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Re: Plywood over light gage shear wall

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Donald-

The experience you describe was what we experienced in the lab....7+ years ago.    I don't recall (It's been a while) the problem of breaking (twisting them off) screws but the entire process was very unpleasant & I was not very satisfied with the result.

The screws had a tendency to pull through the sheathing if the installer was not careful to avoid over driving.  The plywood / OSB would ride up the screw thread if the screw didn't drill into the stud immediately.  We had to push like crazy to keep the sheathing in contact with the framing and we had the luxury of building the walls laying on the floor.  The flanges often flexed away from the screw tip.

Also the flat head screws had a tendency to split the surface ply as the flat head (countersunk) was driven into un-countersunk wood.

Per another post ..........the 2004 AISI Lateral Design standard (adopted by reference by the 2006 IBC)   spec's the flat head screws

but I can't help but wonder how many plywood shear walls (on steel studs) have been built the people responsible for that code section.


Based on my experience the wafer head looks like a better choice.....btw looks like the water head screws with reaming wings must be used on 16 gage or heavier steel studs...so the problem with plywood riding up hasn't been solved for  most installations.


I don't think that the screws are weakened by the driving process .... I believe they're just not strong enough from the "get go".  The torque need to drill & drive appears to be higher than the screw can handle.

For example, I can drive #8 or #10 flat head Phillips sheet metal screws all day long into wood assemblies with properly sized pilot holes.  The #6's are an entirely different story.......even with properly sized (or ever oversized) pilot holes it is very easy to twist off the heads when driving the #6 screws.

the bottom line is.....the torque that can be applied through the Phillips heads & the torque that can be resisted by the engaged threads are both higher (typcally) than the shear strength of the screws shank just below the head.  The problem is just much more noticeable with the #6's  with the #8s' & #10's ......I can feel the larger screws coming up snug

To avoid shank failure there must be some sort of torque limiting element in the driving process

..either a clutch, stalling the driver, operator finesse, or bit cam out.....something must stop the driving process before the screw breaks or strips out.

cheers
Bob



On 5/24/07, Donald Bruckman <bruckmandesign(--nospam--at)verizon.net> wrote:

Correction:

 

That should have read, "… torsional load applied by the screw-gun…"   At the time, anecdotally, we guessed that the screw must have had too much trouble getting through the plywood because perhaps the tip was designed for cutting steel and not plywood, and that when it hit the steel the shank was already hot and fatiqued, but it was at about that point that the dang things just came apart, usually twisting the head clean off.  Finally, if I recall, we're talking about roughly 15% of the screws, which at the time I thought was pretty significant.

 


From: Donald Bruckman [mailto:bruckmandesign(--nospam--at)verizon.net]
Sent: Thursday, May 24, 2007 9:28 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Plywood over light gage shear wall

 

A note from the field.

 

Last time I did a project with plywood shear walls over metal stud, I noticed a lot screws broken off during the drive process. The installer complained that the screw shank was too thin, (I believe they were #8s or #10s although this goes back about 3 or 4 years.) but the EOR claimed bad installation. Whichever it was, as I stood on the scaffold and watched the installers beat up the screws and get frustrated, I got the impression that perhaps a very specific screw should have been spec'd rather than a generic note such as "No. 8 x 1-1/4" screw" (Don't quote me on what was used, I don't recall the exact callout).

 

Perhaps the framer did not have enough experience with the concept and didn't know the proper way to drive the screw (this might be the problem because most metal stud companies are drywall companies and are rarely are given structural work to do) but for whatever the reason, clearly this crew was having big trouble with the screw they were using.

 

First, any experiences with this problem? Second, even though I had them fix all the bad screws, I couldn't help but think about the status of the ones that appeared okay. Has there been testing to find out whether or not a specific screw shank can be weakened by the driving process; specifically by axial load applied to the shank by the screwgun?