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Fasteners for Concrete Masonry

  • August 2018

INTRODUCTION

Buildings use a variety of connectors including anchors, wall ties and fasteners. The distinction between the these types of connectors can be confusing. The broad term “connector” is defined as “a mechanical device for securing two or more pieces, parts, or members together, including anchors, wall ties, and fasteners” (refs. 1, 2). While the terms are often used interchangeably even in technical literature and codes, anchors, wall ties and fasteners each have different purposes. Typical industry usage is:

  • anchors secure masonry to its support. Examples are an anchor bolt or a column flange strap anchor used to connect a masonry wythe to a steel column.
  • Ties, such as adjustable wire ties, are used to connect wythes of masonry in a multiwythe wall.
  • Fasteners connect nonmasonry materials or objects to masonry. An example is a toggle bolt used to install a shelf.

This TEK discusses the use of fasteners in concrete masonry assemblies. TEK 12-01B, Anchors and Ties for Masonry (ref. 3) presents information on anchors and wall ties.

TYPES OF FASTENERS

Many fastener types are available. Fasteners for masonry are typically designed to be inset into a mortar joint, penetrate the face shell of a unit into its hollow core, or bore into a solid unit or solidly grouted wall.

Mortared-In Fasteners

Mortared-in refers to bolts not used for structural purposes, threaded rods and other fasteners that are placed in the masonry mortar joints while the wall is being constructed. This eliminates the need to drill or nail into the masonry, but placement must be exact, as these fasteners cannot be moved or adjusted after placement. Although most fasteners are post-applied rather than mortared in, nailer blocks of pressure-treated wood or metal can be installed during wall construction.

Post-Applied Fasteners

Post-applied fasteners fall into three broad categories: hand-driven mechanical or expansion fasteners, power-actuated fastening systems and chemical/adhesive fasteners.

Hand-Driven Mechanical or Expansion Fasteners

Probably the most familiar fasteners are the hand-driven, mechanical or expansion varieties. These fasteners are offered in several types of metal and, in some cases, plastic.

There are many fastener manufacturers and a large array of mechanical and expansion fastener types (see Figure 1). Some of the most common include:

Self-tapping screws (Figure 1a) that cut threads into the concrete masonry unit or mortar joint through a predrilled hole. Most manufacturers produce these in assorted small diameters and in several lengths.

Toggle fasteners (Figure 1b) frequently called toggle bolts come in several configurations but the most common consists of a threaded bolt and a spring-loaded toggle. Once inserted through a predrilled hole into the core of a hollow concrete masonry unit, the toggle expands and bears against the masonry, holding the bolt in place.

Sleeve fasteners (Figure 1c) consist of a threaded stud with a flared cone-shaped end and an expander sleeve assembled over the stud. A washer and nut are then attached to the end of the stud. After insertion, the nut is tightened, drawing the cone-shaped end into the expander sleeve forcing it to expand and bear against the masonry.

Wedge fasteners (Figure 1d) use a nut, washer and a tapered steel stud bolt. This is surrounded by a steel clip or wedges. As the nut is tightened, the stud is drawn up into the clip or wedge, lodging them against the side of the masonry.

Drop-in fasteners (Figure 1e) typically use steel expansion shells and internal plugs which are forced into the shells, causing them to expand against the substrate.

Strike, hit or split-drive fasteners (Figure 1f) rely on a driving or hammering force on a pin, stud or nail to cause the fastener to expand against the concrete masonry unit.

Figure 1—Typical Hand-Driven Mechanical or Expansion Fasteners
Power-Actuated Fastening Systems

These systems use means such as explosive powder, gas combustion, compressed air or other gas or fuel to embed fasteners into concrete masonry. Of these, powder-actuated systems are most common. Powder-actuated systems use explosive powder to embed the fastener using pressure similar to that of a bullet being fired. The charges used can be more powerful than those in hand guns, so training in the proper use of the tools is critical and in many jurisdictions certification is required. These fastener systems must be fully embedded in masonry (i.e., they cannot extend into hollow areas), so manufacturers recommend that when not used in solid or solid grouted masonry, the concrete masonry face shell thickness be at least 1 ¼ in. (32 mm) thick to accommodate the length of the fastener and withstand the force of the fastener insertion.

When a powder-actuated fastener is driven into concrete masonry, the material around the fastener shank is displaced. This causes the displaced material to compress against the fastener, creating a friction hold. The heat generated during the firing process also causes a sintering, or welding, of the concrete masonry to the fastener (see Figure 2).

There are several types of powder-actuated tools: some shoot the fastener down a barrel while others use pistons to drive the fastener into the wall. The tools are divided into classes according to the velocity of the fastener. The charges also come in a range of power levels.

The fasteners for powder-actuated tools are special heat- treated steel, resulting in a very hard yet ductile fastener, which can penetrate concrete masonry without breaking. The fastener may be threaded or smooth and has a guide to align it in the tool as it is being driven. Fasteners may be packaged in multi-cartridge magazines for rapid repetitive fastening.

Figure 2—Friction Forces in Power-Actuated Fasteners
Chemical/Adhesive Fasteners

These fastener systems consist of smooth or deformed steel bars or rods placed in a predrilled hole and set with chemical bonding compounds such as epoxies, polyesters, vinylesters or cementitious material (see Figure 3). Loads are transferred from the fastener through the bonding compound to the masonry. Surface-mounted adhesive fasteners are available and are typically used for light-duty conditions such as attaching mirrors and frames to a finished masonry surface. Adhesive fasteners can have some advantages over mechanical expansion fasteners, such as the potential for superior strength, especially pull-out. Adhesive systems may also be more resistant to vibration than mechanical expansion anchors, and the adhesive encapsulates the steel fastener providing additional corrosion protection. Closer edge distances may also be possible with adhesive systems.

Figure 3—Adhesive Anchor Systems

DESIGN CONSIDERATIONS AND SELECTION CRITERIA

Because of the variety of fasteners and their applications, fastener design is not addressed in detail in building codes.

Structural Considerations

Structural considerations for fasteners are similar to those for anchors, but the loads on fasteners are typically less. Fastener tension and shear capacities should be considered when selecting a fastener.

Tension is typically transferred from the fastener to the masonry by friction (as for the screw or hit fasteners), keying effects (toggle bolts or expansion systems), bonding (adhesive and chemical systems), or a combination of these mechanisms. Shear is primarily resisted by the fastener itself. As such, shear strength depends on the fastener material and its cross section.

Failure modes for fasteners are also similar to those for anchors and depend on the type of fastener, type of concrete masonry unit, concrete masonry unit compressive strength, depth of embedment, loading conditions, edge distance and fastener load/spacing between fasteners. Typical tension failure modes are fastener breakage, concrete masonry unit cone failure, concrete masonry unit splitting, edge breakouts, pull-out and, in the case of adhesive or chemical fasteners, bond failure. Shear failures include fastener breakage and back pry-out (especially with a group of fasteners or those attached into hollow CMU through the face shell) and edge breakout.

Because fasteners are in most cases proprietary products, it is important to consult the specific manufacturer’s technical data for the fastener being used. Values for pull-out, shear capacity, edge distance and embedment length criteria are given, as well as acceptable substrates and the minimum required concrete masonry unit face shell thickness.

Other Selection Criteria

In addition to the structural requirements, some other basic considerations when selecting a fastener include:

  • the size, especially weight, and configuration of the item being connected to the masonry,
  • whether the fastener will be subject to significant vibration,
  • whether the fastener will be installed in solid or hollow concrete masonry at the attachment point,
  • the minimum edge distance to keep the concrete masonry unit from splitting or spalling,
  • the fastener exposure conditions,
  • whether there is a need for repetitive fastener installation, in which case power-actuated systems offer an advantage,
  • installer qualifications to place adhesive systems or to use powder-actuated fastener tools,
  • restricted access to work areas,
  • power or lighting availability,
  • moisture content of masonry,
  • local availability of fasteners and fastener tools, and
  • other project-specific requirements or conditions.

Codes and Standards

Codes (refs. 1, 2) require that connectors be capable of resisting applied loads and that all pertinent information be included in the project documents. Manufacturer’s literature should be consulted for data pertinent to the fastener and its application. A partial list of national test methods and standards applicable to fasteners includes references 4 through 8.

Corrosion Protection

Specification for Masonry Structures (ref. 9) requires that all metal accessories be stored off the ground and protected from permanent distortions. Since most fasteners include some type of metal, corrosion protection is important. Stainless steel fasteners should conform to ASTM A480, A240 or A580 (refs. 10, 11, 12), as a minimum.

The most common form of corrosion protection for carbon steel fasteners is zinc coating or galvanizing which can be applied in several methods to achieve different coating thicknesses. Table 1 lists minimum corrosion protection requirements (ref. 9).

Table 1—Corrosion Protection Requirements for Connectors

Galvanic Action

Because fasteners connect nonmasonry items to masonry, the potential for corrosion from galvanic action between the fastener and the item being connected to the masonry must be considered when selecting fasteners.

All metals have electrical potential relative to each other. When metals with different potentials come into contact while in the presence of moisture, the more “active” metal—the one with the more negative potential—corrodes and the other metal is galvanically protected. Table 2 presents the ranking of metals based on their electrical potential from anodic (least noble) to cathodic (most noble). The farther apart two metals are in the table, the more severe and faster the galvanic attack. The relative surface areas of the connecting metals also affect the severity of the galvanic action.

To limit galvanic corrosion, use metals that are close in the galvanic series (Table 2). If this is not possible, separate the dissimilar metals with coatings, gaskets, plastic washers, etc. The fastener should also be selected so that it is the most noble, or protected, component. Drainage is also important to ensure the fastener is not subjected to a continually moist or wet condition.

Table 2—Galvanic Series of Metals and Alloys

INSTALLATION

Given the number of fastening options, no one installation method fits all. It is therefore important to follow the specific fastener manufacture’s installation procedures. Some general guidelines include:

  • Place fasteners with proper edge distance and spacing to prevent cracking and spalling of the concrete masonry.
  • Drill holes for insertion anchors the exact diameter specified and to the specified embedment depth.
  • Remove dust from predrilled holes, especially for chemical or adhesive fasteners.
  • For adhesive fasteners, dispense the entire cartridge of adhesive at one time with no interruption in flow.
  • With power-actuated fasteners, use test fastenings to determine the lowest power level that will insert the fastener to the proper depth and position without damaging the concrete masonry.
  • Hold power-actuated tools perpendicular to the masonry surface when firing to avoid ricocheting fasteners.
  • Never fire powder-actuated fasteners into masonry head joints.
  • Store powder loads in separate locked containers away from heat sources. Store the tool unloaded in a locked case.
  • Verify any required installer certification for operation of powder-actuated tools. Sources of information on installation methods include references 17 and 18.
  • Follow all recommended safety procedures.

REFERENCES

  1. International Building Code 2003. International Code Council, 2003.
  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. Anchors and Ties for Masonry, TEK 12-01B. Concrete Masonry & Hardscapes Association, 2011.
  4. Acceptance Criteria for Fasteners Power-Driven into Concrete, Steel and Masonry Elements, ICC Engineering Services Report AC 70 – October 2004. International Code Council Engineering Services Evaluation Committee, Whittier, CA, 2004.
  5. Standard Test Method for Strength of Anchors in Concrete and Masonry Elements, ASTM E488-96 (2003). ASTM International, 2003.
  6. Standard Test Method for Pullout Resistance of Ties and Anchors Embedded in Masonry Mortar Joints, ASTM E754-80 (2000)e1. ASTM International, 2000.
  7. Standard Test Methods for Strength of Power-Actuated Fasteners Installed in Structural Members, ASTM E1190-95 (2000)e1. ASTM International, 2000.
  8. Standard Test Methods for Testing Bond Performance of Bonded Anchors, ASTM E1512-01. ASTM International, 2001.
  9. Specification for Masonry Structures, ACI 530.1-05/ASCE 6-05/TMS 602-05. Reported by the Masonry Standards Joint Committee, 2005.
  10. Standard Specification for General Requirements for Flat-Rolled Stainless and Heat- Resisting Steel Plate, Sheet, and Strip. A480/A480M-05. ASTM International, 2005.
  11. Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications. A240/A240M- 05a. ASTM International, 2005.
  12. Standard Specification for Stainless Steel Wire. A580/A580-98(2004). ASTM International, 2004.
  13. Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy- Coated (Galvannealed) by the Hot-Dip Process, ASTM A653/A653M-05. ASTM International, 2005.
  14. Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, ASTM A153/A153-05. ASTM International, 2005.
  15. Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, ASTM A123/A123M-02. ASTM International, 2002.
  16. Standard Specification for Steel Wire, Epoxy-Coated, ASTM A899-91(2002). ASTM International, 2002.
  17. PATMI Basic Training Manual, Powder Actuated Tool Manufacturers’ Institute, 2005.
  18. Using Powder Activated (Ammunition) Tools – Study Materials for the Certificate of Fitness Exam for E-21. New York City Fire Department, 2001.

 

Steel Reinforcement for Concrete Masonry

  • August 2018

INTRODUCTION

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).

REFERENCES

  1. Post-Tensioned Concrete Masonry Wall Construction, TEK 03-14. Concrete Masonry & Hardscapes Association, 2002.
  2. International Building Code 2003. International Code Council, 2003.
  3. International Building Code 2006. International Code Council, 2006.
  4. Building Code Requirements for Masonry Structures, ACI 530-02/ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee, 2002.
  5. Specification for Masonry Structures, ACI 530.1-02/ASCE 6-02/TMS 602-02. Reported by the Masonry Standards Joint Committee, 2002.
  6. Standard Specification for Deformed and Plain Billet-Steel Bars for Concrete Reinforcement, ASTM A615/A615M-00. ASTM International, Inc., 2000.
  7. Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement, ASTM A706/A706M- 01. ASTM International, Inc., 2001.
  8. Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement, A767/A767M-00b. ASTM International, Inc., 2000.
  9. Standard Specification for Epoxy-Coated Steel Reinforcing Bars, A775/A775M-01. ASTM International, Inc., 2001.
  10. Standard Specification for Rail-Steel and Axle-Steel Deformed Bars for Concrete Reinforcement, A996/A996M-00. ASTM International, Inc., 2000.
  11. Standard Specification for Masonry Joint Reinforcement, ASTM A951-00. ASTM International, Inc., 2000.
  12. Standard Specification for Stainless and Heat-Resisting Steel Wire, ASTM A580-98. ASTM International, Inc., 1998.
  13. Standard Specification for Steel Wire, Deformed, for Concrete Reinforcement, A496/A496M-01. ASTM International, Inc., 2001.
  14. Manual of Standard Practice, MSP 1-01. Concrete Reinforcing Steel Institute, 2001.
  15. Standard Specification for Steel Wire, Plain, for Concrete Reinforcement, ASTM A82-01. ASTM International, Inc., 2001.
  16. Standard Specification for Zinc-Coated (Galvanized) Carbon Steel Wire, ASTM A641-98. ASTM International, Inc., 1998.
  17. Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, ASTM A153-01a. ASTM International, Inc., 2001.
  18. Standard Specification for Epoxy-Coated Steel Wire and Welded Wire Fabric for Reinforcement, ASTM A884/A884M-99. ASTM International, Inc., 1999.
  19. 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.

Anchors and Ties for Masonry

  • August 2018

INTRODUCTION

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:

  1. 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.
  2. 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.
  3. 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)
  1. 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)
  1. 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

  1. Fasteners for Concrete Masonry, TEK 12-05. Concrete Masonry & Hardscapes Association, 2005.
  2. International Building Code. International Code Council, 2012.
  3. Building Code Requirements for Masonry Structures, TMS 402-11/ACI 530-11/ASCE 5-11. Reported by the Masonry Standards Joint Committee, 2011.
  4. Standard Specification for Carbon Structural Steel, A36-ASTM International, 2008.
  5. Standard Specification for Steel, Sheet, Cold-Rolled, Carbon, Structural, High-Strength Low-Alloy with Improved Formability, A1008-11. ASTM International, 2011.
  6. Standard Specification for Steel Wire, Plain for Concrete Reinforcement, A82-07. ASTM International, 2007.
  7. Standard Specification for Steel Welded Wire Reinforcement, Plain, for Concrete, A185-07. ASTM International, 2007.
  8. Specification for Masonry Structures, TMS 602 -11/ACI 530.1-11/ASCE 6-11. Reported by the Masonry Standards Joint Committee, 2011.
  9. Standard Specification for Stainless Steel Wire, ASTM A580-08. ASTM International, 2008.
  10. Standard Specification for General Requirements for Flat Rolled Stainless and Heat-Resisting Steel Plate, Sheet, and Strip, ASTM A480-11a. ASTM International, 2011.
  11. Standard Specification for Annealed or Cold-Worked Austenitic Stainless Steel, Sheet, Strip, Plate and Flat Bar, ASTM A666-10. ASTM International, 2010.
  12. 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.
  13. Standard Specification for Zinc-Coated (Galvanized) Carbon Steel Wire, ASTM A641-09a. ASTM International, 2009.
  14. Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, ASTM A153-09. ASTM International, 2009.
  15. 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.
  16. Standard Specification for Zinc (Hot-Dip Galvanized) Coating on Iron and Steel Products, ASTM A123-09. ASTM International, 2009.
  17. Standard Specification for Epoxy-Coated Steel Wire and Welded Wire Fabric for Reinforcement, ASTM A884-06. ASTM International, 2006.
  18. Standard Specification for Steel Wire Epoxy Coated, ASTM A899-91(2007). ASTM International, 2007.
  19. Multiwythe Concrete Masonry Walls, TEK 16-01A, Concrete Masonry & Hardscapes Association, 2005.
  20. Structural Design of Unreinforced Composite Masonry, TEK 16-02B, Concrete Masonry & Hardscapes Association, 2002.
  21. Design of Concrete Masonry Noncomposite (Cavity) Walls, TEK 16-04A, Concrete Masonry & Hardscapes Association, 2004.
  22. Concrete Masonry Veneer Details, TEK 05-01B, Concrete Masonry & Hardscapes Association, 2003.
  23. Concrete Masonry Veneers, TEK 03-06C, Concrete Masonry & Hardscapes Association, 2012.
  24. Empirical Design of Concrete Masonry Walls, TEK 14-08B, Concrete Masonry & Hardscapes Association, 2008.
  25. Joint Reinforcement for Concrete Masonry, TEK 12-02B, Concrete Masonry & Hardscapes Association, 2005.
  26. Porter, Max L., Lehr, Bradley R., Barnes, Bruce A., Attachments for Masonry Structures, Engineering Research Institute, Iowa State University, February 1992.
  27. Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.