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Re: DEPTH TO THICKNESS RATIO

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> From: =?iso-8859-1?B?SnVhbiBKb3PpIFRyZWZmIERlIGxhIE1vcmE=?=
> <jjtreff(--nospam--at)hotmail.com>

> Can anyone tell me what is the "normal" or a common practice for the maximum
> web to thickness ratio for AISC (prismatic) and for MBMA (tapered)?
> 
> Maybe MBMA doesn't have a standard for this but since some of you design for
> the metal building industry, maybe you can recommend a maximum ratio.

Metal building manufacturers in the US use the AISC, and therefore the
AISC element ratios, for tapered sections as well.

Per ASD 9 clause B5/Table B5.1: Webs in flexural compression

Compact d/t <= 640/sqrt(Fy) (about 90 at 50 ksi)
(reduction in combination with axial loads to d/t <= 257/sqrt(Fy))

Non-compact h/t <= 760/sqrt(Fb) (about 137 at 50 ksi)

"Plate girders" 970/sqrt(Fy) <= h/t <= 14000/sqrt(Fy(Fy+16.5))
                137 <= h/t <= 242 at 50 ksi

Typically, metal building manufacturers will provide h/t = 150+/-25 just
because the design will work out that way to avoid using transverse
stiffeners, but higher/lower ratios are common. Labour for fitting
stiffeners is expensive relative to steel cost in the US.

The MBMA manual does not identify h/t ratios since it is not a
specification for structural steel design. It directs the reader to the
AISC specs or similar.

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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