Home / Technical Resources / cavity wall

Resources

Design of Concrete Masonry Noncomposite (Cavity) Walls

  • August 2018

INTRODUCTION

When selecting a building enclosure, concrete masonry cavity walls are considered to be one of the best solutions available for all types of buildings. From both an initial cost and life-cycle cost perspective, cavity wall construction is highly regarded as the prime choice in many applications.

Cavity walls typically consist of an inner wythe of concrete masonry units that are tied to an exterior wythe of architectural masonry units. The cavity space between the wythes is normally 2 to 4 ½ in. (51 to 114 mm) wide, easily accommodating rigid board insulation. The two wythes together provide a wall that is highly resistant to wind driven rain, absorbs and reflects sound, provides good thermal performance, and has excellent fire resistance characteristics.

Masonry walls constructed of two or more wythes can technically be classified in one of three ways, depending on how the wythes are designed and detailed. These wall types include composite, noncomposite and veneer assemblies. In noncomposite construction, covered in this TEK, each wythe 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. Composite walls are designed so that the wythes act together as a single element to resist structural loads. This requires the masonry wythes to be connected by masonry headers or by a mortar- or grout-filled collar joint and wall ties (see ref. 4). In a veneer wall, the backup wythe is designed as the loadbearing system while the veneer provides a nonloadbearing architectural wall finish that transfers loads to the backup wythe through wall ties (see refs. 5, 6). Although Building Code Requirements for Masonry Structures (ref. 1) 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.

This TEK illustrates the design of noncomposite concrete masonry walls based on Building Code Requirements for Masonry Structures (ref. 1), referred to here as the MSJC code. Each wythe of a noncomposite wall system can be designed to accommodate all types of loads, including gravity loads from roofs, walls and floors, as well as lateral loads from wind or earthquakes. The MSJC code design provisions are used to size these masonry walls.

STRUCTURAL DESIGN

The MSJC code includes noncomposite design provisions for both allowable stress design (Chapter 2) and empirical design (Chapter 5). The assumptions and relevant governing equations for each of these design approaches is given in references 2 and 3 respectively.

Concrete masonry cavity walls can be designed as either reinforced or unreinforced walls. For unreinforced design, flexural tensile stresses in masonry are resisted by bond developed between the masonry units and mortar; axial tension is not permitted (ref. 1). If direct axial tension is encountered in a design, reinforcement must be used. In reinforced masonry design, all tension is assumed to be resisted by reinforcement.

Empirical Design

Empirical design can be an expedient approach for typical loadbearing structures subjected to nominal wind loads (basic wind speed ≤ 110 mph, (177 km/h) (MSJC 5.1.2.2) and located in areas of low seismic risk, as it cannot be used for the design of seismic force resisting systems in SDC (Seismic Design Category) B or higher (MSJC 5.1.2.1). Empirical design utilizes prescriptive provisions, outlining criteria such as wall height to thickness ratios, minimum wall thickness and maximum building height.

References 1 and 3 contain maximum length-to-thickness or height-to-thickness ratios for empirically designed walls. When using these ratios for noncomposite multiwythe walls, the total wall thickness is taken as the sum of the nominal thicknesses of each wythe, neglecting the presence of any cavity thickness. Compressive stress is based on the gross cross-sectional area of all wythes, including hollow cells but not including the cavity between the wythes. When floor or roof loads are carried on only one wythe, only the gross cross-sectional area of that wythe is used to check the axial capacity. In addition, these walls must meet the following requirements for wall ties connecting the wythes:

  • wall ties of wire size W2.8 (3/16 in., MW 18), or metal wire of equivalent stiffness, spaced at a maximum of 24 in. (610 mm) o.c. vertically and 36 in. (914 mm) o.c. horizontally, with at least one wall tie for each 4½ ft² (0.42 m²) of wall area,
  • walls constructed with hollow units must use rectangular ties,
  • walls constructed with solid units must use Z-shaped ties with hooks at least 2 in. (51 mm) long,
  • wall ties may not have drips,
  • additional ties are required within 12 in. (305 mm) of all openings and must be spaced no more than 3 ft (914 mm) apart around the perimeter of the opening.

Requirements for bonding with joint reinforcement are the same as those for wall ties with the following exceptions: cross wire size may not be smaller than W1.7 (9 gage, MW 11) and the supported wall area per cross wire may not exceed 2⅔ ft² (0.25 m²). In addition, the longitudinal wires must be embedded in mortar.

Allowable Stress Design

Similar to empirical design, MSJC allowable stress design includes prescriptive requirements for bonding wythes of noncomposite walls via wall ties, adjustable ties and joint reinforcement.

For rectangular ties, Z ties (for use with other than hollow units) and ladder or tab-type joint reinforcement, ties or cross wires of joint reinforcement, ties must be placed with a maximum spacing of 36 in. (914 mm) horizontally and 24 in. (610 mm) vertically. The minimum number of ties is one per:

  • 2⅔ ft² (0.25 m²) of wall for wire size W 1.7 (9 gage, MW 11), and
  • 4½ ft² (0.42 m²) of wall for wire size W 2.8 (3/16 in., MW 18).

For adjustable ties, one tie must be provided for each 1.77 ft² (0.16 m²) of wall; maximum horizontal and vertical spacing is 16 in. (406 mm); misalignment of bed joints from one wythe to the other may not exceed 1 ¼ in. (31.8 mm); the maximum clearance between connecting parts of the tie is 1/16 in. (1.6 mm); and pintle ties must have at least two pintle legs of wire size W2.8 (3/16 in., MW 18) (see also Figure 1).

For noncomposite masonry walls, the following additional requirements apply.

  • Collar joints are not to contain headers, or be filled with mortar or grout.
  • Gravity loads from supported horizontal members are to be resisted by the wythe nearest the center of the span.
  • Bending moments about the weak axis of the wall and transverse loads are distributed to each wythe according to relative stiffness. This can be determined by:
    Wi = WT [EmIi/(EmIi+ EmI0)]
    Wo = WT [EmI0/(EmIi+ EmI0)]
  • Loads acting parallel to the wall are resisted by the wythe to which they are applied.
  • The cavity width between the wythes is limited to 4½ in. (114 mm) unless a detailed wall tie analysis is performed.
Figure 1 – Allowable Stress Design Noncomposite Adjustable Wall Tie Requirements

DESIGN EXAMPLES

The following examples illustrate the use of noncomposite masonry employing empirical and allowable stress design methods. Although there are no specific provisions in MSJC for noncomposite wall design using strength design, strength design could be used provided the same load distribution principles as presented for allowable stress design are employed.

Empirical Design Design Example:
Design the top story of a two-story noncomposite double wythe masonry wall system supported on continuous footings. Note that the design of the lower story, though not shown, is performed in the same manner, except that the floor live and dead loads from the upper story are also accounted for.

Given:

unsupported wall height= 10 ft (3.01 m)
superimposed gravity dead load= 220 plf (3.2 kN/m)
superimposed gravity live load= 460 plf (6.7 kN/m)
net superimposed uplift from wind= 120 plf (1.8 kN/m)
wind pressure= 24 psf (1,149 Pa)
eccentricity of all gravity loads= 0
f’m= 1,500 psi (10.3 MPa)
Em= 1,350 ksi (9,308 MPa)

 

Wall lateral support requirement: l/t or h/t < 18, so minimum required wall thickness = h/18
= 10 ft (12 in./ft)/18
= 6.7 in. (169 mm)

Try a 4-in. (102 mm) outer wythe and 6-in. (152 mm) inner wythe (providing a total nominal wall thickness of 10 in. (254 mm)), and check allowable axial compressive stress due to dead and live loads (gravity loads are carried by the inner wythe only):

dead:roof220 lb/ft
 wythe = 10 ft x 26 psf (ref. 8)260 lb/ft
live:roof460 lb/ft
total load: 940 lb/ft (13.7 kN/m)

 

Gross area of 6-in. (152-mm) wythe = 67.5 in.²/ft (ref. 7)
fa = 940 lb/ft/(67.5 in.²/ft) = 13.9 psi (0.096 MPa)
Fa = 75 psi (0.52 MPa) for Type M or S mortar, 70 psi (0.48 MPa) for Type N mortar (ref. 1)
fa < Fa (OK for all mortar types)

Per MSJC code section 5.8.3.1, the net uplift on the roof must be resisted by an anchorage system. Use a bond beam at the top of the inner wythe with vertical reinforcement to the foundation to provide this resistance.

ASD Reinforced Design Example:
Given:

unsupported wall height= 18 ft (5.5 m)
wind load, w= 36 psf (1,724 Pa)
net roof uplift at top of wall= 400 plf (5.8 kN/m) )
eccentricity of all vertical loads= 0
f’m= 1,500 psi (0.0718 MPa )
unit density= 115 pcf (1,842 kg/m³)
Grade 60 reinforcement 

Note: The 36 psf (1,724 Pa) wind load is much higher than is generally applicable when using empirical design.

Design the inside wythe first, as it must resist the uplift in addition to the flexural loads. Try two 6-in. (152 mm) wythes with No. 5 (M #16) reinforcement at 32 in. (813 mm) o.c.

Determine reinforcement needed for uplift at midheight:
uplift = 400 lb/ft – 34 lb/ft² (18 ft/2) = 94 lb/ft (1.37 kN/m) (ref. 8)
reinforcement needed = [(94 lb/ft)(32 in.)/(12 in./ft)]/[1.333(24,000 psi)] = 0.0078 in.²
As available for flexure = 0.31 – 0.0078 = 0.3022 in.²
Ms = FsAsjd = 1.333 (24,000 psi) (0.3022 in.²)(0.894)(2.813 in.)
= 24,313 lb-in. for 32 in. width
= 9,117 lb-in./ft (3,378 N⋅m/m) > 8,996 lb-in./ft (3,333 N⋅m/m), therefore Mm controls

Determine applied moment:
Since the wythes are identical, each would carry ½ the lateral load or ½ (36 psf) = 18 psf (124 kPa)
Mmax = wl²/8 = (18 psf)(18 ft)²(12 in./ft)/8
= 8,748 lb-in./ft (3,241 N⋅m/m) < 8,996 lb-in./ft (3,333 N⋅m/m) OK

Check shear:
Vmax = wl/2 = (18psf)(18 ft)/2 = 162 lb/ft (2.36 kN/m)
fv = Vmax/bd = 162 lb/ft/(12 in.)(2.813 in.) = 4.80 psi (33 kPa)
Fv = 37 x 1.333 = 51 psi (351 kPa)
4.80 psi (33 kPa) < 51 psi (351 kPa) OK

A quick check of the outside wythe shows that the same reinforcement schedule will work for it as well. Therefore, use two 6-in. (152-mm) wythes with No. 5 (M #16) vertical reinforcement at 32 in. (813 mm) o.c.

This wall could be designed using an unreinforced 4-in. (102 mm) outside wythe and a reinforced 8-in. (203-mm) inside wythe, with lateral loads distributed to each wythe according to the uncracked stiffness per MSJC section 1.9.2. Experience has shown, however, that the design would be severely limited by the capacity of the unreinforced outside wythe. Additionally, such a design could be used only in SDC A or B since 4-in. (102 mm) concrete masonry does not have cores large enough to reinforce.

Another alternative would be to design this system treating the 4 in. (102 mm) outer wythe as a nonloadbearing veneer. Designing this wall as a 4-in. (102 mm) veneer with an 8-in. (203 mm) reinforced structural backup wythe would result in No. 5 bars at 16 in. (M #16 at 406 mm) on center. This is the same amount of reinforcement used in the example above (two 6-in. (152 mm) wythes with No. 5 (M #16) at 32 in. (813 mm) on center). However, because the 6-in. (152 mm) units have smaller cores, 30% less grout is required.

The design using two 6-in. (152-mm) reinforced wythes has the following advantages over veneer with structural backup:

  • no limitation on SDC as when a veneer or an unreinforced outer wythe is used,
  • no limitation on wind speed as with a veneer,
  • equal mass on both sides of the wall permitting the use of the prescriptive energy tables for integral insulation, and
  • the flexibility of using units with different architectural finishes on each side.

NOMENCLATURE

As          = effective cross-sectional area of reinforcement, in.²(mm²)
b            = width of section, in. (mm)
d            = distance from extreme compression fiber to centroid of tension reinforcement, in. (mm)
Em         = modulus of elasticity of masonry, psi (MPa)
Es          = modulus of elasticity of steel, psi (MPa)
Fa          = allowable compressive stress due to axial load only, psi (kPa)
Fb          = allowable compressive stress due to flexure only, psi (kPa)
Fs          = allowable tensile or compressive stress in reinforcement, psi (kPa)
Fv          = allowable shear stress in masonry, psi (MPa)
fa           = calculated compressive stress in masonry due to axial load only, psi (kPa)
f’m         = specified compressive strength of masonry, psi (kPa)
h            = effective height, in. (mm)
fv           = calculated shear stress in masonry, psi (MPa)
Ii            = average moment of inertia of inner wythe, in.4/ft (m4/m)
Io           = average moment of inertia of outer wythe, in.4/ft (m4/m)
j             = ratio of distance between centroid of flexural compressive forces and centroid of tensile forces to depth d
k           = ratio of distance between compression face of wall and neutral axis to depth d
l            = clear span between supports, in. (mm)
M          = moment at the section under consideration, in.-lb/ft (N⋅m/m)
Mm       = flexural capacity (resisting moment) when masonry controls, in.-lb/ft (N⋅m/m)
Mmax   = maximum moment at the section under consideration, in.-lb/ft (N⋅m/m)
Ms        = flexural capacity (resisting moment) when reinforcement controls, in.-lb/ft (N⋅m/m)
t            = nominal thickness of a member, in. (mm)
Vmax    = maximum shear at the section under consideration, lb/ft (kN/m)
Wi        = percentage of transverse load on inner wythe
Wo       = percentage of transverse load on outer wythe
WT       = total transverse load
w         = wind pressure, psf (Pa)
ρ          = reinforcement ratio

REFERENCES

  1. Building Code Requirements for Masonry Structures, ACI 530-02/ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee, 2002.
  2. ASD of Concrete Masonry (2012 IBC & 2011 MSJC), TEK 14-07C, Concrete Masonry & Hardscapes Association, 2004.
  3. Empirical Design of Concrete Masonry Walls, TEK 1408B, Concrete Masonry & Hardscapes Association, 2003
  4. Structural Design of Unreinforced Composite Masonry, TEK 16-02B, Concrete Masonry & Hardscapes Association, 2001.
  5. Concrete Masonry Veneers, TEK 03-06C, Concrete Masonry & Hardscapes Association, 2012.
  6. Reinforced Composite Concrete Masonry Walls, TEK 1603B, Concrete Masonry & Hardscapes Association, 2006.
  7. Weights and Section Properties of Concrete Masonry Assemblies, CMU-TEC-002-23, Concrete Masonry & Hardscapes Association, 2023.

 

Multi-Wythe Concrete Masonry Walls

  • August 2018

INTRODUCTION

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:

concrete masonry:
Em = 900 f’m = 900(1,500 psi)
= 1,350,000 psi (9,310 MPa)

grout:
Eg = 500fg = 500(2,000 psi)
= 1,000,000 psi (6,890 MPa)

steel:
Es = 29,000,000 psi (200 GPa)

The modular ratio, n, for grout and steel are:

ng = Eg/Em = 1,000,000/1,350,000 = 0.74
ns = Es/Em = 29,000,000/1,350,000 = 21.5

Using these modular ratios, equivalent areas of grout and steel based on a 12-in. (305-mm) width of concrete masonry are:

ng Ag = 0.74 (2 in. x 12 in.) = 17.8 in.² (11,480 mm²)
ns As = 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:

Atr = 24 + 17.8 + 1.08 + 43.5 in.²/ft
= 86.4 in.²/ft (0.18 m²/m)

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

  1. International Building Code, 2003, With Commentary. International Code Council, Inc., 2004.
  2. Building Code Requirements for Masonry Structures, ACI 530-05/ASCE 5-05/TMS 402-05. Reported by the Masonry Standards Joint Committee, 2005.
  3. Concrete Masonry Veneer Details, TEK 05-1B, Concrete Masonry & Hardscapes Association, 2003.
  4. Concrete Masonry Veneers, TEK 03-06C, Concrete Masonry & Hardscapes Association, 2012.
  5. Structural Design of Unreinforced Composite Masonry, TEK 16-02B, Concrete Masonry & Hardscapes Association, 2001.
  6. ASD of Concrete Masonry (2012 IBC & 2011 MSJC), TEK 14-7C, Concrete Masonry & Hardscapes Association, 2004.
  7. Strength Design Provisions for Concrete Masonry, TEK 14-04B, Concrete Masonry & Hardscapes Association, 2008.
  8. Empirical Design of Concrete Masonry Walls, TEK 1408B, Concrete Masonry & Hardscapes Association, 2008.
  9. Design of Concrete Masonry Noncomposite Walls, TEK 16-04A, Concrete Masonry & Hardscapes Association, 2004.
  10. Structural Design of Unreinforced Composite Masonry TEK 16-02B, Concrete Masonry & Hardscapes Association, 2001.
  11. Anchors and Ties for Masonry, TEK 12-01B, Concrete Masonry & Hardscapes Association, 2011.
  12. Weights and Section Properties of Concrete Masonry Assemblies, CMU-TEC-002-23, Concrete Masonry & Hardscapes Association, 2023.

Joint Reinforcement for Concrete Masonry

  • August 2018

INTRODUCTION

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

  1. Standard Specification for Masonry Joint Reinforcement, ASTM A 951-02. ASTM International, 2002.
  2. Standard Specification for Stainless Steel Wire, ASTM A 580/580M-98(2004). ASTM International, 2004.
  3. Specification for Masonry Structures, ACI 530.1-05/ASCE 6 05/TMS 602-05. Reported by the Masonry Standards Joint Committee, 2005.
  4. Building Code Requirements for Masonry Structures, ACI 530 05/ASCE 5-05/TMS 402-05. Reported by the Masonry Standards Joint Committee, 2005.
  5. Anchors and Ties for Masonry, TEK 12-01B, Concrete Masonry & Hardscapes Association 2011.
  6. Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, 2023.
  7. Steel Reinforcement for Concrete Masonry, TEK 12-04D, Concrete Masonry & Hardscapes Association, 2023.
  8. Seismic Design and Detailing Requirements for Masonry Structures, TEK 14-18B, Concrete Masonry & Hardscapes Association, 2003.
  9. Minimum Design Loads for Buildings and Other Structures, ASCE 7-02, American Society of Civil Engineers, 2002.
  10. Concrete Masonry Veneers, TEK 03-06C, Concrete Masonry & Hardscapes Association, 2012.
  11. International Building Code 2003. International Code Council, 2003.

R-Values of Multi-Wythe Concrete Masonry Walls

  • August 2018

INTRODUCTION

Multi-wythe concrete masonry construction lends itself to placing insulation between two wythes of masonry when the wythes are separated to form a cavity. Placing insulation between two wythes of masonry offers maximum protection for the insulation while allowing a vast amount of the thermal mass to be exposed to the conditioned interior to help moderate temperatures. Masonry cavity walls can easily meet or exceed energy code requirements, because the cavity installation allows a continuous layer of insulation to envelop the masonry. When properly sealed, this continuous insulation layer can also increase energy efficiency by mitigating air infiltration/exfiltration.

Cavity wall construction provides hard, durable surfaces on both sides of the assembly, efficiently utilizing the inherent impact resistance and low maintenance needs of concrete masonry. While these needs are most commonly associated with multifamily dwellings, hospitals, schools and detention centers, the benefits of resistance to damage from hail, shopping and loading carts, gurneys, motorized chairs, and even sports make cavity construction ideal for any application.

This TEK lists thermal resistance (R) values of multi-wythe walls. Single wythe R-values are listed in TEK 06-02C, R-Values and U Factors of Single Wythe Concrete Masonry Walls (ref. 1).

The R-values listed in this TEK were determined by calculation using the code-recognized series-parallel (also called isothermal planes) calculation method (refs. 2, 3, 4). The method accounts for the thermal bridging (energy loss) that occurs through the webs of concrete masonry units. The method is fully described on page 4 of this TEK. Alternate code-approved means of determining R-values of concrete masonry walls include two dimensional calculations and testing (ref. 2).

CAVITY WALLS

The term cavity insulation, which in some codes refers to the insulation between studs in lightweight framing systems, should not be confused with the long established term “masonry cavity wall.” Cavity walls are comprised of at least two wythes of masonry separated by a continuous airspace (cavity).

Under current building code requirements a 1 in. (25-mm) clear airspace between the insulation and the outer wythe is required (2 in. (51 mm) is preferred) to help ensure free water drainage (ref. 5).

Cavity walls are typically designed and detailed using actual out-to out dimensions. Thus, a 14-in. (356-mm) cavity wall with a nominal 4 in. (102-mm) exterior wythe and 8-in. (203-mm) backup wythe has an actual cavity width of 23/4 in. (68 mm), allowing for 11 /2 in. (38 mm) of rigid board insulation.

Typical cavity walls are constructed with a 4, 6, 8, 10 or 12 in. (102, 152, 203, 254 or 305 mm) concrete masonry backup wythe, a 2 to 41 /2 in. (51 to 114 mm) wide cavity, and a 4-in. (102-mm) masonry veneer. By reference to Specification for Masonry Structures (ref. 6), the International Building Code (ref. 7) allows cavity widths up to 41/2 in. (114 mm), beyond which a detailed wall tie analysis must be performed. More detailed information on cavity walls can be found in References 8 through 11.

Changing the interior finish materials of a multi-wythe assembly does not typically change the overall assembly R-value significantly, unless the finish material itself is insulative. For cavity assemblies with interior-side finish materials installed on furring, such as wood paneling, the R-values for 1/2 in. (13 mm) gypsum wallboard on furring in Table 4 can be used as a very close approximation.

CONCRETE MASONRY ENERGY PERFORMANCE

Although this TEK presents concrete masonry assembly R-values, it is important to note that R-values or U-factors alone do not fully describe the thermal performance of a concrete masonry assembly.

Concrete masonry’s thermal performance depends on both its steady state thermal characteristics (described by R-value or U-factor) as well as its thermal mass (heat capacity) characteristics. The steady state and mass performance are influenced by the size, type, and configuration of masonry unit, type and location of insulation, finish materials, density of masonry, climate, and building orientation and exposure conditions.

Thermal mass describes the ability of materials to store energy. Because of its comparatively high density and specific heat, masonry provides very effective thermal storage. Masonry walls retain their temperature long after the heat or air-conditioning has shut off. This , in turn, effectively reduces heating and cooling loads, moderates indoor temperature swings, and shifts heating and cooling loads to off-peak hours.

Due to the significant benefits of concrete masonry’s inherent thermal mass, concrete masonry buildings can provide similar energy performance to more heavily insulated light frame buildings.

These thermal mass effects have been incorporated into energy code requirements as well as sophisticated computer models. Due to the thermal mass, energy codes and standards such as the International Energy Conservation Code (IECC) (ref. 12) and Energy Efficient Standard for Buildings Except Low-Rise Residential Buildings, ASHRAE Standard 90.1 (ref. 2), require less insulation in concrete masonry assemblies than equivalent light-frame systems. Although applicable to all climates, the greater benefits of thermal mass tend to be found in warmer climates (lower-numbered Climate Zones).

Although the thermal mass and inherent R-value/U-factor of concrete masonry may be enough to meet energy code requirements (particularly in warmer climates), concrete masonry assemblies may require additional insulation, particularly when designed under more contemporary building code requirements or to achieve above-code thermal performance. For such conditions, there are many options available for insulating concrete masonry construction.

Although in general higher R-values reduce energy flow through a building element, R-values have a diminishing impact on the overall building envelope energy use. In other words, it’s important not to automatically equate higher R-value with improved energy efficiency. As an example, consider a two-story elementary school in Bowling Green, Kentucky. If this school is built using single wythe concrete masonry walls with cell insulation only and a resulting wall R-value of 7 hr. ft2.oF/Btu (1.23 m2.K/W), an estimate of the building envelope energy use for this structure is approximately 27,800 Btu/ft2 (87.7 kW. h/m2), as shown in Figure 1. If we increase the R-value of the wall to R14 by adding additional insulation while holding the other envelope variables constant, the building envelope energy use drops by only 2.5%, which is not in proportion to doubling the wall R-value. Figure 1 illustrates this trend: as wall R-value increases, it has less and less impact on the building envelope thermal performance.

In this example, a wall R-value larger than about R12 no longer has a significant impact on the envelope energy use. At this point, it makes more sense to invest in energy efficiency measures other than wall insulation. The effect of adding insulation to a multi-wythe wall is virtually the same.

When required, concrete masonry can provide assemblies with R values that exceed code minimums. For overall project economy, however, the industry recommends balancing needs and performance expectations with reasonable insulation levels.

ENERGY CODE COMPLIANCE

Compliance with prescriptive energy code requirements can be demonstrated by:

  • the concrete masonry wall by itself or the concrete masonry wall plus a prescribed R-value of added insulation, or
  • the overall U-factor of the wall.

The IECC prescriptive R-value table calls for “continuous insulation” on concrete masonry and other mass walls. This refers to insulation uninterrupted by furring or by the webs of concrete masonry units. Examples of continuous insulation include rigid insulation adhered to the interior of the wall with furring and drywall applied over the insulation, continuous insulation in the cavity of a masonry cavity wall, and exterior insulation and finish systems. These and other insulation options for concrete masonry assemblies are discussed in TEK 06 11A, Insulating Concrete Masonry Walls (ref. 13).

If the concrete masonry assembly will not include continuous insulation, there are several other options to comply with the IECC requirements—concrete masonry assemblies are not required to have continuous insulation in order to meet the IECC, regardless of climate zone.

Other compliance methods include prescriptive U-factor tables and computer programs which may require U-factors and heat capacity (a property used to indicate the amount of thermal mass) to be input for concrete masonry walls. See TEK 06-04B, Energy Code Compliance Using COMcheck, (ref. 14) for more detailed information. Another compliance method, the energy cost budget method, incorporates sophisticated modeling to estimate a building’s annual energy cost. A more complete discussion of concrete masonry IECC compliance can be found in TEK 06-12E (for the 2012 IECC) (ref. 15).

CONCRETE MASONRY UNIT CONFIGURATIONS

Revisions in 2011 to ASTM C90¸ Standard Specification for Loadbearing Concrete Masonry Units (ref. 16) have significantly reduced the minimum amount of web material required for CMU. Values in this TEK are based on concrete masonry units with three webs, with each web being the full height of the unit, and having a minimum thickness as provided in historical versions of ASTM C90 (see Table 1).

The changes in C90, however, allow a much wider range of web configurations, with corresponding changes in R-values and U-factors (because the webs of a CMU act as thermal bridges, reducing the CMU web area increases the R-value of the corresponding concrete masonry assembly). More discussion on the impact of web configuration and thermal performance can be found in CMU-TEC 001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (Ref. 17).

The Thermal Catalog of Concrete Masonry Assemblies (ref.18) lists R-values and U-factors based on traditional units, as included here, as well as units with smaller web areas, as now allowed by ASTM C90. The additional wall assemblies are based on:

  • CMU having two full-height 3 /4 in. (19 mm) thick webs, and
  • a ‘hybrid’ system of CMU, intended to maximize thermal efficiency. The hybrid system uses the two-web units described above for areas requiring a grouted cell, and a one-web unit where grout confinement is not required.

R-VALUE TABLES-TRADITIONAL THREE-WEB UNITS

Table 2 presents R-values of uninsulated concrete masonry cavity walls with 4, 6, 8, 10 and 12 in. (102, 152, 203, 254 and 305 mm) backup wythes and a 4 in. (102 mm) hollow unit concrete masonry veneer. These R-values should be added to the applicable R-values in Tables 3 and 4 to account for cavity insulation and/or interior furring with insulation, respectively. Table 5 contains the thermal data used to develop the tables.

To convert the R-value to U-factor (as may be needed for code compliance), simply invert the R-value, i.e.: U = 1/R. Note that U factors of various wall components cannot be directly added together. To determine the overall cavity wall U-factor, first add the component R-values together, then determine overall U-factor by inverting the total R-value.

As an example, to determine the R-value of a concrete masonry cavity wall with 8 in. (152 mm) 105 pcf (1,682 kg/m3) backup insulated with 2 in. (51 mm) of extruded polystyrene insulation in the cavity, first determine the R-value of the uninsulated wall from Table 2 (4.22 ft2.hr.oF/Btu, 0.74 m2.K/W), then add the cavity insulation R value from Table 3 (10 ft2.hr.oF/Btu, 1.8 m2.K/W), to obtain the total R-value of 14.2 ft2.hr.oF/Btu (2.5 m2.K/W). The corresponding U factor for this wall is:

U = 1/R = 1/14.2 = 0.070 Btu/ hr.oF/Btu (0.4 W/ m2.K)

Note that tables of precalculated R-values and U-factors, including the various insulation and finish systems, are available in Thermal Catalog of Concrete Masonry Assemblies.

The values in Table 2 are based on an ungrouted backup wythe. However, the addition of grout to a hollow concrete masonry backup wythe does not significantly affect the overall R-value of an insulated cavity wall. For example, the R-value of a cavity wall with 8 in. (203 mm) ungrouted 105 pcf (1,682 kg/m3) backup and insulated cavity decreases only about 5% when the backup wythe is solidly grouted. With a partially-grouted backup, the difference in R-value is smaller than 5%.

Calculations are performed using the series-parallel (also called isothermal planes) calculation method (refs. 2, 3, 4). The method accounts for the thermal bridging that occurs through the webs of concrete masonry units. The method is briefly described below, and its use is demonstrated in Appendix C of Thermal Catalog of Concrete Masonry Assemblies.

REFERENCES

  1. R-Values and U-Factors of Single Wythe Concrete Masonry Walls, TEK 06-02C, Concrete Masonry & Hardscapes Association, 2013.
  2. Energy Standard for Buildings Except Low-Rise Residential
    Buildings, ANSI/ASHRAE/IESNA 90.1-2010. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2010.
  3. ASHRAE Handbook, Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2009.
  4. Guide to Thermal Properties of Concrete and Masonry Systems. ACI 122R-02. American Concrete Institute, 2002.
  5. Building Code Requirements for Masonry Structures, TMS 402/ACI 530/ASCE 5. Reported by the Masonry Standards Joint Committee, 2005, 2008, 2011.
  6. Specification for Masonry Structures, TMS 602/ACI 530.1/ASCE 6. Reported by the Masonry Standards Joint Committee, 2005, 2008, 2011.
  7. International Building Code. International Code Council, 2006, 2009, 2012.
  8. Concrete Masonry Veneers, TEK 03-06C, Concrete Masonry & Hardscapes Association, 2012.
  9. Concrete Masonry Veneer Details, TEK 05-01B, Concrete Masonry & Hardscapes Association, 2003.
  10. Design of Concrete Masonry Noncomposite (Cavity) Walls, TEK 16-04A, Concrete Masonry & Hardscapes Association, 2004.
  11. Flashing Details for Concrete Masonry Walls, TEK 19-05A,
    Concrete Masonry & Hardscapes Association, 2008.
  12. International Energy Conservation Code. International Code Council, 2006, 2009, 2012.
  13. Insulating Concrete Masonry Walls, TEK 06-11A, Concrete
    Masonry & Hardscapes Association, 2010.
  14. Energy Code Compliance Using COMcheck, TEK 06-04B,
    Concrete Masonry & Hardscapes Association, 2012.
  15. Concrete Masonry in the 2012 Edition of the IECC, TEK 06-12E, Concrete Masonry & Hardscapes Association, 2012.
  16. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-11. ASTM International, 2011.
  17. Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, CMU-TEC-001-23, Concrete Masonry &
    Hardscapes Association, 2023.
  18. Thermal Catalog of Concrete Masonry Assemblies, Second Edition, CMU-MAN-004-12, Concrete Masonry & Hardscapes Association, 2012.