Masonry Structural design

Strength Design of Unreinforced Panel Walls

Examples of Use of Unreinforced Panel Walls

Panel walls commonly comprise the masonry envelope surrounding reinforced concrete or steel frames. In the context of the 2008 MSJC Code, a panel wall would be termed a “multiwythe, noncomposite” wall. An example of an unreinforced panel wall is shown in Fig. 5.1. The outer wythes of panel walls must span horizontally. They cannot span vertically, because of the open expansion joint under each shelf angle. Support conditions for the horizontally spanning outer wythe can be simple or continuous. An example of the connection of a panel wall to a column is shown in the horizontal section of Fig. 5.2. A simple support condition would be achieved by inserting a vertically oriented expansion joint in the clay masonry wythe on both sides of the column. The inner wythes of panel walls can span horizontally and vertically. As a result, it is convenient to visualize panel walls as being composed of sets of vertical and horizontal crossing strips in each wythe. This is shown schematically in Fig. 5.3. At the end of this section, it will be shown that • Because of their aspect ratio, the inner wythe can almost always be considered to span in the vertical direction only.

• It is simple and only slightly conservative to design single-wythe panel walls as though the vertical strips resisted all out-of-plane load.
• It is simple and only slightly conservative to design two-wythe panel walls as though the vertical strips of the inner wythe resisted all out-of-plane load.

Flexural Design of Panel Walls Using Strength Provisions of 2008 MSJC Code

According to the strength provisions of the 2008 MSJC Code, nominal flexural capacity of unreinforced masonry is computed assuming linear stress-strain relationships. Nominal flexural capacity corresponds to a maximum flexural compressive stress of 0.80 f ′ m, or a maximum flexural tensile stress equal to the modulus of rupture. Because the modulus of rupture is much lower than 0.80 f ′ m it governs. Design actions are factored, and design capacities are computed using those nominal capacities and the appropriate strength-reduction factor.

Load Factors
Load factors are as discussed in Sec. 3.7.1. As prescribed in Sec. 1605.2 of the 2009 IBC, the two loading combinations involving wind are
1. 1.2D + 1.6W + f1L + 0.5 (Lr or S or R)
2. 0.9D + 1.6W + 1.6H
Of these, the second will usually govern. Both combinations have a load factor for W of 1.6.

Modulus of Rupture

For strength design, the nominal flexural strength is to be computed using the modulus of rupture values given in Table 5.1 (Table of the 2008 MSJC Code). Those values are intended to be 2.5 times the corresponding allowable stresses of the 2008 MSJC Code.

Strength-Reduction Factors

Strength-reduction factors are as discussed in Sec. 3.7.2. For combinations of flexure and axial load in unreinforced masonry, φ = 0.60 (Sec. of the 2008 MSJC Code).


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