Transferring diaphragm shears through purlins sitting on beams

[canwesteng]

I'm wondering if it's possible to transfer diaphragm flange forces through purlins running perpendicular to the flange beam sitting on it (sorry if this is confusing, I've attached a sketch). My concerns are:
1 - this is a torque on the purlins
2 - purlins are spaced at 5', is this too great?
3 - connector strength, this required contribution of several connectors on the purlins to safely resist load.
I've see a design that welded HSS to the eaves to avoid transferring shear through joist seats before, but purlins are stronger than joist seats.

More misc info - 30'widex40'longx10'tall building, W16x26 purlins on W16x35 beams. Very low seismic, wind loads
~30 psf net in one direction. Haven't done much roof diaphragm design outside of cookie cutter stuff, it's atypical in the industrial sector I work in

 

[msquared48]

Not really a good idea.

Why do you need to do this? Is the lower beam a drag strut?

Mike McCann, PE, SE (WA)

 

[XR250]

Looks like you would need some sort of blocking to prevent rolling of the purlins and to attach the diaphragm to at the boundary

 

[canwesteng]

It is in fact the drag strut - I have a 6' overhang which is why the purlins are continuous over the beam. I guess the eave could be the chord member if it helps but I assumed this would be both.

 

[KootK]

It's inconventional but, sure, there's a path there so long as rollover capacity is there and shear is low enough that you don't need a collector between purlins. And bearing stiffeners in the purlins would fix the rollover problem if it exists.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[canwesteng]

I guess I'm unsure how to check the rollover problem. Agreed that stiffeners in the purlins provide that resistance, but the additional fabrication probably offsets the savings of not bracing the roof

 

[KootK]

You could treat a section of the purlin as a WT cantilevering up to the deck. Check cross web bending, bolt capacity etc. The trick is figuring out how to combine the rollover capacity needed for deck shear with whatever capacity is also needed to stabilize the purlins for LTB at the girder.

I'd think that the stiffeners would be cheaper than in plane roof bracing or the HSS shear lugs. The stiffeners can be shop installed and you should easily be able to get by with one-sided, partial height stiffeners.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[njkeng]

From what you have shown it appears that the roof sheeting is being used as a sheathing to transfer in plane shear between the drag strut beams. How did you calculate the capacity of the roof sheeting and how did you account for the roof sheet fixings?

 

[msquared48]

You know, I think I would opt to flush out the tops of the beam and purlins and use welded connections here.

Mike McCann, PE, SE (WA)

 

[KootK]

I saw an interesting detail once where a girder was made to pass through some open webbed joists such that the joists were able to cantilever Significantly but the tops of the joists were only 4" above the top of the beam. One OWSJ BC panel was made to be field installed beneath the girder. It was pretty slick.

Another option could be to upset your purlins 6" or so and cantilever out with a channel or HSS. This would get you back to a more conventional rollover height and wouldn't require field welding.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[canwesteng]

Looks this building is getting a braced roof. Oh well, small percent of the cost of the structure and even smaller percent of the project cost.