Masonry Magazine February 1979 Page. 27
NCMA-TEK
An Information series from National Concrete Masonry Association
103
Tall Concrete Masonry Walls
Introduction
The use of concrete masonry walls offers many design opportunities and benefits-low initial costs, speed of construction, structural capability, wide variety of faces and finishes, fire-resistance, thermal insulation and mass, and low maintenance. Since many types of buildings (i.e., factories, warehouses, auditoriums) require the use of tall walls which must also possess the previously mentioned characteristics and advantages, this TEK will discuss the versatility of tall concrete masonry walls to meet a variety of structural conditions often encountered. Such walls may be used as non-bearing panel walls in a structural frame or they may be utilized as bearing walls.
Lateral Support
Maximum concrete masonry wall heights will, of course, depend upon the wall thickness and construction and the magnitude of lateral and vertical loads to be supported. It will also depend upon the applicable building code requirements which must be followed. Most State and local jurisdictions currently permit concrete masonry walls to be designed and constructed in accordance with either the "American Standard Building Code Requirements for Masonry, A41.1-1953(R1970)" or "Specification for the Design and Construction of Load-Bearing Concrete Masonry, NCMA, 1970."
The A41.1 Standard requires the ratio of unsupported wall height to nominal wall thickness (t) or the ratio of unsupported length to nominal thickness (one or the other but not necessarily both) not to exceed 20 if the wall is built of solid masonry units and type M, S, N or O mortar; and 18 if the wall is built of hollow units. Accordingly, if a wall is laterally supported by structural columns or pilasters at intervals not exceeding 20t (for solid units) or 18t (for hollow units), the wall height would not be limited. On the other hand, if the wall is laterally supported by the roof and floor, the wall height could not exceed the values listed in Table 1 for various wall thicknesses.
The NCMA "Specification for the Design and Construction of Load-Bearing Concrete Masonry" limits the slenderness ratio of a non-reinforced bearing wall to 20 where the slenderness ratio is defined as the ratio of effective height to effective thickness. Where a wall is laterally supported at the top and bottom, its effective height is equal to the actual height of the wall. Where there is no lateral support at the top of a wall, its effective height is to be taken as twice the height of the wall above the bottom lateral support. Except for cavity walls, the effective thickness of a wall without pilasters is equal to its actual thickness. For a concrete masonry wall supported at the top and bottom and stiffened by and properly bonded into pilasters at regular intervals, the effective thickness to be assumed in calculating its slenderness ratio is the value given by multiplying its actual thickness by the coefficients given in Table 2. For cavity walls loaded on both wythes, the effective thickness is determined from the following formula:
where to overall thickness of wall, including width of cavity
we width of cavity
For cavity walls loaded on one wythe, the effective thickness is taken as the actual thickness of the loaded wythe only.
For reinforced concrete masonry walls, the NCMA Standard requires bearing walls and non-bearing walls to be laterally supported at distances not exceeding 30t and 36t, respectively. Maximum wall heights for both non-reinforced and reinforced walls of various thicknesses are listed in Table 1.
A new standard covering the design of concrete masonry structures was recently proposed by ACI Committee 531 (ACI Journal, August 1978). Calculation procedures for effective wall thicknesses contained in this proposed standard are the same as those as mentioned above in regard to the NCMA Standard. This proposed standard also requires that all masonry walls shall have a minimum thickness of 1/36 the distance between laterally supported members, either horizontally or vertically, whichever is less except that bearing walls that are nonreinforced shall have a minimum thickness
TEK 103 ©1978 National Concrete Masonry Association