Light gage roof bracing

[vato]

The architect wants trusses supporting purlins and metal roofing which gives me nothing for diaphragm strength. The building is 80x30 with shear walls on each end. I need 500plf on the ends of the "diaphragm". I have been looking at bracing with angles, but I end up needing them across the entire roof, which actually seems reasonable. This seems like very typical construction for metal building with moment frames, but I don't like it in this application. I don't really have a ceiling diaphragm option. Any advice on an efficient bracing pattern or should I switch to the steel deck and ditch the purlins altogether?

 

[paddingtongreen]

Use rolled channels for the purlins. I used to use bottom chord bracing, full width at the end bays and about the same depth to a truss panel point along the edges of the building. Even if not for wind forces, I put some in just to square up the building.

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

 

[asixth]

Do you have intermediate walls between the shear walls spaced at 80'?

 

[vato]

I'm trying to avoid transferring anything into the interior walls. Nothing stacks and the most simple solution is to use the plane of the roof, even if it's 80 feet. I can handle uplift, but the shear is the issue.

 

[Ron]

Agree with Paddingtongreen...use the purlins, forget the deck.

 

[hokie66]

This is typical construction for most buildings in Australia. Don't try to make it work like a diaphragm. Use a discrete member bracing system to form a horizontal truss below the purlins. Rolled steel purlins are too expensive and too difficult to screw fasten the roofing.

 

[rowingengineer]

Agree with hokie

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[vato]

The bracing to create a "horizontal" truss in the roof plane is the approach. It's just that this "truss" is not insignificant with 120mph wind. I suppose I need to add up the weight of steel and compare the systems, trss/purlins or metal deck after I figure out adequate bracing.

 

[MikeE55]

For similar situations I have ditched the purlins and used metal decking with the standing seam fastened down to the deck.

 

[vato]

Thanks mike, that's about where I'm at. I sharpened the pencil and now I'm figuring out the weight of each system. Then we can find out how "green" the architect wants to be.

 

[vato]

So, the 1.5B deck 20 gage comes it at 2 psf,
the purlins, blocking, angle bracing comes in at 2.4 psf.
The angle bracing system adds a lot of axial load to the top chords of the trusses, which will need some checking, where as the deck, in my thinking,
will not increase axial load in the top chords, accept at the gable end/shear wall truss. In addition, I've got some pretty decent loads on my angle bracing connections at the shear walls. I have a meeting later and we will see where we end up. It's all due tomorrow of course and the architect probably doesn't want to update his drawings at this point.

 

[vandede427]

I'd use the deck.

 

[vato]

Thanks to all for the advice. I consult from an office, very isolated from the rest of the earth, and these discussions are really helpful.
The deck it is. The arch didn't care at this point, he just wants the drawings.
thanks again.

 

[hokie66]

So as I understand your proposed system, you would have a structural deck, then fasten the metal roofing to this deck somehow. Is there a recognized method for that? The ribs in both the deck and the roofing go the same way. Maybe top hat battens between?

 

[vato]

The deck and roof are perpendicular since I ditched the purlins. The deck sits on top of the joists now, spanning 6'8 between joists.

 

[hokie66]

Guess I misunderstood your system. When you said trusses in the original post, I thought real trusses, not bar joists.