Masonry Magazine February 1973 Page. 10
Floors must have high fire ratings. Elevator cores must be separated from the rest of the buildings by fire walls that will protect any occupants of elevators. Zones of safety must be accessible anywhere.
More specialized fire-tight compartments are becoming needed for storage of computer tapes. Many companies now have large investments in tapes which could not only be destroyed by fire but which cannot survive heat of more than 150 F. For their safekeeping it has become necessary to provide storage vaults that permit only very low temperature rise. Such vaults are likely to require double-wythe masonry, sometimes with additional protection, and to be designed to provide about twice the fire resistance of the rest of the building. Design is emperical and should be done by experts in the field.
Increasing Fire Resistance
Fire resistance properties of concrete masonry are a function of type of aggregate and solid thickness of the masonry units. With walls of hollow units the thickness of the core space is a minor factor; the principle factor is the total thickness of solid material in the unit rather than the total block thickness. Therefore, with hollow unit construction, the use of a thicker unit will increase the fire resistance only if the amount of solid material is also increased. This fact is recognized in all of the Model Building Codes which list the equivalent thickness method for determining fire resistance of concrete masonry.
For a given total wall thickness and aggregate type the fire resistance of hollow units can be achieved in several ways. Besides increasing the equivalent thickness by use of units having greater net volume, filling the cores, or hollow spaces, with an appropriate material will add to the fire resistance. For example, filling cores with perlite or vermiculite loose-fill insulation will increase the fire resistance of a 2-hour rated wall to at least 4 hours. The same is true if cores are filled with expanded shale or slag.
In reinforced hollow unit construction, where cells are filled with grout, the grout has the effect of adding to the equivalent thickness resulting in a corresponding increase in fire resistance. An 8 inch thick fully grouted wall, for instance, would have an equivalent thickness of 7.6 inches which is the same as if the wall were composed of 100% solid units.
Occasionally, in special or unusual circumstances, the need arises to develop fire resistance ratings in excess of 4 hours, which is the maximum required by building codes; or, the need may exist to estimate the fire resistance period of composite wall assemblies where the rating of each wythe is known but not the composite rating. Methods for solving these problems are discussed in "Fire Resistance Classifications of Building Construction," BMS 92, National Bureau of Standards, and are synopsized in the following paragraphs.
In most cases the fire-resistance period of concrete masonry walls is determined by the temperature rise on the unexposed side and it is on this criterion that this method of estimating the ultimate fire resistance is based. Tests have established that the fire resistance of a wall can be expressed by the formula
R = (cV)
where R fire resistance period
C= coefficient depending
upon the material
V volume of solid material
per unit area of wall
surface
n=exponent depending on
rate of temperature rise
at the exposed side of
the wall.
The fire resistance period of a composite wall may be expressed in terms of the fire resistance periods of the conjoined wythes or laminae of the walls as follows:
If R1, R2, R3, etc. fire resistance periods of walls, or component wythes of walls, having volumes of solid material per unit area of wall surface of V1. V2. Va. etc., respectively, also letting C and n be as defined previously, then for walls in general, R₁ (CV), R2 =