Masonry Magazine August 1978 Page. 10
Sound Transmission
Resistance to transmission of sound is accomplished in two ways: the use of heavy massive walls, or the use of discontinuous construction. The cavity wall employs both of these, i.e., the weight of the two masonry wythes plus the partial discontinuity of the cavity.
In cavity wall construction, the air cavity provides a partial isolation between the two wythes. Sound on one side of the wythe strikes it and causes it to vibrate, but because of the separation and cushioning effect of the cavity, plus the massiveness of the wythes, the vibration of the other wythe is greatly reduced. A 10-in. (254 mm) cavity wall has an STC (Sound Transmission Class) rating of 50, which is usually sufficient for substantially reducing typical outside noises entering the building through the wall. For more information on sound transmission, see Technical Notes 5A.
Fire Resistance
The results of ASTM E 119 fire resistance tests clearly show that masonry cavity walls have excellent fire resistance. Fire resistance ratings of cavity walls range from 2 to 4 h, depending upon the wall thickness and other factors. Due to their high fire resistance properties, brick walls make excellent fire walls for compartmentation in buildings. By using compartmentation, the spread of fire can be halted. Technical Notes 16 Revised describes these fire ratings and applicable design conditions.
Structural Properties
Properly designed, detailed and constructed cavity walls may be used in any building requiring loadbearing or non-loadbearing walls in the same manner as other masonry walls.
The increased flexibility afforded by the separation of the wythes and the use of metal ties permits more freedom of differential movement between the wythes. This is extremely important in today's construction which makes use of increasingly more combinations of dissimilar materials.
DESIGN OF CAVITY WALLS
# General
The successful design of cavity walls depends on proper attention to four elements: appropriate structural design, proper detailing, selection of quality materials, and execution of good workmanship. All four elements must be satisfied to produce a successful cavity wall.
# Structural Design
The structural design of cavity walls can be by either of two methods. The rational design method is based on the properties of the wall materials and engineering analysis. This method may be used for high-rise bearing wall buildings, auditoriums, churches, gymnasiums, warehouses, and other structures where high walls are a necessity. The empirical method is generally satisfactory for one- and two-story buildings consisting of light construction, and limited floor spans and wall heights; and for multi-story buildings where unsupported wall heights are not excessive.
Rational Design. When a cavity wall is composed entirely of brick, it may be designed using the 1969 BIA Standard, Building Code Requirements for Engineered Brick Masonry. This standard is a complete code of minimum requirements for the rational design and construction of brick masonry, both plain (non-reinforced) and reinforced. The standard includes definitions and requirements for materials, structural design and construction. See Technical Notes 24 Series, and BIA's Recommended Practice for Engineered Brick Masonry, for complete guidelines on structural design of engineered brick masonry walls.
Allowable Stresses-Tests of single- and multi-wythe brick walls indicate that the compressive strengths of units, and the type of mortar are the principal factors affecting the overall compressive strength of the wall.
Table 1, based on the 1969 BIA Standard, shows the range of compressive strength values that are possible. The maximum allowable axial compressive design load for an unreinforced brick masonry wall is given by the formula: P 0.20 fCCA, where f' is the 28-day ultimate compressive strength of the brick masonry, C is the eccentricity coefficient, C, is the slenderness coefficient, and A, is the gross cross-sectional area of the wall. The value of 0.20 fCC, is the average allowable compressive stress permitted in the member. not the maximum compressive stress permitted in the extreme fiber.
Effective Thickness-According to the BIA Standard, for cavity walls loaded on one wythe only, the effective thickness shall be taken as the actual thickness of the loaded wythe. When the cavity wall is loaded on both wythes, each wythe shall be considered to act independently and the effective thickness of each wythe shall be taken as its actual thickness.
Lateral Support-The resistance of unit masonry walls to lateral forces or eccentrically applied vertical forces depends not only on the slenderness of the wall but also on the boundary conditions, the degree of end fixity, plate action, arching, the number and location of openings, and other factors.
While all of these factors cannot be determined at the design stage, it is known that lateral forces due to wind vary substantially in different localities and different elevations above the ground. For this reason, a single height-to-thickness ratio for any locality and any elevation is not realistic.
Design wind pressures should be considered in establishing maximum slenderness ratios and lateral support for exterior cavity walls. Table 2, based on the 1969 BIA Standard, may be used as a guide for the design of non-loadbearing brick cavity walls, or they may be more rationally designed.