Masonry Magazine August 1978 Page. 14
Expansion Joints.
The movement of the outer brick wythe due to thermal and moisture expansion may be greater than the movement in solid or composite walls exposed to the same environment. This is due to the greater differences between the mean maximum and mean minimum temperatures of the outer wythe of the cavity wall, and the absence of restraint usually provided by dead and live loads, masonry bonders or filled collar joints in solid walls. This is particularly true for non-loadbearing cavity walls used in skeleton-frame construction, especially if they are insulated. For this reason, it is recommended that expansion joints be provided through the outer wythe of the cavity wall on each side of an external corner where the walls are 50 ft (15.240 m) or more in length. Where possible this joint may be placed in line with the jambs of the windows nearest the corner.
In certain situations it may prove advantageous to use continuous horizontal joint reinforcement as temperature steel in the exterior brick wythe. This will provide increased tensile strength and help reduce the movement of the wythe between expansion joints. This reinforcement may consist of No. 9 gage continuous masonry wall reinforcement spaced 16 in. (406 mm) o.c. vertically (see Fig. 6).
When brick or hollow brick is used in the exterior wythe with solid or hollow concrete masonry backup in the inner wythe, special care must be exercised in the detailing. Control joints must be placed in the concrete masonry wythe, along with suitable joint reinforcement, to control cracking from shrinkage due to initial drying. Control joints may be placed 20 ft (6.096 m) o.c., or as recommended by the National Concrete Masonry Association. The locations of these control joints may or may not coincide with those of the expansion joints in the brick wythe.
For further discussion on the design, location, and construction of expansion joints, see Technical Notes 18 Series.
Anchorage.
The flexible anchorage of masonry walls to skeleton frames, to permit differential movement of the walls and frame, is discussed in Technical Notes 18 Series. It is suggested that exterior walls carry their own dead weight to the foundation unless heights [in excess of 100 ft (30.480 m)] or masonry openings make it mandatory that the walls be vertically supported by the structural frame. This suggestion may be in conflict with the local building codes, so it is advisable to check before designing in this manner.
A modification of this recommendation is used for cavity walls enclosing skeleton-frame structures. The exterior wythe may be supported on the foundation, with lateral support provided by anchoring the outer wythe to the inner wythe and the frame on which the inner wythe is supported. The anchors or ties, providing lateral support for the outer wythe, should be capable of resisting both tension and compression, but should be designed to permit movement parallel to the plane of the wall in both a vertical and horizontal direction (see Fig. 9).
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When the wall contains openings, it is recommended that reinforced brick lintels be used to avoid stress concentrations and movement problems.
Caps and Copings.
Where caps or copings are used at the top of cavity walls, they should be properly anchored, grouted and flashed to provide for any movement and prevent any moisture penetration into the wall from the top. See Technical Notes 18 Series for detailing.
Selection of Materials
The selection of quality materials is essential to the satisfactory performance of a brick masonry cavity wall. No amount of design, detailing or construction can compensate for poor materials. The selection of materials is even more important if the design is based on the 1969 BIA Standard.
Masonry Units.
The exterior masonry wythe of a brick masonry cavity wall may be brick or hollow brick. The inner wythe may be brick, hollow brick, structural clay tile, or hollow or solid concrete masonry units. If the wall is designed under the 1969 BIA Standard, both wythes must be brick.
Brick
Brick or hollow brick must conform to the requirements for Grade MW or SW in ASTM C 62, С 216 or C 652. For insulated cavity walls in freezing climates, only Grade SW should be used in the exterior wythe. In addition, when the wall is designed according to 1969 BIA Standard, brick used in loadbearing or shear walls must also meet the dimension and distortion tolerances for Type FBS of ASTM C 216. If the brick do not comply with these tolerance requirements, they may be used provided the ultimate compressive strength of the masonry (f') is determined by prism tests. Used or salvaged brick are not permitted in engineered brick masonry construction.
Other Units
Structural clay tile must conform to ASTM C 34, С 56 or C 126. Hollow or solid concrete masonry units must conform to ASTM C 90, C 129, С 145 or C 55.
Mortar.
The mortar has an important effect on the strength of a cavity wall. Transverse strength tests of full scale walls have shown that the bond between mortar and brick units is the most important single factor affecting wall strength when the load is applied such that failure occurs through a horizontal joint.
These tests indicate that portland cement-lime mortars, under ASTM C 270 or BIA Designation M1-72, Type M or Type S, provide maximum bond between masonry units and mortar. Type S mortar is recommended for use in cavity walls in locations where wind velocities are expected to exceed 80 mph (128.748 km/ h). For other locations, Type N may be used. The designer, in any case, should select the lowest strength mortar that is compatible with the structural requirements. Portland cement-lime mortars for brick masonry are discussed in Technical Notes 8 Series.
Ties and Anchors.
Wire or metal anchors which serve as ties in multi-wythe masonry walls should comply with ASTM A 82 or A 185.