platform framed gable ends

[AlpineEngineer]

I design in high wind region and platform framed gable ends are a big no-no unless you have the proper gable end bracing detail, we typically spec baloon framed gable ends.

I have a situation where the roof is stick built and the gable end is platform framed but there are ceiling joists (at the same elevation as the hinge point on the gable end) which run perpindicular to the gable end. Don't these ceiling joists and the ceiling diaphram provide the necessary bracing since they are running perpindicular. The building was designed prescriptively so I am trying to decide if this is allowed without getting into engineeing it.

thanks

 

[msquared48]

The ceiling sheetrock is not recognized as a structural diaphragm and this would be the only way to get the load from the wall top plate to the sidewalls unless you had flat diagonal struts across the top of the ceiling joists extending to the outside walls.

Best to use periodic diagonal struts from the top plate of the wall (bottom of the endwall truss) to the roof diaphragm. Some intermediate blocking will be required at the roof. All the diagonal struts would be hidden in the attic space.

Mike McCann
MMC Engineering

 

[Ron]

I agree with Mike. The bracing is not that difficult to put in and certainly is necessary in high wind areas.

I'm curious....you actually specify balloon framing on gable ends? I'm in a high wind area and we almost never see balloon framing. Are you actually getting what you specify?

 

[msquared48]

Ron:

Without the ceiling joists, being stick framed, the endwall would have to be platform framed with full height studs and no intermediate plate. I always spec this. I don't make too many friends with the contractors here, but I can sleep at night.

The problem is that most I do not get the chance to inspect, as you suggest here.. Therein lies the rub...

Mike McCann
MMC Engineering

 

[Ron]

Mike, I agree. That's a better way to do it and you're right...they'll give you the wobbly ladder to inspect it!

Full balloon framing is extremely rare in my area.

 

[AlpineEngineer]

Thanks guys. Mike thanks for explaining. I didn't realize the ceiling couldn't be used. I knew in analysis it wouldn't work but I thought it could be used in a prescriptive situation. I usually spec baloon framed gable ends, this happens to be a remodel. The contractors hate it but we have seen failures on gable ends in my area so I have some ammo when they start to argue with me.

On another note; have you guys seen the detail in the WFCM for 130mph exp B wind design for "ceiling bracing gable endwall"? They are showing a 20 gauge strap which comes vertically up the gable end wraps over the gable end truss and then bends and nails down flat to 2x4 bracing which rests on top of the ceiling rafters. I am unclear what this accomplishes. It seems to me it is tieing the ceiling and gable endwall together but it isn't taking the load to the roof diaphragm like Mike said. Any ideas on what this accomplishes?

See attached dwg

 

[DaveAtkins]

I had never seen that detail before--interesting!

Even though a GWB ceiling cannot "officially" resist wind load, we all know it is able to act somewhat like a diaphragm. And the detail shows bottom chord bracing extending the full length of the building, engaging all of the truss bottom chords in weak axis bending. That is why this "prescriptive" detail works, in my opinion.

DaveAtkins