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Deflection Design

Deflection Design

Deflection Design

I am designing steel roof members for a church.  The roof is full of gables and also some flat areas.  The gables are framed with light gauge steel trusses while the flat areas are framed with precast plank.  In certain areas, the trusses and the plank are both supported by a single steel beam.  My maximum span is around 30' and the ceiling structure is suspended acoustical tiles.

I am curious to see what people would use for deflection limits based on IBC.  Are there reasons to put an absolute limit on deflection of a beam supporting plank?   

RE: Deflection Design

I'm assuming that your beam has a large deflection.

In the case of a roof deflecting you have to consider the consequences of deflection. the L/360 limit, if I remember  correctly, was to prevent the cracking of ceiling plaster (or GWB). I have seen structures where the rotation of beams at the support ends cracked finishes. As long as the deflection is not causing roof ponding or damaging some precious architectural finish your probably okay. The ponding senario (water and/or ice) can be progressive, ie more deflection, more weight, more deflection etc.

RE: Deflection Design

If your support beam sags, what picks up the load?  This may be a secondary member, or the plank / beam due to rigid connections, etc.

RE: Deflection Design

Judgement time.  L/360 is a good "all purpose" limit to at least begin the process of checking deflection.  I would also look at all the other building elements that are supported by the beam, or would be affected by the beam's deflection.  For 30' (L/360 = 1") can 1" of deflection be tolerated by the other attached elements without undue damage or cracks, etc?  If not, then perhaps you can allow for the beams movements with detailed slip connections.  Or limit the deflection further to 1/2" or whatever you deem appropriate.  There's no absolute answer here other than your knowledge of what all the affected elements are.

RE: Deflection Design

Looking at IBC, I could actually use L/240 for snow.  IN table 1604.3, the last column is actually D+L but snow is specifically called out for in column two as S.  Do people use the D+L for snow as well or make it more stingent at L/360?  

RE: Deflection Design

Like JAE says, it is a judgment call - building codes are not "recommendations" on what to do, just minimum requirements. Since you are talking about one beam, how much money can be saved by going with the MINIMUM code requirement? Suggest limiting deflection to L/360 (or less) under worst case load. Of course the "real" deflection during normal use will be much less. Differential cost, compared to a "minimum" design will likely be trivial.

www.SlideRuleEra.net reading

RE: Deflection Design

Why not build camber into the support beam equal to dead load deflection?  Then, only snow and live loading will cause ponding or below horizontal deflection.  Not much cost for a pre-cambered steel beam.

RE: Deflection Design

I have more than one beam for this system, but I was trying to wrap it up in a simple discussion.  The much larger discussion is what the IBC code deflection would be for the snow.  I am curious to see what others think.

RE: Deflection Design

Check out design guide 3 from AISC.  They discuss the limitations of L/360, when you need to establish an absolute deflection limit and what that limit might be. Acoustical ceiling tiles are generally tollerant of roof deflection, except you might have problems around the perimeter or other partitions where the ceiling tiles could be attached to the wall with a perimeter angle.

If during a heavy snow your roof deflects a lot, the occupants will become alarmed and you will get a call from them.  Even if the roof framing has plenty of strength, they will be spooked and worry that their building will fall down.  We are looking at such a case right now (designed by others).  

RE: Deflection Design

I am following this post, with no comment. My fealing is that deflection is an engineering problem and not a code problem. I would only concern myself with pooling, and nothing else.

RE: Deflection Design

Search 'Deflection of Structural Members' with Google. This is a PDF file published by the University of Michigan. Slide 4 has a table that should be of interest to you. I still think this should be addressed by design, not by code.

RE: Deflection Design

I agree the code is minimum requirements for design, but you need to be the judge on what is framing into it to determine the stiffness required.  Also, cambering was brought up, and cambering has a tolerance of 1/2-3/4" so this would probably be a case that cambering would not be utilized, but as you will see opinions vary.

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