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Re: screws in wood diaphragm

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On 1/29/07, Jim Lutz <Jim.Lutz(--nospam--at)bhcconsultants.com> wrote:

I also seem to recall hearing that nails are a better choice than screws because screws are more brittle. It also seems intuitive to me that a nailed structure would tend to have a much higher damping ratio due to the nature of pullout versus a screw , which is also a good thing.

Jim Lutz, P.E., S.E .

720 3rd Avenue, Suite 1200

Seattle, WA 98104-1820

206 505 3400 Ext 126

206 505 3406 (Fax)



Agreed that nailed diaphragms would have better damping & screws are often more brittle.

But I believe that this thread began talking by about horizontal diaphragms with screws....my guess is that the designer is looking  to avoid floor squeaks. 

Since in a typical wood framed structure the floors are much stuff than the lateral system, I would posit that floor diaphragm damping Iif it was even activated) would not contribute much to the dynamic response.   A glued & ring shank nailed diaphragm would be very stiff as well.

wrt respect to nailed vs screwed plywood shear walls...we tested some (a while ago) and found

dry wall screws are a no-no...too brittle (IMO probably to brittle for drywall shear walls)
nails bend & yield repeatedly....good hysteresis / damping less pinching of the cyclic load / deflection curves

larger diameter screws (#8 & #10) tend to perform pretty well, they tend to damage the plywood locally & curves pinch

thicker plywood requires larger dia nails....thick plywood can fail smaller nails
#8's in 3/8    #10's in 1/2 when in doubt use larger dia screws

you've got to balance local plywood capacity against screw strength & make sure that the screws "win" .....otherwise the plywood will break the  screws.


all of this (screws in shear walls) is pretty much (IMO) an academic exercise, a real world designer should stick with proven techniques unless there is some huge performance benefit to be had.

we did tons of shear wall tests & I never really developed the confidence to recommend screws in shear walls.  

we even tested a glued & screwed narrow shear wall....it was very strong, right up until it failed suddenly 

If one is depending on yielding behavior (shear walls) I would stick with nails, if you're looking to build a rigid / squeak free floor diaphragm (& IMO residential timber framed floors are rigid)....... I would use ring shanks & adhesive.

cheers
Bob