For unreinforced solid or grouted masonry bearing walls, the ratio of unsupported height to nominal thickness, or the ratio of unsupported length to nominal thickness, should not exceed 20. For hollow walls or walls of hollow masonry units, the ratio should be 18 or less. For cavity or stone walls, the ratio should not exceed 14. See ANSI Standard Building Code Requirements for Masonry, 41.1, American National Standards Institute.
In calculating the ratio of unsupported length to thickness for cavity walls, you can take the thickness as the sum of the nominal thickness of the inner and outer wythes. For walls composed of different kinds or classes of units or mortars, the ratio should not exceed that allowed for the weakest of the combinations. Veneers should not be considered part of the wall in computing thickness for strength or stability.
For nonbearing, unreinforced exterior walls, the thickness ratio should not exceed 20. For unreinforced partitions, the ratio should be 36 or less.
Cantilever walls and masonry walls in locations exposed to high winds should not be built higher than 10 times their thickness unless adequately braced or designed in accordance with engineering principles. Backfill should not be placed against foundation walls until they have been braced to withstand horizontal pressure.
In determining the unsupported length of walls, existing cross walls, piers, or buttresses may be considered as lateral supports, if these members are well bonded or anchored to the walls and capable of transmitting forces perpendicular to the plane of the wall to connected structural members or to the ground.
In determining the unsupported height of walls, the floors and roofs may be considered as lateral supports, if they can resist a lateral force of at least 200 lb / lin ft and provision is made to transmit the lateral forces to the ground. Ends of floor joists or beams bearing on masonry walls should be securely fastened to the walls (Fig. 11.7). (See also Arts. 11.6 and 11.11.) Interior ends of anchored joists should be lapped and spiked, or the equivalent, so as to form continuous ties across the building. When lateral support is to be provided by joists parallel to walls, anchors should be spaced no more than 6 ft apart and engage at least three joists which should be bridged solidly at the anchors.
Unsupported height of piers should not exceed 10 times the least dimension.
However, when structural clay tile or hollow concrete units are used for isolated piers to support beams or girders, unsupported height should not exceed 4 times the least dimension unless the cellular spaces are filled solidly with concrete or either Type M or S mortar (Art. 4.16).
Anchors for Masonry Facings. Support perpendicular to its plane may be provided an exterior masonry wythe, whether it is a veneer (non-load-bearing) or the outer wythe of a hollow wall, by anchoring it to construction capable of furnishing the required lateral support. Accordingly, a masonry veneer may be tied with masonry bonders or metal ties to a backup masonry wall that is given lateral support or the veneer may be anchored directly to structural framing. Methods of bonding wythes together are described in Art. 11.3.2. The following applies to anchorage of masonry walls to structural framing.
Several types of anchors are illustrated in Fig. 11.8. They should be corrosion resistant. Also, they should be able to resist tension and compression applied by forces acting perpendicular to the wall. Yet, the anchors should be flexible enough to permit, between walls and framing, small differential horizontal and vertical movements parallel to the plane of the wall. The anchors should be embedded at one end in the mortar of bed joints and extend almost to the face of the wall. The other end should be securely attached to framing providing lateral support. The type of anchor to use depends on the construction to which the wall is to be anchored.
Figure 11.8a shows a corrugated metal tie for attachment of masonry walls to wood studs. Such ties should be fastened to studs with corrosion-resistant nails that are driven through sheathing to penetrate at least 11â„2 in into the studs. The ties should have a thickness of at least 22 ga, width of 7â„8 in, and length of 6 in.
Anchors shown in Fig. 11.8b and c may be used to attach masonry walls to metal studs. The wires of these anchors should be at least 9 ga. The anchor shown in Fig. 11.8d is suitable for tying masonry walls to structural steel framing, as illustrated in Fig. 11.9b.
Dovetail anchors for anchorage of masonry walls into dovetail slots in concrete framing are illustrated in Fig. 11.8e and f. Applications of the type shown in Fig. 11.8e are shown in Fig. 11.9c and d. Wires in these anchors should be at least 6 ga and should be spread to a width of at least 4 in for embedment at least 2 in into bed joints in the wall. The flat-bar type (Fig. 11.8f ) should have a minimum thickness of 16 ga and width of 7â„8 in. The end to be embedded in a bed joint should be turned upward at least 1â„4 in.
