Masonry Magazine March 1967 Page. 32
Concrete Masonry Walls in High-Rise Buildings
Bearing stresses will govern required wall dimensions. Where wall thickness is limited, bearing stresses will usually determine maximum permissible building height.
Stability Against Overturning-Overturning resistance is achieved through dead load; however, only the dead load carried by the shear walls is considered effective in developing resisting moment. This conservative assumption favors the use of shear walls that are also load-bearing, such as load-bearing interior cross walls.
Shear and Distribution of Wind Loads-Wind loads are transmitted through the floors to those shear walls parallel to the direction assumed for the wind flow (projected section for nonparallel walls). The percentage of total lateral load carried by a shear wall is assumed proportional to its stiffness or "relative rigidity" with respect to the sum of the rigidities of all participating shear walls.
Flexural Resistance of Walls-Flexural stress in walls should be checked. These stresses can be caused by wind loads on exterior walls, by eccentricity of loading, and by insufficiently rigid floor diaphragms.
Floor-Wall Connections-Finally, because the strength and stability of the high-rise building depends upon the inter interaction of the connecting floor and roof elements, anchorage must be adequate to transfer the horizontal forces.
Buildings up to 13 stories in height have been designed and built of both reinforced and nonreinforced load-bearing concrete masonry construction. Selection between the two types of construction depends upon (1) requirements of the local building code, and (2) anticipated loading, especially wind and seismic forces.
Reinforced concrete masonry
Reinforced concrete masonry is construction in which steel reinforcement is embedded and so placed that the masonry and the reinforcement act together in resisting forces. Solid or hollow concrete masonry units are laid to form continuous vertical and horizontal cavities within the construction. These cavities receive the steel reinforcement and grout which form a bonded composite construction in resisting compressive, tensile, and shearing stresses.
Originally developed in areas subject to earthquakes, reinforced concrete masonry today is finding new application in other areas for use in high-rise load-bearing construction. In some instances, it is employed only in sections of a high-rise structure where high stresses are encountered which would require heavier wall section without reinforcement. In other instances, it may be economical where greater wall height to thickness ratio is needed. Design criteria for reinforced concrete masonry is contained at present in all major codes, such as the Uniform Building Code of the International Conference of Building Officials, and Building Code Requirements for Reinforced Masonry, promulgated by the National Bureau of Standards. The latter document forms the basis of requirements on reinforced masonry in other major codes, such as the SBCC Southern Standard Building Code, BOCA Basic Building Code, and the National Building Code, sponsored by the American Insurance Association.
In the ASA Standard referred to in Table 1, compressive strength (fim) of masonry may be determined by two methods: (1) An assumed value depending on the compressive strength of the units, or (2) on the basis of strength tests performed on prisms built of the same materials and under the same conditions as the structure. Table 2 shows assumed values used in accordance with method (1). The prism test, also permitted in the Uniform Building Code, enables the designer to take greater advantage of the high compressive-strength potential of concrete masonry.
Reinforced concrete masonry is similar in many respects to its counterpart reinforced concrete. And like its counterpart, reinforced concrete masonry seems destined to play an ever increasing role in the construction of high-rise load-bearing structures.
Nonreinforced concrete masonry
The requirements listed in ASA A41.1 "American Standard Building Code Requirements for Masonry," provide the basis for design of nonreinforced concrete masonry in most communities in the U. S. In some respects, requirements in this document are ultra-conservative and should be modified so that the designer can make more efficient use of the excellent strength properties of concrete masonry. The question is asked, "Under present code requirements, what is available to the designer who wishes to use nonreinforced load-bearing concrete masonry in high-rise construction?"
One option that has been sucessfully employed in many 6-8 story buildings is the use of the 12-in. load-bearing concrete masonry wall. Buildings up to 70 feet in height are permitted with 12-in. solid concrete masonry units (ASTM Designation: C 145) providing walls are stiffened at distances not greater than 12 feet by concrete floors or by masonry cross walls.
Additional building height is permitted if the wall thickness is increased