Bracing Design for Steel Joists Skewed 5% from Vertical Plane

[efsinc]

The architect is sloping the roof 5%, 7' in 140'. the 30K series steel joists @48" o.c. will span the 50' bay from a 5% downward sloping wide flange beam to a 5% downward sloping steel ledger on a cmu wall. thus the direction of the slope is perpendicular to the span of the joists.

the architect wants the joists not to be plumb to the world, but to be perpendicular to the roof. thus each joist is skewed from the vertical plane 5%.

I would appreciate any advisement regarding the bracing of these joists so the lines are sharp.

Thank you.

 

[BAretired]

Assume a load of w psf. Each steel joist is 5% off vertical, so the joist makes an angle of 2.86 degrees with a vertical plane.

The vertical load on each truss is 4w plf. Each joist carries 3.995w parallel to its web and 0.19975w perpendicular to the web at the top chord.

In 140', you will have 35 spaces or 34 joists. Total slope component = 34*0.19975w = 6.79w plf which has to be taken out by a truss in the plane of the roof or diaphragm action of the steel deck.

If you opt for a truss at the high end, bracing members in the plane of the roof will carry a tension of 6.79w*sp where sp is the spacing of the braces.

BA

 

[paddingtongreen]

Are they going to hang equipment from the bottom of the joists? There is no diaphragm there.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JAE]

BAretired is on the right track I think.

Also - check with local joist manufacturer's as they can offer good advice. Probably need some additional bridging (maybe X-bridging).

 

[BAretired]

You need bridging between joists, even if the joists are vertical. If you hang anything from the underside of a sloping joist, resolve the vertical forces into components parallel and perpendicular to the plane of the joist.

Bridging members must carry the perpendicular component to one or both ends where you will need diagonal members to get the force into the roof truss or diaphragm.

You could, depending on the magnitude of the perpendicular force, add another truss in the plane of the bottom chords to span 50' and carry the force to the endwalls.


BA

 

[efsinc]

this forum is terrific. thank you.

there will likely be some suspended materials including such things as lighting, mechanical ducting, and fire sprinkler lines.