Roof Framing
Roof Framing
(OP)
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...






RE: Roof Framing
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.
RE: Roof Framing
RE: Roof Framing
RE: Roof Framing
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
RE: Roof Framing
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.
RE: Roof Framing
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Roof Framing
Are you assuming fully braced (via decking?)
RE: Roof Framing
RE: Roof Framing
RE: Roof Framing
RE: Roof Framing