In the 2015 code, Section 715.4.2 was added to address the joint
between a rated fire barrier wall and a non-fire-resistance-rated
exterior curtain wall. This topic will be discussed in more detail when
the technical requirements of Section 715.4 are discussed.
The means by which the exterior curtain wall is constructed and
attached to the floor will dictate the fire risk associated with the
exterior curtain wall/floor intersection. Figure 5.4 shows several
different arrangements for the exterior wall and floor intersection.
In Condition A, the exterior walls bear on the floor slab and run
from the top of the floor below to the bottom of the floor above. In
this scenario, there is no void at the edge of the floor slab, and as
such, the only path for flame propagation is via leap-frogging.
Under Conditions B and C, the exterior curtain wall skirts past the
outer edge of the floor slab, creating a void between the wall and
the edge of the floor slab. That void, if left unprotected, will allow
for the passage of flames from the floor below to the floor above.
In Condition B, the cavities of the exterior curtain wall are open.
As such, the void extends from the interior surface of the exterior
sheathing, siding or finish material to the edge of the floor slab. In
this situation, the two paths for flame propagation are through the
void and via leap-frogging.
In Condition C, the exterior curtain wall contains a membrane on the
interior surface of the wall so the void would only include the gap
between the edge of the horizontal assembly and the interior face
of the exterior wall. However, this wall construction contains a void
within the wall cavity. As such, in this situation, the paths of flame
propagation include through the void, through the cavity and via
leap-frogging.