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Steel deck roof diaphragm plus sheathing

Steel deck roof diaphragm plus sheathing

Steel deck roof diaphragm plus sheathing

(OP)
My current project has two buildings that are apparently required to be 'non-combustible' and as such have CMU walls, l.g. steel roof trusses and metal roof deck. The architect wants asphalt shingles which require 3/4" sheathing for a nailing surface.

I'm comfortable designing for lateral forces assuming the wood sheathing is my diaphragm. However, I am not sure how to fasten the sheathing through the decking to ensure diaphragm action, let alone what to use for the decking as its only purpose here is to shield the sheathing from fire.

This, in my opinion, could be easily accomplished by 2 layers of sheetrock on the roof truss bottom chords or cement board, but the architect says it is required.

The questions are:

1. How should the sheathing be fastened? (I'd like to emulate typical 6"oc/12"oc with self tapping screws, but that would seem quite a bit more labor intensive than using 16d nails driven pneumatically).
2. Can the steel deck serve as the diaphragm? I assume it can, but where can shear values be obtained for this? Obviously they are not in the NDS.
3. If it is easier to use the sheathing as the diaphragm as it is required anyway, what should be required for the deck if it is simply serving a purpose of non-combustibility?
 

RE: Steel deck roof diaphragm plus sheathing

Devil's advocate, have you asked if fire retardent treated plywood can be used in lieu of the metal deck?

RE: Steel deck roof diaphragm plus sheathing

(OP)
OHIO - They are adamant about the steel deck. I think your idea would be fine. I wonder what that would cost though - and if it would affect the shear value. I assume it would, but it would be stamped prior to Fire Treating.

EDIT: 16d nails obviously not necessary for sheathing.

RE: Steel deck roof diaphragm plus sheathing

I have come across this issue several times.  I design the metal decking as my diaphragm and ignore any benefits provided by the plywood sheathing.  I have typically called for 5/8" plywood or OSB to provide a substrate for the shingles.  I have it fastened nominally to prevent uplift off the deck with self drilling wood to light gage metal screws.

From a cost perspective, I have been told that it is often more expensive to provide FRT plywood instead of sheathing over metal deck.  

Another cause for concern with FRT plywood is the corrosive nature of the FRT chemicals.  I really don't like the idea of placing a material of this nature against the light gauge truss material.

Nick Deal, P.E.
Michael Brady Inc.
http://www.michaelbradyinc.com
 

RE: Steel deck roof diaphragm plus sheathing

I've used the same approach as TTU outlined often with a layer of insulation between the plywood and steel deck.  As far a non-combustible requirement I find it difficult to believe the roof system has to be completely non-comb. Are the shingles non combustible?  FMGlobal requires a layer of densglass sheathing on top of the steel deck to separate combustible insulation and roofing materials from the envelope to achieve a certain insurance rating/credit.

RE: Steel deck roof diaphragm plus sheathing

(OP)
Thanks for the replies. I got a call from a truss fabricator I work with on a lot of projects and while I had him on the phone I asked him his thoughts.

He sent me some helpful links with allowable shears for different products.

Based on my loads and 2'oc truss spacing I'm fine with a 1.5" 24g deck with #10's at every rib. I could probably go every second rib, but I'm not going to.

Then 5/8" OSB with 2" #6's at 12" o.c.

 

RE: Steel deck roof diaphragm plus sheathing

VTEIT,

I think you are on the right track.  I would prefer to use 22g deck and increase the truss spacing to five or six feet.

BA

RE: Steel deck roof diaphragm plus sheathing

I would agree with BAretired, in order to use less trusses and maximize what the decking can span, use 22 gauge decking and space light gauge trusses at 48" o.c.  You'll get much better diaphragm values from 22 gauge decking as well.

Nick Deal, P.E.
Michael Brady Inc.
http://www.michaelbradyinc.com
 

RE: Steel deck roof diaphragm plus sheathing

What is the roof pitch?
 

RE: Steel deck roof diaphragm plus sheathing

Lodgepole Pine.

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

RE: Steel deck roof diaphragm plus sheathing

Huh? Beautiful Trees.

I was just curious how yins handle a diaphragm on a sloped/gable roof since the sheathing is discontinuous in the middle and how the shears are transferred to the end walls.
 

RE: Steel deck roof diaphragm plus sheathing

Use a continuous bent plate under the deck at the ridge.   

BA

RE: Steel deck roof diaphragm plus sheathing

At the ridge, I specify intermittent blocking to provide continuity for the diaphragm.  Continuous blocking would be better, but would prevent use of a ridge vent if needed.

For load transfer at the side walls, I use sway blocking/panels to transfer forces at the truss heels to the supporting wall below.  For gable end walls , I either run the sheathing up the face of the gable truss or specify a drag truss (designed for in-plane shear) if it occurs along an internal shear wall.

Nick Deal, P.E.
Michael Brady Inc.
http://www.michaelbradyinc.com
 

RE: Steel deck roof diaphragm plus sheathing

Gable ends considered in the shearwall design?

Just pickin' brains here, I don't get involved in this kind of design and would like to know more.  

RE: Steel deck roof diaphragm plus sheathing

TTU-
Amazing, you answered me prior to my post.
You're like my wife....

RE: Steel deck roof diaphragm plus sheathing

It's called ESP.

BA

RE: Steel deck roof diaphragm plus sheathing

I'm having a nightmare...

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

RE: Steel deck roof diaphragm plus sheathing

Mike-
Do you need a cold one or something?  

RE: Steel deck roof diaphragm plus sheathing

No.  

I'm just relaxing and codgitating prior to addressing a contractual difference with a client.  The drink(s) will come later.  (smile)

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

RE: Steel deck roof diaphragm plus sheathing

(OP)
yins? are you from Pittsburgh? It's an 8-pitch.

I think shear transfer trusses are required wherever the diaphragm is responsible for transfering lateral loads to shear walls, right?

I require blocking at heals.

I had never considered the issue of discontinuous sheathing at the ridge. Thanks for bringing that up.

 

RE: Steel deck roof diaphragm plus sheathing

My parents are yinzer's from the 'Burgh...I'm not too far way. It slips out sometimes.

I'm not familiar with the heel blocking details or Shear transfer trusses.  

RE: Steel deck roof diaphragm plus sheathing

(OP)
I wasn't familiar with shear transfer trusses until I recently got a truss shop drawing submital with a note that said "trusses not designed to function as part of the lateral force resisting system". It was a medium sized, rectangular stick framed building and the only shear walls were the load bearing exterior ones and the gable walls.

I marked it "Revise and Resubmit" and noted that the trusses had to act as part of the LFRS.

I got a call from the fabricator and he educated me that he could easily accommodate this, and on smaller buildings the design may not even change, but that it had to be noted on drawings (it was in the spec anyway). I think that designing trusses for transfer of in-plane forces is pretty standard. Out-of-plane is another story and is more complicated as it must be achieved with bracing.

In terms of heel blocking - a lot of builders use 2x blocking cut to the truss/rafter spacing (minus .75") because it makes layout really easy as they are setting trusses.  

RE: Steel deck roof diaphragm plus sheathing

Mike,
Codgitating?  I assume that is deep thinking by old codgers.

RE: Steel deck roof diaphragm plus sheathing

I see.
Yes, we used to use blocking on trusses with taller heels (when I worked as a nail bender) only up to the point where they didn't suffocate the soffit/ridge vents.

I also never cared for "gable" trusses that only had vertical web members.
To avoid using them, I simply used regular trusses at the ends but set them back the wall thickness and balloon framed the gable end wall.
Then, the last truss nailed directly to the wall studs and also served as a drywall nailer.
I hated the gable trusses because they made the gable sheathing very wavy and screwed the siding up.  

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