fire walls

huangyhg

fire walls
i have a design that consists of 3-story storage with steel frame and concrete composite deck. the size of the building requires a fire wall that separates the structure in two. i have designed the floor framing so that the adjecent steel line resides 10 feet from the masonry wall, resulting in the floor deck to span between the steel support and a shelf angle bolted to the masonry wall. this allows me to use the wall for bearing as well as a shear wall.
the code for fire walls requires that a collapse on one side shall not cause a collapse on the other side (and the wall to remain).  my feeling is that the bolts and/or angle on the fire side would fail and that the bolts on the "good" side would prevent wall damage.
it seems terribly excessive to build a steel line immediately on either side of the masonry wall.
thanks for any input.
zulak
normally, to meet the code explicitly on this matter, you do have to essentially build two separate structures with a wall between that is fire rated.  you then have to consider that the fire can occur on either side and if so, with a building collapse, ensure that the wall/floor connection will fail before the wall itself will fail.  
this can be somewhat difficult as you do not know the exact mechanism by which the structure fails.
alternatively, you build two walls separated completely so that collapse on one side doesn't affect the other.
we designed a single story warehouse in pennsylvania a few years ago that used a single cmu wall and separate steel framing on each side (the wall was therefore non-load bearing).  the connections of the wall to the steel was just adequate to support the required lateral loads on the walls (in this case a 5 psf lateral code pressure perpendicular to the wall).  lateral bracing for the building was added, and the wall did not serve as a shearwall.
i agree with you zulak that it seems excessive to build two structures next to each other. from a purely engineering point of view and not a code point of view i would say that the fire wall should be stable in the event of a fire on one side. this is the case as long as the failure of the first line of steel does not redistribute so much floor load to the wall that the wall fails. you have to assume that the first line of steel could be affected by the fire and fail before the bolts are affected. in which case you may need to check the wall for additional redistributed floor load and ensure failure of the bolts before failure of the wall.
carl bauer
a reference is nfpa 221: standard for fire walls and fire barrier walls.  structural stability and strength is discussed for the various types of fire walls (cantilevered, tied, double).  i think it costs about $20.