A wall constructed with two or more wythes of masonry can technically be classified in one of three ways, depending on how each individual wythe is designed and detailed. These three wall systems are composite, noncomposite or veneer walls. A true veneer is nonstructural—any contribution of the veneer to the wall’s out-of plane load resistance is neglected.
Building Code Requirements for Masonry Structures (ref. 1) defines veneer as a masonry wythe which provides the exterior finish of a wall system and transfers out-of-plane loads directly to the backing, but is not considered to add load resisting capacity to the wall system.
Noncomposite walls, on the other hand, are designed such that each wythe individually resists the loads imposed on it. Bending moments (flexure) due to wind or gravity loads are distributed to each wythe in proportion to its relative stiffness.
Composite walls are designed so that the wythes act together as a single member to resist structural loads. This requires that the two masonry wythes be connected by masonry headers or by a mortar or grout filled collar joint and wall ties to help ensure adequate load transfer between the two wythes.
The primary function of anchored veneers is to provide an architectural facade and to prevent water penetration into the building. As such, the structural properties of veneers are neglected in veneer design. The veneer is assumed to transfer out-of-plane loads through the anchors to the backup system. Building Code Requirements for Masonry Structures Chapter 6 (ref. 1) includes requirements for design and detailing anchored masonry veneer.
A masonry veneer with masonry backup and an air space between the masonry wythes is commonly referred to as a cavity wall. The continuous air space, or cavity, provides the wall with excellent resistance to moisture penetration and wind driven rain as well as a convenient location for insulation. This TEK addresses concrete masonry veneer with concrete masonry backup.
DESIGN CONSIDERATIONS
Masonry veneers are typically composed of architectural units such as: concrete or clay facing brick; split, fluted, glazed, ground face or scored block; or stone veneer. Most commonly, anchored masonry veneers have a nominal thickness of 4 in. (102 mm), although 3 in. (76 mm) veneer units may be available as well.
Although structural requirements for veneers are minimal, the following design considerations should be accounted for: crack control in the veneer, including deflection of the backup and any horizontal supports; adequate anchor strength to transfer applied loads; differential movement between the veneer and backup; and water penetration resistance.
The continuous airspace behind the veneer, along with flashing and weeps, must be detailed to collect any moisture that may penetrate the veneer and direct it to the outside. A minimum 1 in. (25 mm) air space between wythes is required (ref. 1), and is considered appropriate if special precautions are taken to keep the air space clean (such as by beveling the mortar bed away from the cavity or by placing a board in the cavity to catch and remove mortar droppings and fins while they are still plastic). Otherwise, a 2 in. (51 mm) air space is preferred. As an alternative, proprietary insulating drainage products can be used.
Although veneer crack control measures are similar to those for other concrete masonry wall constructions, specific crack control recommendations have been developed for concrete masonry veneers. These include: locating control joints to achieve a maximum panel length to height ratio of 11/2 and a maximum spacing of 20 ft (6,100 mm), as well as where stress concentrations occur; incorporating joint reinforcement at 16 in. (406 mm) on center; and using Type N mortar for maximum flexibility. See CMU-TEC-009-23, Crack Control Strategies for Concrete Masonry Construction (ref. 2) for more detailed information.
Because the two wythes in a veneer wall are designed to be relatively independent, crack control measures should be employed as required for each wythe. It is generally not necessary for the vertical movement joints in the veneer wythe to exactly align with those in the backup wythe, provided that the ties allow differential in-plane lateral movement.
Wall ties may be joint reinforcement or wire wall ties. Wall ties for veneers transfer lateral loads to the structural wythe and also allow differential inplane movement between wythes. This second feature is particularly important when the two wythes are of materials with different thermal and moisture expansion characteristics (such as concrete masonry and clay brick), or in an insulated cavity wall which tends to have differential thermal movement between the wythes. When horizontal joint reinforcement is used to tie the two wythes together, hot-dipped ladder type reinforcement is preferred over truss type, because the ladder shape accommodates differential in-plane movement and facilitates placing vertical reinforcement, grout and loose fill insulation. Because veneers rely on the backup for support, wall ties must be placed within 12 in. (305 mm) of control joints and wall openings to ensure the free ends of the veneer are adequately supported. More information on ties for veneers can be found in TEK 03-06C, Concrete Masonry Veneers (ref. 4).
The distance between the inside face of the veneer and the outside face of the masonry backup must be a minimum of 1 in. (25 mm) and a maximum of 4 1/2 in. (114 mm). For glazed masonry veneer, because of their impermeable nature, a 2 in. (51 mm) wide airspace is recommended with air vents at the top and bottom of the wall to enhance drainage and help equalize air pressure between the cavity and the exterior of the wall. Vents can also be installed at the top of other masonry veneer walls to provide natural convective air flow within the cavity to facilitate drying. For vented cavities, it is prudent to create baffles in the cavity at the building corners to isolate the cavities from each other. This helps prevent suction being formed in the leeward cavities.
REFERENCES
Building Code Requirements for Masonry Structures, ACI 530-02/ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee, 2002.
CMU-TEC-009-23, Crack Control Strategies for Concrete Masonry Construction, Concrete Masonry and Hardscapes Association, 2023.
TEK 03-06C, Concrete Masonry Veneers, Concrete Masonry and Hardscapes Association, 2012.
Multiwythe masonry walls can take one of several forms: composite, noncomposite or veneer walls. The primary differences between these wall systems are in construction details and how applied loads are assumed to be carried and distributed through the loadbearing system.
In composite masonry, multiwythe masonry members act with composite action (refs. 1, 2). That is, composite walls are designed so that the wythes act together as a single structural member to resist loads. This requires that the masonry wythes be connected by masonry headers (which are rarely used due to cost and detailing restrictions) or by a mortar- or grout-filled collar joint and wall ties to help ensure adequate load transfer between wythes.
In contrast, each wythe of a noncomposite masonry wall (also referred to as a cavity wall) is connected to the adjacent wythe with metal wall ties, but they are designed such that each wythe individually resists the loads imposed on it. Transverse bending moments (flexure), such as those due to wind, are distributed to each wythe in proportion to its relative stiffness. Loads acting parallel to the plane of a noncomposite wall (in-plane) are resisted only by the wythe on which the loads are applied, neglecting stress transfer between wythes.
In a veneer wall, the backup wythe is designed as the loadresisting system, with the veneer providing the architectural wall finish. The anchored veneer transfers all out-of-plane loads to the backup through wall ties, while supporting its own weight inplane. Veneer walls are not covered in this TEK. Architectural detailing is covered in Concrete Masonry Veneer Details, TEK 0510B (ref. 3). Prescriptive design and detailing requirements are included in Concrete Masonry Veneers, TEK 03-06C, and (ref. 4), while engineered design procedures are outlined in Structural Design of Unreinforced Composite Masonry, TEK 16-02B (ref. 5). Note that although Building Code Requirements for Masonry Structures defines a cavity wall as a noncomposite masonry wall, the term cavity wall is also commonly used to describe a veneer wall with masonry backup.
Although Building Code Requirements for Masonry Structures includes design provisions for noncomposite and composite masonry walls, these design approaches are rarely taken with masonry walls, as they require two structural walls to be constructed adjacent to one another. In other words, if the structural design dictates the use of a 12-in. (305-mm) thick wall, it is often easier and more cost effective to use a single 12-in. (305-mm) wythe rather than a composite system consisting of 4-in. and 8-in. (102- and 203-mm) units. The primary advantage of using composite and noncomposite construction is in applications where different architectural features are desired on each side of a fully exposed concrete masonry wall. Greater flexibility in moisture control and insulation, as well as increased fire resistance rating and sound transmission class, can also be realized when compared to single wythe walls.
Information on the allowable stress design method, the strength design method and empirical design can be found in references 6, 7 and 8, respectively. The criteria specific to noncomposite and composite masonry walls are discussed in this TEK. Design tables are included in Design of Concrete Masonry Noncomposite Walls, TEK 16-04A, and Structural Design of Unreinforced Composite Masonry, TEK 16-02B (refs. 9, 10).
NONCOMPOSITE WALLS
In noncomposite construction, the wythes are connected by wall ties, as opposed to being rigidly bonded as in composite walls. The joint between wythes of noncomposite walls are not permitted to contain headers, grout or mortar.
With the exception of structural load paths and wall tie spacing requirements, architectural details for noncomposite masonry walls are nearly indistinguishable from those for masonry veneer on masonry backup. See Concrete Masonry Veneer Details, TEK 05-01B and Concrete Masonry Veneers, TEK 03-06C (refs. 3, 4).
Structural Design
Noncomposite walls are designed as follows: imposed vertical loads are carried by the wythe closest to the center of span of the supported member; bending moments are distributed to each wythe in proportion to its relative stiffness; and loads acting parallel to the plane of the wall (shear loads) are carried only by the wythe affected. In addition, the cavity width is limited to 4 ½ in. (114 mm) unless a detailed wall tie analysis is performed.
Transverse (out-of-plane) loads are distributed based on the wythe flexural stiffness as indicated by the moment of inertia, as follows:
Building Code Requirements for Masonry Structures includes prescriptive wall tie spacing requirements to aid compatible lateral deflection between wythes (see Figure 1). Wire wall ties, which may also include cross wires of horizontal joint reinforcement, are used to connect the wythes. Unless a detailed analysis is performed, the prescriptive requirements apply. In addition to the provisions shown in Figure 1, these prescriptive requirements include:
Collar joints may not contain headers, grout, or mortar.
Where the cross wires of joint reinforcement are used as ties, the joint reinforcement is required to be ladder-type or tab-type, as the truss-type restricts differential in-plane movement between the two wythes. Adjustable joint reinforcement assemblies are also permitted, and are considered to be a type of adjustable tie.
Additional requirements for wall ties can be found in Anchors and Ties for Masonry, TEK 12-01B (ref. 11).
Figure 1—Noncomposite Wall Detailing Requirements
COMPOSITE WALLS
Composite walls are multiwythe walls where both wythes act structurally as one unit. They depend on sufficient stress transfer across the joint between wythes for composite action. In addition to the general design requirements to ensure sufficient structural capacity that are applicable to all structural masonry walls, Building Code Requirements for Masonry Structures contains prescriptive requirements for bonding the wythes of composite walls as well as allowable shear stresses for the collar joint. While not prohibited by the code (ref. 2), wythes of composite masonry walls should not be constructed with dissimilar materials, such as clay and concrete masonry, as rigidly bonding such materials together does not permit differential movement between wythes.
Composite walls are most often designed with the axial load from floor slabs or the roof carried by the inner wythe of masonry. The vertical joint between wythes may contain either vertical or horizontal reinforcement, or reinforcement may be placed in either wythe. The thickness of the joint between adjacent wythes is not limited in thickness but is typically sized to accommodate modular layout and any reinforcement that may be placed in the joint. Stresses in each wythe due to axial load and flexure are calculated using the modular ratio, n, to transform sections using elastic analysis and assuming no slip at the collar joint, as shown in the following example.
Example: Reinforced Composite Wall Transformed Section and Neutral Axis
Consider a composite masonry wall constructed of 6-in. (152-mm) concrete masonry, a 2-in. (51-mm) grouted collar joint containing vertical No. 4 (M#13) bars at 48 in. (1,219 mm) on center, and 4-in. (102-mm) concrete brick. Moduli of elasticity for the materials are:
Using these modular ratios, equivalent areas of grout and steel based on a 12-in. (305-mm) width of concrete masonry are:
ngAg = 0.74 (2 in. x 12 in.) = 17.8 in.² (11,480 mm²) nsAs = 21.5 (0.20 in.²/bar x 0.25 bar/ft) = 1.08 in.² (697 mm²)
The resulting transformed section is shown in Figure 2.
Figure 2—Transformed Section for Example (based on a 12-in. (305-mm) section)
The net cross-sectional areas of the 6-in. (152-mm) and 4-in. (102-mm) concrete masonry wythes are 24.0 in.²/ft (0.051 m²/m) and 43.5 in.c/ft (0.092 m²/m), respectively (ref. 12). Determine the total transformed area, Atr:
Next, determine the neutral axis location of the transformed section, by calculating x̄, the distance from the neutral axis of the 6-in. (152-mm) concrete masonry to the neutral axis of the transformed section.
Moments of inertia of the three wall elements are: (Icm) = 130.0 in.4/ft (1.78 x 108 mm4/m) (ref. 12) Ig = (1/12) bh³ = (1/12)(8.9)(2)³ = 5.9 in.4/ft (8.10 x 107 mm4/m) Is = (1/12) bh³ = (1/12)(2.2)(0.5)³ = 0.023 in.4/ft (3.13 x 104 mm4/m) (Icm)4-in. = 47.6 in.4/ft (6.50 x 107 mm4/m) (ref. 12)
Using the parallel axis theorem, the moment of inertia of the transformed section, Itr, is:
Stresses in each element are then determined using: the transformed moment of inertia, Itr: the modular ratio, n; the area of the transformed section, Atr; and the distance from the extreme fiber to the neutral axis of the composite section, c. For example, the calculated tension in the steel due to flexure is:
Bonding the Wythes
To ensure shear transfer, Building Code Requirements for Masonry Structures requires that the joint between wythes either be filled with mortar or grout and connected by wall ties or be crossed by connecting masonry headers.
Wall tie spacing requirements are illustrated in Figure 3.
Although allowed, the use of masonry headers is an outdated method of connecting masonry wythes and is not recommended for several reasons. Headers are less ductile than metal wall ties, making accommodation for differential movement a critical issue. Differential movement can shear the headers, effectively eliminating the composite action, particularly with the combination of concrete masonry and clay masonry wythes. Also, walls bonded by headers are also more susceptible to water penetration.
When headers are used, they must be uniformly spaced and have a total cross-sectional area not less than four percent of the total wall surface area. Headers are also required to be embedded at least 3 in. (76 mm) into each wythe. See Figure 3.
Construction Considerations
In composite masonry construction, insulation and vapor retarders, if required, can not be located in the joint between wythes, as is commonly done in noncomposite construction. Insulation can be located either in the cores of the inner wythe or on the wall interior.
Because the two wythes of a composite wall act as one structural unit, vertical movement joints, including fire-rated control joints, should extend through both wythes at the same location across the cavity joint.
Figure 3—Composite Wall Detailing Requirements
NOTATIONS
An = net cross-sectional area of a wall element, in.²/ft (mm²/m) Atr = area of the transformed section, in.²/ft (mm²/m) c = the distance from the extreme fiber to the neutral axis of the composite section, in. (mm) d = distance from extreme compression fiber to centroid of tension reinforcement, in. (mm) Eg = modulus of elasticity of grout, psi (MPa) Em = modulus of elasticity of masonry in compression, psi (MPa) Es = modulus of elasticity of steel, psi (MPa) f’g = specified compressive strength of grout, psi (MPa) f’m = specified compressive strength of masonry, psi (MPa) fs = calculated tensile or compressive stress in reinforcement, psi (MPa) Icm = moment of inertia of concrete masonry, in.4/ft (mm4/m) Ig = moment of inertia of the grout, in.4/ft (mm4/m) Is = moment of inertia of the steel, in.4/ft (mm4/m) Ii = average moment of inertia of inner wythe, in.4/ft (mm4/m) Io = average moment of inertia of outer wythe, in.4/ft (mm4/m) Itr = moment of inertia of transformed section, in.4/ft (mm4/m) M = maximum moment at the section under consideration, in-lb/ft (N-mm/m) n = modular ratio Wi = transverse load on inner wythe, psf (kPa) Wo = transverse load on outer wythe, psf (kPa) wT = total transverse load, psf (kPa) x̄ = distance from the neutral axis of an element to the neutral axis of the transformed section, in. (mm)
REFERENCES
International Building Code, 2003, With Commentary. International Code Council, Inc., 2004.
Building Code Requirements for Masonry Structures, ACI 530-05/ASCE 5-05/TMS 402-05. Reported by the Masonry Standards Joint Committee, 2005.
Empirical design is a procedure of proportioning and sizing unreinforced masonry elements based on known historical performance for a given application. 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.
This TEK is based on the provisions of Section 2109 of the International Building Code (IBC) (ref. 1). These empirical design requirements do not apply to other design methods such as allowable stress or limit states design. For empirical design of foundation walls, see TEK 15-01B, Allowable Stress Design of Concrete Masonry Foundation Walls (ref. 2)
APPLICABILITY OF EMPIRICAL DESIGN
The IBC allows elements of masonry structures to be designed by empirical methods when assigned to Seismic Design Category (SDC) A, B or C, subject to additional restrictions described below. 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 (0.003 times the wall’s vertical cross-sectional area and spaced not more than 48 in. (1,219 mm) apart).
In addition, buildings that rely on empirically designed masonry walls for lateral load resistance are allowed up to 35 ft (10.7 m) in height.
The 2003 IBC restricts empirical design to locations where the basic wind speed (three-second gust, not fastest mile) is less than or equal to 110 mph (79 m/s), as defined in Minimum Design Loads for Buildings and Other Structures, ASCE 7 (ref. 3). A wind speed of this velocity generally applies along the East and Gulf coasts of the United States.
The 2006 IBC further refines the empirical design limitations. Whereas with the 2003 IBC, the designer need only check the SDC and basic wind speed, with the 2006 IBC, to use empirical design the designer must check:
SDC,
basic wind speed,
building height, and
location of gravity loads resultant.
The limitations based on SDC are the same as in the 2003 IBC, described above. Building height and basic wind speed conditions where empirical design is permitted under the 2006 IBC are summarized in Table 1.
The 2006 IBC also requires the resultant of gravity loads to fall within the kern of the masonry element, to avoid imparting tension to the element. This area is defined as: within the center third of the wall thickness, or, for foundation piers, within the central area bounded by lines at one-third of each cross-sectional dimension of the pier.
Table 1—2006 IBC Empirical Design Limitations Based on Building Height and Basic Wind Speed
DESIGN PROVISIONS
Minimum Wall Thickness
Empirically designed (unreinforced) bearing walls of one story buildings must be at least 6 in. (152 mm) thick. For buildings more than one story high, walls must be at least 8 in. (203 mm) thick. The minimum thickness for unreinforced masonry shear walls and for masonry foundation walls is also 8 in. (203 mm). Note that the 2003 IBC allows shear walls of one-story buildings to have a minimum thickness of 6 in. (152 mm).
Lateral Support
Lateral support for walls can be provided in the horizontal direction by cross walls, pilasters, buttresses and structural frame members, or in the vertical direction by floor diaphragms, roof diaphragms and structural frame members, as illustrated in Figure 1. 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.
Figure 1—Lateral Support of Empirically Designed (Unreinforced) Concrete Masonry Walls
Table 2—Wall Lateral Support Requirements (ref. 1)
Table 3—Maximum Unreinforced Wall Spans, ft (m)
Allowable Stresses
Allowable stresses in empirically designed masonry due to building code prescribed vertical (gravity) dead and live loads (excluding wind or seismic) are given in Table 4.
Table 4 includes two sets of compressive stresses for hollow concrete masonry units (CMU). The first set, titled “Hollow Unit Masonry (Units Complying With ASTM C 90- 06 or Later)” apply to most CMU currently available. The 2006 edition of the CMU specification, Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90 (ref. 7), included slightly reduced minimum face shell thickness requirements for CMU 10 in. (254 mm) and greater in width. These smaller face shells require a corresponding adjustment to the allowable compressive stresses. The values currently published in the 2006 IBC (“Hollow Unit Masonry (Units Complying With Previous Editions of ASTM C 90)” in Table 4), apply to the previous face shell thicknesses, and should only be used if the CMU to be used have the thicker face shells listed in previous editions of ASTM C 90. 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. The area is based on the specified dimensions of masonry, rather than on nominal dimensions. In multiwythe walls, the allowable stress is determined by the weakest combination of units and mortar shown in Table 4.
In addition, the commentary to Building Code Requirements for Masonry Structures (refs. 6, 8) contains additional guidance for concentrated loads. According to the commentary, when concentrated loads act on empirically designed masonry, the course immediately under the point of bearing should be a solid unit or be filled solid with mortar or grout. 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.
Table 4—Allowable Compressive Stress for Empirical Design of Masonry
Anchorage for Lateral Support
Where empirically designed masonry walls depend on cross walls, roof diaphragms, floor diaphragms or structural frames for lateral support, it is essential that the walls be properly anchored so that the imposed loads can be transmitted from the wall to the supporting element. Minimum anchorage requirements for intersecting walls and for floor and roof diaphragms are shown in Figures 2 and 3, respectively.
Masonry walls are required to be anchored to structural frames that provide lateral support by ½ in. (13 mm) diameter bolts spaced at a maximum of 4 ft (1.2 m), or with other bolts and spacings that provide equivalent anchorage. The bolts must be embedded a minimum of 4 in. (102 mm) into the masonry.
In addition, the 2006 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. In addition, roofs must be designed and constructed in a manner such that they will not impose thrust perpendicular to the shear walls to which they are attached.
The height of empirically designed shear walls is not permitted to exceed 35 ft (10.7 m). The minimum nominal thickness of shear walls is 8 in. (203 mm), except under the 2003 IBC, which allows shear walls of one-story buildings to have a minimum thickness of 6 in. (152 mm).
Figure 4—Empirically Designed Shear Wall Requirements
Wythes of multiwythe masonry walls are required to be bonded together. 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. Where masonry headers are used to bond wythes of solid masonry construction, at least 4 percent of the wall surface of each face must be composed of headers, which must extend at least 3 in. (76 mm) into the backing. The distance between adjacent full-length headers may not exceed 24 in. (610 mm) in either the horizontal or vertical direction. In walls where a single header does not extend through the wall, headers from opposite sides must overlap at least 3 in. (76 mm), or headers from opposite sides must be covered with another header course which overlaps the header below by at least 3 in. (76 mm).
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. (864 mm) by lapping at least 3 in. (76 mm) over the unit below, or by lapping at vertical intervals not exceeding 17 in. (432 mm) with units that are at least 50 percent greater in thickness than the units below.
Bonding with metal wall ties (other than adjustable ties). Wire size W2.8 (MW18) wall ties, or metal wire of equivalent stiffness, may be used to bond wythes. Each 4½ ft² (0.42 m²) of wall surface must have at least one tie. Ties must be spaced a maximum of 24 in. (610 mm) vertically and 36 in. (914 mm) horizontally. Hollow masonry walls must use rectangular wall ties for bonding. In other walls, ends of ties must be bent to 90° angles to provide hooks no less than 2 in. (51 mm) long. Additional bonding ties are required at all openings, and must be spaced a maximum of 3 ft (914 mm) apart around the perimeter and located within 12 in. (305 mm) of the opening. 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 1.77 ft² (0.164 m²) of wall area, with maximum horizontal and vertical spacings of 16 in. (406 mm). The ties must have a maximum clearance between connecting parts of 1/16 in. (1.6 mm), and, when pintle legs are used, at least two legs with a minimum wire size of W2.8 (MW18). The bed joints of the two wythes may have a maximum vertical offset of no more than 1¼ in. (32 mm). (See Reference 9 for an illustration of these requirements.)
Bonding with prefabricated joint reinforcement. Where adjacent wythes of masonry are bonded with prefabricated joint reinforcement, there must be at least one cross wire serving as a tie for each 2⅔ ft² (0.25 m²) of wall area. The joint reinforcement must be spaced 24 in. (610 mm) or closer vertically. Cross wires on prefabricated joint reinforcement must be at least wire size W1.7 (MW11) and shall be without drips. 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.
Miscellaneous Empirical Requirements
Following are additional empirical requirements in Building Code Requirements for Masonry Structures. Although not included explicitly in IBC Section 2109, the IBC includes a direct reference to Building Code Requirements for Masonry Structures.
Chases and Recesses Masonry directly above chases or recesses wider than 12 in. (305 mm) must be supported on lintels.
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. (102 mm), although 8 in. (203 mm) is typical.
Support on Wood Empirically designed masonry is not permitted to be supported by wood girders or other forms of wood construction, due to expected deformations in wood from deflection and moisture, causing distress in the masonry, and due to potential safety implications in the event of fire.
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.
Figure 6—Prescriptive Requirements for Corbelling
EMPIRICALLY DESIGNED PARTITION WALLS
In many cases, the building structure is designed using traditional engineered methods, such as strength design or allowable stress design, but the interior nonloadbearing masonry walls are empirically designed. In these cases, the partition walls are supported according to the provisions listed in Tables 2 and 3, but it is important that the support conditions provide isolation between the partition walls and the building’s structural elements to prevent the building loads from being transferred into the partition. The anchor, or other support, must provide the required lateral support for the partition wall while also allowing for differential movement. This is in contrast to the “Anchorage for Lateral Support” section, which details anchorage requirements to help ensure adequate load transfer between the building structure and the loadbearing masonry wall.
Figure 7 shows an example of such a support, using clip angles. C channels or adjustable anchors could be used as well. The gap at the top of the wall should be between ½ and 1 in. (13 and 25 mm), or as required to accommodate the anticipated deflection. The gap is filled with compressible filler, mineral wool or a fire-rated material, if required. Fire walls may also require a sealant to be applied at the bottom of the clip angles. This joint should not be filled with mortar, as it may allow load transfer between the structure and the partition wall.
Figure 7—Example of Support for Empirically Designed Masonry Partition Wall
REFERENCES
International Building Code. International Code Council, 2003 and 2006.
Allowable Stress Design of Concrete Masonry Foundation Walls, TEK 15-01B. Concrete Masonry & Hardscapes Association, 2001.
Minimum Design Loads for Buildings and Other Structures, ASCE 7-02. New York, NY: American Society of Civil Engineers, 2002.
Minimum Design Loads for Buildings and Other Structures, ASCE 7-05. New York, NY: American Society of Civil Engineers, 2005.
Masonry Designer’s Guide, 5th Edition. Council for Masonry Research and The Masonry Society, 2007.
Building Code Requirements for Masonry Structures, ACI 530-08/ASCE 5-08/TMS 402-08. Reported by the Masonry Standards Joint Committee, 2008.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90-06. ASTM International, Inc., 2006.
Building Code Requirements for Masonry Structures, ACI 530/ASCE 5/TMS 402. Reported by the Masonry Standards Joint Committee, 2002 and 2005.
Anchors and Ties for Masonry, TEK 12-01B. Concrete Masonry & Hardscapes Association, 2008.
Floor and Roof Connections to Concrete Masonry Walls, TEK 05-07A. Concrete Masonry & Hardscapes Association, 2001.
Reinforcement in concrete masonry walls increases strength and ductility, increases resistance to applied loads, and in the case of horizontal reinforcement, also provides increased resistance to shrinkage cracking. This TEK covers non-prestressed reinforcement for concrete masonry construction. Prestressing steel is discussed in Post-Tensioned Concrete Masonry Wall Construction, TEK 03-14 (ref. 1). Unless otherwise noted, the information is based on the 2003 International Building Code (IBC) (ref. 2). For masonry design and construction, the IBC references Building Code Requirements for Masonry Structures and Specification for Masonry Structures (MSJC Code and Specification) (refs. 4, 5). In some cases, the IBC has adopted provisions different from the MSJC provisions. These instances have been noted where applicable.
MATERIALS
Reinforcement used in masonry is principally reinforcing bars and cold-drawn wire products. Wall anchors and ties are usually formed of wire, metal sheets or strips. Table 1 lists applicable ASTM Standards governing steel reinforcement, as well as nominal yield strengths for each steel type.
Table 1—Reinforcement Used in Masonry (ref. 2)
Reinforcing Bars
Reinforcing bars are available in the United States in 11 standard bar sizes designated No. 3 through 11, No. 14 and No. 18 (M#10-36, M#43, M#57). The size of a reinforcing bar is designated by a number corresponding to its nominal diameter. For bars designated No. 3 through No. 8 (M#10-25), the number indicates the diameter in eighths of an inch (mm), as shown in Table 2.
To help address potential problems associated with reinforcement congestion and grout consolidation, the IBC limits the reinforcing bar diameter to the lesser of one-eighth the nominal member thickness, and one-fourth the least dimension of the cell, course or collar joint into which it is placed. For typical single wythe walls, this corresponds to a maximum bar size of No. 8, 9 and 11 for 8-, 10- and 12- in. walls, respectively (M#25, 29 and 36 for 203-, 254- and 305-mm walls). In addition, the following limits apply:
maximum bar size is No. 11 (M#36),
the area of vertical reinforcement may not exceed 6% of the grout space area (i.e., about 1.26 in.² , 1.81 in.² , or 2.40 in.² of vertical reinforcement for 8-, 10- and 12-in. concrete masonry, respectively (815, 1,170 or 1,550 mm² for 203-, 254- and 305-mm units, respectively), and
for masonry designed using strength design procedures, the maximum bar size is No. 9 (M#29) and the maximum area of reinforcement is 4% of the cell area (i.e., about 0.84 in.² , 1.21 in.² , or 1.61 in.² of vertical reinforcement for 8-, 10- and 12-in. concrete masonry, respectively (545, 781 or 1,039 mm² for 203-, 254- and 305-mm units, respectively).
The prescriptive limits on reinforcement sizes, above, are construction-related. Additional design limits to prevent over-reinforcing and brittle failures may also apply depending on the design method used and the design loads resisted. Manufacturers mark the bar size, producing mill identification and type of steel on reinforcing bars (see Figure 1). Note that the bar size indicates the size in SI units per ASTM standards.
The ASTM standards include minimum requirements for various physical properties including yield strength and stiffness. While not all reinforcing bars have a well-defined yield point, the modulus of elasticity, Es , is roughly the same for all reinforcing steels and for design purposes is taken as 29,000,000 psi (200 GPa).
When designing by the allowable stress design method, allowable tensile stress is limited to 20,000 psi (138 MPa) for Grade 40 or 50 reinforcing bars and 24,000 psi (165 MPa) for Grade 60 reinforcing bars. For reinforcing bars enclosed in ties, such as those in columns, the allowable compressive stress is limited to 40% of the specified yield strength, with a maximum of 24,000 psi (165 MPa). For strength design, the nominal yield strength of the reinforcement is used to size and distribute the steel.
Table 2—Reinforcing Bar Nominal Properties
Figure 1—ASTM Standard Bar Identifi cation Marks
Cold-Drawn Wire
Cold-drawn wire for joint reinforcement, ties or anchors varies from W1.1 to W4.9 (MW7 to MW32) with the most popular size being W1.7 (MW11). Table 3 shows standard wire sizes and properties. Because the IBC limits the size of joint reinforcement to one half the joint thickness, the practical limit for wire diameter is 3/16 in. (W2.8, 4.8 mm, MW18) for a ⅜ in. (9.5 mm) bed joint. Wire for masonry is plain with the exception that side wires for joint reinforcement are deformed by means of knurling wheels.
Stress-strain characteristics of reinforcing wire have been determined by extensive testing programs. Not only is the yield strength of cold-drawn wire close to its ultimate strength, but the location of the yield point is not clearly indicated on the stress-strain curve. ASTM A 82 (ref. 15) defines yield as the stress determined at a strain of 0.005 in./in. (mm/mm).
Table 3—Properties of Wire For Masonry
CORROSION PROTECTION
Grout, mortar and masonry units usually provide adequate protection for embedded reinforcement provided that minimum cover and clearance requirements are met. Reinforcement with a moderate amount of rust, mill scale or a combination is allowed to be used without cleaning or brushing, provided the dimensions and weights (including heights of deformations) of a cleaned sample are not less than those required by the applicable ASTM standard. When additional corrosion protection is needed, reinforcement can be galvanized or epoxy coated.
Joint Reinforcement
Carbon steel can be protected from corrosion by coating the steel with zinc (galvanizing). The zinc protects in two ways: first, as a barrier separating the steel from oxygen and water, and second during the corrosion process, the zinc is sacrificed before the steel is attacked. Increasing the zinc coating thickness improves the level of corrosion protection.
Required levels of corrosion protection increase with the severity of exposure. When used in exterior walls or in interior walls exposed to a mean relative humidity over 75%, carbon steel joint reinforcement must be hot-dip galvanized or epoxy-coated, or stainless steel joint reinforcement must be used. When used in interior walls exposed to a mean relative humidity less than or equal to 75%, it can be mill galvanized, hot-dip galvanized, or be stainless steel. The corresponding minimum protection levels are:
Mill galvanized—ASTM A 641 (ref. 16) 0.1 oz/ft² (0.031 kg/m²)
Hot-dip galvanized—ASTM A 153 (ref. 17), Class B, 1.5 oz/ft² (458 g/m²)
Epoxy-coated—ASTM A 884 (ref. 18) Class A, Type 1 ≥ 7 mils (175 µm) (ref. 3). Note that both the 2003 IBC and 2002 MSJC code incorrectly identify Class B, Type 2 epoxy coated joint reinforcement, which is not applicable for masonry construction.
In addition, joint reinforcement must be placed so that longitudinal wires are embedded in mortar with a minimum cover of ½ in. (13 mm) when not exposed to weather or earth, and ⅝ in. (16 mm) when exposed to weather or earth.
Reinforcing Bars
A minimum amount of masonry cover over reinforcing bars is required to protect against steel corrosion. This masonry cover is measured from the nearest exterior masonry surface to the outermost surface of the reinforcement, and includes the thickness of masonry face shells, mortar and grout. The following minimum cover requirements apply:
masonry exposed to weather or earth bars larger than No. 5 (M#16) …………………….2 in. (51 mm) No. 5 (M#16) bars or smaller……………………1½ in. (38 mm)
masonry not exposed to weather or earth … 1½ in. (38 mm)
PLACEMENT
Installation requirements for reinforcement and ties help ensure that elements are placed as assumed in the design, and that structural performance is not compromised due to mislocation. These requirements also help minimize corrosion by providing for a minimum amount of masonry and grout cover around reinforcing bars, and providing sufficient clearance for grout and mortar to surround reinforcement and accessories so that stresses can be properly transferred.
Reinforcing Bars
Tolerances for placing reinforcing bars are:
variation from d for walls and fl exural elements: d ≤ 8 in. (203 mm) ………………………. ±½ in. (13 mm) 8 in. (203 mm) < d ≤ 24 in. (610 mm) ±1 in. (25 mm) d > 24 in. (610 mm) ……………………. ±1¼ in. (32 mm)
for vertical bars in walls ………..±2 in. (51 mm) from the specified location along the length of the wall.
In addition, a minimum clear distance between reinforcing bars and the adjacent (interior of cell) surface of a masonry unit of ¼ in. (6.4 mm) for fine grout or ½ in. (13 mm) for coarse grout must be maintained so that grout can flow around the bars.
DEVELOPMENT
Development length or anchorage is necessary to adequately transfer stresses between the reinforcement and the grout in which it is embedded. Reinforcing bars can be anchored by embedment length, hook or mechanical device. Reinforcing bars anchored by embedment length rely on interlock at the bar deformations and on sufficient masonry cover to prevent splitting from the reinforcing bar to the free surface. Detailed information and requirements for development, splice and standard hooks are contained in TEK 12-06A, Splices, Development and Standard Hooks for CM Based on the 2009 & 2012 IBC (ref. 19).
International Building Code 2003. International Code Council, 2003.
International Building Code 2006. International Code Council, 2006.
Building Code Requirements for Masonry Structures, ACI 530-02/ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee, 2002.
Specification for Masonry Structures, ACI 530.1-02/ASCE 6-02/TMS 602-02. Reported by the Masonry Standards Joint Committee, 2002.
Standard Specification for Deformed and Plain Billet-Steel Bars for Concrete Reinforcement, ASTM A615/A615M-00. ASTM International, Inc., 2000.
Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement, ASTM A706/A706M- 01. ASTM International, Inc., 2001.
Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement, A767/A767M-00b. ASTM International, Inc., 2000.
Standard Specification for Epoxy-Coated Steel Reinforcing Bars, A775/A775M-01. ASTM International, Inc., 2001.
Standard Specification for Rail-Steel and Axle-Steel Deformed Bars for Concrete Reinforcement, A996/A996M-00. ASTM International, Inc., 2000.
Standard Specification for Masonry Joint Reinforcement, ASTM A951-00. ASTM International, Inc., 2000.
Standard Specification for Stainless and Heat-Resisting Steel Wire, ASTM A580-98. ASTM International, Inc., 1998.
Standard Specification for Steel Wire, Deformed, for Concrete Reinforcement, A496/A496M-01. ASTM International, Inc., 2001.
Manual of Standard Practice, MSP 1-01. Concrete Reinforcing Steel Institute, 2001.
Standard Specification for Steel Wire, Plain, for Concrete Reinforcement, ASTM A82-01. ASTM International, Inc., 2001.
Standard Specification for Zinc-Coated (Galvanized) Carbon Steel Wire, ASTM A641-98. ASTM International, Inc., 1998.
Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, ASTM A153-01a. ASTM International, Inc., 2001.
Standard Specification for Epoxy-Coated Steel Wire and Welded Wire Fabric for Reinforcement, ASTM A884/A884M-99. ASTM International, Inc., 1999.
Reinforcement Detailing Requirements for Concrete Masonry, TEK 12-06A. Concrete Masonry & Hardscapes Association, 2007.
TEK 12-04D, Revised 2006. Disclaimer: Although care has been taken to ensure the enclosed information is as accurate and complete as possible, CMHA does not assume responsibility for errors or omissions resulting from the use of this TEK.
Standard joint reinforcement for concrete masonry is a factory fabricated welded wire assembly consisting of two or more longitudinal wires connected with cross wires forming a truss or ladder configuration. It was initially conceived primarily to control wall cracking associated with thermal or moisture shrinkage or expansion and as an alternative to masonry headers when tying masonry wythes together. Note that horizontal steel requirements for crack control can be met using joint reinforcement or reinforcing bars. See Crack Control Strategies for Concrete Masonry Construction, CMU TEC-009-23 (ref. 6).
Joint reinforcement also increases a wall’s resistance to horizontal bending, but is not widely recognized by the model building codes for structural purposes. In some instances, it may be used in design for flexural resistance or to meet prescriptive seismic requirements.
This TEK discusses the code and specification requirements for joint reinforcement and presents a general discussion of the function of joint reinforcement in concrete masonry walls. Detailed information on additional uses for joint reinforcement can be found in other TEK as referenced throughout this publication.
MATERIALS
Reinforcement types used in masonry principally are reinforcing bars and cold-drawn wire products. Joint reinforcement is governed by Standard Specification for Masonry Joint Reinforcement, ASTM A 951 (ref. 1), or Standard Specification for Stainless Steel Wire, ASTM A 580/580M Type 304 or Type 316 (ref. 2), if the joint reinforcement is stainless steel according to the Specification for Masonry Structures (ref. 3). Cold-drawn wire for joint reinforcement varies from W1.1 to W4.9 (11 gage to 1/4 in. diameter; MW7 to MW32), the most popular size being W1.7 (9 gage, MW11). Wire for masonry is plain, except side wires for joint reinforcement are deformed by means of knurling wheels.
Because Building Code Requirements for Masonry Structures (ref. 4) limits the size of joint reinforcement to one half the joint thickness, the practical limit for wire diameter is W2.8, (3/16 in., MW17) for a 3/8 in. (9.5 mm) bed joint. Joint reinforcement of this thickness may be difficult to install however, if a uniform mortar joint thickness of 3/8 in. (9.5 mm) is to be maintained.
Types of Joint Reinforcement
Reflecting its multiple purposes in masonry walls, joint reinforcement comes in several configurations. One longitudinal wire is generally required for each bed joint (i.e., two wires for a typical single wythe wall), but code or specification requirements may dictate otherwise. Typical joint reinforcement spacing is 16 in. (406 mm) on center. Adjustable ties, tabs, third wires and seismic clips are also available in combination with joint reinforcement for multi-wythe and veneer walls.
Ladder-type joint reinforcement (Figure 1) consists of longitudinal wires flush welded with perpendicular cross wires, creating the appearance of a ladder. It is less rigid than truss type joint reinforcement and is recommended for multi-wythe walls with cavity spaces or unfilled collar joints. This permits the two wythes to move independently, yet still transfers outof-plane loads from the exterior masonry to the interior masonry wall. Cross wires 16 in. (406 mm) on center should be used for reinforced concrete masonry construction, to keep cross wires out of the core spaces, thus preventing them from interfering with the placement of vertical reinforcement and grout.
Truss-type joint reinforcement (Figure 2) consists of longitudinal wires connected with diagonal cross wires. This shape is stiffer in the plane of the wall than ladder-type joint reinforcement and if used to connect multiple wythes restricts differential movement between the wythes. For this reason, it should be used only when differential movement is not a concern, as in single wythe concrete masonry walls. Because the diagonal cross wires may interfere with the placement of vertical reinforcing steel and grout, truss type joint reinforcement should not be used in reinforced or grouted walls.
Tabs, ties, anchors, third wires and seismic clips of assorted configurations are often used with the joint reinforcement to produce a system that works to: control cracking; bond masonry wythes together; anchor masonry; and, in some cases, resist structural loads. Tie and anchor spacing and other requirements are included in Anchors and Ties for Masonry, TEK 12-01B (ref.5).
Recommendations for the use of some of the different types of joint reinforcement are listed in Table 1.
CORROSION PROTECTION
Grout, mortar and masonry units usually provide adequate protection for embedded reinforcement, provided that minimum cover and clearance requirements are met.
Coating Requirements
The carbon steel in joint reinforcement can be protected from corrosion by coating with zinc (galvanizing). The zinc protects steel in two ways. First, it provides a barrier between the steel and oxygen and water. Second, during the corrosion process, the zinc provides a sacrificial coating. The protective value of the zinc coating increases with increased coating thickness; therefore the required amount of galvanizing increases with the severity of exposure, as listed below (refs. 3, 4):
Interior walls exposed to a mean relative humidity less than or equal to 75%: Mill galvanized, ASTM A 641 (0.1oz/ft2) (0.031 kg/m2) Hot-dip galvanized, ASTM A 153 (1.5 oz/ft2) (458 g/m2) Stainless steel AISI Type 304 or Type 316 conforming to ASTM A 580
Exterior walls or interior walls exposed to a mean relative humidity > 75%: Hot-dip galvanized, ASTM A 153 (1.5 oz/ft2 (0.46 kg/m2) Epoxy coated, ASTM A 884 Class A Type 1, > 7 mils (175 mm) Stainless steel AISI Type 304 or Type 316 conforming to ASTM A 580
Cover Requirements
Specification for Masonry Structures also lists minimum cover requirements for joint reinforcement as a further means of corrosion protection. It must be placed so that longitudinal wires are embedded in mortar with a minimum cover of:
1/2 in. (13 mm) when not exposed to weather or earth,
5/8 in. (16 mm) when exposed to weather or earth.
PRESCRIPTIVE CODE REQUIREMENTS
Building Code Requirements for Masonry Structures includes prescriptive requirements for joint reinforcement. There are multiple uses for joint reinforcement in masonry structures. Joint reinforcement can be used to provide crack control, horizontal reinforcement, and bond for multiple wythes, corners and intersections. The following list highlights only those requirements specific to joint reinforcement. Crack control topics are covered in CMU-TEC-009-23 (ref. 6). For information on anchors and ties, see Anchors and Ties for Masonry, TEK 12-01B (ref. 5). There is also a useful discussion on joint reinforcement as structural reinforcing in Steel Reinforcement for Concrete Masonry, TEK 12-04D (ref. 7).
General Requirements for Joint Reinforcement
For masonry in other than running bond: Horizontal reinforcement shall be 0.00028 times the gross vertical cross-sectional area of the wall. This requirement can be met with joint reinforcement placed in the horizontal bed joints. For 8in. (203-mm) masonry walls, this amounts to W1.7 (9 gage, MW11) joint reinforcement every other course. There are additional criteria for stack bond masonry in Seismic Design Categories D, E and F.
Seismic Requirements: In Seismic Design Category C and higher (for concrete masonry other than veneer), horizontal joint reinforcement spaced not more than 16 in. (406 mm) on center vertically with at least two wires of W1.7 (MW11) is required. Horizontal reinforcement also must be provided at the bottom and top of all wall openings and must extend at least 24 in. (610 mm) past the opening. Additional details on seismic requirements, including shear walls, are covered in Seismic Design and Detailing Requirements for Masonry Structures, CMHA TEK 14-18B (ref. 8).
Allowable Stress Design Requirements
In addition to the requirements above, concrete masonry walls designed by the allowable stress method and bonded by wall ties must have a maximum tie spacing of 36 in. (914 mm) horizontally and 24 in. (610 mm) vertically. Joint reinforcement cross wires can be used in place of wall ties to meet this requirement.
When the walls are designed for noncomposite action, truss-type joint reinforcing is not to be used for tying the wythes.
Combination joint reinforcement with tabs or adjustable ties are popular options for bonding multiwythe walls and are governed by additional code requirements.
Empirical Design Requirements
When two wythes of masonry are bonded with joint reinforcement, at least one cross wire must serve as a tie for each 22/3 ft2 (0.25 m2) of wall area. The vertical spacing of the joint reinforcement can not exceed 24 in. (610 mm), and the cross wires must be W1.7 (9 gage, MW11) minimum, without drips, and embedded in mortar.
Intersecting walls, when depending on each other for lateral support, can be anchored by several prescriptive methods including the use of joint reinforcement spaced no more than 8 in. (203 mm) on center vertically. The longitudinal wires must extend at least 30 in. (762 mm) in each direction at the intersection and be at least W1.7 (9 gage, MW11).
Interior nonloadbearing wall intersections may be anchored by several prescriptive methods, including joint reinforcement at a maximum spacing of 16 in. (406 mm) o.c. vertically.
Requirements for Use in Veneer
Prescriptive requirements for joint reinforcement in masonry veneer are included in Building Code Requirements for Masonry Structures, Chapter 6. These provisions are limited to areas where the basic wind speed does not exceed 110 mph (177 km/hr) as listed in ASCE 7-02 (ref. 9). Additional limitations are covered in the Code. The information below is for joint reinforcement or the joint reinforcement portion of a tie/anchor system. For information on anchor and tie requirements see Concrete Masonry Veneers, TEK 03-06C (ref. 10).
Ladder-type or tab-type joint reinforcement is permitted in veneer construction with the cross wires used to anchor the masonry veneer. Minimum longitudinal and cross wire size is W1.7 (9 gage, MW11), and maximum spacing is 16 in. (406 mm) on center vertically.
Adjustable anchors combined with joint reinforcement may be used as anchorage with the longitudinal wire of the joint reinforcement being W1.7 (9 gage, MW11) minimum.
Joint reinforcement may also be used to anchor masonry veneer to masonry provided the maximum distance between the inside face of the veneer and the outside face of the concrete masonry backup wythe is 4 1/2 in. (114 mm).
In Seismic Design Categories E and F, the 2005 edition of Building Code Requirements for Masonry Structures requires continuous single wire joint reinforcement, W1.7 (9 gage, MW11) minimum, in the veneer wythe at a maximum spacing of 18 in. (457 mm) on center vertically. Clips or hooks must attach the wire to the joint reinforcement. The International Building Code 2003 (ref. 11) also mandates this requirement for Seismic Design Category D.
Anchor spacings, and, as a result, possibly joint reinforcement spacing, are reduced for Seismic Design Categories D, E and F and in high wind areas.
Requirements for Use in Glass Unit Masonry
Horizontal joint reinforcement is to be spaced no more than 16 in. (406 mm) on center, located in the mortar bed joint, and must not span across movement joints.
Minimum splice length is 6 in. (152 mm).
Joint reinforcement must be placed immediately above and below openings in the panel.
Joint reinforcement must have at least 2 parallel, longitudinal wires of size W1.7 (9 gage, MW11) and have welded cross wires of W1.7 (9 gage, MW11) minimum.
INSTALLATION
Joint reinforcement installation is a routine task for masons. The joint reinforcement is placed on the face shells and mortar is placed over it. Cover requirements must be maintained. Installing the correct type of joint reinforcement with the specified corrosion resistant coating is important, as is making sure it is installed at the proper spacings and locations. Quality assurance provisions related to joint reinforcement generally include:
Submittals
Material Certificate indicating compliance should include:
material meets specified ASTM standard,
corrosion protection specified has been supplied,
configuration specified has been supplied, and
other criteria as required or specified.
Inspection
Oil, dirt and other materials detrimental to bond should be removed. Light rust and mill scale are permissible.
Cover requirements are met.
Splices are a minimum of 6 in. (152 mm) (see Figure 3) to properly transfer tensile stresses. Tying is not necessary. Construction documents may specify longer splices, especially if the joint reinforcement is being used as part of the structural horizontal reinforcing steel.
Verify that joint reinforcement utilized for crack control does not continue through movement joints.
If ties or anchors are part of the joint reinforcement, check that embedment in the adjoining wythe, alignment and spacing are within specified values.
REFERENCES
Standard Specification for Masonry Joint Reinforcement, ASTM A 951-02. ASTM International, 2002.
Standard Specification for Stainless Steel Wire, ASTM A 580/580M-98(2004). ASTM International, 2004.
Specification for Masonry Structures, ACI 530.1-05/ASCE 6 05/TMS 602-05. Reported by the Masonry Standards Joint Committee, 2005.
Building Code Requirements for Masonry Structures, ACI 530 05/ASCE 5-05/TMS 402-05. Reported by the Masonry Standards Joint Committee, 2005.
Anchors and Ties for Masonry, TEK 12-01B, Concrete Masonry & Hardscapes Association 2011.
Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, 2023.
Steel Reinforcement for Concrete Masonry, TEK 12-04D, Concrete Masonry & Hardscapes Association, 2023.
Seismic Design and Detailing Requirements for Masonry Structures, TEK 14-18B, Concrete Masonry & Hardscapes Association, 2003.
Minimum Design Loads for Buildings and Other Structures, ASCE 7-02, American Society of Civil Engineers, 2002.
Masonry connectors can be classified as wall ties, anchors or fasteners. Wall ties connect one masonry wythe to an adjacent wythe. Anchors connect masonry to a structural support or frame. Fasteners connect an appliance to masonry. This TEK covers metal wall ties and anchors. Fasteners are discussed in TEK 12-05 (ref. 1).
The design of anchors and ties is covered by the International Building Code and Building Code Requirements for Masonry Structures (refs. 2, 3).These provisions require that connectors be designed to resist applied loads and that the type, size and location of connectors be shown or indicated on project drawings. This TEK provides a guide to assist the designer in determining anchor and tie capacity in accordance with the applicable standards and building code requirements.
DESIGN CRITERIA
Connectors play a very important role in providing structural integrity and good serviceability. As a result, when selecting connectors for a project, designers should consider a number of design criteria. Connectors should:
Transmit out-of-plane loads from one wythe of masonry to another or from masonry to its lateral support with a minimum amount of deformation. It is important to reduce the potential for cracking in masonry due to deflection. There is no specific criteria on connector stiffness, but some authorities suggest that a stiffness of 2,000 lb/in. (350 kN/m) is a reasonable target.
Allow differential in-plane movement between two masonry wythes connected with ties. This is especially significant as more insulation is used between the outer and inner wythes of cavity walls and where wythes of dissimilar materials are anchored together. On the surface, it may appear that this criterion is in conflict with Item 1, but it simply means that connectors must be stiff in one direction (out-of-plane) and flexible in the other (in plane). Note that some connectors allow much more movement than unreinforced masonry can tolerate (see ref. 27 for a discussion of potential masonry wall movements). In order to preserve the in-plane and out-of-plane wall tie stiffness, current codes (refs. 2, 3) allow cavity widths up to 4 1/2 in. (114 mm) without performing wall tie analysis. With an engineered analysis of the wall ties, cavity widths may be significantly increased to accommodate thicker insulation.
Meet applicable material requirements:
plate and bent-bar anchors—ASTM A36 (ref. 4)
sheet-metal anchors and ties—ASTM A1008 (ref. 5)
wire anchors and ties—ASTM A82 (ref. 6), and adjustable wire ties must also meet the requirements illustrated in Figure 1
wire mesh ties – ASTM A185 (ref. 7)
Provide adequate corrosion protection. Where carbon steel ties and anchors are specified, corrosion protection must be provided by either galvanizing or epoxy coating in conformance with the following (ref. 8):
A. Galvanized coatings:
Joint reinforcement in interior walls exposed to a mean relative humidity of 75% or less—ASTM A641 (ref. 13), 0.1 oz zinc/ft2 (0.031 kg zinc/m2)
Joint reinforcement, wire ties and wire anchors, exterior walls or interior walls exposed to a mean relative humidity greater than 75%—ASTM A153 (ref. 14), 1.5 oz zinc/ft2 (458 g/m2)
Sheet metal ties or anchors, interior walls exposed to a mean relative humidity of 75% or less—ASTM A653 (ref. 15) Coating Designation G60
Sheet metal ties or anchors, exterior walls or interior walls exposed to a mean relative humidity greater than 75%—ASTM A153 Class B
Steel plates and bars, exterior walls or interior walls exposed to a mean relative humidity greater than 75%—ASTM A123 (ref. 16) or ASTM A153 Class B
Plate and bent-bar anchors—ASTM A480 and ASTM A666 (refs. 10, 11)
Sheet metal anchors and ties—ASTM A480 and ASTM A240 (refs. 10, 12)
Wire ties and anchors—ASTM A580
B. Epoxy coatings:
Joint reinforcement—ASTM A884 (ref. 17) Class A Type 1 > 7 mils (175 µm)
Wire ties and anchors—ASTM A899 (ref. 18) Class C 20 mils (508 µm)
Sheet metal ties and anchors—20 mils (508 µm) per surface or per manufacturer’s specification
Where stainless steel anchors and ties are specified, Specification for Masonry Structures (ref. 8) requires that AISI Type 304 or 316 stainless steel be provided that complies with:
Joint reinforcement—ASTM A580 (ref. 9)
Accommodate construction by being simple in design and easy to install. Connectors should not be so large and cumbersome as to leave insufficient room for mortar in the joints, which can result in a greater tendency to allow water migration into the wall. In the same way, connectors should readily accommodate insulation in wall cavities.
WALL TIE AND ANCHOR REQUIREMENTS
Multiwythe Masonry Wall Types
Wall ties are used in all three types of multiwythe walls (composite, noncomposite and veneer), although some requirements vary slightly depending on the application. The primary differences between these wall systems are in construction details and how the applied loads are assumed to be distributed.
Composite walls are designed so that the masonry wythes act together as a single structural member. This requires the masonry wythes to be connected by masonry headers or by a mortar- or grout filled collar joint and wall ties to help ensure adequate load transfer. TEKs 16-01A and 16-02B (refs. 19, 20) more fully describe composite walls.
In noncomposite masonry (also referred to as a cavity wall), wythes are connected with metal wall ties, but they are designed such that each wythe individually resists the loads imposed on it. Noncomposite walls are discussed in TEKs 16-01A and 16-04A (refs. 19, 21).
In a veneer wall, the backup wythe is designed as the load-resisting system, with the veneer providing the architectural wall finish. Information on veneer walls can be found in TEKs 05-01B and 03 06C (refs. 22, 23). Note that although a cavity wall is defined as a noncomposite masonry wall (ref. 3), the term cavity wall is also commonly used to describe a veneer wall with masonry backup.
Building Code Requirements for Masonry Structures also includes empirical requirements for wire wall ties and strap-type ties used to connect intersecting walls. These requirements are covered in TEK 14-08B (ref. 24).
Wall Ties
Wire wall ties can be either one piece unit ties, adjustable two piece ties, joint reinforcement or prefabricated assemblies made up of joint reinforcement and adjustable ties (see Figure 2). Note that the 2011 edition of Specification for Masonry Structures allows adjustable pintle ties to have only one leg (previously, two legs were required for this type of wall tie).
Wall ties do not have to be engineered unless the nominal width of the wall cavity is greater than 4 1/2 in. (114 mm). These wall tie analyses are becoming more common as a means to accommodate more thermal insulation in the wall cavity. Masonry cavities up to 14 in. (356 mm) have been engineered. Of note for these analyses is that the span of wire is a more critical factor than cavity width, i.e. the span length of the pintel component typically controls the mode of failure.
The prescribed size and spacing is presumed to provide connections that will be adequate for the loading conditions covered by the code. These wall tie spacing requirements can be found in TEK 03-06C (for veneers) and TEK 16-01A (for composite and noncomposite walls). Note that truss-type joint reinforcement is stiffer in the plane of a wall compared to ladder-type, so it is more restrictive of differential movement. For this reason, laddertype joint reinforcement is recommended when significant differential movement is expected between the two wythes or when vertical reinforcement is used. See TEK 12-02B (ref. 25) for more information.
Additional tests are needed for adjustable anchors of different configurations and for one piece anchors. Proprietary anchors are also available. Manufacturers of proprietary anchors should furnish test data to document comparability with industry-tested anchors.
Anchors are usually designed based on their contributory area. This is the traditional approach, but some computer models suggest that this approach does not always reflect the actual behavior of the anchorage system. However, there is currently no accepted computer program to address this point, so most designers still use the contributory area approach with a factor of safety of three. The use of additional anchors near the edges of wall panels is also recommended and required around large openings and within 12 in. (305 mm) of unsupported edges.
CONSTRUCTION
When typical ties and anchors are properly embedded in mortar or grout, mortar pullout or pushout will not usually be the controlling mode of failure. Specification for Masonry Structures requires that connectors be embedded at least 1 1/2 in. (38 mm) into a mortar bed of solid units. The required embedment of unit ties in hollow masonry is such that the tie must extend completely across the hollow units. Proper embedment can be easily attained with the use of prefabricated assemblies of joint reinforcement and unit ties. Because of the magnitude of loads on anchors, it is recommended that they be embedded in filled cores of hollow units. See TEK 03-06C for more detailed information.
REFERENCES
Fasteners for Concrete Masonry, TEK 12-05. Concrete Masonry & Hardscapes Association, 2005.
International Building Code. International Code Council, 2012.
Building Code Requirements for Masonry Structures, TMS 402-11/ACI 530-11/ASCE 5-11. Reported by the Masonry Standards Joint Committee, 2011.
Standard Specification for Carbon Structural Steel, A36-ASTM International, 2008.
Standard Specification for Steel, Sheet, Cold-Rolled, Carbon, Structural, High-Strength Low-Alloy with Improved Formability, A1008-11. ASTM International, 2011.
Standard Specification for Steel Wire, Plain for Concrete Reinforcement, A82-07. ASTM International, 2007.
Standard Specification for Steel Welded Wire Reinforcement, Plain, for Concrete, A185-07. ASTM International, 2007.
Specification for Masonry Structures, TMS 602 -11/ACI 530.1-11/ASCE 6-11. Reported by the Masonry Standards Joint Committee, 2011.
Standard Specification for Stainless Steel Wire, ASTM A580-08. ASTM International, 2008.
Standard Specification for General Requirements for Flat Rolled Stainless and Heat-Resisting Steel Plate, Sheet, and Strip, ASTM A480-11a. ASTM International, 2011.
Standard Specification for Annealed or Cold-Worked Austenitic Stainless Steel, Sheet, Strip, Plate and Flat Bar, ASTM A666-10. ASTM International, 2010.
Standard Specification for Chromium and Chromium Nickel Stainless Steel Plate, Sheet and Strip for Pressure Vessels and for General Applications, ASTM A240-11a. ASTM International, 2011.
Standard Specification for Zinc-Coated (Galvanized) Carbon Steel Wire, ASTM A641-09a. ASTM International, 2009.
Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, ASTM A153-09. ASTM International, 2009.
Standard Specification for Steel Sheet, Zinc-Coated Galvanized or Zinc-Iron Alloy-Coated Galvannealed by the Hot-Dip Process, ASTM A653-10. ASTM International, 2010.
Standard Specification for Zinc (Hot-Dip Galvanized) Coating on Iron and Steel Products, ASTM A123-09. ASTM International, 2009.
Standard Specification for Epoxy-Coated Steel Wire and Welded Wire Fabric for Reinforcement, ASTM A884-06. ASTM International, 2006.
Standard Specification for Steel Wire Epoxy Coated, ASTM A899-91(2007). ASTM International, 2007.
Empirical Design of Concrete Masonry Walls, TEK 14-08B, Concrete Masonry & Hardscapes Association, 2008.
Joint Reinforcement for Concrete Masonry, TEK 12-02B, Concrete Masonry & Hardscapes Association, 2005.
Porter, Max L., Lehr, Bradley R., Barnes, Bruce A., Attachments for Masonry Structures, Engineering Research Institute, Iowa State University, February 1992.
Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.
Masonry is often specified because of its aesthetic versatility. Combining masonry units of different size, color and finish provides a virtually limitless palette. Often, exterior concrete masonry walls incorporate clay brick, or concrete masonry is used in clay brick walls as accent bands. The bands add architectural interest to the wall and can also help hide horizontal elements such as flashing and expansion joints. However, combining these two materials within one wythe of masonry requires special detailing due to their different material properties.
In general, all masonry walls should be designed and detailed to accommodate anticipated movement resulting from volume changes in the masonry materials themselves. For example, vertical control joints and horizontal joint reinforcement can be incorporated into concrete masonry walls to control cracking and still allow horizontal shrinkage of the concrete masonry units to occur without introducing undue stress into the wall. Similarly, clay masonry walls incorporate vertical and horizontal expansion joints to allow the clay to expand without distress. When both clay and concrete masonry units are used in the same masonry wythe, detailing is required to accommodate concrete masonry shrinkage and clay masonry expansion occurring side by side. Concrete masonry is a hydraulic cement product and as such requires water for cement hydration, which hardens the concrete. Therefore, concrete masonry units are relatively wet at the time of manufacture and from that time on tend to shrink as the units dry. Conversely, clay masonry units are very dry subsequent to firing during the manufacturing process and then tend to expand as they pick up moisture from the atmosphere and from mortar as they are laid. Without due consideration of these opposing movements, cracking can result. In veneers, the cracking is primarily an aesthetic issue, as any water that penetrates the veneer through cracks between the two materials drains down the cavity and is directed out of the wall via flashing and weep holes.
BANDING DETAILS
When detailing a wall to accommodate movement, the design goal is to allow the movement to occur (as restraint will cause cracking) while providing appropriate support. The recommendations that follow are based on a record of successful performance in many locations across the United States. These can be adjusted as needed to suit local conditions and/or experience.
In general, several strategies are used to accommodate movement. These include movement joints (control joints in concrete masonry and expansion joints in clay masonry); horizontal joint reinforcement to take tension due to concrete masonry shrinkage and help keep any cracks that occur closed; and sometimes horizontal joints to allow longitudinal movement. In veneers, it is particularly important that the band, as well as the wall panel above and below the band be supported by wall ties. Wall ties should be installed within 12 in. (305 mm) of the top and bottom of the band to help ensure the surrounding masonry is adequately supported.
In addition, using a lower compressive strength mortar helps ensure that if cracks do occur, they occur in the mortar joint rather than through the unit. Type N mortar is often specified for veneers, because it tends to be more flexible than other mortar types.
Concrete Masonry Band in Clay Brick Wall
Figure 1a shows a two-course high concrete masonry band in a clay brick exterior wythe of a cavity wall. With this type of construction, the following practices are employed to minimize the potential for cracking.
Horizontal joint reinforcement is placed in the mortar joints above and below the band to take stress from the differential movement in that plane. For bands higher than two courses, joint reinforcement should also be placed within the band itself at a spacing of 16 in. (406 mm) on center vertically. Ideally, the joint reinforcement and ties should be placed in alternate joints so that one does not interfere with placement of the other. Some designers, however, prefer placing joint reinforcement in every bed joint in the concrete masonry band, particularly if the aspect ratio of the band is high. In this case, a tie which accommodates both tie and wire in the same mortar joint should be used, such as a seismic clip type wall tie.
Although the detail in Figure 1a has demonstrated good performance in many areas of the United States, there are locations where use of bond breaks at the top and bottom of the band is preferred (see Figure 1b) A local masonry industry representative should be contacted for further information on which detail has been more successful in a given location.
Figure 1b shows a slip plane incorporated into the interfaces between the concrete and clay masonry to allow unrestrained longitudinal movement between the two materials. This can be accomplished by placing building paper, polyethylene, flashing or a similar material in the horizontal bed joints above and below the band. When hollow masonry units are used for the band, the slip plane below the band should incorporate flashing, so that any water draining down the cores of the band can be directed out of the wall at that point.
When slip planes are used, joint reinforcement should be incorporated into the concrete masonry band. The exposed mortar joint at the top and bottom of the band should be raked back and sealed with an appropriate sealant to prevent water penetration at these joints. Note that this construction is typically more expensive than the detail shown in Figure 1a.
In addition to joint reinforcement, reduced spacing of expansion joints in the wall is recommended to reduce the potential for cracking. Experience has shown that vertical expansion joints in the clay masonry should extend through the concrete masonry band as well, and be placed at a maximum of 20 ft (6.1 m) along the length of the wall. Although concrete masonry construction typically requires control joints rather than expansion joints, control joints should not be used in the concrete masonry band at the expansion joint locations.
Note that local experience may require reducing the expansion joint spacing to 16 ft (4.9 m). If brick vertical expansion joint spacing does exceed 20 ft (6.1 m), consider placing an additional vertical movement joint through the concrete masonry accent band near mid panel with joint reinforcement continuous through that joint. The continuous joint reinforcement in this location helps keep the clay brick above and below the band from cracking as the concrete masonry shrinks.
Bands only one course high must be detailed to incorporate joint reinforcement and wall ties in the joints above and below the band (see Figure 2).
When concrete masonry banding is used over a wood stud backup, similar provisions apply (see Figure 3). It is imperative that joint reinforcement be used in the concrete masonry band, even if it is not used in the surrounding clay brick masonry.
Clay Brick Band in Concrete Masonry Wall
The recommendations to control differential movement for clay brick masonry bands in concrete masonry are very similar to those for a concrete masonry band in clay brick veneer: joint reinforcement above and below the band and wall ties within the band. Seismic clip type wall ties are recommended, as they provide an adjustable wall tie and joint reinforcement in one assembly.
With this construction, it is imperative that the veneer control joint not contain mortar as it goes through effectiveness. Note that although control joints in structural masonry walls must permit free longitudinal movement while resisting lateral or out-of plane shear loads, veneers are laterally supported by the backup and do not require a shear key.
In single wythe construction as shown in Figure 5, flashing and weep holes are used above the accent band to facilitate removal of any water that may accumulate in the wall. The use of two reduced thickness concrete masonry units allows flashing to be placed within the wall without causing a complete horizontal bond break at the flashing.
In reinforced walls (Figure 5b), flashing and weeps are also used. On the wall interior, rather than using reduced thickness units, a full size unit is cut to fit to allow adequate space for the reinforcement and grout.
In addition to its structural use or as the exterior wythe of composite and noncomposite walls, concrete brick and architectural facing units are also used as veneer over various backing surfaces. The variety of surface textures, colors, and patterns available makes concrete masonry a versatile and popular exterior facing material. Architectural units such as split-face, scored, fluted, ground face, and slump are available in a variety of colors and sizes to complement virtually any architectural style.
VENEER—DESIGN CONSIDERATIONS
Veneer is a nonstructural facing of brick, stone, concrete masonry or other masonry material securely attached to a wall or backing. Veneers provide the exterior wall finish and transfer out-of-plane loads directly to the backing, but they are not considered to add to the load-resisting capacity of the wall system. Backing material may be masonry, concrete, wood studs or steel studs.
There are basically two types of veneer—anchored veneer and adhered veneer. They differ by the method used to attach the veneer to the backing, as illustrated in Figure 1. Unless otherwise noted, veneer requirements are those contained in the International Building Code (IBC) and Building Code Requirements for Masonry Structures (refs. 2, 3).
For the purposes of design, veneer is assumed to support no load other than its own weight. The backing must be designed to support the lateral and in some instances the vertical loads imposed by the veneer in addition to the design loads on the wall, since it is assumed the veneer does not add to the strength of the wall.
Masonry veneers are typically designed using prescriptive code requirements that have been developed based on judgement and successful performance. The prescriptive requirements relate to size and spacing of anchors and methods of attachment, and are described in the following sections. The assembly can be designed as a noncomposite cavity wall where the out-of-plane loads are distributed to the two wythes in proportion to their relative stiffness. Design criteria are provided in IBC Chapter 16 as well as in TEK 16-04A, Design of Concrete Masonry Noncomposite (Cavity) Walls, (ref. 4).
In addition to structural requirements, differential movement between the veneer and its supports must be accommodated. Movement may be caused by temperature changes, moisture-volume changes, or deflection. In concrete masonry, control joints and horizontal joint reinforcement effectively relieve stresses and accommodate small movements. For veneer, control joints should generally be placed in the veneer at the same locations as those in the backing, although recommended control joint spacing can be adjusted up or down based on local experience, the aesthetic requirements of the project, or as required to prevent excessive cracking. See CMU-TEC 009-23, Crack Control for Concrete Brick and Other Concrete Masonry Veneers (ref. 5), for further information.
For exterior veneer, water penetration into the cavity is anticipated. Therefore, the backing system must be designed and detailed to resist water penetration and prevent water from entering the building. Flashing and weeps in the veneer collect any water that penetrates the veneer and redirects it to the exterior. Partially open head joints are one preferred type of weep. They should be at least 1 in. (25 mm) high and spaced not more than 32 in. (813 mm) on center. If necessary, insects can be thwarted by inserting stainless steel wool into the opening or by using proprietary screens. For anchored veneer, open weeps can also serve as vents, allowing air circulation in the cavity to speed the rate of drying. Additional vents may be installed at the tops of walls to further increase air circulation. More detailed information is contained in TEK 05-01B, Concrete Masonry Veneer Details, TEK 19-04A, Flashing Strategies for Concrete Masonry Walls, and TEK 19-05A, Flashing Details for Concrete Masonry Walls (refs. 1, 6, 7).
ANCHORED VENEER
Anchored veneer is veneer which is supported laterally by the backing and supported vertically by the foundation or other structural elements. Anchors are used to secure the veneer and to transfer loads to the backing. Anchors and supports must be noncombustible and corrosion-resistant.
The following prescriptive criteria apply to anchored veneer in areas with velocity pressures, qz, up to 40 psf (1.92 kPa). Modified prescriptive criteria is available for areas with qz greater than 40 psf (1.92 kPa) but not exceeding 55 psf (2.63 kPa) with a building mean roof height up to 60 ft (18.3 m). These modified provisions are presented in the section High Wind Areas. In areas where qz exceeds 55 psf (2.63 kPa), the veneer must be designed using engineering philosophies, and the following prescriptive requirements may not be used.
In areas where seismic activity is a factor, anchored veneer and its attachments must meet additional requirements to assure adequate performance in the event of an earthquake. See the section Seismic Design Categories C and Higher for details.
Masonry units used for anchored veneer must be at least 2 ⅝ in. (67 mm) thick.
A 1 in. (25 mm) minimum air space must be maintained between the anchored veneer and backing to facilitate drainage. A 1 in. (25 mm) air space is considered appropriate if special precautions are taken to keep the air space clean (such as beveling the mortar bed away from the cavity). Otherwise, a 2 in. (51 mm) air space is preferred. As an alternative, proprietary insulating drainage products can be used.
The maximum distance between the inside face of the veneer and the outside face of the backing is limited to 4 ½ in. (114 mm), except for corrugated anchors used with wood backing, where the maximum distance is 1 in. (25 mm).
When anchored veneer is used as an interior finish supported on wood framing, the veneer weight is limited to 40 lb/ft2 (195 kg/m2).
Deflection Criteria
Deflection of the backing should be considered when using masonry veneer, in order to control crack width in the veneer and provide veneer stability. This is primarily a concern when masonry veneer is used over a wood or steel stud backing. Building Code Requirements for Masonry Structures, however, does not prescribe a deflection limit for the backing. Rather, the commentary presents various recommendations for deflection limits.
For anchored veneer, Chapter 16 of the International Building Code requires a deflection limit of l/240 for exterior walls and interior partitions with masonry veneer.
Support of Anchored Veneer
The height and length of the veneered area is typically not limited by building code requirements. The exception is when anchored veneer is applied over frame construction. For wood stud backup, veneer height is limited to 30 ft (9.14 m) (height at plate) or 38 ft (11.58 m) (height at gable). Similarly, masonry veneer over steel stud backing must be supported by steel shelf angles or other noncombustible construction for each story above the first 30 ft (9.14 m) (height at plate) or 38 ft (11.58 m) (height at gable). This support does not necessarily have to occur at the floor height, for example it can be provided at a window head or other convenient location.
Exterior anchored veneer is permitted to be supported on wood construction under the following conditions:
the veneer has an installed weight of 40 psf (195 kg/m2) or less,
the veneer has a maximum height of 12 ft (3.7 m),
a vertical movement joint in the veneer is used to isolate the veneer supported on wood construction from that supported by the foundation,
masonry is designed and constructed so that the masonry is not in direct contact with the wood, and
the horizontally spanning member supporting the masonry veneer is designed to limit deflection due to unfactored dead plus live loads to l/600 or 0.3 in. (7.5 mm).
Over openings, the veneer must be supported by non- combustible lintels or supports attached to noncombustible framing, as shown in Figure 2.
The following requirements assume that the veneer is laid in running bond. When other bond patterns are used, the veneer is required to have joint reinforcement spaced no more than 18 in. (457 mm) on center vertically. The joint reinforcement need only be one wire, with a minimum size of W1.7 (MW11).
Figure 2—Steel Lintel Flashing Detail (ref. 8)
(backing not shown for clarity)
Anchors
Veneers may generally be anchored to the backing using sheet metal anchors, wire anchors, joint reinforcement or adjustable anchors, although building codes may restrict the use of some anchors. Corrugated sheet metal anchors are permitted with masonry veneer attached to wood backing only. Requirements for the most common anchor types are summarized in Figures 3 through 5 and Table 1. As an alternative, adjustable anchors of equivalent strength and stiffness may be used. Cavity drips are not permitted. See TEK 12-01B, Anchors and Ties for Masonry, (ref. 9) for detailed information on anchor materials and requirements.
Attachment to Backing
When masonry veneer is anchored to wood backing, each anchor is attached to the backing with a corrosion- resistant 8d common nail, or a fastener with equivalent or greater pullout strength. For proper fastening of corrugated sheet metal anchors, the nail or fastener must be located within ½ in. (13 mm) of the 90° bend in the anchor. The exterior sheathing must be either water resistant with taped joints or be protected with a water- resistant membrane, such as building paper ship-lapped a minimum of 6 in. (152 mm) at seams, to protect the backing from any water which may penetrate the veneer.
When masonry veneer is anchored to steel backing, adjustable anchors must be used to attach the veneer. Each anchor is attached with corrosion-resistant screws that have a minimum nominal shank diameter of 0.19 in. (4.8 mm), or an anchor with equivalent pullout strength. Cold-formed steel framing must be corrosion resistant and should have a minimum base metal thickness of 0.043 in. (1.1 mm). Sheathing requirements are the same as those for wood stud backing.
Masonry veneer anchored to masonry backing may be attached using wire anchors, adjustable anchors or joint reinforcement. Veneer anchored to a concrete backing must be attached with adjustable anchors.
Figure 3—Adjustable Anchors
Figure 4—Corrugated Sheet Metal Anchor Requirements
Anchor Placement
When typical ties and anchors are properly embedded in mortar or grout, mortar pullout or pushout will not usually be the controlling mode of failure. For this reason, connectors must be embedded at least 1 ½ in. (38 mm) into a mortar bed of solid units, and the mortar bed joint must be at least twice the thickness of the embedded anchor. The required embedment of unit ties in hollow masonry is such that the tie must extend completely across the hollow units (Figure 6). Proper embedment can be easily attained with the use of prefabricated assemblies of joint reinforcement and unit ties. Because of the magnitude of loads on anchors, it is recommended that they be embedded in filled cores of hollow units. To save mortar, screens can be placed under the anchor and 1 to 2 in. (25 to 51 mm) of mortar can be built up into the core of the block above the anchor (Figure 7).
Figure 5—Requirements for Joint Reinforcement Used to Anchor Veneer
Figure 6—Minimum Embedment of Joint Reinforcement and Wire Ties
Figure 7—Use of Mesh to Facilitate Embedment of Anchor in Mortar
High Wind Areas
In areas with qz greater than 40 psf (1.92 kPa) but not exceeding 55 psf (2.63 kPa) with a building mean roof height up to 60 ft (18.3 m), the following modified prescriptive provisions may be used.
The modified prescriptive provisions are:
the maximum wall area supported by each anchor must be reduced to 70% of the value listed in Table 1,
anchor spacing is reduced to a maximum of 18 in. (457 mm), both vertically and horizontally, and
around openings larger than 16 in. (406 mm) in either direction, anchors must be placed within 12 in. (305 mm) of the opening and spaced at 24 in. (610 mm) on center or less.
In areas where qz exceeds 55 psf (2.63 kPa), the veneer must be designed using engineering philosophies.
Seismic Design Categories C and Higher
To improve veneer performance under seismic loading in Seismic Design Category (SDC) C, the sides and top of the veneer must be isolated from the structure, so that vertical and lateral seismic forces are not transferred to the veneer. This reduces accidental loading and allows more building deflection without causing damage to the veneer.
In SDC D, in addition to this isolation, the maximum wall area supported by each anchor must be reduced to 75% of the value listed in Table 1, although the maximum spacings are unchanged. In addition, when the veneer is anchored to wood backing, the veneer anchor must be attached to the wood using a corrosion-resistant 8d ring-shank nail, a No. 10 corrosion- resistant screw with a minimum nominal shank diameter of 0.190 in. (4.8 mm), or with a fastener having equivalent or greater pullout strength.
In SDC E and F, the requirements listed above for SDC C and D must be met, as well as the additional requirements listed here. The weight of each story of anchored veneer must be supported independently of other stories to help limit the size of potentially damaged areas. In addition, to improve veneer ductility the veneer must have continuous W1.7 (MW11) single wire joint reinforcement at 18 in. (457 mm) o.c. or less vertically, with a mechanical attachment to the anchors, such as clips or hooks.
ADHERED VENEER
Conventional adhered veneer is veneer secured and supported through adhesion with a bonding material applied over a backing that both meets required deflection limits and provides for necessary adhesion. When applied to a masonry or concrete backing, the veneer may be applied directly to the backing substrate using layers of neat cement paste and Type S mortar, as illustrated in Figure 1. When applied over steel or wood framing, the adhered masonry veneer is applied to a metal lath and portland cement plaster backing placed against the sheathing element and attached to the stud framing members.
Alternative design of adhered veneer is permitted under the International Building Code when in compliance with Building Code Requirements for Masonry Structures (MSJC), where the requirements of unit adhesion (shear stress > 50 psi, 345 kPa) are met, out-of-plane curvature of the backing is limited to prevent the veneer from separating from the backing, and freeze thaw cycling, water penetration, and air and water vapor transmission are considered. Although the MSJC does not stipulate a deflection limit to control out-of-plane curvature, the Tile Council of America limits the deflection of backing supporting ceramic tiles to l/360 (ref. 11). Similarly, IBC Chapter 16 (for engineered design) requires a deflection limit of l/360 for exterior walls and interior partitions with plaster or stucco, which would be similar to an adhered veneer application.
Proprietary polymer-fortified adhesive mortars exist that meet the adhesion requirements and are used as a mortar setting bed to adhere the masonry veneers directly to a masonry or concrete backing, or to a lath and plaster backing system over wood or steel studs.
In addition, several proprietary systems are available to aid in placement of adhered masonry veneer on suitable exterior or interior substrates. These typically take the form of galvanized steel support panels that are mechanically anchored to a masonry or concrete backing, or placed against the sheathing element and attached to stud framing members. These products essentially take the place of the metal lath in the adhered veneer application. The metal panels contain support tabs and other features to facilitate the veneer application, carry the dead load of the veneer, and improve bonding of the veneer to the panel. In some cases, metal panel systems provide drainage or air flow channels as well. In lieu of mortar, construction adhesives having a shear bond strength greater than 50 psi (345 kPa) are used to bond the masonry veneer to the panel and masonry pointing mortar is used to fill the joint space between the masonry units. Installation using these products should follow manufacturer’s instructions.
Masonry units used in this application are limited to 2 ⅝ in. (67 mm) thickness, 36 in. (914 mm) in any face dimension, 5 ft2 (0.46 m2) in total face area and 15 lb/ft2 (73 kg/m2 ) weight. In addition, the International Building Code (ref. 4) stipulates: a minimum thickness of 0.25 in. (6.3 mm) for weather-exposed adhered masonry veneer; and, for adhered masonry veneers 2 used on interior walls, a maximum weight of 20 lb/ft2 (97 kg/ m2).
When an interior adhered veneer is supported by wood construction, the wood supporting member must be designed for a maximum deflection of 1/600 of its span.
Adhered veneer and its backing must also be designed to either:
have sufficient bond to withstand a shearing stress of 50 psi (345 kPa) based on the gross unit surface area when tested in accordance with ASTM C482, Standard Test Method for Bond Strength of Ceramic Tile to Portland Cement Paste (ref. 10), or
be installed according to the following:
A paste of neat portland cement is brushed on the backing and on the back of the veneer unit immediately prior to applying the mortar coat. This neat cement coating provides a good bonding surface for the mortar.
Type S mortar is then applied to the backing and to each veneer unit in a layer slightly thicker than ⅜ in. (9.5 mm). Sufficient mortar should be used so that a slight excess is forced out the edges of the units.
The units are then tapped into place to eliminate voids between the units and the backing which could reduce bond. The resulting thickness of mortar between the backing and veneer must be between ⅜ and ¼ in. (9.5 and 32 mm).
Mortar joints are tooled with a round jointer when the mortar is thumbprint hard.
When applied to exterior stud walls, the IBC requires adhered masonry veneer to include a screed or flashing at the foundation. In addition, minimum clearances must be maintained between the bottom of the adhered veneer and paved areas, adjacent walking surfaces and the earth.
Backing materials for adhered veneer must be continuous and moisture-resistant (wood or steel frame backing with adhered veneer must be backed with a solid water repellent sheathing). Backing may be masonry, concrete, metal lath and portland cement plaster applied to masonry, concrete, steel framing or wood framing. Note that care must be taken to limit deflection of the backing, which can cause veneer cracking or loss of adhesion, when adhered masonry veneer is used on steel frame or wood frame backing. The surface of the backing material must be capable of securing and supporting the imposed loads of the veneer. Materials that affect bond, such as dirt, grease, oil, or paint (except portland cement paint) need to be cleaned off the backing surface prior to adhering the veneer.
NOTATIONS:
l = clear span between supports, in. (mm)
qz = velocity pressure evaluated at height z above ground, in.-lb/ft2 (kPa)
