Slope roof perpendicular to joist span 1

[knelli]

Hi,

We are designing a big box store with outdoor storage. The prototype drawings show the joists spanning perpendicular to the roof slope so that the roof deck ribs can be used for drainage to a gutter. The project manager wants to increase the slope of the roof, I'm suggesting 1/2" per ft. How do you specify joist seats for this condition? As the slope of the support beams increase, the joists will go out of plumb. For 1/4" per foot, should there be a special joist seat?

I have the Vulcraft manual and Vulcraft's "Designing with Steel Joist, Joist Girders and Steel Deck". They address sloping joists parallel to the axis, but not sloping the joist support.

Thank you!

 

[civilperson]

Use tapered shims to keep the joists webs vertical on sloping support ends or have flat spots on the CMU top course bondbeam steel plate inserts.

 

[JAE]

The sloping seats might be a bit expensive. It is a viable option.

However, I would check with a local joist manufacturer and see what sort of tilt the joists can tolerate. You can use standard SJI bridging (horizontal) except near the ends use X bridging to help the joists remain tied together laterally and not be affected by the slope.

The 1/2" in 1 foot doesn't sound like it would be an issue with the joists....and much cheaper than having the joist manufacturer custom make side-sloping seats.

 

[SteveGregory]

The bridging should be installed to keep the keep the bottom and top chords of the joists perpendicular to the roof decking. This assumes there is a good support for the bridging at the top of the slope.

 

[strguy11]

If you intend for the joist to Perp to the deck, make sure the details clearly show this show that joist manuf. clearly knows they are to design for bending out of plane on the joists.

 

[tngolfer]

This actually came up on a project I was working on a couple of months ago. (CMU bearing walls, open web steel joists) The project was a music performance hall and they wanted the joists to be visible and turned 90° to the roof slope. Luckily the acoustic sub recommended a hanging ceiling so we were able to turn the joists to parallel to the roof slope as they weren't visible anymore.

Say you did have joists spanning perpendicular to the roof slope... my questions:

1. On your drawings, do you all give the joist bearing elevation for each joist?

2. How do you account for the roof slope with the bearing being consistent (top of a cmu coursing)?

 

[knelli]

1. Our joists are bearing on steel beams so I call out top of steel at the grid lines and constant slope between.
2. If we were bearing on CMU, I think I would call out each joist bearing and space them at an interval that gives a good number, i.e. for 1/4" per foot space at 6' for 1.5" drop at each bearing point. Then would have to accomodate the slope at the seat or at the deck bearing. Still not sure about that.

 

[civilperson]

CMU bond beams are built with level courses, the slope between adjacent joist's ends must be made up with shims or additional block course after 8" or 4" (half course). Good masons can slope the top course by cutting the block to the varying heights required, and then place the weld plate with embedded studs into the bond beam concrete fill.