Acceptable deflections

Awl--

When choosing beams for supporting roofs, floors, etc, what is an acceptable deflection over a given length?

I'm looking at a 12 ft beam, supported at 0, 6, 12 ft, and depending on the beam I choose (and various point/distributed loads), I'm winding up w/ deflections of between .010 and .033 between supports, ie, at about 3 and 9 ft.

What is acceptable? Is this a matter of local codes, or do rules-of-thumb apply here?

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

Reply to
Proctologically Violated©®
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0.010 and 0.033 inches? My gut reaction is that this is fine. Some overhead rolling cranes deflect by this much. But someone will probably shoot me for saying this without knowing every detail of your design :-).

Chris

Reply to
Christopher Tidy

For tile, specs usually call for L/360 or L/720.

Reply to
Richard J Kinch

Most building codes for RESIDENTIAL use specify 1/180 deflection for roof loads (asphalt shinges) either 1/240 or 1/360 for LIVE load ONLY on floor joists, 1/720 deflection for special cases like tile floors. Some codes alow derating for snow loads and the like.

A 1/240 deflection will feel like a cheap building when you walk on it, pretty bouncy. 1/180 will feel slightly unsafe.

All of these are at the center of the joist span which includes the sum of deflection on the support beams plus the floor joists.

Steel beam aplications tend to be limited by the strength of the beam, wood beam applications tens to wimp out on deflection first. You have to calcualte both.

.033" on 6' span says you are using a steel beam, getting 1/2200 deflection. That is a VERY stiff beam.

Christ> Proctologically Violated©® wrote:

Reply to
RoyJ

Some interesting stuff comes out of BeamBoy.

That .033 deflection was for a 4x13 beam (13, for the uninitiated, is the lbs per foot of the section--the stock book from a decent steel house gives the actual dimensions--Ryerson or Thypin, for example, good books to have). The deflection is even smaller in a 6x9 beam, despite the much lower wt per foot. In 6x12, smaller still. And a mere .009 for the same wt per foot, but in a 8x13 wide flange.

Unfortunately, I have some head room constraints, so the 4x13 beam may be the only option. But fortunately still a viable one, deflection-wise.

It seems, from some preliminary pricing, that steelhouses charge basically by the pound, so a 6x9 beam would in fact be substantially cheaper--and stiffer--than a 4x13. Go figger.

Cost adds up quickly w/ this stuff. I'm looking at a few grand, just in goddamm beams! Might be worth bumping my head, once in a while. :)

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Proctologically Violated©®

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