Page 357 - Design of Reinforced Masonry Structures

WALLS UNDER GRAVITY AND TRANSVERSE LOADS 6.9

The vertical reinforcement in the walls is usually provided to develop the required
flexural strength in the vertically spanning walls. However, this vertical reinforcement
is not counted upon to carry any load in compression. This is because the vertical rein-
forcing bars are not easily laterally restrained by ties and are therefore vulnerable to
buckling and consequent loss of their compression strength. Horizontal reinforcement
is provided either to resist flexure in horizontally spanning walls, and as shear reinforce-
ment in shear walls.
Both horizontal and vertical wall reinforcement are provided based on structural
requirements. Walls with reinforcement spaced at large intervals are referred to as lightly
or partially reinforced walls. In this type of construction, the walls are considered to
consist of strips of reinforced masonry with un-reinforced masonry spanning between
them. The reinforced masonry strips may be horizontal or vertical depending on the posi-
tion of reinforcing bars. Some common forms of walls of this type are shown in Fig. 6.8.
In Figure 6.8a, vertical reinforcement is grouted in some of its vertically aligned cells.
The reinforcement, along with the grouted masonry units, acts as a vertical beam. The
un-reinforced masonry spans horizontally between these strips to resist vertical loads.
Similarly, in Fig. 6.8b, the bond beams provided at the top and bottom of the wall act as
horizontal beams, and the un-reinforced masonry spans between these beams to resist
lateral loads. Figure 6.8c, shows a system with both horizontally and vertically reinforced
strips. In this case, the un-reinforced masonry is supported on all the four sides, resisting
lateral loads by two-way flexural action similar to a plate supported on all four sides.
6. Multiple-wythe walls can be designed as noncomposite or composite walls. MSJC Code
defines composite masonry as a multicomponent masonry member acting with com-
posite action. Wythes in noncomposite walls act independently of each other in resist-
ing loads. The term composite action implies an internal mechanism in a member so
designed that transfer of stresses occurs between the components, so that the combined
components act together as an integral unit in resisting loads. A masonry wall in which
two or more wythes are bonded together so that it acts as a structural unit is often
referred to as bonded wall. A composite wall is simply a multiple-wythe wall in which
at least one of the wythes is different from the other wythe or wythes with respect to
type or grade of masonry unit or mortar (i.e., strength or composition characteristics).

FIGURE 6.8 Vertical and horizontal reinforcing strips in the masonry walls. (Courtesy: NCMA.)