When you do any structural calculation, you need to check all the
different modes that it can fail. For a simple beam you need to
do deflection, max stress, and end shear. For economy of time,
you pick the one that is most likely to fail for a given material
and do that first. If it works out, do the others. Steel is quite
stiff so you normally do the stress calc first, then the shear,
deflection last. If the first two work ok, the deflection is
usually ok also. Wood is the opposite: deflection is usually the
problem, especially with longer spans. I always do the deflection
calc first on wood unless I don't care if the structure bounces.

You asked why if the deflection is ok, why can't you use it?
Because you exceeded the elastic limit and the beam does not
spring back to the original shape. In fact, it will just keep
bending if you keep pushing the jack handle.

The beam you selected is an 'S' shape which is getting to be
relatively uncommon. Makes me think that this is a salvaged beam
rather than a new "A36" beam. As such, you have no idea what kind
of steel is in it. If it is salvage (or worse, salvage from a
fire), I would not assume it was up in the 45kpsi yield area.

Sounds like you are making a standard shop press. You will want
the extra tonnage so keep the 20 ton jack. Beam strength is a
1/x^2 function so shortening it up a little makes a big
difference. Take it down to 30" and you increase the theoretical
strength by 60%. The posts look fine for tension loads but you
will need to use large pins or reinforce the holes where the pins
go through to keep them from deforming. 1/2" pins will shear,
5/8" will want to bend.

BTW: 'bend like a pretzil' was overstating it. It would just bend
until you quit honking on the handle!

Cheers.

Steve wrote:

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.