The intent of this Article is to ensure that the control of fire spread by the interior fire separations between adjacent fire compartments is not defeated through the spread of fire by thermal radiation outside the building. Minimum separations (Do) are specified between unprotected openings in separate fire compartments of the building where the exterior faces of these compartments are deemed to expose each other to a thermal radiation hazard. This situation may arise where the angle, θ, between the intersecting planes of the exposing building faces is less than 135° and both of these faces have unprotected openings. The exterior walls of the fire compartments do not have to intersect in order to apply Article 3.2.3.14. Rather, the critical factor is the angle, θ, between the planes formed by the exterior walls. Examples are shown in Figures A-3.2.3.14.-A, A-3.2.3.14.-B and A-3.2.3.14.-C of situations which would be addressed by this Article.
Openings in Walls at a Right-Angle Corner
Openings in Walls which are Parallel to One Another
Openings in Walls with an Included Angle of 45 Degrees
The exterior unprotected openings in the fire compartments are not deemed to expose each other to a thermal radiation hazard if:
- the angle formed between the planes of the exposing building faces is 135° or more, or
- the fire compartments are sprinklered.
In order to apply Sentence 3.2.3.14.(1), both of the exterior walls must have unprotected openings. Sentence (1) does not apply where the exterior wall of only one fire compartment has unprotected openings. The separation of exterior unprotected openings in adjacent fire compartments is not required if the openings in both compartments are in the same plane (θ = 180°).
Sentence 3.2.3.14.(2) requires the exterior walls of each fire compartment within the distance, Do to have a fire-resistance rating. The fire-resistance rating must be at least equal to the required interior separation between the fire compartments.
Thermal radiation from an opening is substantially reduced at angles less than 45° from the plane of the unprotected opening.
This may be shown schematically in Figure A-3.2.3.14.-D.:
Thermal Radiation
Sentence 3.2.3.14.(2) requires the portion of an exterior wall (containing an unprotected opening) of any other fire compartment that falls within the shaded area of the Figure above to have a fire-resistance rating.
Figure A-3.2.3.14-E illustrates the application of Sentence 3.2.3.14.(2) where the exterior walls of 2 fire compartments contain unprotected openings and the planes of the openings meet at 90°.
In order to apply Sentence 3.2.3.14.(2), the fire-resistance rating of each of the exterior walls exposed to significant thermal radiation at all angles of less than 135° within the distance, Do, from the opening in the opposite wall must be investigated.
In the example shown in Figure A-3.2.3.14.-E, the effect of Opening A on the exterior wall of Fire Compartment “B” must be investigated as follows.
In this example, a portion of the exterior wall of Fire Compartment “B” happens to fall within the distance, Do, and within the angle, θ less than 135°. However, only that part of the wall which is shown shaded is required to have a fire-resistance rating.
