Steel Joist Side Slope in Commercial Building

[McQSE]

Does anyone orient their steel joists perpendicular to the roof slope so that the joist has a side slope? I have a commercial building with a 1% slope. I would like to run the steel joists perpendicular to the slope so they can bear on the frame and I won’t need additional framing (as I would if the joists ran parallel to the slope). I talked to someone at Vulcraft and they said they would not do any special design for this case. I also found an old question and answer article from Modern Steel Construction that talks about the issue. Does anyone have some experience in this application? Thanks.

 

[MiketheEngineer]

You'd be lucky to get the joists installed within 1% on a dead flat roof!!

If Vulcraft likes it - then OK

 

[jike]

1% is about 1/8" per foot. What Vulcraft told you is what we have also experienced.

I would generally recommend a 1/4" per foot slope. Less requires a ponding analysis.

 

[strguy11]

A while back we had a project where we did this. We were told by the joist manufacturer to show a detail, depicting one of the following situations:

Show the Joist vertically with a bent plate across the top, so that there was no point bearing on the decking.

Show the joist perpindicular to the deck, and require the joist to be designed for the biaxial bending that would ensue.

Either way, just make sure you cleary show what is required of the joist manufacturer. My recollection from the conversation I had with another supplier, was that for an 1/4" per foot slope, they may only require the bracing to be installed closer together.

 

[ajh1]

Positioning joists as you've described is typical of the metal building industry for longer bays. Same orientation generally applies for the more typical Z-purlin as well.

 

[MikeE55]

For 1/4" or less slope, the manufacturer ignores the slope and you can safely ignore it also. I once designed a gym this way with slope up to 1/2", and regretted it because the joists tended to have sweep in them. Lesson learned - if you are going to run joists perpendicular to the slope, stay at 1/4" per foot or less.

 

[McQSE]

Thanks for all of your input. I am going to increase the slope to 1/4" and run the joists perpendicular to the slope. This should cut some cost by eliminating the extra steel beams or girder joists. I also plan to add a big note for the joist manufacturer to design the joists for the side slope condition.