Roof Framing

[bpstruct]

Have a project where we are installing a new roof on a building (currently doesn't have one). It is an old load bearing brick masonry building. Planning to use C-purlins at the existing joist pockets with some modified connectors....problem I am running into is this. The contractor who was hired before me (I know - wrong order), quoted the job with 8" C's (14 gage) at 3 feet. I have performed calcs for this and checked vs light gage framing tables and it just doesn't work....not even close. Calcs match what I see in tables and RISA, but the contractor brings up a good point. You see this size purlin in metal buildings all the time spaced at around 5 feet. Normally Z purlins, but the section modulus is very close. I know that there is some flexural reduction due to continuity of the roof joists over main frames, but it really doesn't reduce moment that much. Am I missing something really obvious? Maybe I've miscalculated or something...

 

[a2mfk]

What are your bracing assumptions? Multiple spans with moment splices or all simple-supported?

Run the calcs and stick to your numbers, I always say I know the way I do things to be correct and I cannot explain every other building out there nor can I justify others designs... And it has been said several times on this forum about PEMBs, they are designed with very low factors of safety and almost no additional capacity.

 

[bpstruct]

Our building is a single span. I see roof purlins lapped over interior supports on metal buildings, but even if those are "continuous", the reduction isn't enough to justify such a wide discrepancy in spacing. I just don't even see how these metal buildings meet code. They don't seem to be close.

 

[a2mfk]

Maybe they don't, your average plan checker would never know.

 

[msquared48]

On a brick building, I would expect the roof dead load to be in the order of 12 to 15 psf, but only 2 to 3 psf max for a metal building. Big difference.

Also, I assume that this roof is to serve as a diaphragm to support the brick walls laterally for wind and seismic. Is the decking rated to deliver the required shears at the 3 foot spacing?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[bpstruct]

The dead load will be no different than a metal building in this case. No MEP being hung from the roof, metal deck will be exposed....Building is one story, but pretty tall. And very long.

Yes the deck will serve as the diaphragm to transfer lateral to the end walls. That was the other thing...he wanted to use 26 gage deck....just doesn't work for lateral. He will also moan when he sees how he is going to have to connect this deck.

The guy builds a lot of metal buildings around here, but there is not much (if any) code enforcement. Kind of a rural place.

 

[msquared48]

The roof deck in a metal building does not work as a diaphragm structurally. That's the reason for the commonly used "X" bracing in the roof structure - to lower the weight of the building and save $$$$. I'm sure that he does not comprehend that fact though, then again, it's all about the $$$$$$$$.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAE]

What sort of span are your joists?
Are you assuming fully braced (via decking?)

 

[bpstruct]

Yes. Assuming fully braced. Attaching with a good deck and two rows of bridging/blocking. Clear span is 24 feet.

 

[a2mfk]

I have had this exact same problem on a conventional building with a contractor who mainly did PEMBs. You have to explain to them these buildings have completely different structural systems, and a PEMB is not a conventional shear diaphragm (roof and wall) system. That does not explain the roof member size difference but who knows why the other roofs in your area appear to be under-designed...

 

[bpstruct]

Thanks to all...feel better about telling them what I've told them. Don't feel better about PEB's in general though.

 

[hokie66]

Single span cold rolled sections will generally have only half or less the capacity of lapped continuous sections, as the double thickness of the lapped part draws the moment to the support.