Calc'g deflections using BeamBoy

Without getting into a lot of "stuff" one would expect less deflection in the "I" position. For the beam to deflect, the upper plate has to stretch and the lower plate has to compress. This is the primary contributor to the rigidity of the beam. The secondary contributor is the distributed stretching and compression of the center web. In the "H" position, you only have the distributed stretching and compression of the sides and the center web contributes very little.

Jerry

Wide Flanged beams are designed so that they can carry maximum load with the flanges top and bottom with the web between. The objective is to locate the maximum amount of material at the positions of maximum tension and compression. You don't need the material concentrated at the center of the beam which is what you do when you arrange a beam horizontally in an "H" position. The web is at the neutral axis. A similar mistake in understanding occurs when people try to stiffen a sheet steel wall with angle iron. They stitch it with the "V" against the skin rather than with on leg of the "L" touching the skin and the other leg as far away from the skin as possible to increase the moment arm. Randy

Been fooling around w/ this program/utility. Pretty neat.

Quick Qs:

For a W beam, you can orient it so that it looks like an "I" or like an "H". Which orientation does the X-X value for the moment correspond to? I believe X-X corresponds to the "I" orientation, judging from comparisons w/ channels, whose relative deflections is intuitively clear.

For a "4x13" W beam, whose flange width (4.060) is about equal to the main web depth (4.160), and considerably thicker (flange thickness .345 vs web thickness of .280), the program indicates deflection in the H position to be

Does this make sense? I would have expected the deflection in the H position to be only 1/2 the deflection in the I position.

Whazzup?

-- Mr. P.V.'d formerly Droll Troll

Thank you for a very illuminating explanation!

I was thinking only in terms of rigidity of the one dimension, which I know varies as the square. ie, a 2x8 is 4 times as stiff as a 2x4, etc. Thus, I thought the H position of a beam would be "additively" stiffer than the I position, in the case where the flanges were the same dimension as the web.

But the compression/stretching aspect of the flanges is an eye-opener, and makes a lot of sense! Bless you! I mighta gotten myself into a fair amount of trouble down the road, as in: Damn, what are all those cracks doing in my new concrete floor--and why are the beams sagging???? :)

The scary thing about wide flange is that you can make a set-up that is quite strong enough but downright scary when you see the amount of deflection. Stiffness is separate from strength.... almost. A real "eye opener" is a stop action photo of a large aluminum airplane wing during rough weather. The wing is certainly strong enough but flexes several feet up and down at the wing tip. Often building codes provide a maximum deflection value for floors when loaded simply because the occupiers would freak otherwise. It is prudent to ask yourself if the structure you are building has to be rigid or just strong. I was involved in building a free standing jib crane for loading some type of heavy dies. The dies weighed around a ton. The new crane was modified once then taken out and replaced simply because the flex was not acceptable. We built a monster that I am sure would fail at something like twenty times what its service load was. The modern steel wide flange is a very efficient design. In the years I have been fabricating structural I have seen total weights of buildings go down yet the strength is still there. This is especially so when large tubing started to appear for columns and bracing. It caught some contractors by surprise who were bidding on the weight of steel rather than the complexity of joints and type of members. Randy

I was thinking only in terms of rigidity of the one dimension, which I know varies as the square. ie, a 2x8 is 4 times as stiff as a 2x4, etc. Thus, I thought the H position of a beam would be "additively" stiffer than the I position, in the case where the flanges were the same dimension as the web.

But the compression/stretching aspect of the flanges is an eye-opener, and makes a lot of sense! Bless you! I mighta gotten myself into a fair amount of trouble down the road, as in: Damn, what are all those cracks doing in my new concrete floor--and why are the beams sagging???? :)

Just a comment, but the wings on the B-52 typically flex 15 feet at the tips. Doesn't sound like much until you see marks on the wall at the lowest and highest positions, with a B-52 wing clamped at the midpoint position. Don't recall where I saw that particular exhibit, but it was impressive.

I understand that the flexing of the wings alone is sufficient to serve as a wing deicing system.