Wood Diaphragm Design In Florida

[JedClampett]

I'm reviewing a project done by another company in Florida. I'm not liking the shear transfer at the top of the walls, but I accept the fact I might not understand the design.

Basic facts:
Gable Style Building about 55 ft. by 70 ft. in plan.
Eave Height is 21 ft.
Roof slope is 4:12.
Wood trusses every 2'-0"
Tie Beams at the top of CMU walls.
120 mph wind
3/4 inch plywood on the top and bottom of the trusses.

My problems are:
I don't see any blocking at the bearing ends of the truss.
At the Gable ends there are vertical studs at 4'-0" covered with vertical piece of 3/4 inch plywood. There is bracing between the gable end and the top and bottom plywood. The connection between the plywood and the tie beams has a piece of pressure treated timber between it and the tie beam. It's not too direct in my mind, especially between the top plywood and the wall.
My question is, can the bottom piece of plywood be the diaphragm? It's more directly attached to the tie beam, (2 x 6 continuous; concrete anchored to the tie beam) although I'm still not crazy about its attachment.

 

[Ron]

JC...not likely. Under the Florida Building Code, there has to be specific gable bracing of the trusses, typically as a diagonal member, perpendicular to the trusses.

Neither a floor diaphragm nor roof sheathing meet this requirement...looks like something's missing.

 

[hokie66]

The ceiling diaphragm would be stiffer and more direct than the roof diaphragms either side of the ridge. The connections still have to develop the forces into the walls.

 

[JedClampett]

To clarify, there are braces at the gable ends, perpendicular to the truss span. They consist of crossed 2 x 4's at 4'-0" spacing. They span the first four spaces (8'-0").
Hokie, it seems to me that what you say is exactly right. But one of the advantages of roof diaphragms is you can walk on them during installation. For a ceiling diaphragm, you're going to be driving a lot of nails overhead.

 

[Ron]

JC...not a problem with nail guns.

How are they developing the ceiling diaphragm at the walls? Did you mean there is no perimeter blocking between the trusses at the diaphragm termination as well? Any clips between the ceiling plywood joints?

 

[JedClampett]

As it's hard to explain, I'll scan in the detail when I get back to the office. As I said, I don't like it, but my opinion is "If it's not wrong, its right."

 

[msquared48]

Even though they are allowed, the ceiling diaphragme I know are sheetrock. Not real good in a high seismic area. Besides, many are broken with vertical steps. I trust the roof, not the ceiling.

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

 

[JedClampett]

Here's the details, both gable and bearing ends. I tried to circle up the connection between the bottom plywood and the tie beam.

 

[msquared48]

JC:

Looks like you are detailing a flat plywood ceiling diaphragm instead of gyp, correct?

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

 

[hokie66]

Jed,

I know little about Florida specific practice, but I've never liked supporting trusses directly on concrete. I would have used a timber plate bolted to the concrete, and would have provided blocking between the trusses. The adhesive anchors look too close to the corner of the concrete beam.

 

[JedClampett]

To be clear, I'm reviewing a design done by others. It is plywood, not gypsum board.
As far as the trusses being supported directly on the concrete, they do have a waterproof membrane, not shown on the detail.
I'll make the suggestions mentioned. Thank all of you for your help.

 

[Ron]

JC...looks OK to me, assuming perimeter built as detailed, though I would prefer a 2x6 at the diaphragm termination rather than a 2x4...edge distance on tie beam not good.

 

[dhengr]

JC:
I agree with Ron, those look like pretty good details to me. The trusses are all tied down, all of the plywood diaphragm edges are blocked, making for stronger diaphragms. Edge distances, fastener spacings, fastener capacities should all be checked against load demand in various directions. The gable end framing, the roof trusses and the roof diaphragm are braced by the 2x4 x-bracing system at each end, but the capacity and exact load path of this system should be reviewed too. All the right details are generally shown if fastener spacing and capacities, etc. check out. Furthermore, the plywood roof diaphragm loads are carried down to the 70' long conc. tie beam, through the cont. 2x4 eave joint blocking and the cont. 4x6 nailer blocking at the bottom of the facia plywood, if the nailing is all done correctly and the bolting into the conc. beam check out.

 

[dhengr]

JC:
In fact the ceiling diaphragm is quite appropriate here, it potentially does a good job of lateral support for the top of the conc. blk. wall and for the bottom of the gable end truss, or the whole truss and roof system for that matter. This elevation in the gable wall framing has always been a bit suspect, in that, the way it is usually framed it kinda forms a hinge point in the wall framing, except for whatever lateral support the ceiling framing (usually sht.rk.) provides. Furthermore, it’s the most efficient diaphragm in getting the loads to the gable end walls when thinking about wind loads perpendicular to the long dimension of the bldg. Otherwise, both the roof and the ceiling diaphragms will distribute some of the lateral loads. How is that 12x20 conc. tie beam reinforced?

 

[a2mfk]

I agree with the last few posters reviews. I have used a plywood sheathing ceiling diaphragm before with a steep-sloped gable roof where bracing was not reasonable.

Anyone else thing the 3/4" plywood is odd? I have never needed that for diaphragm design, even in FL... Especially since you have two diaphragms..

Even if allowed by code I would never use a gyspum board diaphragm for a ceiling in wind country, too many forensic jobs of leaky roofs, especially once the wind rips off a couple of shingles or worse, a sheet of plywood... Specifically not allowed in the FBC under the HVHZ (S FL only but may be in the general code somewhere):

FBC 2319.17.1.2 The drywall ceiling is not to be considered a ceiling diaphragm.

Jed- think you were worried about a moisture barrier on the trusses. Problem is the highest capacity uplift connections for CMU or concrete are usually direct embeds, plus these sit on a channel normally:

(HVHZ) FBC 2319.3.2.2 Joists and rafters may bear on a Product Approved channel-shaped metal saddle and fastened to the masonry by a steel strap anchor embedded into a grout filled cell of the masonry or concrete.

 

[a2mfk]

Ron- where can I find that section in the FBC that specifically prohibits that? I would think if its engineered then its allowed... Maybe you mean prescriptively?

Ron (Structural)
5 Nov 10 23:29
"JC...not likely. Under the Florida Building Code, there has to be specific gable bracing of the trusses, typically as a diagonal member, perpendicular to the trusses.

Neither a floor diaphragm nor roof sheathing meet this requirement...looks like something's missing. "

Regards,
Andrew Kester, PE
Florida