RE: Fire wall/shear wall on property line

Ah, the joy of firewalls!

 

In my somewhat limited experience (two firewalls that I can actively recall), you end up having to choose between three basic options: 1) design the wall as a cantilever wall to resist the forces of the falling floor and temporary wind forces; 2) design the connections of the floor to the wall to be "break away" or "melt away" style connections and the wall as a cantilever to resist temporary wind forces; or 3) design a double wall system where if one wall falls with the floor, the remain wall is still a full firewall that is designed to resist the temporary wind forces.

 

Now, Jordan has given some good thought on option #1.  Your original thought fits within option #3.  I am not sure if option #2 would work for you or not as it is can be relatively easy to design "break away" connections for gravity loads (for typical wood construction this can be done with so-called "fire cuts"), but can get real messy when dealing with diaphram transfer connections if the wall is also a lateral resistance wall.

 

Regards,

 

Scott

Adrian, MI

-----Original Message-----
From: Jeff Hedman [mailto:jeff_h@lrpope.com]
Sent: Tuesday, February 05, 2008 2:23 PM
To: seaint@seaint.org
Subject: Fire wall/shear wall on property line

We are currently working on a 24’-0” tall masonry building whose rear wall is on the property line and must be a 2 hour fire wall.  12’-0” of the rear wall is a concrete retaining wall.  The owner is hell bent on wood trusses and diaphragm.  If a ledger is used to tie the wood roof to the masonry fire wall/shear wall, we must use straps to eliminate cross grain bending.  According to IBC 2006 section 705.2, the wall must be designed to resist the un rated diaphragm catching fire and collapsing and bringing the fire wall with it.  The easy thing to do would be to use a rated roof system but the architect is telling me that the system would consist of steel trusses and b deck with concrete or 4 layers of exterior gyp board, and the owner does not want to do either.  The rear 12’-0” tall concrete retaining wall is 12” thick due to the fact that it is supporting the building as well as a surcharge of another 12’-0” retaining wall only 6 feet away.  The best idea we can come up with is to place a 6” fully grouted masonry wall on top of the 12’-0” concrete retaining wall to act as the 2 hour fire wall only.  We would then frame a wood framed shear wall on top of the 12’-0” concrete retaining wall next to the masonry fire wall to transfer all lateral and gravity loads down to the 12’-0” concrete retaining wall.  If in the event of a structure fire the unrated roof collapses, the fire wall would still be intact and protect the adjacent lot and satisfy IBC 2006 705.2.  The wood framed shear walls are 12’-0” tall and all sheathing panel edges are blocked so they conform to the exceptions of IBC 2006 2305.1.5. As far as simply moving the building off of the property line, the lot is flag shaped and if the building is moved to the larger area of the lot, it will destroy any kind of storage yard.  The way the lot is laid out, if the building is moved off of the property line, there will not be sufficient room to have a two way drive into the property, so the building has to be where it is.  We are wondering if any one has run into this before and solved it with a better idea.

 

Jeff Hedman , E.I.T.

L.R. Pope Engineers & Surveyors, Inc.

1240 East 100 South Suite # 15B

St. George, Utah  84790

Office: 435-628-1676

Fax: 435-628-1788

 

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