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Author Topic: International Bldg code structural member deflection limits  (Read 3914 times)

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Offline beetle

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International Bldg code structural member deflection limits
« on: February 14, 2008, 11:13:32 AM »
I need help and thought maybe someone one this forum could help, in reviewing the the subject building codes table 1604.3 which provides the limits of structural member deflection, I do not understand what the values mean?

For example; the table indicates a deflection limit of "I/360" for floor members. What does this I/360 mean?

Thanks,
Jeff
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Offline Jim_Rogers

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Re: International Bldg code structural member deflection limits
« Reply #1 on: February 14, 2008, 01:13:39 PM »
What it means is you take any length divide it by 360 parts and you're allowed one part deflection...
For example if you had a beam 360 inches long that would be 30' you would be allowed to let it deflect 1". Something that I wouldn't like to see.

Here is another example take a beam 10' long that's 120 inches divided by 360 = .333 or one third of an inch. To convert any decimal into fractions for better understanding; take .333 x 16 (for sixteenths) = 5.328 sixteenths so that is 5/16" a little more than 1/4" deflection.

What you have to be careful of in deflections is that the allowed deflection won't crack your sheet rock ceiling.

Some designer like 1/480 for a stiffer second floor and to eliminate the risk of the sheet rock ceiling from cracking.

Jim Rogers
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Offline scgargoyle

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Re: International Bldg code structural member deflection limits
« Reply #2 on: February 14, 2008, 04:47:24 PM »
Is the deflection at the rated live load, such as 30 psf, or just as it sits?
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Offline Don P

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Re: International Bldg code structural member deflection limits
« Reply #3 on: February 14, 2008, 06:17:30 PM »
Typically you just use the rated live load and neglect the dead load since these codes came from trying to keep plaster from cracking, the dead load had already caused its deflection and would not affect a plaster job where a varying live load would affect the sag and cracking potential.
I would strongly suggest using 40 psf and L/480 though. People are very sensitive to vibration in the 8-10 Hz range. Normally using the "enhanced" numbers the frequency gets above 15 Hz.

Deflection for a simple beam= 5WL3/384EI
The simple beam calc I have in the toolbox uses that formula as does the bldg code.
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Offline Don P

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Re: International Bldg code structural member deflection limits
« Reply #4 on: February 16, 2008, 09:53:50 PM »
There is a copy of the typical beam situations and design formulas for each here;
http://www.awc.org/pdf/DA6-BeamFormulas.pdf

This is a link to a detailed article on floor vibration and why many designers are specifying L/480 deflections.
http://www.gostructural.com/article.asp?id=1818
A laborer works with his hands
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