Masonry Magazine August 1978 Page. 12
TABLE 3
Empirical Design
Allowable Compressive Stresses in Cavity Walls
ANSI A41.1-1953 (R 1970)
| the following table shows the allowable compressive stresses in cavity walls according to ANSI A41.1-1953 (R 1970).
`
Allowable Compressive Stresses,
Net Cross-Sectional Area, psf (MPa)
| Mortar Type | Compressive Strength of Units | M | S | N |
|---|---|---|---|---|
| Solid units | 2 500 psi (17.237 MPa) plus | 140 (0.965) | 130 (0.896) | 120 (0.827) |
| | 1 500 (10.342 MPa) to 2 500 psi (17.237 MPa) | 100 (0.689) | 90 (0.621) | 80 (0.552) |
| Hollow units | | 70 (0.483) | 60 (0.414) | 55 (0.379) |
Net cross-sectional area of cavity walls is the nominal gross cross-sectional area of the wall minus area of cavity between wythes. The allowable compressive stresses in Table 3 are based upon the assumption that floor and roof loads bear upon but one of the two wythes. Where hollow walls are concentrically loaded, the allowable stresses shown in Table 3 may be increased 25%.
`
Empirical Design. For cavity walls composed of dissimilar units, the only available design standard is the American Standard Building Code Requirements for Masonry, ANSI A41.1-1953 (R 1970) hereinafter referred to as ANSI A41.1 (see Technical Notes 42). This standard provides minimum requirements for masonry construction including definitions, requirements for materials, allowable stresses, and general requirements for all types of masonry except reinforced masonry and thin veneers.
`
Allowable Stresses-The allowable compressive stresses included in Table 3, based on ANSI A41.1, are adequate for the design of most structures in which loadbearing cavity walls are used. However, for buildings in which higher compressive stresses are developed, it may be desirable to design the interior wythe of the cavity wall as the loadbearing element and to consider the exterior wythe as a veneer or curtain wall.
`
Thickness-ANSI A41.1 states that for loadbearing walls. "... 10-inch [254 mm] cavity walls shall not exceed 25 feet [7.620 m) in height above the support of such walls. The facing and backing of cavity walls shall each have a thickness of at least 4 inches [102 mm] and the cavity shall be not less than 2 inches [51 mm] (actual) nor more than 3 inches [76 mm] in width."
`
Lateral Support-ANSI A41.1 requires that the ratio of unsupported height to nominal thickness or the ratio of unsupported length to nominal thickness (one or the other, but not necessarily both) should not exceed 18 for cavity walls. In computing this ratio, the value for the thickness of the cavity wall is taken as the sum of the nominal thicknesses of the inner and outer wythes.
`
Lateral support in the vertical direction may be provided by floors, roofs, or spandrel beams. Lateral support in the horizontal direction, where the length of the wall is the controlling factor, may be provided by cross walls, columns, buttresses or pilasters.
`
Detailing
Regardless of the method of structural design, proper detailing of cavity walls is very important to insure that the wall acts as it is intended with regard to the various properties listed in the beginning of this Technical Notes. Areas of concern for detailing are bonding. joint reinforcement, flashing, weep holes, shelf angles, expansion joints, anchorage, caps and copings.
`
Bonding. The facing and backing (adjacent wythes) of cavity walls should be tied together with corrosion resistant 3/16-in. (4.8 mm) diameter steel ties, or metal tie wire of equivalent stiffness, embedded in the horizontal mortar joints. There should be at least one metal tie for each 4½ sq ft (0.418m²) of wall area. Individual ties in alternate courses should be staggered; the maximum vertical distance between ties should not exceed 24 in. (610 mm) and the maximum horizontal distance should not exceed 36 in. (914 mm) (see Fig. 2). The ends of the ties should be bent to 90 deg angles to provide hooks not less than 2 in. (51 mm) long, or ties bent to a rectangular shape should be used. Additional ties should be provided at all openings spaced not more than 3 ft (914 mm) apart around the perimeter and within 12 in. (305 mm) of the opening. Where the cavity width exceeds 3/2 in. (89 mm) but is less than 4/2 in. (114 mm), there should be at least one metal tie for each 3 sq ft (0.279 m²) of wall area. It should be noted that crimping of the metal ties in the middle to form a drip is not necessary, and could decrease the strength of the tie.
`
When continuous wall ties (joint reinforcement) are used, they may be either truss or ladder type with at least one side wire in each wythe. Where the cavity wall is composed of a brick exterior wythe and a 6-in. (152 mm) or greater hollow concrete block inner wythe, the reinforcement should consist of three wires, or two wires with rectangular ties, as shown in Fig. 3. In both