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Re: Wood on Steel Studs

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Now here's a subject that has been troubling me. Steel stud shear walls.
Check out Chapter 22, Div VIII, Lateral Resistance for Steel Stud Wall
Systems in the 97 UBC. Also see the recent tests from Santa Clara Univ. If
you need copies, contact Hank Martin at AISI, 916-887-8335. Wood structural
panels on steel studs behave quite a bit differently than on wood studs.
Screws are essentially pinned and rotate at the face of steel studs after
first yield. 

After additional cycles of loading, the screws have very little or no initial
stiffness as reloading for the next cycle begins. It takes quite a bit of
drift before the screws are again engaged and then the lateral load
resistance picks up quite rapidly only after a considerable drift.  

Despite having very little initial stiffness after the first cycle,  design
values for panels on steel studs can end up being quite a bit higher than
comparable test assemblages on wood studs. The flexural capacity of screws is
not really being tapped. Screws essentially rock at the face of studs and
then are engaged in a tension/prying action after considerable drift. Since
tension in screws is higher than bending, the design values can be higher. Be
sure to specify screws with the largest/deepest threads to reduce the chances
that screws will simply strip and walk out of their holes. I wonder if steel
studs are taking advantage of a loophole in the way design values are
established.
 

I think this means that after future earthquakes steel stud buildings will be
swaying around and replumbed with very little lateral force. Its not a matter
of instability so much, but it could be unsettling to owners who prefer that
their walls not creak in the wind and not move when leaned against. To repair
damaged steel stud walls, owners may have to plan to remove and replace the
screws into newly drilled holes in the studs to restore the initial stiffness
that was lost in the first or few cycles. So the life cycle costs for steel
stud shear walls in high seismic areas may be higher than wood studs. 

In contrast, nails into wood studs are essentially fixed just below the face
of the stud and take advantage of the flexural capacity of nails to dissipate
energy. After many cycles, wood stud walls with wood structural panels still
have considerable initial stiffness for the next cycle. For owners, that
means damaged wall will not creak or drift as easily when pushed against
compared to comparably damaged steel stud walls. In theory, wood stud walls
dissipate much more energy because their hysteretic curves are not as pinched
as steel stud walls.

Have others out there thought about this concern?