From: John Rose <jrose36(--nospam--at)earthlink.net>
Date: Wed, 02 Jun 1999 12:13:20 -0700
The double-side shear wall is just like a one-side wall with the same nailing,
etc. as far as deflection is considered. Each side resists one-half of the total
shear force. So if wall is designed for 900 plf, check deflection based on 450
plf per side. The cyclic load shear wall test project sponsored by APA at
UC-Irvine in 1995 showed that UBC deflection formula predicts deflection
accurately for shear loads up to 40% over UBC allowable values, which is still
less than yield limit state. Two other factors need to be considered: the
hold-down will now be required to resist the total force (based on 900 plf for
example); and check bearing stress of end posts on plates. The end posts may
have to be increased in size or number so that bearing does not control. (I
recommend using 1977 NDS bearing stress f-c perp. = 385 psi for Douglas fir.
Current values have bearing capacity margin - max./ult. vs. design - of only
about 1.7, whereas old values were better matched to strength limit state of
shear wall, about 2.5 or higher times design value.)
SEAOSD developed recommended construction guidelines for double-side shear
walls, some of which are reflected in UBC provisions for this application. These
include offsetting vertical panel joints on opposite sides of wall, so they
don't occur on same stud. Also APA Help Desk (253-620-7400 or
www.help(--nospam--at)apawood.org) has suggested details for ofsetting nails in multiple rows
on opposite sides of plates and end posts.
John Rose/APA, Tacoma, WA
Jeff Smith wrote:
> For double sided shear walls I specify 3x sill plates and usually a 3x plus
> a 2x top plate, as well as 3x or 4x's at the panel edges. Plate fasteners
> are usually with lag or wood screws. Of course I get nothing but grief from
> the builders. Their biggest complaint is that they can not end nail the
> studs in a conventional fashion with anything less than a 30d nail.
> For some conditions, usually a retrofit, with a double sided 4" nailing I
> have specified double 2x6 plates and alternated the edge nails from the
> upper plate on one side and the lower top plate on the other side and
> additional plate nails, but technically this is not allowable.
> Does the UBC voodoo deflection formula apply for double sided shear walls? I
> would say no, unless it has been tested, and tested for a full range of
> aspect ratios. But since that is all we have, would you simply double the
> thickness "t"?
> I do not recall seeing any official detailing guidelines for double sided
> shearwalls, yet I have been detailing them since I started out. William
> Vaughn from the SEAOC Code Committee was editor for a real nice guideline in
> 1997. However, it did not address double sided walls. Does the new Breyer
> book have any design examples? I hope the ICBO code examples will address
> some of these issues.
> Anyone have any comments?
> Jeff Smith