Steel deck roof diaphragm plus sheathing

[HeavyCivil]

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?

 

[OHIOMatt]

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

 

[HeavyCivil]

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.

 

[TTUengr51]

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

 

[CTSeng]

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.

 

[HeavyCivil]

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.

 

[BAretired]

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

 

[TTUengr51]

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

 

[ToadJones]

What is the roof pitch?

 

[msquared48]

Lodgepole Pine.

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

 

[ToadJones]

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.

 

[BAretired]

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

BA

 

[TTUengr51]

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

 

[ToadJones]

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.

 

[ToadJones]

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

 

[BAretired]

It's called ESP.

BA

 

[msquared48]

I'm having a nightmare...

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

 

[ToadJones]

Mike-
Do you need a cold one or something?

 

[msquared48]

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

 

[HeavyCivil]

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.