Several years ago I did a supply complex warehouse for the Corps that
was divided into four areas with 4-hour area separation walls. After
much discussion of progressive collapse during and following the fire
the following options were allowed by the Corps. 1) Two 4-hour walls
side by side anchored only to its roof diaphragm with a seismic joint
between the walls. 2) A single free standing cantilever 4-hour wall not
attached to the diaphragm on either side, and therefore a seismic jont
on both sides of the wall. This option would then require that lateral
load bracing be placed along the 4-hour wall for each side.
The argument at the time was that to prevent the spread of fire to the
adjacent areas a rated wall must remain standing. It is similar to the
rating required at a property line, each building providing its own
protection. If the rated wall was attached to only one diaphragm and
that side burned the protection was lost to the remaining structure. If
the rated wall were attached to both diaphragms and at the time anyway
there were no reliable fusible links that would support the
vertical/lateral loads, the burned area may very well pull the rated
wall down also, again breaching the protection.
I ended up using a double wall since the walls were between 26 and 36
feet tall and cantilevering concrete walls was out of the question in
seismic zone 3.
Michael Zaitz wrote:
> Hello All;
> Sometime back the issue of 4 hour firewalls that allows the structure to
> collapse on either side of the wall without affecting the structure on
> the other side of the wall. I have a wall that is now required to be a
> 4 hour wall (due next week) and as such I need to do something to allow
> the collapse. To make matters worse the wall is a shear wall (or at the
> moment it is). I remember talk of anchors that will melt of allowing
> the structure to fall but did not see anything in the archives that had
> any manufacturers or the like. Coould someone provide me with some
> information or some leads as to where to find information on these types
> of anchors?
Forrest T. Braun, P.E.
BBFM Engineers, Inc.