On Thu, 19 Mar 2009 07:36:41 -0700, "Stuart Fields"
wrote:


Oops! Ned you are right. I grabbed the wrong beam condition. BTW I
designed a sail boat trailer for a full keel sail boat using the deflections
as the limiting condition for obvious reasons. Also a spiral staircase, a
hydraulic engine hoist and a building addition, and a helicopter trailer
modification. As well as the deflection calcs for a 30' non standard I beam
with both distributed and concentrated loads. All had some safety factors
added and all have been in use for some time with no failures to perform.
With all that said, the table 4 on page 5-35 does have an easier to use
formula for safe loads.

Is that titled "Approximate Safe Loads in Pounds on Steel Beams?" (I'm
looking at the 5th edition, which I suspect is older than yours.) Note
that this is an approximation, and uses a very conservative value for
allowable stress. It'd be interesting to compare results from that
table to the AISC code.

Given that the ends of the I beam are restricted
from rolling or departing from their location, I fail to see that using
deflection, and keeping it small, doesn't yield a safe condition.

Here's an example: Take the OP's 8" beam (let's assume it's a W8x13)
and set it up as a simply supported beam 5 feet long and apply a 12
ton point load at the middle. It'll deflect .090", which is less than
1/600 of the span, but the stress in the flanges will be over the
yield of A36 steel, and about 1.6 times the allowable stress.

Make the beam even shorter and you'll get to a point where shear
rather than bending stress is the limitation on loading.

This
technique was used on the building addition, as well as the 30' non standard
I beam with the concurrance of a P.E.

Quite often deflection *is* the controlling factor. Presumably your PE
evaluated the design and signed off on your deflection calculations as
appropriate in that instance. He might not have agreed if the beam was
unbraced or being used for overhead hoisting.

--
Ned Simmons