Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Firewall Design Philosophy - - Was "The weakest link"

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Title: Message
Kevin, and fellow engineers,
 
        Sorry to be coming into this thread so late but I was otherwise engaged.
 
        About five years ago I was faced with the task of designing a fire wall between an existing building and a new building.  I agonized my way through many of the same thoughts as I see appearing in the 17 other posts recently submitted.  The City planning department and the Fire Marshal's office were of very little help other than to tell me that "It has to be designed by a Professional Engineer."  I did some research; and, of course, my research raised more questions than it answered.
 
        In reviewing the material in Commentary M, in the Structural Commentaries for the National Building Code of Canada, and finding some of the material in that publication not making a lot of sense to me I reached the conclusion that there is a gap in the field of building technology.  Perhaps WE should do something to fill that gap (If WE don't who will?).
 
        Following are some thoughts I have on the problem.
 
Purpose of a Firewall
 
        The general purposes of a firewall are
  1. It must withstand a specified fire for a specified length of time.
  2. It must remain standing throughout the fire and for some specified time afterwards even if the building which is on fire undergoes a complete structural failure.
  3. It must resist certain specified loads which may result from the fire or from other causes closely related to the fire.
Types of Firewall
 
        Commentary M of the Canadian NBC defines four types of firewall.  There may be others; but this is a good start.
  1. Double wall, which is, in fact, two separate walls each designed for at lease half of the combined required fire rating.
  2. Cantilevered firewall, which is a free-standing wall not connected to the building on either side of the wall.
  3. Tied firewall, where the structures on either side of the firewall are tied to each other but not to the firewall.
  4. Weak Link firewall, where the structures on each side of the firewall are both tied to the firewall with a "week ling" that is expected to fail in the event of a fire or structural collapse due to a fire and the failing structure is allowed to fall away without bringing down the firewall.
        Of these, the last two seem to me as well as to others who have posted recently to be unreliable due to the fact that the "weak link" may not fail if the fire is not close enough cause failure of the link. 
 
        On further reflection the "size of the wall may make a difference to how it will perform.  For instance, a "long" wall may have only a "short" portion under "attack" by fire at any one time.  To extrapolate this a little further, for a "long" weak link system to fail structurally the failing structure would have to pull down not only the weak link which was supported to fail but didn't; it would also have to pull down the similar link on the other side of the wall PLUS the inherent strength of the wall itself which is connected to other parts of the wall not under duress.  This would seem to make the tied firewall and/or the weak link firewall where instantaneous, complete failure is more probable but a good detail where a progressive collapse failure is more probable.
 
Situation Classification of Firewalls
 
        Here I am thinking of firewalls which have other uses than just serving as a fire separation.  These include the following.
  1. Load bearing vs, non load bearing.
  2. Shear walls vs. non shear walls.
  3. Damaged vs. undamaged.  Here I am thinking of earthquake country.  I know that fire and earthquake are unlikely to happen at the same time; but an earthquake could cause a fire.  The performance of damaged firewalls should probably not be ignored.  Unreinforced masonry, which might be satisfactory in one location might be completely unacceptable in another.
        There are some thoughts I have which may be useful for starters.
 
Regards,
 
H. Daryl Richardson       
----- Original Message -----
Sent: Wednesday, April 26, 2006 1:23 PM
Subject: "The weakest link"

In wood structures separated by a masonry firewall, the attachments of the wood to the masonry wall should be designed so as not to cause collapse of the wall if the wood structure on one side burns and collapses.
In the Canadian Commentary M to the NBC 1995, they show a system using a "liaison faible" which I guess is translated from "weak link".  The idea is to provide lateral support to the masonry wall at each floor level, and to resist certain lateral loads such as seismic and wind, as well as loads which may occur during the fire, such as the force of the water jet.  But in the case of fire, to be weak enough to break under the force of the falling structure. 
 
There is not much detail as to how to implement this.
 
Does anyone have any examples ?
 
Kevin