SECTION 704  EXTERIOR WALLS704.7 Unexposed surface temperature. Where protected openings are not limited by Section 704.8, the limitation on the rise of temperature on the unexposed surface of exterior walls as required by ASTM E 119 shall not apply. Where protected openings are limited by Section 704.8, the limitation on the rise of temperature on the unexposed surface of exterior walls as required by ASTM E 119 shall not apply provided that a correction is made for radiation from the unexposed exterior wall surface in accordance with the following formula:
(Equation 7-1) 
where:
Ae  =  Equivalent area of protected openings.
A   =   Actual area of protected openings.
Af  =   Area of exterior wall surface in the story under consideration exclusive of openings, on which the temperature limitations of ASTME119 for walls are exceeded.
Feo =  An “equivalent opening factor” derived from Figure 704.7 based on the average temperature of the unexposed wall surface and the fire-resistance rating of the wall.

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