Resources

Fire Resistance Ratings of Concrete Masonry Assemblies

September 2020

INTRODUCTION

Concrete masonry is widely specified for fire walls and fire barriers because concrete masonry is noncombustible, provides durable fire resistance, and is economical to construct. Chapter 7 of the International Building Code (IBC) (ref. 2) governs materials and assemblies used for structural fire resistance and fire-rated separation of adjacent spaces. This TEK is based on the provisions of Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, ACI 216.1/TMS 216 (ref. 1) , which outlines a procedure to calculate the fire resistance ratings of concrete masonry assemblies. The 2014 edition of the ACI 216.1/TMS 216 is referenced in the 2015 IBC for concrete and masonry materials. This TEK is based on both prescriptive details and tables as well as the calculated fire resistance procedure, which is practical, versatile and economical. The calculation procedure allows the designer virtually unlimited flexibility to incorporate the excellent fire-resistive properties of concrete masonry into a design. Included are methods for determining the fire resistance rating of concrete masonry walls, columns, lintels, beams, and concrete masonry fire protection for steel columns. Also included are assemblies composed of concrete masonry and other components, including plaster and gypsum wallboard finishes, and multi-wythe masonry components including clay or shale masonry units.

METHODS OF DETERMINING FIRE RESISTANCE RATINGS

Because full-scale fire testing of representative test specimens is not practical in daily practice due to time and financial constraints, the IBC outlines multiple options for fire rating determination:

standardized calculation procedures, such as those in the ACI 216.1/TMS 216 and in Section 722 of the IBC;
prescriptive designs such as those in Section 721 of the IBC;
engineering analysis based on a comparison with tested assemblies;
third party listing services, such as Underwriters Laboratory; and
alternative means approved by the building official per Section 104.11 of the IBC.

Of these, the calculation method is an economical and commonly used method of determining concrete masonry fire resistance ratings. The calculations are based on extensive research, which established relationships between the physical properties of materials and the fire resistance rating. The calculation method is fully described in ACI 216.1/TMS 216 and IBC Section 722, and determines fire resistance ratings based on the equivalent thickness of concrete masonry units and the aggregate types used to manufacture the units. Private commercial listing services allow the designer to select a fire rated assembly that has been previously tested, classified and listed in a published directory of fire rated assemblies. The listing service also monitors materials and production to verify that the concrete masonry units are and remain in compliance with appropriate standards, which usually necessitates a premium for units of this type. The system also is somewhat inflexible in that little variation from the original tested wall assembly is allowed, including unit size, shape, mix design, constituent materials, and even the plant of manufacture. More information on listing services for fire ratings is provided in CMU-FAQ 015-23 (ref. 16).

For prescriptive designs, the IBC provides a series of tables that describes requirements of various assemblies to meet the fire resistance ratings specified. The last two options listed above require justification to the building official that the proposed design is at least the equivalent of what is prescribed in the code.

CALCULATED FIRE RESISTANCE RATINGS

Background

The calculated fire resistance method is based on extensive research and testing of concrete masonry walls. Fire testing of wall assemblies is conducted in accordance with Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E119 (ref. 3), which measures four performance criteria, as follows:

resistance to the transmission of heat through the wall assembly;
resistance to the passage of hot gases through the wall, sufficient to ignite cotton waste;
load-carrying capacity of loadbearing walls; and
resistance to the impact, erosion and cooling effects of a hose stream on the assembly after exposure to the standard fire.

The fire resistance rating of concrete masonry is typically governed by the heat transmission criteria. From the standpoint of life safety (particularly for fire fighters) and reuse, this failure mode is certainly preferable to a structural collapse endpoint, characteristic of many other building materials.

The calculated fire resistance rating information presented here is based on the IBC and ACI 216.1/TMS 216 (refs. 1, 2).

Equivalent Thickness

Extensive testing has established a relationship between fire resistance and the equivalent solid thickness of concrete masonry walls, as shown in Table 1. Equivalent thickness is essentially the solid thickness that would be obtained if the volume of concrete contained in a hollow unit were recast without core holes (see Figure 1). The equivalent thickness is determined in accordance with Standard Methods of Sampling and Testing Concrete Masonry Units, ASTM C140 (ref. 4), and is reported on the C140 test report. If the equivalent thickness is unknown, but the percent solid of the unit is, the equivalent thickness of a hollow unit can be determined by multiplying the percent solid by the unit’s actual thickness.

The equivalent thickness of a 100% solid unit or a solid grouted unit is equal to the actual thickness. For partially grouted walls where the unfilled cells are left empty, the equivalent thickness for fire resistance rating purposes is equal to that of an ungrouted unit. For partially grouted walls with filled cells, see the following section. Loadbearing units conforming to ASTM C90 (ref. 5) that are commonly available include 100% solid units, 75% solid units, and hollow units meeting minimum required face shell and web dimensions. Typical equivalent thickness values for these units are listed in Table 2.

Filling Cells with Loose Fill Material

If all cells of hollow unit masonry are filled with an approved material, the equivalent thickness of the assembly is the actual thickness. This also applies to partially grouted concrete masonry walls where all ungrouted cells are filled with an approved material.

Applicable fill materials are: grout, sand, pea gravel, crushed stone, or slag that comply with ASTM C33 (ref. 6); pumice, scoria, expanded shale, expanded clay, expanded slate, expanded slag, expanded fly ash, or cinders that comply with ASTM C331 (ref. 7); perlite meeting the requirements of ASTM C549 (ref. 8); or vermiculite complying with C516 (ref. 9).

Wall Assembly Fire Ratings

The fire resistance rating is determined in accordance with Table 1 utilizing the appropriate aggregate type used in the masonry unit and the equivalent thickness.

Units manufactured with a combination of aggregate types are addressed by footnote C, which may be expressed by the following equation (see also the blended aggregate example, below):

Blended aggregate example:

The required equivalent thickness of an assembly constructed of units made with expanded shale (80% by volume), and calcareous sand (20% by volume), to meet a 3-hour fire resistance rating is determined as follows. From Table 1:

Multi-Wythe Wall Assemblies

The fire resistance rating of multi-wythe walls (Figure 2) is based on the fire resistance of each wythe and the air space between each wythe using the following equation:

For multi-wythe walls of clay and concrete masonry, use the values in Table 3 for the brick wythe in the above equation.

Reinforced Concrete Masonry Columns

Concrete masonry column fire testing evaluates the ability of the column to carry design loads under standard fire test conditions. Based on a compendium of fire tests, the fire resistance rating of reinforced concrete masonry columns is based on the least plan dimension of the column as indicated in Table 4. The minimum required cover over the vertical reinforcement is 2 in. (51 mm).

Concrete Masonry Lintels

Fire testing of concrete masonry beams and lintels evaluates the ability of the member to sustain design loads under standard fire test conditions. This is accomplished by ensuring that the temperature of the tensile reinforcement does not exceed 1,100°F (593°C) during the rating period. The calculated fire resistance rating of concrete masonry lintels is based on the nominal thickness of the lintel and the minimum cover of longitudinal reinforcement (see Table 5). The cover requirements protect the reinforcement from strength degradation due to excessive temperature during the fire exposure period. Cover requirements may be provided by masonry units, grout, or mortar. Note that for 3 and 4 hour requirements, not enough cover is available for 6-in. (152 mm) masonry; however, if a special analysis indicates that the reinforcement is not necessary or not needed, such as when conditions for arching action are present, the cover requirements may be waived. See TEK 17-01D (ref. 11) for lintel design and conditions for arching action.

Control Joints

Figure 3 shows control joint details in fire-rated wall assemblies in which openings are not permitted or where openings are required to be protected. Maximum joint width is 1/2 in. (13 mm). Although these details are not directly in the IBC, they are included by reference in ACI 216.1/TMS 216.

In addition to these prescriptive fire resistance rated control joints, other control joints may be permitted in fire rated masonry walls. For example, the IBC and ACI 216.1/1/TMS 216 include provisions for ceramic fiber joint protection for precast panels, which are similar to concrete masonry walls in that both rely on concrete for fire protection, and both are governed by the ASTM E119 heat transmission criteria (see Figure 4). The first two categories of aggregate types in Table 1 would correspond to the carbonate or siliceous aggregate concrete curve and the last two aggregate categories of Table 1 would correspond to the semi-lightweight or lightweight concrete curve. For example, for an 8-in. (203-mm) limestone aggregate concrete masonry wall with a maximum control joint width of 1/2 in. (13 mm), a 1 in. (25 mm) thickness (measured perpendicular to the face of the wall) of ceramic fiber in the joint can be used in walls with fire resistance ratings up to 3 hours, while a 2 in. (51 mm) thickness can be used in the joints of a 4-hour wall.

Steel Columns Protected by Concrete Masonry

Fire testing of a steel column protected by concrete masonry evaluates the structural integrity of the steel column under fire test conditions, by measuring the temperature rise of the steel. The calculated fire resistance rating of steel columns protected by concrete masonry, as illustrated in Figure 5, is determined by:

Effects of Finish Materials on Fire Resistance Ratings

In many cases, drywall, plaster or stucco finishes are used on concrete masonry walls. While finishes are normally applied for architectural reasons, they can also provide additional fire resistance. The IBC and ACI 216.1/TMS 216 include provisions for calculating the additional fire resistance provided by these finishes.

Note that when finishes are used to achieve the required fire rating, the masonry alone must provide at least one- half of the total required rating and the contribution of the finish on the non-fire-exposed side cannot be more than one-half of the contribution of the masonry alone. This is to assure structural integrity during a fire. The finish material must also be continuous over the entire wall.

Certain finishes deteriorate more rapidly when exposed to fire than when they are on the non-fire side of the wall. Therefore, two separate tables are required. Table 7 applies to finishes on the non fire-exposed side of the wall, and Table 8 applies to finishes on the fire-exposed side. For finishes on the non-fire exposed side of the wall, the finish is converted to an equivalent thickness of concrete masonry by multiplying the finish thickness by the factor given in Table 7. The result, Tef, is then added to the concrete masonry wall equivalent thickness, Te, and used in Table 1 to determine the wall’s fire resistance rating (i.e., the equivalent thickness of concrete masonry assemblies, Tea = Te Tef).

For finishes on the fire-exposed side of the wall, a time (from Table 8) is assigned to the finish. This time is added to the fire resistance rating determined for the base wall and nonfire-exposed side finish, if any. The times listed in Table 8 are essentially the length of time the various finishes will remain intact when exposed to fire (i.e., on the fire-exposed side of the wall).

When calculating the fire resistance rating of a wall with finishes, two calculations are performed, assuming each side of the wall is the fire exposed side. The fire rating of the wall assembly is the lower of the two. Typically, for an exterior wall with a fire separation distance greater than 5 ft (1,524 mm), fire needs be considered on the interior side only

Installation of Finishes

Finishes that contribute to the total fire resistance rating of a wall must meet certain minimum installation requirements. Plaster and stucco are applied in accordance with the provisions of the building code without further modification. Gypsum wallboard and gypsum lath are to be attached to wood or metal furring strips spaced a maximum of 16 in. (406 mm) o.c., and must be installed with the long dimension parallel to the furring members. All horizontal and vertical joints must be supported and finished.

UNCONVENTIONAL AGGREGATES

In recent years, manufacturers of concrete masonry products have been exploring the use of alternative materials in the production of concrete masonry units. Some of these materials have not been evaluated using standardized fire resistance test methods or have been evaluated only to a limited degree. Such unconventional materials, which are typically used as a replacement for conventional aggregates, may not be covered within existing codes and standards due to their novelty or proprietary nature.

While test methods such as ASTM E119 define procedures for evaluating the fire resistance properties of concrete masonry assemblies, including those constructed using unconventional constituent materials, there has historically been no defined procedure for applying the results of ASTM E119 testing to standardized calculation procedures available through ACI 216.1/TMS 216. To provide consistency in applying the results of full scale ASTM E119 testing to established calculation procedures, CMHA has developed CMU-FAQ-013-23 (Ref. 15).

This guideline stipulates that when applying the fire resistance calculation procedure of ACI 216.1/TMS 216 to products manufactured using aggregate types that are not listed in ACI 216.1/TMS 216, at least two full scale ASTM E119 tests must be conducted on assemblies containing the unconventional material. Based on the results of this testing, an expression can be developed in accordance with this industry practice that permits the fire resistance of units produced with such aggregates to be calculated for interpolated values of equivalent thickness and proportion of non listed aggregate.

REFERENCES

Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, ACI 216.1- 14/TMS216-14. American Concrete Institute and The Masonry Society, 2014.
International Building Code 2015. International Code Council, 2015.
Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E119-16a. ASTM International, Inc., 2016.
Standard Methods for Sampling and Testing Concrete Masonry Units, ASTM C140-16. ASTM International, Inc., 2016.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-16. ASTM International, Inc., 2016.
Standard Specification for Concrete Aggregates, ASTM C33-16e1. ASTM International, Inc., 2016.
Standard Specification for Lightweight Aggregates for Concrete Masonry Units, ASTM C331-14. ASTM International, Inc., 2014.
Standard Specification for Perlite Loose Fill Insulation, ASTM C549-06(2012). ASTM International, Inc., 2012.
Standard Specification for Vermiculite Loose Fill Thermal Insulation, ASTM C516-08(2013)e1. ASTM International, Inc., 2013.
Steel Column Fire Protection, TEK 07-06A. Concrete Masonry & Hardscapes Association, 2009.
ASD of Concrete Masonry Lintels Based on the 2012 IBC/2011 MSJC, TEK 17-01D. Concrete Masonry & Hardscapes Association, 2011.
Standard Specification for Concrete Building Brick, ASTM C55 14a. ASTM International, Inc., 2014.
Standard Specification for Calcium Silicate Brick (SandLime Brick), ASTM C73-14. ASTM International, Inc., 2014.
Standard Specification for Prefaced Concrete and Calcium Silicate Masonry Units, ASTM C744-16. ASTM International, Inc., 2016.
How is the fire resistance of a concrete masonry assembly calculated when using unconventional aggregates?, CMU-FAQ-013-23. Concrete Masonry & Hardscapes Association, 2023.
What is the difference between fire resistance ratings for masonry assemblies obtained through IBC versus a listing service such as UL or FM?, CMU-FAQ-015-23. Concrete Masonry & Hardscapes Association, 2023.

Construction of Low-Rise Concrete Masonry Buildings

November 2018

INTRODUCTION

The current trend of urban renewal and infill has sparked a high volume of new low-rise masonry residences. These structures come in many forms, but quite often they employ the use of load-bearing concrete masonry walls supporting a wood floor system. These new buildings are largely derivative of the historic load bearing masonry “brownstone” or “three flat” structures of old. This guide is intended to assist contractors and architects to give this building type a modern approach to detailing.

FLOOR SYSTEM CONNECTIONS

When designing low-rise loadbearing structures, the connection detail between the floor system and the wall system is critical for achieving a watertight structure. Much of this TEK will deal with which strategy should be utilized in connecting a wood floor system to a masonry load-bearing wall. Connection methods covered are joist hangers, beam pockets and ledger beam details. Other floor systems are used in low-rise construction that are not addressed here – see 05-07A for further information (ref. 2).

BRICK AND BLOCK COMPOSITE WALL DETAILS

Quite often, the front facade of these structures is composed of brick to give the building a more residential, more human scale. One way to construct a brick and block wall is to separate the two wythes with an airspace, creating a cavity wall. Another is to use a composite wall design. The composite wall consists of an exterior wythe of brick directly mortared or grouted and tied to an inner wythe of CMU. The collar joint between the two wythes should be 100% solid as it is the only defense against water penetration. Minimum tie requirements are one tie per 22/3ft2 of wall area for W1.7 (MW11)(9 gauge) wire or one tie per 41/2ft2 of wall area using W2.8 (MW19)(3/16 in.)wire (ref. 2). A W1.7 (MW11)(9 gauge) joint reinforcement @16 in. (406 mm) on center would meet this requirement and is often used. Details covered for this system are base flashing, window head and window sill details.

EXTERIOR CONCRETE MASONRY

The use of water repellent admixtures in concrete masonry and mortars can greatly reduce the amount of water entering the masonry. In addition, they inhibit any water that penetrates the face from wicking to the back of the wall.

Proper selection and application of integral water repellents and surface treatments can greatly enhance the water resistive properties of masonry, but they should not be considered as substitutes for good fundamental design including flashing details and crack control measures. See TEKs 19-01, 19-02A, and 19-04A (refs. 6, 3, & 5) for more information on water resistant concrete masonry construction.

Because a 4 in. (102 mm) concrete masonry veneer will shrink over time, a 4 in. (102 mm) hot-dipped galvanized ladder type joint reinforcement should be placed in bed joints spaced 16 in. (406 mm) vertically.

Compared to type N or O, type S mortar tends to be less workable in the field and should only be specified when dictated by structural requirements. Sills, copings and chimney caps of solid masonry units, reinforced concrete, stone, or corrosion resistant metal should be used. Copings, sills and chimney caps should project beyond the face of the wall at least 1 in. (25 mm) and should have functional flashing and weep holes.

In addition, all sills, copings and chimney caps should have a minimum slope of 1:4, be mechanically anchored to the wall, and should have properly sized, sealed, and located movement joints when necessary.

Flashing should be installed at locations shown on the plans and in strict accordance with the details and industry standard flashing procedures. Functional, unpunctured flashing and weep holes are to be used at the base of wall above grade, above openings, at shelf angles, lintels, wall-roofing intersections, chimneys, bay windows, and below sills and copings. The flashing should be extended past the face of the wall. The flashing should have end dams at discontinuous ends, and properly sealed splices at laps.

JOIST HANGER DETAILS

The use of a joist hanger system can greatly simplify the bearing detail. The floor system does not interrupt the continuity of the bearing wall. Installation is quicker and easier resulting in a more economical installation.

BEAM POCKET DETAILS

The traditional beam pocket detail still can be effective. Stepped flashing above the bearing line is critical to the performance of this system. Without the flashing, any water present in the wall has an unobstructed path inside the building and has the potential to deteriorate the floor structure.

LEDGER BEAM DETAILS

The use of a ledger beam which is bolted to a bond beam is also a good option for this bearing condition. Through wall flashing is still required to maintain a watertight wall. Any water that penetrates the block with run down the inner cores of the block until it hits the flashing. The flashing and weep holes will allow the water to exit without damaging the structure.

PARAPETS AND WINDOW SILLS

Below are details for a parapet condition and a window sill condition. The parapet is reinforced with No. 4 bars at 48 in. (No.13M @1219 mm) on center or as required for wind resistance. If a metal cap is used, it should extend down the face of the wall at least 3 in. (76 mm) with continuous sealant at the joint on both sides of the wall. The sill detail shows the arrangement of flashing, end dam, weep holes and drip edge and how they all form a watertight

WINDOW HEAD DETAILS

These two window head details show the relationship between the steel lintel, drip edge, flashing, end dams, and weep holes. The first option shows the use of a concrete masonry lintel which is grouted solid and reinforced. The second detail shows two steel lintels used for spanning the opening.

CONTROL JOINT DETAILS

Control joints simply are weakened planes placed at approximately 20 ft. (6 m) on center in concrete masonry walls and at changes in wall elevation/thickness. Notice that the joint reinforcement is discontinuous at the joint. Cores are shown grouted adjacent to the joints as well to ensure structural stability in taller walls and/or high load situations.

COMPOSITE WALL BASE FLASHING DETAILS

Figure 14 shows a stair-stepped ﬂashing detail with the exposed drip edge and weep holes. Figure 15 shows a straight through wall ﬂashing detail. The ﬂashing must be set in mastic on top of the concrete foundation, or the ﬂashing must be self adhesive. The ﬂashing should be turned up on the inner side of the wall to direct water to the outside of the wall.

COMPOSITE WALL WINDOW DETAILS

Here steel lintels back-to-back create the above window span. Stepped ﬂashing turned up on the inside, and folded to form an end dam protects the head condition from moisture. The sill detail also uses ﬂashing, end dams and weep holes to keep moisture out of the wall. The use of a precast concrete or stone sill is highly suggested over using brick rowlock sills.

CONCRETE MASONRY VENEER DETAILING

Figure 18 shows the detailing of a 4 in. (102 mm) concrete masonry veneer used in conjunction with a 8 in. (205 mm) CMU backup wall.

Three types of joint reinforcement are shown including tri-rod, tab and adjustable types. It is imperative that the veneer have a continuous wire embedded in every other course to control movement. With the tri-rod system, the joint reinforcement satisﬁes this requirement. With the other two systems, an additional ladder type joint reinforcement is used to provide this movement control for the veneer.

REFERENCES

Building Code Requirements for Masonry Structures,
ACI 530-05/ASCE 6-05/TMS-402-05. Reported by the
Masonry Standards Joint Committee, 2005.
Floor and Roof Connections to Concrete Masonry
Walls, TEK 05-07A, Concrete Masonry & Hardscapes
Association, 2001.
Design for Dry Single-Wythe Concrete Masonry
Walls, TEK 19-02B, Concrete Masonry & Hardscapes
Association, 2004.
Flashing Details for Concrete Masonry Walls, TEK 19-05A,
Concrete Masonry & Hardscapes Association, 2004.
Flashing Strategies for Concrete Masonry Walls, TEK 19-
04A, Concrete Masonry & Hardscapes Association, 2003.
Water Repellents for Concrete Masonry Walls, TEK 19-01,
Concrete Masonry & Hardscapes Association, 2002.

Precast Concrete Lintels for Concrete Masonry Construction

August 2018

INTRODUCTION

Lintels function as beams to support the wall weight and other loads over an opening, and to transfer these loads to the adjacent masonry. Because of their rigidity, strength, durability, fire resistance and aesthetics, the most common types of lintels for concrete masonry construction are those manufactured of precast reinforced concrete or reinforced concrete masonry units (ref. 3). The color and surface texture of these lintels can be used as an accent or to duplicate the surrounding masonry.

LINTEL DIMENSIONS

Precast lintel dimensions are illustrated in Figure 1. Precast concrete lintels are manufactured to modular sizes, having specified dimensions corresponding to the concrete masonry units being used in the construction.

A modular lintel length should be specified, with a minimum length of the clear span plus 8 in. (203 mm), to provide at least 4 in. (102 mm) bearing at each end (ref. 1). Additionally, if lintels are subjected to tensile stresses during storage, transportation, handling, or placement, it is recommended that steel reinforcement be provided in both the top and bottom to prevent cracking. Minimum concrete cover over the steel should be 1 ½ in. (13 mm). The lintel width, or width of the combination of side-by-side lintels, should equal the width of the supported masonry wythe.

Lintels should be clearly marked on the top whenever possible to prevent the possibility of improper installation in the wall. In the event the top of the lintel is not marked and may be installed upside down, the same size bars should be used in both the top and bottom.

LINTEL DESIGN

Precast concrete lintels are designed using the strength design provisions of Building Code Requirements for Structural Concrete, ACI 318-99 (ref. 2). In strength design, service loads are increased to account for variations in anticipated loads, becoming factored loads. The lintel is then sized to provide sufficient design strength. Further information on determining design loads for lintels is included in ASD of CM Lintels Based on 2012 IBC/2011 MSJC, TEK 17-01D (ref. 3).

Nominal lintel strength is determined based on the strength design provisions of ACI 318 and then reduced by strength reduction factors, called phi (Φ) factors. These factors account for any variability in materials and construction practices. The resulting capacity needs to equal or exceed the factored loads. Precast concrete strength reduction factors are 0.9 and 0.85 for flexure and shear, respectively (ref. 2).

Tables 1 through 4 list design moment and shear strengths for various precast lintel sizes and concrete strengths, based on the following criteria (ref. 2).

Flexural strength:

Shear strength, no shear reinforcement:

ACI 318 contains requirements for minimum and maximum reinforcing steel areas to ensure a minimum level of performance. Minimum reinforcement area for lintels is A_{s min} = 3(f’_c)^½bd/f_y but not less than 200bd/f_y. In addition, the reinforcement ratio is limited to 75% of the balanced reinforcement ratio, ρ_max = 0.75ρ_b.

Deflection criteria for lintels is based on controlling cracking in the masonry being supported. Consequently, less deflection is allowed when the lintel supports unreinforced masonry. In this case, lintel deflection is limited to the effective span of the lintel (measured in inches) divided by 600 (L/600) (ref. 1). In addition, ACI 318 limits precast lintel deflection to L/240 when the element supported by the lintel is not likely to be damaged by large deflections, and L/480 when the element supported by the lintel is likely to be damaged by large deflections. Lintel deflection is calculated based on the effective moment of inertia, I_e, as follows (ref. 2, Section 9.5.2.3).

Shrinkage and creep due to sustained loads cause additional long-term deflections over and above those occurring when loads are first applied. ACI 318 requires that deflections due to shrinkage and creep are included, and provides an expression to estimate this additional deflection (ACI 318 Section 9.5.2.5):

λ = ξ/(1+50ρ’)

where ξ = 2.0 for exposures of 5 years or more.

Figure 2 – Strength Design Structural Model

DESIGN EXAMPLE

The residential basement wall shown in Figure 3 needs a lintel over the window opening. The floor live load is 400 lb (1.8 kN) per joist and the floor dead load is 100 lb (0.44 kN) per joist. Consider the floor joist loads, spaced at 16 in. (406 mm) on center, as uniformly distributed. Use a lintel self-weight of 61 lb/ ft (0.89 kN/m) and weight of 77.9 lb/ft2 (3.73 kPa) for the bond beam at the top of the wall over the lintel.

Determine effective depth, d: Assuming an 8 in. (203 mm) high lintel with two No. 4 (13M) bars,
d = 7.625 in. – 1.5 in. – 0.5/2 in.
= 5.88 in. (149 mm)

Check for arching action: The effective span length, L = 96 + 5.88 = 101.9 in. (2588 mm). Since the height of masonry above the opening is less than L/2, arching of the masonry over the opening cannot be assumed (see ref. 4 for detailed information about determining arching action).

Determine design loads:
LL = (400 lb)(12/16 in.) = 300 lb/ft (4.4 kN/m)
Dead loads include floor, wall, and lintel self-weight.
D_floor = 100 lb (12/16 in.) = 75 lb/ft (1.1 kN/m)
D_lintel = 61 lb/ft (0.89 kN/m)
D_{b beam} = (77.9lb/ft²)(7.625/12 ft)= 50 lb/ft (0.31 kN/m)
D_total = (75 + 61 + 50) = 186 lb/ft (3.2 kN/m)

For deflection calculations use loads as given above. For strength design multiply live loads by 1.7 and dead loads by 1.4. Maximum moment and shear for strength design:

M_max = wL²/8
= {[(1.7)(300)+(1.4)( 186 ) lb/ft](101.9 in.)²/8}(ft/12 in.)
= 83,328 in.-lb (9.4 kN m)

V_max = wL/2 (at distance “d” from support) (ref.2)
= [(1.7)(300)+(1.4)(186 lb/ft)](101.9/2-5.88 in.)(ft/12 in.)
= 2,893 lb (12.9 kN)

From Table 3, an 8 x 8 in. (203 x 203 mm) lintel with two No. 4 (13M) bars and f ‘_c = 4000 psi (20.7 MPa) has sufficient strength.

Check deflection: Deflection is determined using the effective moment of inertia of the lintel, I_e, calculated as follows (ref. 2).

E_c = w_c^1.533(f’_c)^½ = (150 pcf)^1.533(4000 psi)^½
= 3,834,000 psi (26,400 MPa)
f_r = 7.5(f’_c)^½ = 474 psi (3.3 MPa)
y_t = 7.625 in./2 = 3.81 in. (97 mm)
I_g = bh³/12 = (7.625 in.)(7.625 in.)³/12
= 282 in.⁴ (11,725 cm⁴)
M_cr = f_rI_g/y_t = 474 psi(282 psi)/3.81 in.
= 35,083 in.-lb (4.0 kN⋅m)
M_{max uf} = wL²/8 = [(300+186 lb/ft)(101.9 in.)²/8](ft/12 in.)
= 52567 in.-lb (5.9 kN⋅m)
(M_cr/M_{max uf})³ = (35,083/52567)³ = 0.297
n = E_s/E_c = 29,000,000/3,834,000 = 7.6
ρ = A_s/bd = 0.40 in.²/(7.625 in.)(5.88 in.) = 0.00892
nρ = 7.6(0.00892) = 0.0678
c = nρd[(1 + 2/nρ)^½ – 1]
= 0.0678(5.88 in.)[(1+ 2/0.0678)^½-1] = 1.80 in. (45 mm)
I_cr = bc³/3 + nA_s (d – c)²
= 7.625 in.(1.8 in.)³/3 + 7.6(0.4 in.²)(5.88 – 1.8)²
= 65.4 in.⁴ (2714 cm⁴)
I_e = (M_cr/M_{max uf})³ I_g + [1- (M_cr/M_{max uf})³]I_cr
= 0.297(282) + [1-0.297]65.4 in.⁴
= 130 in.⁴ (5411 cm⁴) < I_g OK

For a simply supported beam under uniform load,

∆_max = 5wL⁴/384E_cI_e
= 5(300 + 186 lb/ft)(101.9 in.)⁴/[384(3,834,000 psi)(130 in.⁴)]/(12 in./ft)
= 0.114 in. (2.9 mm)

Long-term deflection multiplier,
λ = ξ/(1+50ρ’) = 2/[1 + 50(0)] = 2

Long-term deflection,
∆_LT = λ∆_max = 2(0.114 in.) = 0.228 in. (5.8 mm)

Total deflection,
∆_tot = ∆_max + ∆_LT = 0.114 + 0.228 = 0.342 in. (8.7 mm)

Deflection limit for this case is L/240 = 101.9 in./240
= 0.42 in. (10.7 mm) > 0.342 in. (8.7 mm) OK

Figure 3 – Wall Configuration for Design Example

NOTATIONS

a = depth of equivalent rectangular stress block, in. (mm)
A_s = area of tension reinforcement, in.² (mm²)
b = actual width of lintel, in. (mm)
c = distance from extreme compression fiber to neutral axis, in. (mm)
C = resultant compressive force in concrete, lb (kN)
d = distance from extreme compression fiber to centroid of tension reinforcement, in. (mm)
D_{b beam} = dead load of bond beam, lb/ft (kN/m)
D_floor = dead load of floor, lb/ft (kN/m)
D_lintel = dead load of lintel, lb/ft (kN/m)
D_tot = total design dead load, lb/ft (kN/m)
E_c = modulus of elasticity of concrete, psi (MPa)
f ‘_c = specified compressive strength of concrete, psi (MPa)
f_r = modulus of rupture of concrete, psi (MPa)
f_y = specified yield strength of reinforcement, psi (MPa) (60,000 psi, 413 MPa)
I_cr = moment of inertia of cracked section transformed to concrete, in.⁴ (cm⁴)
I_e = effective moment of inertia, in.⁴ (cm⁴)
I_g = moment of inertia of gross concrete section about centroidal axis, in.⁴ (cm⁴)
L = effective length, clear span plus depth of member, not to exceed the distance between center of supports, in. (mm)
LL = live load, lb/ft (kN/m)
M_cr = cracking moment, in.-lb (kN⋅m)
M_max = maximum factored moment on section, in.-lb (kN⋅m)
M_{max uf} = maximum unfactored moment on section, in.-lb (kN⋅m)
M_n = nominal moment strength, in.-lb/ft (kN⋅m/m)
n = modular ratio, E_s/E_c
T = resultant tensile force in steel reinforcement, lb (kN)
V_max = maximum factored shear on section, lb (kN)
V_n = nominal shear strength, lb (kN)
w = uniform load, lb/in. (kN/m)
w_c = density of concrete, pcf (kN/m³)
y_t = distance from centroidal axis of gross section to extreme fiber in tension, in. (mm)
∆_max = maximum immediate deflection, in. (mm)
∆_LT = long-term deflection, in. (mm)
∆_tot = total deflection, in. (mm)
ε_c = strain in concrete, in./in. (mm/mm)
ε_s = strain in steel reinforcement, in./in. (mm/mm)
ξ = time-dependent factor for sustained load
λ = multiplier for additional long-term deflection
Φ = strength reduction factor
ρ = reinforcement ratio, A_s/bd
ρ’ = reinforcement ratio for nonprestressed compression reinforcement, A_s‘/bd
ρ_b = reinforcement ratio producing balanced strain conditions
ρ_max = limit on reinforcement ratio

REFERENCES

Building Code Requirements for Masonry Structures, ACI 530-99/ASCE 5-99/TMS 402-99. Reported by the Masonry Standards Joint Committee, 1999.
Building Code Requirements for Structural Concrete, ACI 318-99. American Concrete Institute, 1999.
ASD of CM Lintels Based on 2012 IBC/2011 MSJC, TEK 17-01D, Concrete Masonry & Hardscapes Association, 2011.

ASD of Concrete Masonry Lintels Based on the 2012 IBC/2011 MSJC

August 2018

INTRODUCTION

Lintels and beams are horizontal structural members designed to carry loads above openings. Although lintels may be constructed of grouted and reinforced concrete masonry units, precast or cast-in-place concrete, or structural steel, this TEK addresses reinforced concrete masonry lintels only. Concrete masonry lintels have the advantages of easily maintaining the bond pattern, color, and surface texture of the surrounding masonry and being placed without need for special lifting equipment.

Concrete masonry lintels are sometimes constructed as a portion of a continuous bond beam. This construction provides several benefits: it is considered to be more advantageous in high seismic areas or areas where high winds may be expected to occur; control of wall movement due to shrinkage or temperature differentials is more easily accomplished; and lintel deflection may be substantially reduced.

The content presented in this TEK is based on the requirements of the 2012 IBC (ref. 1a), which in turn references the 2011 edition of the MSJC Code (ref. 2a).

Significant changes were made to the allowable stress design (ASD) method between the 2009 and 2012 editions of the IBC. These are described in detail in TEK 14-07C, ASD of Concrete Masonry (2012 IBC & 2011 MSJC) (ref. 3), along with a detailed presentation of all of the allowable stress design provisions of the 2012 IBC.

DESIGN LOADS

Vertical loads carried by lintels typically include:

distributed loads from the dead weight of the lintel, the dead weight of the masonry above, and any floor and roof loads, dead and live loads supported by the masonry; and
concentrated loads from floor beams, roof joists, or other beams framing into the wall. Axial load carried by lintels is negligible.

Most of these loads can be separated into the four types illustrated in Figure 1: uniform load acting over the effective span; triangular load with apex at mid-span acting over the effective span; concentrated load; and uniform load acting over a portion of the effective span.

The designer calculates the effects of each individual load and then combines them using superposition to determine the overall effect, typically by assuming the lintel is a simply supported beam.

^A Effective span length is the center-to-center distance between supports.

Arching Action

For some configurations, the masonry will distribute applied loads in such a manner that they do not act on the lintel. This is called arching action of masonry. Arching action can be assumed when the following conditions are met (see also Figure 2):

masonry wall laid in running bond,
sufficient wall height above the lintel to permit formation of a symmetrical triangle with base angles of 45° from the horizontal as shown in Figure 2,
at least 8 in. (203 mm) of wall height above the apex of the 45° triangle,
minimum end bearing (4 in. (102 mm) typ.) is maintained,
control joints are not located adjacent to the lintel, and
sufficient masonry on each side of the opening to resist lateral thrust from the arching action.

Lintel Loading

The loads supported by a lintel depend on whether or not arching action can occur. When arching is not present, the lintel self-weight, the full weight of the wall section above the lintel and superimposed loads are considered. Self weight is a uniform load based on lintel weight (see Table 1).

When arching occurs, the wall weight supported by the lintel is taken as the wall weight within the triangular area below the apex (see Figure 2 and Table 2). This triangular load has a base equal to the effective span length of the lintel and a height of half the effective span. Any superimposed roof and floor live and dead loads outside this triangle are neglected, since they are assumed to be distributed to the masonry on either side of the lintel. Loads applied within the triangle need to be considered, however.

Concentrated loads are assumed to be distributed as illustrated in Figure 3. The load is then resolved onto the lintel as a uniform load, with a magnitude determined by dividing the concentrated load by this length. In most cases, this results in a uniform load acting over a portion of the lintel span.

The MSJC (ref. 2) does not address how to apply uniform loads that are applied within the 45° triangle. There are two schools of thought (see Figure 4):

Apply the full uniform load directly to the lintel without further distribution just as though there was no arching for those loads.
Distribute the portions of uniform loads that are applied within the 45o triangle to the lintel. These uniform loads within the 45o triangle may be dispersed and distributed at a 45o angle onto the lintel (ref. 5).

Lintels are required to be designed to have adequate stiffness to limit deflections that would adversely affect strength or serviceability. In addition, the deflection of lintels supporting unreinforced masonry is limited to the clear lintel span divided by 600 to limit damage to the supported masonry (ref. 2).

Table 1—Lintel Weights per Foot, D(lintel), lb/ft (kN/m) (A)

^A Face shell mortar bedding. Unit weights: grout = 140 pcf (2,242 kg/m³); lightweight masonry units = 100 pcf (1602 kg/m³); normal weight units = 135 pcf (2,162 kg/m³).

Figure 3—Distribution of Concentrated Load for Running Bond Construction

Figure 4—Methods of Applying Uniform Loads that Occur Within the 45° Triangle

DESIGN TABLES

Tables 3 and 4 present allowable shear and moment, respectively, for various concrete masonry lintels, with various amounts of reinforcement and bottom cover based on a specified compressive strength of masonry, f’m = 1,500 psi (10.3 MPa) and the allowable stress design provisions of the 2011 MSJC (ref. 2a) and the 2012 IBC (ref.1a).

Table 3—Allowable Shear Capacities for Concrete Masonry LintelsA

DESIGN EXAMPLE

Design a lintel for a 12 in. (305 mm) normal weight concrete masonry wall laid in running bond with vertical reinforcement at 48 in. (1.2 m) o.c. The wall configuration is shown in Figure 5.

Case 1—Arching Action

Check for Arching Action. Determine the height of masonry required for arching action. Assuming the lintel has at least 4 in. (102 mm) bearing on each end, the effective span is:

L = 5.33 + 0.33 = 5.67 ft (1.7 m).

The height of masonry above the lintel necessary for arching to occur in the wall (from Figure 2) is h + 8 in. (203 mm) = L/2 + 8 in. = 3.5 ft (1.1 m).
Based on an 8-in. (203-mm) high lintel, there is 18.0 – (3.33 + 4.0 + 0.67) = 10.0 ft (3.0 m) of masonry above the lintel. Therefore, arching is assumed and the superimposed uniform load is neglected.

Design Loads. Because arching occurs, only the lintel and wall dead weights are considered. Lintel weight, from Table 1, for 12 in. (305 mm) normal weight concrete masonry units assuming an 8 in. (203 mm) height is D_lintel = 88 lb/ft (1.3 kN/m).

For wall weight, only the triangular portion with a height of 3.5 ft (1.1 m) is considered. From Table 2, wall dead load is:

D_wall = 63 lb/ft² (3.5 ft)
= 221 lb/ft (3.2 kN/m) at the apex.

Maximum moment and shear are determined using simply supported beam relationships. The lintel dead weight is considered a uniform load, so the moment and shear are,

M_lintel = D_lintelL²/8
= (88)(5.7)²/8
= 357 lb-ft (0.48 kN-m)
V_lintel = D_lintelL/2
= (88)(5.7)/2 = 251 lb (1.1 kN)

For triangular wall load, moment and shear are,

M_wall = D_wallL²/12
= (221)(5.7)²/12
= 598 lb-ft (0.81 kN-m)
V_wall = D_wallL/4
= (221)(5.7)/4 = 315 lb (1.4 kN)

Because the maximum moments for the two loading conditions occur in the same locations on the lintel (as well as the maximum shears), the moments and shears are superimposed and summed:

M_max = 357 + 598
= 955 lb-ft = 11,460 lb-in (1.3 kN-m)
V_max = 251 + 315
= 566 lb (2.5 kN)

Lintel Design. From Tables 3 and 4, a 12 x 8 lintel with one No. 4 (M#13) bar and 3 in. (76 mm) or less bottom cover has adequate strength (M_all = 22,356 lb-in. (2.53 kN-m) and V_all = 2,152 lb (9.57 kN)). In this example, shear was conservatively computed at the end of the lintel. However, Building Code Requirements for Masonry Structures (ref. 2) allows maximum shear to be calculated using a distance d/2 from the face of the support.

Case 2—No Arching Action

Using the same example, recalculate assuming a 2 ft (0.6 m) height from the bottom of the lintel to the top of the wall. For ease of construction, the entire 2 ft (0.6 m) would be grouted solid, producing a 24 in. (610 mm) deep lintel.

Because the height of masonry above the lintel is less than 3.5 ft (1.1 m), arching cannot be assumed, and the superimposed load must be accounted for.

D_lintel = 264 lb/ft (3.9 kN/m), from Table 1. Because the lintel is 24 in. (610 mm) deep, there is no additional dead load due to masonry above the lintel.

W_total = 264 lb/ft + 1,000 lb/ft
= 1,264 lb/ft (18.4 kN/m)

M_max = wL²/8
= (1,264)(5.7)²/8 x 12 in./ft
= 61,601 lb-in (7.0 kN-m)

V_max = wL/2 = (1,264)(5.7)/2
= 3,602 lb (16.0 kN)

From Tables 3 and 4, a 12 x 24 lintel with one No. 4 (M#13) reinforcing bar and 3 in. (76 mm) or less bottom cover is adequate (M_all = 122,872 lb-in. (13.88 kN-m) and V_all = 10,256 lb (45.62 kN).

Figure 5—Wall Configuration for Design Example

NOTATIONS

b = width of lintel, in. (mm)
D_lintel = lintel dead load, lb/ft (kN/m)
D_wall = wall dead load, lb/ft (kN/m)
d = distance from extreme compression fiber to centroid of tension reinforcement, in. (mm)
f’_m = specified compressive strength of masonry, psi (MPa)
h = half of the effective lintel span, L/2, ft (m)
L = effective lintel span, ft (m)
M_all = allowable moment, in.-lb (N⋅m)
M_lintel = maximum moment due to lintel dead load, in.-lb (N⋅m)
M_max = maximum moment, in.-lb (N⋅m)
M_wall = maximum moment due to wall dead load moment, in.-lb (N⋅m)
V_all = allowable shear, lb (N)
V_lintel = maximum shear due to lintel dead load, lb (N)
V_max = maximum shear, lb (N)
V_wall = maximum shear due to wall dead load, lb (N)
W_total = total uniform live and dead load, lb/ft (kN/m)
w = uniformly distributed load, lb/in. (N/mm)

REFERENCES

International Building Code. International Code Council.
1. 2012 Edition
Building Code Requirements for Masonry Structures. Reported by the Masonry Standards Joint Committee. a. 2011 Edition: TMS 402-11/ACI 530-11/ASCE 5-11
ASD of Concrete Masonry (2012 IBC & 2011 MSJC), TEK 14-07C, Concrete Masonry & Hardscapes Association, 2011.
Weights and Section Properties of Concrete Masonry Assemblies, CMU-TEC-002-23, Concrete Masonry & Hardscapes Association, 2023.
Openings in Concrete Masonry Walls (Part 1), Masonry Chronicles Winter 2008-09, Concrete Masonry Association of California and Nevada, 2009.

Design and Construction of Dry-Stack Masonry Walls

August 2018

INTRODUCTION

Construction of masonry wall systems is possible without the use of mortar. The use of standard CMU units laid dry and subsequently surface bonded with fiber reinforced surfaced bonding cement has been well documented in the past. (ref. 16) With the use of specially fabricated concrete masonry units known as “dry-stack units,” construction of these mortarless systems is simple, easy and cost effective. This TEK describes the construction and engineering design of such mortarless wall systems.

The provisions of this TEK apply to both specialty units manufactured specifically for dry-stack construction and conventional concrete masonry units with the following system types:

Grouted, partially grouted or surface bonded
Unreinforced, reinforced, or prestressed

Note that dry-stacked prestressed systems are available that do not contain grout or surface bonding. The provisions of this TEK do not apply to such systems due to a difference in design section properties (ref 8).

Specially designed units for dry-stack construction are available in many different configurations as shown in Figure 1. The latest and most sophisticated designs incorporate face shell alignment features that make units easier and faster to stack plumb and level. Other units are fabricated with a combination of keys, tabs or slots along both horizontal and vertical faces as shown in Figure 1 so that they may interlock easily when placed. Physical tolerances of dry-stack concrete units are limited to ±¹/₁₆ in. (1.58 mm.) which precludes the need for mortaring, grinding of face shell surfaces or shimming to even out courses during construction. Interlocking units placed in running bond resist flexural and shear stresses resulting from out-of-plane loads as a result of the keying action: (a) at the top of a web with the recess in the web of the unit above, (b) at two levels of bearing surface along each face shell at the bed joint, and (c) between adjacent blocks along the head joint. The first of these two interlocking mechanisms also ensures vertical alignment of blocks.

The interlocking features of dry-stack units improve alignment and leveling, reduce the need for skilled labor and reduce construction time. Floor and roof systems can be supported by mortarless walls with a bond beam at the top of the wall which expedites the construction process.

Wall strength and stability are greatly enhanced with grouting which provides the necessary integrity to resist forces applied parallel, and transverse to, the wall plane. Vertical alignment of webs ensures a continuous grout column even when the adjacent cell is left ungrouted. Grouting is necessary to develop flexural tensile stress normal to the bed joints, which is resisted through unit-mortar bond for traditional masonry construction. Strength of grouted dry-stack walls may also be enhanced by traditional reinforcement, prestressing, post-tensioning or with external fiber-reinforced surface coatings (surface bonding) as described in the next section.

Typical applications for mortarless concrete masonry include basement walls, foundation walls, retaining walls, exterior above-grade walls, internal bearing walls and partitions. Dry-stack masonry construction can prove to be a cost-effective solution for residential and low-rise commercial applications because of it’s speed and ease of construction, strength and stability even in zones of moderate and high seismicity. More information on design and construction of dry-stack masonry can be found in Reference 5.

CONSTRUCTION

Dry-stack concrete masonry units can be used to construct walls that are grouted or partially grouted; unreinforced, reinforced or prestressed; or surface bonded. With each construction type, walls are built by first stacking concrete masonry units.

For unreinforced construction as shown in Figure 2a, grouting provides flexural and shear strength to a wall system. Flexural tensile stresses due to out-of-plane bending are resisted by the grout cores. Grout cores also interlace units placed in running bond and thus provide resistance to in-plane shear forces beyond that provided by friction developed along horizontal joints. Grout cores can also be reinforced to increase flexural strength.

Reinforcement can be placed vertically, in which case only those cells containing reinforcement may be grouted as shown in Figure 2b, as well as horizontally, in which case the masonry must be fully grouted. Another version is to place vertical prestressing tendons in place of reinforcement. Vertical axial compressive stress, applied via the tendons, increases flexural and shear capacity. Tendons may be bonded to grout, or unbonded, based upon the design. Placement of grout may be optional. Horizontally reinforced bond beam lintels can be created using a grout stop beneath the unit to contain grout.

As an alternative to reinforcing or prestressing, wall surfaces may be parged (coated) with a fiber-reinforced surface bonding cement/stucco per ASTM C887(ref. 14) as illustrated in Figure 2c. This surface treatment, applied to both faces of a wall, bonds concrete units together without the need for grout or internal reinforcement. The parging material bridges the units and fills the joints between units to provide additional bonding of the coating to the units through keying action. The compressive strength of the parging material should be equal to or greater than that of the masonry units.

Figure 2–– Basic Dry-Stack Masonry Wall Types

Laying of Units

The first course of dry-stack block should be placed on a smooth, level bearing surface of proper size and strength to ensure a plumb and stable wall. Minor roughness and variations in level can be corrected by setting the first course in mortar. Blocks should be laid in running bond such that cells will be aligned vertically.

Grout and Reinforcement

Grout and grouting procedures should be the same as used in conventional masonry construction (ref. 1, 10) except that the grout must have a compressive strength of at least 2600 psi (190 MPa) at 28 days when tested in accordance with ASTM C 1019 (ref.12). Placement of grout can be accomplished in one lift for single-story height walls less than 8 ft (2.43 m). Grout lifts must be consolidated with an internal vibrator with a head size less than 1 in. (25 mm).

Vertical Reinforcing

As for conventional reinforced masonry construction, good construction practice should include placement of reinforcing bars around door and window openings, at the ends, top and bottom of a wall, and between intersecting walls. Well detailed reinforcement such as this can help enhance nonlinear deformation capacity, or ductility, of masonry walls in building systems subjected to earthquake loadings – even for walls designed as unreinforced elements. Additional information on conventional grouting and reinforced masonry wall can be found in TEK 09-04A and TEK 03-03B (refs. 9 & 6).

Pre-stressed Walls

Mortarless walls can also be prestressed by placing vertical tendons through the cores. Tendons can be anchored within the concrete foundation at the base of a wall or in a bottom bond beam and are tensioned from the top of a wall.

Surface Bonded Walls

For walls strengthened with a surface bonding, a thin layer of portland cement surface bonding material should be troweled or sprayed on to a wall surface. The thickness of the surface coating should be at least ⅛ in. (3.2 mm.) or as required by the material supplier.

ENGINEERING PROPERTIES

Walls constructed with mortarless masonry can be engineered using conventional engineering principles. Existing building code recommendations such as that produced by the building code (ref. 1) can serve as reference documents, but at the time of this printing it does not address mortarless masonry directly. It is thus considered an alternate engineered construction type. The International Building Code (ref. 7) does list allowable stresses based on gross-cross-sectional area for dry-stacked, surface-bonded concrete masonry walls. These values are the same as presented in TEK 03-05A (ref. 16). Suggested limits on wall or building height are given in Table 1.

Test data (refs. 2, 3 and 4) have shown that the strength of drystack walls exceeds the strength requirements of conventional masonry, and thus the recommended allowable stress design practices of the code can be used in most cases. When designing unreinforced, grouted masonry wall sections, it is important to deduct the thickness of the tension side face shell when determining the section properties for flexural resistance.

Table 1 –– Summary of Wall Heights for 8” (203 mm) Dry-stacked Units (ref. 5)

* Laterally supported at each floor

Unit and Masonry Compressive Strength

Units used for mortarless masonry construction are made of the same concrete mixes as used for conventional masonry units. Thus, compressive strength of typical units could vary between 2000 psi (13.79MPa) and 4000 psi. (27.58 MPa) Standard Methods of Sampling and Testing Concrete Masonry Units (ref. 11) can be referred to for determining strength of dry-stack units.

Masonry compressive strength f’_m can conservatively be based on the unit-strength method of the building code (ref . 15), or be determined by testing prisms in accordance with ASTM C1314 (ref. 4). Test prisms can be either grouted or ungrouted depending on the type of wall construction specified.

Solid Grouted, Unreinforced Construction

Out-of-Plane & In-Plane Allowable Flexural Strength

Because no mortar is used to resist flexural tension as for conventional masonry construction, flexural strength of mortarless masonry is developed through the grout, reinforcement or surface coating. For out-of-plane bending of solid grouted walls allowable flexural strength can be estimated based on flexural tensile strength of the grout per Equation 1.

Consideration should be given to the reduction in wall thickness at the bed joints when estimating geometrical properties of the net effective section.

Correspondingly, flexural strength based on masonry compressive stress should be checked, particularly for walls resisting significant gravity loads, using the unity equation as given below.

Buckling should also be checked. (Ref. 8)

In-Plane Shear Strength

Shear strength for out-of-plane bending is usually not a concern since flexural strength governs design for this case. For resistance to horizontal forces applied parallel to the plane of a wall, Equation 3 may be used to estimate allowable shear strength.

F_v is the allowable shear strength by the lesser of the three values given in Equation 4.

Grouted, Reinforced Construction

Mortarless masonry that is grouted and reinforced behaves much the same as for conventional reinforced and mortared construction. Because masonry tensile strength is neglected for mortared, reinforced construction, flexural mechanisms are essentially the same with or without the bed joints being mortared provided that the units subjected to compressive stress are in good contact. Thus, allowable stress design values can be determined using the same assumptions and requirements of the MSJC code. (ref.1)

Out-of-Plane & In-Plane Allowable Flexural Strength

Axial and flexural tensile stresses are assumed to be resisted entirely by the reinforcement. Strains in reinforcement and masonry compressive strains are assumed to vary linearly with their distance from the neutral axis. Stresses in reinforcement and masonry compressive stresses are assumed to vary linearly with strains. For purposes of estimating allowable flexural strengths, full bonding of reinforcement to grout are assumed such that strains in reinforcement are identical to those in the adjacent grout.

For out-of-plane loading where a single layer of vertical reinforcement is placed, allowable flexural strength can be estimated using the equations for conventional reinforcement with the lower value given by Equations 5 or 6.

In-Plane Shear Strength

Though the MSJC code recognizes reinforced masonry shear walls with no shear, or horizontal reinforcement, it is recommended that mortarless walls be rein- forced with both vertical and horizontal bars. In such case, allowable shear strength can be determined based on shear reinforcement provisions (ref. 1) with Equations 7, 8 and 9.

Where F_v is the masonry allowable shear stress per Equations 8 or 9.

Solid Grouted, Prestressed Construction

Mortarless masonry walls that are grouted and pre- stressed can be designed as unreinforced walls with the prestressing force acting to increase the vertical compres- sive stress. Grout can be used to increase the effective area of the wall. Flexural strength will be increased because of the increase in the f_a term in Equation 1. Shear strength will be increased by the N_v term in Equation 4.

Because the prestressing force is a sustained force, creep effects must be considered in the masonry. Research on the long-term behavior of dry-stacked masonry by Marzahn and Konig (ref. 8) has shown that creep effects may be accentuated for mortarless masonry as a result of stress concentrations at the contact points of adjacent courses. Due to the roughness of the unit surfaces, high stress concentrations can result which can lead to higher non-proportional creep deformations. Thus, the creep coefficient was found to be dependent on the degree of roughness along bed-joint surfaces and the level of applied stress. As a result, larger losses in prestressing force is probable for dry-stack masonry.

Surface-Bonded Construction

Dry-stack walls with surface bonding develop their strength through the tensile strength of small fiberglass fibers in the 1/8” (3.8mm) thick troweled or surface bonded cement-plaster coating ASTM C-887(Ref. 14). Because no grouting is necessary, flexural tension and shear strength are developed through tensile resistance of fiberglass fibers applied to both surfaces of a wall. Test data has shown that surface bonding can result in a net flexural tension strength on the order of 300 psi.(2.07 MPa) Flexural capacity, based on this value, exceeds that for conventional, unreinforced mortared masonry construction, therefore it is considered conservative to apply the desired values of the code (ref. 1) for allowable flexural capacity for portland cement / lime type M for the full thickness of the face shell.

Out-of-Plane and In-Plane Flexural Strength

Surface-bonded walls can be considered as unreinforced and ungrouted walls with a net allowable flexural tensile strength based on the strength of the fiber-reinforcement. Flexural strength is developed by the face shells bonded by the mesh. Allowable flexural strength can be determined using Equation 1 with an F_t value determined on the basis of tests provided by the surface bonding cement supplier. Axial and flexural compressive stresses must also be checked per Equation 2 considering again only the face shells to resist stress.

Surface Bonded In-Plane Shear Strength

In-plane shear strength of surface-bonded walls is attributable to friction developed along the bed joints resulting from vertical compressive stress in addition to the diagonal tension strength of the fiber coating. If the enhancement in shear strength given by the fiber reinforced surface parging is equal to or greater than that provided by the mortar-unit bond in conventional masonry construction, then allowable shear strength values per the MSJC code (ref. 1) may be used. In such case, section properties used in Equation 3 should be based on the cross-section of the face shells.

Figure 3 – A Mortarless Garden Wall Application

Figure 4 – A Residential, Mortarless, Single-Family Basement – Part of a 520 Home Development

REFERENCES

Building Code Requirements for Masonry Structures), ACI 530-02/ ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee (MSJC), 2002.
Drysdale, R.G., Properties of Dry-Stack Block, Windsor, Ontario, July 1999.
Drysdale, R.G., Properties of Surface-Bonded Dry-Stack Block Construction, Windsor, Ontario, January 2000.
Drysdale, R.G., Racking Tests of Dry-Stack Block, Windsor, Ontario, October 2000.
Drysdale, R.G., Design and Construction Guide for Azar Dry-Stack Block Construction, JNE Consulting, Ltd., February 2001.
Grout for Concrete Masonry, TEK 09-04A, Concrete Masonry & Hardscapes Association, 2002.
2000 International Building Code, Falls Church, VA. International Code Council, 2000.
Marzahn, G. and G. Konig, Experimental Investigation of Long-Term Behavior of Dry-Stacked Masonry, Journal of The Masonry Society, December 2002, pp. 9-21.
Hybrid Concrete Masonry Construction Details, TEK 0303B. Concrete Masonry & Hardscapes Association, 2009.
Specification for Masonry Structures, ACI 530.1-02/ASCE 6-02/ TMS 602-02. Reported by the Masonry Standards Joint Committee (MSJC), 2002.
Standard Methods of Sampling and Testing Concrete Masonry Units, ASTM C140-02a, ASTM International, Inc. , Philadelphia, 2002.
Standard Method of Sampling and Testing Grout, ASTM C1019-02, ASTM International, Inc., Philadelphia, 2002.
Standard Specification for Grout for Masonry, ASTM C 476-02. ASTM International, Inc., 2002
Standard Specification for Packaged, Dry, Combined Materials for Surface Bonding Mortar, ASTM C 887-79a (2001). ASTM International, Inc., 2001.
Standard Test Method for Compressive Strength of Masonry Assem blages, ASTM C1314-02a, ASTM International, Inc., Philadelphia, 2002.
Surface Bonded Concrete Masonry Construction, TEK 03-05A. Concrete Masonry & Hardscapes Association, 1998.

NOTATION

A_n net cross-sectional area of masonry, in² (mm²)
A_s effective cross-sectional area of reinforcement, in2 (mm2)
b width of section, in. (mm)
d distance from extreme compression fiber centroid of tension reinforcement, in. (mm)
F_a allowable compressive stress due to axial load only, psi (MPa)
F_b allowable compressive stress due to ß exure only, psi (MPa)
F_s allowable tensile or compressive stress in reinforcement, psi (MPa)
F_t flexural tensile strength of the grout, psi(MPa)
F_v allowable shear stress in masonry psi (MPa)
f_a calculated vertical compressive stress due to axial load, psi (MPa)
f_b calculated compressive stress in masonry due to ß exure only, psi (MPa)
f’ specified compressive strength of masonry, psi (MPa)
I moment of inertia in.⁴ (mm⁴)
j ratio of distance between centroid of flexural compressive forces and centroid of tensile forces to depth, d
k ratio of the distance between compression face of the wall and neu tral axis to the effective depth d
M maximum moment at the section under consideration, in.-lb (N-mm)
N_v compressive force acting normal to the shear surface, lb (N)
Q first moment about the neutral axis of a section of that portion of the cross section lying between the neutral axis and extreme fiber in.³ (mm³)
S_g section modulus of uncracked net section in.³ (mm³)
V shear force, lb (N)