Composite Beam - Defl limit under wet weight

[vandede427]

We use an in-house rule of thumb to limit an uncambered beam to L/800 for the steel beam under the wet weight of concrete (anything > L/800 requires camber).

I'm one of those anal people that likes to see things written down in a code, or a recommendation from a technical guide, etc. I don't like things out of left field; I like to know the methodology behind them.

Does anybody know where this info could be found?
And what are everybody else's limit before you decide a beam needs camber?

 

[Lion06]

We limit the precomposite deflection to 3/4".

 

[slickdeals]

We limit the precomposite deflections to 3/4" as well or L/360.

 

[vandede427]

again, can you direct me to where that is written? or is it just your rule of thumb.

 

[BAretired]

There is nothing in CSA S16-01 pertaining to your exact question, but Article 17.11 "Unshored Beams" requires that the stresses in the tension flange due to load applied before concrete strength reaches 0.75f'c plus the stresses in the same location due to the remaining specified loads considered to act on the composite section shall not exceed Fy.

I think I like your rule better, except I would think anything over L/800 or 3/4" requires cambering or shoring. I assume that shoring costs less than cambering.

Best regards,

BA

 

[slickdeals]

I think it is more of a rule of thumb. You can look at this paper for some reference.
ftp://imgs.ebuild.com/woc/C970734.pdf

Also look for a paper by John Ruddy for Ponding of Concrete Deck Floors

 

[WillisV]

Review AISC's Design Guide 3 on Serviceability which provides guidance on deflections. The recommend L/360 or 1" max for the instantaneous dead load + 50% of expected creep load.

 

[Dinosaur]

I believe your answer may be found in ASTM A6, the specification for most steel rolled shape dimensions and mill tolerances.

I believe they will show a mill tolerance for some allowable camber in the as rolled condition. Therefore, since beams will come with some curvature, designing for less than that amount is not productive as whatever it is is still within tolerance.

If you use the AISC steel manual, you will probably still have excerpts from ASTM A6 at the end of the first chapter and the notes on mill camber are reprinted there (they are in my slightly older version which was printed on papyrus).

Good luck.

 

[vandede427]

The auther of the article posted by slickdeals references a max camber for different beam depths/lengths given by AISC. I can't find any AISC document that says that specifically.

 

[WillisV]

The camber table you are referring to was last printed in the 2nd Edition LRFD Manual. It was not put in the 3rd or 13th edition. This table didn't have anything to do with deflection though.