Page 358 - Design of Reinforced Masonry Structures
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6.10                       CHAPTER SIX
























            FIGURE 6.9  Composite walls. (Courtesy: NCMA.)


           Composite masonry walls may consist of brick-to-brick, block-to-block, or brick-to-
           block wythes, with the collar joint filled with mortar or grout, and the wythes connected
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           by metal ties. The width of collar joint may vary from  / 8 to 4 in. Joints may be reinforced
           horizontally or vertically, or the reinforcement may be placed in either the brick or the
           block wythe. Alternatively, the two wythes may be connected by headers (Fig. 6.9).
             Composite walls are particularly advantageous for resisting high loads, both in-plane
           or out-of plane. They may be preferred for architectural or economic reasons. Typically,
           composite walls are used as exterior walls in which the exterior-facing wythe consists of
           concrete brick or split block, and backing wythe is composed of hollow or solid concrete
           block. When built as load-bearing walls, axial loads from the roof or the floor slabs are
           applied on the inner wythes. The two wythes of the composite wall will act integrally,
           if adequate shear-transfer capacity is present at the interface between the two.
             Composite masonry walls may be analyzed based on the elastic transformed area
           of the net section. Careful consideration should be given to the question of allowable
           stresses when two dissimilar materials are used to develop composite action in a masonry
           wall. The maximum stresses in composite walls or other structural members composed
           of different kinds or grades of units or mortars should not exceed the allowable stresses
           for the weakest of the combination of units and mortars of which the member is com-
           posed. Alternatively, the maximum compressive stress permitted in composite walls or

           other structural members may be based on prism tests.
           Design of composite walls must comply with the pertinent provisions of the applicable
         code, such as the MSJC Code [6.1], which specify design requirements for masonry walls
         consisting of more than one wythe. According to MSJC Code Section 2.1.5.2 [6.1], some
         of the important requirements are as follows:
         1. The Multiple-wythe walls designed for composite action shall have collar joints either:
           (a)  Crossed by connecting headers, or
           (b)  Filled with mortar or grout and connected by wall ties.
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