Alt Design Edmonds99-SMS-Branham-CKC Letter-Wood Frame Firewall Detailing-11-21-19M, "
STRUCTURAL ENGINEERS
November 21, 2019
Leif Bjorback
City of Edmonds Development Services Department
121 511, Ave N.
Edmonds, WA 98020
RE: Edmonds Highway 99 — Wood Frame Firewall Detailing;
Plan Check No: BLD2018-1622
Dear Leif,
Cary Kopczynski & Company
10500 NE 8th Street, Ste 800
Bellevue, WA 98004-4351
Phone (425) 455-2144
www.ckcps.com
The Edmonds Highway 99 project will require fire walls in the wood framed portion of the building
(reference drawings and response letters provided by Studio Meng Strazzara). The traditional approach for
these types of assemblies has been to construct two independent wood stud walls with a gap between them
(similar to a party wall / demising wall), and then to achieve the fire rating through the addition of gypsum
wall board sheathing, thickened rimboards, fire retardant treatments, etc. In recent years, fire protection codes
have added new language that would remove the floor and roof structural sheathing spanning across the gap
between the walls. CKC takes the position that this approach is neither necessary nor appropriate.
If the sheathing is discontinued across the gap between the walls, then a building separation is
effectively created. Accordingly, this gap will now need to be increased substantially in order to meet the
structural separation requirements as outlined in the building code. As an example, a 5-story wood structure
with wood -framed shearwalls could reasonably be expected to drift 1.5% in a design level seismic event. At
approximately 10'-0" per floor, this amounts to 9" of drift and a 1'-1" building separation, far in excess of the
1" to 3" typically utilized for this gap between walls. The Structural Engineers Association of Southern California
issued a letter in 2008 which addressed this continuity issue in further detail, which has been included for
reference.
Our position is that the intent of the fire protection code can still be met by maintaining the
sheathing across the gap between the walls. Three key points in support of this assertion are as follows:
1) Self -Supporting Structure Each Side of the Gap. The gravity framing each side of the gap is
independent of one another and self-supporting. The removal of one does not compromise the
structural integrity of the other. Furthermore, shear walls occur each side of the gap which can
perform independently of one another, ensuring lateral stability of both sides as well.
2) Char Rate of Sheathing Relative to the Remainder of the Structural Frame. Regardless of
the intended fire rating, the sheathing typically varies in thickness from 1 /2" to 3/4", and should
be expected to be compromised structurally well before the gravity frame is compromised to the
point of collapse. As such, a collapse on one side of the gap due to fire should not be expected
to produce a collapse on the other side, since the connection required to facilitate this collapse
(the structural sheathing) would have become too weakened to impart any significant load.
3) Strength of Sheathing & Connections Relative to the Remainder of the Structural Frame.
In the instance that a collapse did occur on one side of the gap, the mechanism for transmitting
the force of this collapse across the gap would be through the sheathing and sheathing fastening.
LEADERS IN BUILDABLE DESIGN
S e a t t l e I S a n F r a n c i s c o I L o s An g e I e s I C h i c a g o
Assuming a standard combined weight of the wall and floor framing of 40 psf, the force across
this gap through the sheathing would be on the order of 500 plf without consideration of impact
or inertia. Consideration of inertia would at a minimum increase this load to 1,000 plf. The
strength across this gap will be limited by the strength of the sheathing nailing (10d common @
6" oc edges & 12" oc field), which amounts to 840 plf. Consequently, it should not be expected
that even competent sheathing will have adequate strength to transmit load across the gap that
would result in a building frame collapse.
All of these points taken into consideration together provide a logical argument for maintaining the
sheathing across the gap in manner that satisfies both the structural and fire -rating requirements as outlined in
their respective codes.
Of further note is that in the 30+ years that CKC has designed these types of buildings we have
never once discontinued the structural sheathing in this manner, nor have we seen any of our peers do so.
Maintaining the sheathing continuity has recently and historically been accepted in multiple jurisdictions along
the West coast of the United States, including Seattle, Tacoma, and the San Francisco Bay area. We are of the
strong opinion that removing this continuity not only violates sound engineering judgement, but also
standard of care.
Should you require any further clarification, feel free to call or email.
Regards,
Joel Kipple, PE, SE
Associate / Project Manager
LEADERS IN BUILDABLE DESIGN
Seattle / San Francisco / Chicago