But aren't we supposed to be looking at shear (or at least the Yield)
stress max, not tensile? I assumed that because the yield limit is
quite clearly shown as 18000 in the little programme I use. I looked
Use the yield point, that is where it will bend and not spring
back. Actual values to use vary by grade of steel and may have
some safety factor inserted. 18kpsi is a resonably conservative
number for building construction. For new A36 steel I would
expect to see something like 40kpsi (or more) when I put a test
strip cut from the outer web in the pull test machine at school.
We usually see something like 50kpsi for hot rolled strap in the
same test.
Your program shows A36 at 18kpsi. My program comes preloaded at
24kpsi. Actual test is in the 40kpsi range. Yours has a bigger
safety factor loaded in.
As a practical matter, you won't see these low numbers in A36. we
needed some special steel for a very agressive forming operation.
Paying (lots of)extra money we could get some fully annealed,
fine grain stock that had a yield under 30kpsi but elongation of
around 40% in two inches without breaking.
up some other data, and I have one steel that has a Yield of 300 MPa
and a Tensile of 450. The Yield is what I assumed was used to measure
bending strength.
Again plugging in the figures, if I halve the length, with the same
force and same beam, I halve the stress. If I double the depth,
leaving the length at its original value, the stress is one quarter,
from what I am seeing. So as far as I can see, depth (square) is more
useful than shortening (linear?).
Deflection is a cubed function of the depth, stress is a squared
function, length is a LINEAR function. I didn't have enough
coffee on the first response, I used the linear loading function,
not the point load function. Which is why good engineers double
check their calculations the next day. My oops.
I am not just being argumentative here, I thought I had this sorted
out, and I see a couple of other posts to the same thread with
different opinions.
Cheers.
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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