Masonry Magazine March 1975 Page. 20
Noise Resistance
The cavity wall also possesses superior resistance to passage of sound. Recent tests, performed in accordance with ASTM E 90-70, indicate that the Sound Transmission Class of a cavity wall will improve on the average of 3 over its composite counterpart. In Table 2 this information is presented along with the wall weight in pounds per square foot.
Fire Resistance
Results from fire tests indicate that cavity walls perform in similar manner to other concrete masonry wall types. That is, the fire resistance is a function of aggregate type and equivalent solid thickness. Equivalent thickness is calculated for each wythe and added together to determine equivalent thickness for the wall. Equivalent thickness may be calculated for each wythe by multiplying the actual thickness of the wythe by the decimal equivalent of the percentage of solid material in the wythe. For example, the equivalent thickness of a 12" cavity wall consisting of 4" solid unit, 2" air space and 6" hollow block having a net area 55% solid is determined as follows:
4" solid 100% solid = 3.62" (actual thickness)
6" hollow, 55% solid: 5.62 x 0.55 = 3.09" (equiv. thickness)
Total equivalent thickness = 6.71"
The equivalent thickness is then used in conjunction with the concrete block aggregate type to determine the fire resistance rating. Appropriate tables can be found in the local building code.
Design
A review of structural test data reveals that for flexural strength, racking strength, and compressive strength (both wythes loaded), the performance of concrete masonry cavity walls is approximately equivalent to that of other concrete masonry wall types using the same type and quantity of material. Design criteria in building codes, however, contains certain provisions which differ for cavity walls when compared with other wall types. The summary which follows is based on ANSI Standard A 41.1-American Standard Building Code Requirements for Masonry. Provisions of this Standard are widely used and referenced by most of the building codes around the nation.
The first restriction for cavity walls encountered in ANSI A 41.1 is concerned with mortar type to be used: when exterior cavity walls less than 12" in thickness are used where the design wind pressure exceeds 20 lbs. per square foot, the types of mortar permitted are type M or S, ASTM C 270.
Allowable compressive stresses for cavity walls are about 25% less than for other wall types of the same masonry units. This is due to the fact that cavity wall allowable stresses are based on the assumption that floor loads bear upon only 1 wythe. Provisions are made to increase the allowable stresses by 25% when both wythes are loaded. Cross-sectional area of cavity walls is considered to be the gross area minus the area of the cavity. See Table 3.
Lateral support requirements for cavity walls are the same as for other wall types except that in computing the wall height or length to thickness ratio, the cavity width is not included. In other words, the wall thickness is considered to be the sum of the nominal thickness of each wythe.