Masonry Magazine March 1967 Page. 33
TABLE 1
RECOMMENDED ALLOWABLE STRESSES IN REINFORCED CONCRETE MASONRY BASED ON THE BUILDING CODE REQUIREMENTS FOR REINFORCED MASONRY (AMERICAN STANDARD A41.2-1960)
Allowable Stresses*
Type of Stress | Factor | f'm = 1200 psi | f'm = 1600 psi
------- | -------- | -------- | --------
Compressive: Axial | fm = 0.20 fm 1 | 2401 | 3201
Flexural | fm = 0.33 fim | 400 | 528
Shear: Beams with no web reinforcement | Vm = 50 psi: | 50 psi | 50 psi
Beams with web reinforcement | V | 150 psi | 150 psi
Bond: Plain bars | | 80 psi | 80 psi
Deformed bars (ASTM A305) | : | 160 psi | 160 psi
Bearing | 'm = 0.25 fm | 300 psi | 400 psi
Modulus of Elasticity | Em = 1000 fim | 1,200,000 psi | 1,600,000 psi
Tensile Stress in Longitudinal Reinforcement-latermediate and Hard Grade Steel Bars | fs= | 20,000 psi | 20,000 psi
Combined Stresses: fm fa not to exceed 1.0 | Fm Fa | |
Where:
f = Computed axial unit stress.
fm = Computed flexural unit stress.
Fa = Axial unit stress permitted by Code.
Fm = Flexural unit stress permitted by Code.
* Allowable stresses may be increased by 33-1/3 per cent where designing for wind load in combination with dead and other loads providing the calculated stresses due to dead and other loads alone do not exceed the allowable stresses.
+ In reinforced bearing walls reinforced with an area of steel not less than 0.002 times the gross cross-sectional area of the wall, not more than 2/3 of which is used in the principal direction and where the principal reinforcement is spaced not more than 48 in. apart. Where the ratio of wall height to thickness exceeds 10, the allowable stress should be reduced proportionally to 0.15 f'm for walls having a ratio of height to thickness of 25. Where the reinforcement is designed, placed and anchored in position for columns, higher stresses are permitted.
Does not apply to continuous or restrained members which are so constructed as not to provide T-beam or equivalent action.
in 4-in. increments for every 70 feet of additional height. This type of construction has proven to be very economical and popular in buildings such as apartments where the interior cross walls are load-bearing concrete masonry, and the architect has more freedom in selection of materials for nonload-bearing exterior walls.
A second option is the provision in most building codes that permits the designer to exceed the allowable stress and height requirements if he can justify the design to the satisfaction of the building authority. Design justification requires more detailed structural analysis than is common, and recognition of the factors that influence wall strength. One code, the 1965 "National Building Code of Canada," now contains specific provisions for detailed structural analysis of load-bearing masonry. Other code groups are studying similar provisions at present.
The most important variables that influence the load-carrying capacity of nonreinforced concrete masonry walls are:
1. Strength of masonry unit
2. Slenderness ratio of wall (h/t)
3. Eccentricity of vertical load
4. Properties of mortar
5. Quality of workmanship
6. Type of wall construction
Traditional masonry codes provide for these variables in an arbitrary manner: For a given wall type, masonry unit, and type of mortar, the maximum compressive stress permitted is a fixed value; no provision is made for walls of different height-thickness ratios and different degrees of load eccentricity. It generally is assumed the allowable stresses in the code provide an adequate factor of safety under the worst of conditions, ie., a wall with the greatest slenderness ratio permitted by the code, and with eccentric application of load. At wall heights less than maximum, the safety factor increases to values that are uneconomical and unwarranted. Likewise, the code which provides an adequate safety factor when loads are applied eccentrically, becomes unnecessarily conservative when compressive loads are axial. Conversely, if the code requirements for allowable stress were established to satisfy a reasonable factor of safety when loads were concentric and walls low in height, an unsafe condition would exist within most construction. The rational solution would appear a code which recognizes the influence of h/t and eccentricity, and varies allowable stresses accordingly.
Such an approach is introduced here with the expectation that more building codes will eventually adopt the rational procedure as an alternate for design of nonreinforced masonry.
Table 3 shows recommended allowable shear, tensile, and compressive stresses for use where design is based on rational structural analysis. These values, referred to as "basic allowable stresses" are related to type of mortar used, and for allowable compressive stress, to ultimate strength of unit. The allowables recommended result from evaluation of considerable research data involving wall strength as related to: (1) unit strength, (2) mortar strength, (3) wall slenderness, and (4) eccentricity of vertical load. The basic allowable stresses apply to concentrically-looded walls with a height-to-thickness ratio of 20 and reflect a safety factor of 4. Where rational structural analysis and proper workmanship is obtained, this factor of safety is considered adequate.
Correction factors in Table 4 should be used to determine the allowable compressive stress where slenderness ratio is less than 20 and/or walls are subject to eccentric loading. Maximum stress should not exceed the product of the basic allowable stress and the appropriate correction factor.
*See also "Recommended Building Code Requirements for Engineered Concrete Masonry" published by National Concrete Masonry Association