Issue: Appears on pages s : Keywords: AAC; masonry; anchors; autoclaved aerated concrete AAC masonry; clay brick; clay tile; concrete block; concrete brick; construction; construction materials; curing; grout; grouting; inspection; joints; materials handling. The Code covers the design and construction of masonry structures while the Specification is concerned with minimum construction requirements for masonry in structures. Some of the topics covered in the Code are: definitions, contract documents; quality assurance; materials; placement of embedded items; analysis and design; strength and serviceability; flexural and axial loads; shear; details and development of reinforcement; walls; columns; pilasters; beams and lintels; seismic design requirements; glass unit masonry; veneers; and autoclaved aerated concrete masonry. An empirical design method and a prescriptive method applicable to buildings meeting specific location and construction criteria are also included. The Specification covers subjects such as quality assurance requirements for materials; the placing, bonding and anchoring of masonry; and the placement of grout and of reinforcement. This Specification is meant to be modified and referenced in the Project Manual.
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Empirical provisions preceded the development of engineered masonry design, and can be traced back several centuries. This approach to design is based on historical experience in lieu of analytical methods. It has proven to be an expedient design method for typical loadbearing structures subjected to relatively small wind loads and located in areas of low seismic risk. Empirical design has also been used extensively for the design of exterior curtain walls and interior partitions.
Using empirical design, vertical and lateral load resistance is governed by prescriptive criteria which include wall height to thickness ratios, shear wall length and spacing, minimum wall thickness, maximum building height, and other criteria, which have proven to be effective through years of experience.
These empirical design requirements do not apply to other design methods such as allowable stress or limit states design. When empirically designed elements are part of the seismic lateral force resisting system, however, their use is limited to SDC A. Empirical design has primarily been used with masonry laid in running bond.
When laid in stack bond, the IBC requires a minimum amount of horizontal reinforcement 0. In addition, buildings that rely on empirically designed masonry walls for lateral load resistance are allowed up to 35 ft A wind speed of this velocity generally applies along the East and Gulf coasts of the United States.
Building height and basic wind speed conditions where empirical design is permitted under the IBC are summarized in Table 1. The IBC also requires the resultant of gravity loads to fall within the kern of the masonry element, to avoid imparting tension to the element. For buildings more than one story high, walls must be at least 8 in.
The minimum thickness for unreinforced masonry shear walls and for masonry foundation walls is also 8 in. Note that the IBC allows shear walls of one-story buildings to have a minimum thickness of 6 in. For empirically designed walls, such support must be provided at the maximum intervals given in Tables 2 and 3. Note that the span limitations apply to only one direction; that is, the span in one direction may be unlimited as long as the span in the other direction meets the requirements of Tables 2 or 3.
Table 4 includes two sets of compressive stresses for hollow concrete masonry units CMU. These smaller face shells require a corresponding adjustment to the allowable compressive stresses. This distinction is not applicable to masonry that will be solidly grouted.
Calculated compressive stresses for both single and multiwythe walls are determined by dividing the design load by the gross cross-sectional area of the wall, excluding areas of openings, chases or recesses. In multiwythe walls, the allowable stress is determined by the weakest combination of units and mortar shown in Table 4. Further, when the concentrated load acts on the full wall thickness, the allowable stresses under the load may be increased by 25 percent.
The allowable stresses may be increased by 50 percent when concentrated loads act on concentrically placed bearing plates that are greater than one-half but less than the full area. The bolts must be embedded a minimum of 4 in. In addition, the IBC requires the designer to check the roof loading for net uplift and, where net uplift occurs, to design the anchorage system to entirely resist the uplift. Figure 2—Empirical Anchorage Requirements for Lateral Support of Intersecting Masonry Walls Figure 3—Empirical Anchorage Requirements for Floor and Roof Diaphragms Shear Walls Where the structure depends on masonry walls for lateral stability against wind or earthquake forces, shear walls must be provided parallel to the direction of the lateral forces as well as in a perpendicular plane, for stability.
Requirements for empirically designed masonry shear walls are shown in Figure 4. Shear wall spacing is determined empirically by the length-to-width aspect ratio of the diaphragms that transfer lateral forces to the shear walls, as listed in Table 5. The height of empirically designed shear walls is not permitted to exceed 35 ft The minimum nominal thickness of shear walls is 8 in.
Bonding can be achieved using masonry headers, metal wall ties, or prefabricated joint reinforcement, as illustrated in Figure 5. Various empirical requirements for each of these bonding methods are given below. Bonding of solid unit walls with masonry headers.
The distance between adjacent full-length headers may not exceed 24 in. In walls where a single header does not extend through the wall, headers from opposite sides must overlap at least 3 in. Bonding of hollow unit walls with masonry headers. Where two or more hollow units are used to make up the thickness of a wall, the stretcher courses must be bonded at vertical intervals not exceeding 34 in.
Bonding with metal wall ties other than adjustable ties. Wire size W2. Ties must be spaced a maximum of 24 in. Hollow masonry walls must use rectangular wall ties for bonding. Additional bonding ties are required at all openings, and must be spaced a maximum of 3 ft mm apart around the perimeter and located within 12 in.
Note that wall ties may not include drips, and that corrugated ties may not be used. Bonding with adjustable ties. Adjustable ties must be spaced such that there is one tie for each 1. See Reference 9 for an illustration of these requirements.
Bonding with prefabricated joint reinforcement. The joint reinforcement must be spaced 24 in. Cross wires on prefabricated joint reinforcement must be at least wire size W1. The longitudinal wires must be embedded in the mortar. Figure 5—Types of Bonding Change in Wall Thickness Whenever wall thickness is decreased, at least one course of solid masonry, or special units or other construction, must be placed under the thinner section to ensure load transfer to the thicker section below.
Chases and Recesses Masonry directly above chases or recesses wider than 12 in. Lintels Lintels are designed as reinforced beams, using either the allowable stress design or the strength design provisions of Building Code Requirements for Masonry Structures.
End bearing must be at least 4 in. Corbelling When corbels are not designed using allowable stress design or strength design, they may be detailed using the empirical requirements shown in Figure 6.
Only solid or solidly grouted masonry units may be used for corbelling. The anchor, or other support, must provide the required lateral support for the partition wall while also allowing for differential movement. Figure 7 shows an example of such a support, using clip angles. C channels or adjustable anchors could be used as well. Fire walls may also require a sealant to be applied at the bottom of the clip angles.
International Code Council, and National Concrete Masonry Association, Reported by the Masonry Standards Joint Committee, Reported by the Masonry Standards Joint Committee, and NCMA and the companies disseminating this technical information disclaim any and all responsibility and liability for the accuracy and the application of the information contained in this publication.
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An empirical design method and a prescriptive method applicable to buildings meeting specific location and construction criteria are also included. The Specification covers subjects such as quality assurance requirements for materials; the placing; bonding and anchoring of masonry; and the placement of grout and of reinforcement. This Specification is meant to be modified and referenced in the Project Manual. The Code and Specification are written as legal documents so that may be adopted by reference in general building codes. The Code covers the design and construction of masonry structures, with subjects covered ranging from quality assurance to the details and development of reinforcement. Compliance with the Specification is required by the Code to control materials, labor and construction. The Code Commentary addresses many subjects covered by the Code, give some of the considerations in developing the provisions.
ACI 530-08 Building Code Requirements for Masonry
EMPIRICAL DESIGN OF CONCRETE MASONRY WALLS