Field house roof deflection criteria

[WWTEng]

I am used to using LL/360 & TL/240 for long span roof joists. I have a field house with a width of 70’ and the price difference between LL/360 & LL/240 is enormous. The joist is going to have a significant pitch to it, so I am not worried about ponding effects. There is no ceiling; the roof deck is 1.5Bx22ga with tapered insulation.

My question to the more informed folks here is if I am conservative with LL/360 & TL/240?

Thanks and merry Christmas.

 

[hokie66]

If the joists are pitched, why the tapered insulation?

 

[msquared48]

OP:

I think you are within the guidelines of the IBC, but I wonder, as Hokie, why the tapered insulation... Are you attempting to channel the water laterally across the top of the joists to artificially created drainage channels rather than varying the height of the joists?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[WWTEng]

Sorry, its not tapered insulation (its tapered in other areas). Its 4" of rigid insulation is what the arch is calling out.

 

[WWTEng]

FYI- I may not have been clear in my post. I am looking for opinions on LL/360 & TL/240 VS LL/240 & TL/180.

 

[MiketheEngineer]

I like L/240 if you don't have any drywall, plaster, glass, etc. I think the IBC even says that. Go check.

 

[msquared48]

What roofing material are you using - is it flexible enough not to develop leaks at the L/240 level?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[JedClampett]

In my copy of the Standard specifications for Longspan Steel joists, it restricts the deflection to L/240 for live load. I wouldn't exceed that. There is no total load restriction.
By the way, this is less than the allowable deflection from IBC Table 1604.3. I assume that the open web joist people have a good reason for limiting the deflection beyond code. Maybe the roofs got too bouncy.

 

[Ron]

L/240 is fine, provided you have no rigid appurtenances (ceiling, piping, etc.) attached to the underside of the joists.