higher values. So it boils down to what kind of safety factor
do you want to build in.
For new, prime A36 steel I would expect to see numbers up in the
40-45kpsi yield/55k tensile as measured in a good testing
machine. A36 is a pretty loose spec plus many steel yards call
any standard shape beam "A36" to distinguish it from HSLA or
other stock. Once you get to seconds, old, used, fire recovery,
or other salvage, all bets are off. And it usually goes down,not
up.
The OP is talking about an odd 'S' shape which is not a common
size in new prime stock. So I would guess old material and fudge
the number downward accordingly. On the other hand, if he bends
the top beam on a shop press, he sees it instantly and can just
quit pumping the handle, little harm done.
Increasing the depth of the beam has the same proportional effect
as shortening it. In this case, I suspect the OP has the chunk of
steel and wants to use it.
Cheers.
Old Nick wrote:
On 8 Aug 2003 23:41:30 -0700, (Steve) wrote
something
......and in reply I say!:
I think I can explain.
I am not sure where you get the figure of 45000 for mild steel.
I only play around with this stuff from time to time, for projects
that I build. But I had a bit of learning to do.
Standard steel (which you have to assume this is) can go as low as
15000 psi stress max. beam.exe quotes 18000 psi.
Beam depth is far more effective than shortening for this. I tried
your beam even at 24", and beam.exe says it will still fail at 40000
lb if standard steel (18000 psi from beam.exe). But an 8x4 beam will
_just_ sneak in at full length. A 10" by 5 " beam is well under.
I can see a danger of the beam still failing if any side force is
applied while it's under extreme strain. I have a huge press frame
that I picked up at an auction. It has twin beams, side by side, I
think for this reason.
I posted a question about an I beam with a Moment of Inertia of 26.49.
It is a mild steel I-Beam and is 3.5" wide and 6" high. The beam
length is 38" and will have a 40,000 lb load placed in the middle of
the beam with a .5" spreader plate
where the jack will attach (This is going to be the top beam for a
hydraulic press). The deflection at load is .05". The beam will be
supported by two 60" posts that are 2.5"x2.5"x.25" thick square posts.
Someone states that this would "bend like a pretzel" under full load.
If the deflection is only .05" at maximum load how will the beam "bend
like a pretzel"? I know that the elastic yield strength of mild steel
is around 36,000 psi but I have read that most steel these days is
around the 45,000 psi. The same poster stated that the max stress will
be 43.6kpsi. I could truss the beam or could make the beam shorter in
length or could lower the hydraulic jack to a 10 ton model. I tried to
post this using the original post but was unsuccessful. Sorry for top
posting. Thanks, Steve.
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