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Concrete Masonry Basement Wall Construction

  • November 2018

Introduction

Basements allow a building owner to significantly increase usable living, working, or storage space at a relatively low cost. Old perceptions of basements have proven outdated by stateofthe-art waterproofing, improved drainage systems, and natural lighting features such as window wells. Other potential benefits of basements include room for expansion of usable space, increased resale value, and safe haven during storms.

Historically, plain (unreinforced) concrete masonry walls have been used to effectively resist soil loads. Currently, however, reinforced walls are becoming more popular as a way to use thinner walls to resist large backfill pressures. Regardless of whether the wall is plain or reinforced, successful performance of a basement wall relies on quality construction in accordance with the structural design and the project specifications.

Materials

Concrete Masonry Units: Concrete masonry units should comply with Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90 (ref. 8). Specific colors and textures may be specified to provide a finished interior to the basement. Drywall can also be installed on furring strips, if desired. A rule of thumb for estimating the number of concrete masonry units to order is 113 units for every 100 ft2 (9.3 m2) of wall area. This estimate assumes the use of 3/8 in. (9.5 mm) mortar joints.

Mortar: Mortar serves several important functions in a concrete masonry wall; it bonds the units together, seals joints against air and moisture penetration, and bonds to joint reinforcement, ties, and anchors so that all components perform as a structural element.

Mortar should comply with Standard Specification for Mortar for Unit Masonry, ASTM C 270 (ref. 9). In addition, most building codes require the use of Type M or S mortar for construction of basement walls (refs. 2, 4, 5, 9, 13), because Type M and S mortars provide higher compressive strengths. Table 1 lists mortar proportions.

Typical concrete masonry construction uses about 8.5 ft3 (0.24 m3) of mortar for every 100 ft2 (9.3 m2) of masonry wall area. This figure assumes 3/8 in. (9.5 mm) thick mortar joints, face shell mortar bedding, and a 10% allowance for waste.

Grout: In reinforced concrete masonry construction, grout is used to bond the reinforcement and the masonry together. Grout should conform to Standard Specification for Grout for Masonry, ASTM C 476 (ref. 10), with the proportions listed in Table 2. As an alternative to complying with the proportion requirements in Table 2, grout can be specified to have a minimum compressive strength of 2000 psi (13.8 MPa) at 28 days. Enough water should be added to the grout so that it will have a slump of 8 to 11 in. (203 to 279 mm). The high slump allows the grout to be fluid enough to flow around reinforcing bars and into small voids. This initially high water-to-cement ratio is reduced significantly as the masonry units absorb excess mix water. Thus, grout gains high strengths despite the initially high waterto-cement ratio.

Construction

Prior to laying the first course of masonry, the top of the footing must be cleaned of mud, dirt, ice or other materials which reduce the bond between the mortar and the footing. This can usually be accomplished using brushes or brooms, although excessive oil or dirt may require sand blasting.

Masons typically lay the corners of a basement first so that alignment is easily maintained. This also allows the mason to plan where cuts are necessary for window openings or to fit the building’s plan.

To make up for surface irregularities in the footing, the first course of masonry is set on a mortar bed joint which can range from 1/4 to 3/4 in. (6.4 to 19 mm) in thickness. This initial bed joint should fully bed the first course of masonry units, although mortar should not excessively protrude into cells that will be grouted.

All other mortar joints should be approximately 3/8 in. (9.5 mm) thick and, except for partially grouted masonry, need only provide face shell bedding for the masonry units. In partially grouted construction, webs adjacent to the grouted cells are mortared to restrict grout from flowing into ungrouted cores. Head joints must be filled solidly for a thickness equal to a face shell thickness of the units.

Tooled concave joints provide the greatest resistance to water penetration. On the exterior face of the wall, mortar joints may be cut flush if parging coats are to be applied.

When joint reinforcement is used, it should be placed directly on the block with mortar placed over the reinforcement in the usual method. A mortar cover of at least 5/8 in. (15.9 mm) should be provided between the exterior face of the wall and the joint reinforcement. A mortar cover of 1/2 in. (12.7 mm) is needed on the interior face of the wall. For added safety against corrosion, hot dipped galvanized joint reinforcement is recommended.

See Figures 1-4 for construction details.

Reinforced Masonry: For reinforced masonry construction, the reinforcing bars must be properly located to be fully functional. In most cases, vertical bars are positioned towards the interior face of basement walls to provide the greatest resistance to soil pressures. Bar positioners at the top and bottom of the wall prevent the bars from moving out of position during grouting. A space of at least 1/2 in. (12.7 mm) for coarse grout and 1/4 in. (6.4 mm) for fine grout should be maintained between the bar and the face shell of the block so that grout can flow completely around the reinforcing bars.

As mix water is absorbed by the units, voids can form in the grout. Accordingly, grout must be puddled or consolidated after placement to eliminate these voids and to increase the bond between the grout and the masonry units. Most codes permit puddling of grout when it is placed in lifts less than about 12 in. (305 mm). Lifts over 12 inches (305 mm) should be mechanically consolidated and then reconsolidated after about 3 to 10 minutes.

Surface Bonding: Another method of constructing concrete masonry walls is to dry stack units (without mortar) and then apply surface bonding mortar to both faces of the wall. The surface bonding mortar contains thousands of small glass fibers. When the mortar is applied properly to the required thickness, these fibers, along with the strength of the mortar itself, help produce walls of comparable strength to conventionally laid plain masonry walls. Surface bonded walls offer the benefits of excellent dampproof coatings on each face of the wall and ease of construction.

Dry-stacked walls should be laid in an initial full mortar bed to level the first course. Level coursing is maintained by using a rubbing stone to smooth small protrusions on the block surfaces and by inserting shims every two to four courses.

Water Penetration Resistance: Protecting below grade walls from water entry involves installation of a barrier to water and water vapor. An impervious barrier on the exterior wall surface can prevent moisture entry.

The barrier is part of a comprehensive system to prevent water penetration, which includes proper wall construction and the installation of drains, gutters, and proper grading.

Building codes (refs. 2, 4 , 5, 9, 13) typically require that basement walls be dampproofed for conditions where hydrostatic pressure will not occur, and waterproofed where hydrostatic pressures may exist. Dampproofing is appropriate where groundwater drainage is good, for example where granular backfill and a subsoil drainage system are present. Hydrostatic pressure may exist due to a high water table, or due to poorly draining backfill, such as heavy clay soils. Materials used for waterproofing are generally elastic, allowing them to span small cracks and accommodate minor movements.

When choosing a waterproof or dampproof system, consideration should be given to the degree of resistance to hydrostatic head of water, absorption characteristics, elasticity, stability in moist soil, resistance to mildew and algae, impact or puncture resistance, and abrasion resistance. A complete discussion of waterproofing, dampproofing, and drainage systems is included in TEK 19-03A (ref. 6).

All dampproofing and waterproofing systems should be applied to walls that are clean and free from dirt, mud and other materials which may reduce bond between the coating and the concrete masonry wall.

Draining water away from basement walls significantly reduces the pressure the walls must resist and reduces the possibility of water infiltration into the basement if the waterproofing (or dampproofing) system fails. Perforated pipe has historically proven satisfactory when properly installed. When placed on the exterior side of basement walls, perforated pipes are usually laid in crushed stone to facilitate drainage. To prevent migration of fine soil into the drains, filter fabrics are often placed over the gravel.

Drainage pipes can also be placed beneath the slab and connected into a sump. Pipes through the footing or the wall drain water from the exterior side of the basement wall.

The drainage and waterproofing systems should always be inspected prior to backfilling to ensure they are adequately placed. Any questionable workmanship or materials should be repaired at this stage since repairs are difficult and expensive after backfilling.

Backfilling: One of the most crucial aspects of basement construction is how and when to properly backfill. Walls should be properly braced or have the first floor in place prior to backfilling. Otherwise, a wall which is designed to be supported at the top may crack or even fail from the large soil pressures. Figure 5 shows one bracing scheme which has been widely used for residential basement walls. More substantial bracing may be required for high walls or large backfill pressures.

The backfill material should be free-draining soil without large stones, construction debris, organic materials, and frozen earth. Saturated soils, especially saturated clays, should generally not be used as backfill materials since wet materials significantly increase the hydrostatic pressure on the walls.

Backfill materials should be placed in several lifts and each layer should be compacted with small mechanical tampers. Care should be taken when placing the backfill materials to avoid damaging the drainage, waterproofing or exterior insulation systems. Sliding boulders and soil down steep slopes should thus be avoided since the high impact loads generated can damage not only the drainage and waterproofing systems but the wall as well. Likewise, heavy equipment should not be operated within about 3 feet (0.9 m) of any basement wall system.

The top 4 to 8 in. (102 to 203 mm) of backfill materials should be low permeability soil so rain water is absorbed into the backfill slowly. Grade should be sloped away from the basement at least 6 in. (152 mm) within 10 feet (3.1 m) of the building. If the ground naturally slopes toward the building, a shallow swale can be installed to redirect runoff.

Construction Tolerances

Specifications for Masonry Structures (ref. 8) specifies tolerances for concrete masonry construction. These tolerances were developed to avoid structurally impairing a wall because of improper placement.

  1. Dimension of elements in cross section or elevation
    …………………………………….¼ in. (6.4 mm), +½ in. (12.7 mm)
  2. Mortar joint thickness: bed………………………..+⅛ in. (3.2 mm)
    head………………………………..-¼ in (6.4 mm), +⅜ in. (9.5 mm)
  3. Elements
    • Variation from level: bed joints……………………………………….
      ±¼ in. (6.4 mm) in 10 ft (3.1 m), ±½ in. (12.7 mm) max
      top surface of bearing walls……………………………………………..
      ±¼ in.(6.4 mm), +⅜ in.(9.5 mm), ±½ in.(12.7mm) max
    • Variation from plumb………….±¼ in. (6.4 mm) 10 ft (3.1 m)
      ………………………………………±⅜ in. (9.5 mm) in 20 ft (6.1 m)
      ……………………………………………±½ in. (12.7 mm) maximum
    • True to a line…………………..±¼ in. (6.4 mm) in 10 ft (3.1 m)
      ………………………………………±⅜ in. (9.5 mm) in 20 ft (6.1 m)
      ……………………………………………±½ in. (12.7 mm) maximum
    • Alignment of columns and bearing walls (bottom versus top)
      ……………………………………………………………..±½ in (12.7 mm)
  4. Location of elements
    • Indicated in plan……………..±½ in (12.7 mm) in 20 ft (6.1 m)
      …………………………………………….±¾ in. (19.1 mm) maximum
    • Indicated in elevation
      ……………………………………….±¼ in. (6.4 mm) in story height
      …………………………………………….±¾ in. (19.1 mm) maximum

Insulation: The thermal performance of a masonry wall depends on its R-value as well as the thermal mass of the wall. Rvalue describes the ability to resist heat flow; higher R-values give better insulating performance. The R-value is determined by the size and type of masonry unit, type and amount of insulation, and finish materials. Depending on the particular site conditions and owner’s preference, insulation may be placed on the outside of block walls, in the cores of hollow units, or on the interior of the walls.

Thermal mass describes the ability of materials like concrete masonry to store heat. Masonry walls remain warm or cool long after the heat or air-conditioning has shut off, keeping the interior comfortable. Thermal mass is most effective when insulation is placed on the exterior or in the cores of the block, where the masonry is in direct contact with the interior conditioned air.

Exterior insulated masonry walls typically use rigid board insulation adhered to the soil side of the wall. The insulation requires a protective finish where it is exposed above grade to maintain durability, integrity, and effectiveness.

Concrete masonry cores may be insulated with molded polystyrene inserts, expanded perlite or vermiculite granular fills, or foamed-in-place insulation. Inserts may be placed in the cores of conventional masonry units, or they may be used in block specifically designed to provide higher R-values.

Interior insulation typically consists of insulation installed between furring strips, finished with gypsum wall board or panelling. The insulation may be fibrous batt, rigid board, or fibrous blown-in insulation.

Design Features

Interior Finishes: Split faced, scored, burnished, and fluted block give owners and designers added options to standard block surfaces. Colored units can be used in the entire wall or in sections to achieve specific patterns.

Although construction with staggered vertical mortar joints (running bond) is standard for basement construction, the appearance of continuous vertical mortar joints (stacked bond pattern) can be achieved by using of scored units or reinforced masonry construction.

Natural Lighting: Because of the modular nature of concrete masonry, windows and window wells of a variety of shapes and sizes can be easily accommodated, giving basements warm, natural lighting. For additional protection and privacy, glass blocks can be incorporated in lieu of traditional glass windows.

References

  1. Basement Manual-Design and Construction Using Concrete Masonry, CMU-MAN-002-01, Concrete Masonry & Hardscapes Association, 2001.
  2. BOCA National Building Code. Country Club Hills, IL: Building Officials and Code Administrators International, Inc. (BOCA), 1999.
  3. Building Code Requirements for Masonry Structures, ACI 530-02/ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee, 2002.
  4. International Residential Code. Falls Church, VA: International Code Council, 2000.
  5. International Building Code. Falls Church, VA: International Code Council, 2000.
  6. Preventing Water Penetration in Below-Grade Concrete Masonry Walls, TEK 19-03A. Concrete Masonry & Hardscapes Association, 2001.
  7. Seismic Design Provisions for Masonry Structures, TEK 14-18B, Concrete Masonry & Hardscapes Association, 2009.
  8. Specifications for Masonry Structures, ACI 530.1-02/ASCE 6-99/TMS 602-02. Reported by the Masonry Standards Joint Committee, 2002.
  9. Standard Building Code. Birmingham, AL: Southern Building Code Congress International, Inc. (SBCCI), 1999.
  10. Standard Specification for Grout for Masonry, ASTM C 476-01. American Society for Testing and Materials, 2001.
  11. Standard Specification for Load-Bearing Concrete Masonry Units, ASTM C 90-01. American Society for Testing and Materials, 2001.
  12. Standard Specification for Mortar for Unit Masonry, ASTM C 270-00. American Society for Testing and Materials, 2000.
  13. Uniform Building Code. Whittier, CA: International Conference of Building Officials (ICBO), 1997.

Concrete Masonry Gravity Retaining Walls

  • August 2018

INTRODUCTION

Retaining walls support soil and other materials laterally. That is, retaining walls “retain” earth, keeping it from sliding. Retaining walls must resist overturning and sliding, and the pressure under the toe (front bottom edge of footing) should not exceed the bearing capacity of the soil. Finally, the wall must be strong enough to prevent failure at any point in its height due to the pressure of the retained material. Concrete masonry retaining walls meet these requirements admirably.

Three different types of concrete masonry retaining walls are illustrated in Figure 1. They are the simple unreinforced vertical face gravity retaining wall, the steel reinforced cantilever retaining wall, and the segmental retaining wall. This TEK addresses unreinforced gravity retaining walls only. Each of these retaining wall systems has its advantages, and the choice may depend on a number of factors including aesthetics, constructibility, cost, and suitability for a particular project. The gravity wall is much simpler in design and construction, and can be an effective choice for smaller projects. It is thicker at the base than cantilever and segmental walls, and hence could cost more to construct on larger projects. Gravity retaining walls resist sliding by means of their large mass, whereas cantilever retaining walls are designed to resist sliding by using reinforcement. Because of their large mass, gravity retaining walls may not be appropriate for use on soils with low bearing capacities.

An engineer who is familiar with local conditions can assist in the choice of retain ing wall type. Where especially unfavorable soil conditions occur or where piling is required under a retaining wall, the assistance of an engineer is essential for design and construction.

Figure 1—Concrete Masonry Retaining Walls

DESIGN

The primary force acting on a retaining wall is the pressure exerted by the retained material at the back of the wall and on the heel of the footing. The magnitude and direction of this pressure depends on the height and shape of the surface and on the nature and properties of the backfill. One common method of estimating backfill pressure is the equivalent fluid pressure method. In this method, it is assumed that the retained earth will act as a fluid in exerting pressure on the wall. Assumed equivalent fluid pressures vary with the type of soil. Representative soil types with their equivalent fluid pressures are shown in Table 1.

Since the stability of the gravity type retaining wall depends mainly on its weight, the thickness required at its base will increase with height of backfill, or wall height. Uplift pressure at the back of the wall (the heel) is avoided by designing the gravity retaining wall thick enough at the base so that the resultant of all forces (overturning force and vertical loads) falls within a zone called the kern, which is the middle one third of the base. The eccentricity of the resultant force is equal to or less than one-sixth of the base width. When the eccentricity, e, is equal to one-sixth the base width exactly, the maximum footing pressure on the soil at the front edge of the base (toe) will be twice the average pressure on the soil.

The horizontal force of the retained material causes the overturning moment on the gravity retaining wall. For a given wall height, the required thickness at the base will depend not only on height, but also on the magnitude of the equivalent fluid pressure of the retained soil. The two forces act in opposition; the horizontal force tends to overturn the wall, while the vertical forces tend to stabilize it via gravity. The ratio of wall height to base width will vary with the ratio of vertical pressure to horizontal pressure. More properly, the relationship between thickness of base and wall height can be expressed:

where:
H = height of gravity retaining wall, in. (mm)
L = width of gravity retaining wall at base, in. (mm)
Q = equivalent fluid pressure of retained material acting horizontally as overturning moment, pcf (kg/m³)
W = average weight of masonry, soil and other material acting vertically to retain soil, pcf (kg/m³)

This relationship between wall height and base width for gravity retaining walls is shown in Figure 2 for different ratios of horizontal to vertical unit loads. The relationship shown in Figure 2 is employed in the selection of dimensions for gravity retaining walls up to eight ft (1.8 to 2.4 m) high.

Having selected the height-base proportions from Figure 2, the trial design is analyzed for safety against overturning and sliding, bearing pressure on the soil, and flexural and shear stress in the wall.

Figure 2—Relationship of Gravity Retaining Wall Height to Width at Base

CONSTRUCTION AND MATERIALS

Each course of the retaining wall should be constructed with full-size concrete masonry units, with an overlapping bond pattern between courses, as shown in Figure 3.

Hollow or solid concrete masonry units used in gravity retaining walls should meet the requirements of ASTM C 90 (ref. 2) and preferably have an oven-dry density of 125 lb/ft³ (2002 kg/m³) or more. Cores of hollow units are typically filled to increase the weight of the wall. The fill should be granular in areas subject to freezing. Bond is important to ensure sufficient shear resistance to withstand the pressure exerted by the retained earth. Type M or S mortars complying to ASTM C 270 (ref. 3) are recommended.

Concrete footings should be placed on firm undisturbed soil. In areas where freezing is expected, the base of the footing should be placed below the frost line. If the soil under the footing consists of soft or silty clay, it is usually advisable to place 4 to 6 in. (102 to 152 mm) of well compacted sand or gravel under the footing before pouring the concrete. It is usually not necessary to reinforce the footing.

If heavy equipment is employed for backfilling, it should not be allowed to approach closer to the top of the wall than a distance equal to the wall height. Care should also be taken to avoid large impact forces on the wall as could occur by a large mass of moving earth.

Provision should be made to pre vent water accumulation behind the retaining wall. Accumulated water causes increased pressure, seep age, and in areas subject to frost action, an expansive force of considerable magnitude near the top of the wall. In most instances, weep holes located at 5 to 10 foot (1.5 to 3 m) spacing along the base of the wall are sufficient.

Figure 3—Overlapping Bond Between Courses

DESIGN EXAMPLES

  1. 4-foot (1.2 m) high gravity retaining wall
    equivalent fluid pressure of soil = 30 pcf (4.7 kN/m³)
    soil weight = 100 pcf (15.7 kN/m³)
    soil friction coefficient = 0.55
    soil bearing capacity = 2000 lb/ft² (0.096 MPa)
    100% solid concrete masonry units, 120 pcf (18.9 kN/m³)
    concrete footing, 150 pcf (23.6 kN/m³)

First, determine the width of the wall base:

From Figure 2, the base of the wall is 24 in. (610 mm), which can be accomplished using three 8-inch (203 mm) block. Note that the footing weight was not included in the calculation of average unit weight of the materials acting vertically, so that the width determined from Figure 2 would be the width of the masonry wall at its base.

Determine overturning moment:
pressure at the base of the wall, p = total soil height x equivalent fluid pressure of soil
p = (4.67 ft)(30 pcf) = 140 lb/ft² (6703 Pa)
resultant pressure, P = ½ (p)(total soil height)
P = ½ (140 lb/ft²)(4.67 ft) = 327 lb/ft (4.8 kN/m)

Determine resisting moment (about the toe):
First, determine the weight of each element, then determine the resisting moment of each weight, then sum the resisting moments to determine the total resisting moment.

 

Element:Weight 
S1(0.67 ft)(1.33 ft)(100 pcf)= 89 lb (396 N)
S2(0.67 ft)(2.67 ft)(100 pcf)= 179 lb (796 N)
S3(0.33 ft)(4.0 ft)(100 pcf)= 132 lb (587 N)
M1(0.67 ft)(4.0 ft)(120 pcf)= 322 lb (1432 N)
M2(0.67 ft)(2.67 ft)(120 pcf)= 214 lb (952 N)
M3(0.67 ft)(1.33 ft)(120 pcf)= 107 lb (476 N)
F(2.67 ft)(0.67 ft)(150 pcf)= 268 lb (1192 N)

 

Element:Weight, lb (N) xArm, ft (m) =Moment, ft-lb (N-m)
S189 (396)1.33 (0.41)118.5 (161)
S2179 (796)2.00 (0.61)357.8 (485)
S3132 (587)2.50 (0.76)330.0 (447)
M1322 (1432)0.67 (0.20)215.5 (292)
M2214 (952)1.33 (0.41)285.5 (387)
M3107 (476)2.00 (0.61)213.9 (290)
F268 (1192)1.33 (0.41)356.4 (483)
Total1311 (5832) 1878 (2546)

 

Determine the overturning moment about the base, M:
M = (P)(⅓ x total height of soil)
M = (327 lb/ft)(⅓ x 4.67 ft) = 509 ft-lb/ft (2.28 kN-m/m)

Check safety factors:
overturning moment safety factor = 1878/509 = 3.7
3.7 > 2 OK
sliding safety factor = (1311 lb)(0.55)/(327 lb/ft) = 2.2
2.2 > 1.5 OK

Check pressure on soil:

Since the concrete masonry used in this example is assumed solid or fully grouted, the calculations do not include a check of shear stresses and flexural stresses in the wall. Flexural and shear stresses are checked in the second design example, and it is seen that the magnitudes are very low. Flexural and shear stresses in gravity retaining walls will almost always be of minor importance.

  1. 6-foot (1.8 m) high gravity retaining wall
    equivalent fluid pressure of soil = 40 pcf (7.1 kN/m³)
    soil weight = 100 pcf (15.7 kN/m³)
    soil friction coefficient = 0.55
    soil bearing capacity = 2000 lb/ft² (0.096 MPa)
    hollow concrete masonry units, 130 pcf (20.4 kN/m³), units will be filled with sand, resulting in a combined weight of 115 pcf (18.1 kN/m³)
    f’m = 1500 psi (10.3 MPa)

Type S portland cement-lime mortar concrete footing, 150 pcf (23.6 kN/m³)

First, determine the width of the wall base:

From Figure 2, try a base width of 42 in. (1067 mm), with a footing width of 50 in. (1270 mm)

Determine overturning moment:
p = (6.67 ft)(40 pcf) = 267 lb/ft² (0.013 MPa)
P = ½ (267 lb/ft²)(6.67 ft) = 890 lb/ft (13 kN/m)
M = (890 lb/ft)(⅓ x 6.67 ft) = 1978 ft-lb/ft (8.81 kN-m/m)

Element:Weight, lb (N) xArm, ft (m) =Moment, ft-lb (N-m)
S122 (98)1.50 (0.46)33 (45)
S244 (196)1.83 (0.56)80 (108)
S366 (294)2.17 (0.66)143 (194)
S488 (391)2.50 (0.76)220 (298)
S5110 (489)2.83 (0.86)311 (422)
S6132 (587)3.17 (0.97)418 (566)
S7154 (685)3.50 (1.07)539 (731)
S8176 (783)3.83 (1.17)674 (914)
S9198 (881)4.17 (1.27)826 (1120)
M1690 (3070)0.83 (0.25)575 (780)
M2202 (899)1.50 (0.46)303 (411)
M3177 (787)1.83 (0.56)325 (441)
M4152 (676)2.17 (0.66)329 (446)
M5126 (560)2.50 (0.76)316 (428)
M6101 (449)2.83 (0.86)287 (389)
M776 (338)3.17 (0.97)241 (327)
M850 (222)3.50 (1.07)177 (240)
M925 (111)3.83 (1.17)97 (132)
F419 (1864)2.08 (0.63)872 (1182)
Total3008 (13,380)6766 (9173)

 

Check safety factors:
overturning moment safety factor = 6766/1978 = 3.4
3.4 > 2 OK
sliding safety factor = (3008 lb)(0.55)/(890 lb/ft) = 1.9
1.9 > 1.5 OK

Check pressure on soil:
location of P and eccentricity, e:

Check flexural stresses:
At 6 ft (1.8 m) depth:
P = ½ (6 ft)(40 pcf)(6 ft) = 720 lb (3203 N)
M = (720 lb)(⅓ x 6 ft) = 1440 ft-lb (1952 N-m)

Assume mortar bed is 50% of gross area:

Check shear stresses:

REFERENCES

  1. Building Code Requirements for Masonry Structures, ACI 530-95/ASCE 5-95/TMS 402-95. Reported by the Masonry Standards Joint Committee, 1995.
  2. Standard Specification for Load-Bearing Concrete Masonry Units, ASTM C 90-94. American Society for Testing and Materials, 1994.
  3. Standard Specification for Mortar for Unit Masonry, ASTM C 270-92a. American Society for Testing and Materials, 1992.

Designing Concrete Masonry Walls for Wind Loads

  • August 2018

INTRODUCTION

Traditionally, empirical requirements have been used for the selection of masonry wall dimensions and lateral support spacing for resistance to wind pressures. These empirical requirements provide satisfactory results for buildings less than 35 ft (11 m) in height where the basic wind pressure does not exceed 25 psf (1197 Pa). This TEK addresses those cases where it is necessary or desirable to undertake a more thorough structural analysis of the lateral wind resistance of a concrete masonry wall or wall-pilaster combination.

Such analysis involves a knowledge of the magnitude and distribution of the wind force to various elements of a masonry structure and the flexural and shear strength of these elements. The information in this TEK provides guidelines for the design of masonry walls supported in both the vertical and horizontal directions.

WALLS

The need to investigate the lateral wind resistance capacity of a wall is usually greater in the case of plain (unreinforced) nonbearing or lightly loaded masonry walls because the vertical load on the wall may be insufficient to completely offset the development of flexural tension. Analysis of masonry walls is often based on the assumption that lateral loads are transmitted in the vertical direction with no end fixity at the lateral supports. Although this approach is straightforward, it may be overly conservative when the ratio of horizontal to vertical distances between lateral supports is relatively small and end fixity is developed. In such cases, end fixity and two-way bending can be utilized.

When wind loads are applied normal to a masonry wall surface, the loads are transmitted to horizontal supports (floors, roofs, beams) and/or vertical supports (cross walls, pilasters). Wall panels are usually assumed to function structurally as thin plates or slabs. For simplicity, walls are often designed to span horizontally between vertical supports or to span vertically between horizontal supports. However, walls can be designed assuming two-way bending using pilasters or cross walls as well as the roof structure and footing as supports. Assuming that the flexural resistance and rigidity of the walls in both the vertical and horizontal spans are known, the lateral load capacity and the proportion of the lateral load transmitted vertically and horizontally to the edge supports will depend on the restraint developed at the edges, the horizontal to vertical span ratio of the panel, and the distribution of the loads applied to the wall panels.

The curves in Figure 1 can be used to approximate the proportion of wind load transmitted in the vertical and horizontal directions. These curves are based on the assumption that the moment of inertia and modulii of elasticity of the walls will be the same in both the horizontal and vertical directions. The curves were derived by equating the theoretical formulas for calculating the maximum deflection for a strip of wall in both directions. It was further assumed that the walls either have no openings, or that any wall openings are located so that their effect on the stiffness of the wall panel is the same in both directions, and that the wall panels on each side of the support are similar in length and height.

When calculating the wind load to be carried by a vertical support, such as a pilaster, a value for K corresponding to the assumed support conditions at the edges of the wall panels and the appropriate wall panel length-to-height ratio is selected from the curves. Then, the value of wp is determined from the formula given at the top of Figure 1. This value, wp, represents the load which, when applied as a uniformly distributed load over the height of the pilaster, will approximate the actual wind load transmitted to the pilaster by the walls under the design conditions.

Figure 1—Approximate Horizontal and Vertical Wind Load Distribution

Design Example

To illustrate the use of the curves and formula given in Figure 1, assume a building with exterior walls spanning 12 ft (3.7 m) vertically between the floor and the roof is designed to resist a wind pressure of 20 psf (958 Pa). The walls are also supported horizontally at 18 ft (5.5 m) by pilasters which are built integrally with the wall. The roof loads will be carried by trusses simply supported on the pilasters, so the walls will be considered free at the top and fixed at the bottom and at the pilasters.

Selecting the appropriate value for K from the curve given for Case 1-A and a wall length-to-height ratio of 18/12 or 1.50, the wind load per foot of height to be carried by the pilasters, wp, may be calculated as follows:

wp =Kwd X
wp = (0.91) (20 psf) (18 ft)
= 328 lb/ft (4787 N/m)

The value of 328 lb/ft (4787 N/m) represents the uniformly distributed load which, when considered to be applied over the full height of the pilaster, will approximate the actual load transmitted to the pilasters by the adjacent walls under the design conditions. The moment and shear developed in the pilasters as a result of this load will depend on the assumed top and bottom support conditions for the pilaster.

The wall construction consists of 12 in. (305 mm) hollow concrete masonry units laid in running bond with face shell mortar bedding, using Type N portland cement lime mortar. Additional design information includes:

Section modulus, S = 159.9 in.³/ft (0.009 m³/m)
Net area, An = 36 in.²/ft (0.08 m²/m)
Allowable tensile stress parallel to bed joints = 1.33 x 38 psi = 50.5 psi (0.35 MPa) (ref. 1)
Allowable tensile stress normal to bed joints = 1.33 x 19 psi = 25.3 psi (0.17 MPa) (ref. 1)

As already determined, the horizontal span carries 91% of the wind load. With the wall fixed at the ends, the maximum moment, M, in the horizontal span (from Figure 2) is:

The flexural tensile stress in the horizontal span, ft, is:

The allowable tensile stress for hollow units, Type N mortar, tension parallel to bed joints, was determined to be 50.5 psi (0.35 MPa). Since the calculated tensile stress is less than the allowable, the design meets the code criteria.

In the vertical span, the wall described above carries 9% (1 – 0.91) of the wind load. Since the wall is free at the top and fixed at the base, the maximum moment is:

The flexural tensile stress in the vertical span is:

This value can be reduced by the dead load stress on the wall at the point of maximum moment. Assuming that the wall weighs 50 lb/ft² (2394 N/m²):

This results in a net axial compressive stress of 7 psi (48.3 Pa).

PILASTERS

A pilaster is a thickened wall section or vertical support built contiguous with and forming a part of the masonry wall. Pilasters are often used to stiffen masonry walls and to provide all or part of the lateral support. They may be built of hollow or solid units (manufactured in one or two pieces), grouted hollow units or reinforced hollow units. Pilasters function primarily as flexural members when used only for lateral support although they can also be used to support vertical compressive loads.

When designing pilasters, the lateral loads transmitted to the pilasters by the adjacent wall panels must be determined. Figure 1 can be used to approximate the proportion of wind load which is transmitted horizontally to pilasters and to calculate the approximate wind load carried by a pilaster.

The formulas given in Figure 2 can be used to calculate the maximum moment and shear on a pilaster after wp and the support conditions for the pilaster have been determined.

Consider the design described in the previous design example. From Figure 1, it was determined that for Case 1-A with span ratio of 1.5, approximately 91% of the wind load is transmitted in the horizontal span. If the pilasters in the above example are assumed to be fixed at the bottom and simply supported at the top, the maximum moment and shear values are as follows:

The pilaster, therefore, should be designed to provide an allowable moment and shear resistance equal to or greater than the above values.

Figure 2—Formulas For Maximum Moment and Shear on Walls and Pilasters Subjected To Uniform Lateral Loads

NOTATION:

An     = net cross-sectional area of masonry, in.²/ft (m²/m)
ft       = flexural tension in masonry, psi (MPa)
H      = height of wall, ft (m)
K       = proportion of wind load transmitted horizontally to pilasters or cross walls
M      = moment, in.-lb/ft (N•m/m)
S        = section modulus, in.³/ft (m³/m)
Vmax = maximum shear, lb/ft (N/m)
w       = uniformly distributed wind load, psf (Pa)
wd     = design wind load on wall, psf (Pa)
wp     = uniform lateral load which approximates the actual wind load transmitted by the walls to the pilasters or cross walls, lb/ft of height (N/m)
X       = horizontal span of wall, from center to center of pilasters or cross walls, ft (m)

REFERENCES

  1. Building Code Requirements for Masonry Structures, ACI 530-92/ASCE 5-92/TMS 402-92. Reported by the Masonry Standards Joint Committee, 1992.

TEK 14-03A, Revised 1995.

Concrete Masonry Foundation Wall Details

  • August 2018

INTRODUCTION

Concrete masonry is used to construct various foundation wall types, including full basement walls, crawlspace walls, stem walls and piers. Concrete masonry is well suited for below grade applications, because of its strength, durability, economy, and resistance to fire, insects and noise. The modular nature of concrete masonry allows floor plan and wall height changes to be easily accommodated as well. Concrete masonry can be used to provide a strong, durable, energy efficient and insect resistant foundation for all building types.

This TEK contains details for various types of concrete masonry foundation walls, with accompanying text as appropriate. The reader is referred to TEK 03-11, Concrete Masonry Basement Wall Construction, TEK19-03B, Preventing Water Penetration in Below Grade Concrete Masonry Walls and CMHA’s Basement Manual for more detailed design and construction information (refs. 2, 3, 4, respectively).

Footings

Footings lie under the basement, crawlspace or stem wall and transfer structural loads from the building to the supporting soil. Footings are typically cast-in-place concrete, placed beneath the frost depth to prevent damage resulting from heaving caused by freezing of water in the soil.

Footings should be placed on undisturbed native soil, unless this soil is unsuitable, weak or soft. In this case, the soil should be removed and replaced with compacted soil, gravel or concrete. Similarly, tree roots, construction debris and ice should be removed prior to placing footings.

Unless otherwise required, footings should be carefully aligned so that the concrete masonry wall will be near the center line of the footing. Although the top surface of poured concrete footings should be relatively level, it should generally not be troweled smooth, as a slightly roughened surface enhances the bond between the mortar and concrete. Concrete footing design is governed by Building Code Requirements for Structural Concrete, ACI 318 (ref. 5), and concrete foundations are constructed with tolerances conforming to the requirements of Standard Specifications for Tolerances for Concrete Construction and Materials, ACI 117 (ref. 9).

BASEMENT WALLS

Basements are typically built as conditioned space so that they can be used for storage, work or living space. Because of this, water penetration resistance is of paramount importance to basement wall design and construction.

Following recommended backfill procedures will help prevent basement wall cracking during this operation. Walls should always be properly braced to resist backfill soil loads or have the first floor diaphragm in place prior to backfilling. Otherwise, a wall designed to be supported at the top may crack or even fail from overstressing the wall. Similarly, heavy equipment, such as bulldozers or cranes, should not be operated over the backfill during construction unless the basement walls are appropriately designed for the higher resulting loads.

The top 4 to 8 in. (102 to 203 mm) of backfill should be low permeability soil so rain water absorption into the backfill is minimized. Finished grade should be sloped away from the building.

Control joints are not typically used in foundation walls due to concerns with waterproofing the joint and the fact that shrinkage is less significant in below grade walls due to relatively constant temperature and moisture conditions. If warranted, horizontal joint reinforcement can be installed as a crack control measure.

The foundation drain shown in Figures 1 and 2 can also be located on the interior side of the footing, or on both sides if necessary. The drain should be placed below the top of the footing. The optional footing drain shown, such as 2 in. (51 mm) PVC pipe at 8 ft (2400 mm) on center, allows water on the interior to reach the foundation drain. Footing drains can either be cast into the footing or constructed using plastic pipes through the bottom of the first course of masonry, directly on top of the footing.

For reinforced construction (Figure 2), reinforcing bars must be properly located to be fully functional. In most cases, vertical reinforcement is positioned towards the interior face of below grade walls to provide the greatest resistance to soil pressures.

A solid top course on the below grade concrete masonry wall spreads loads from the building above and also improves soil gas and termite resistance. Where only the top course is to be grouted, wire mesh or another equivalent grout stop material can be used to contain the grout to the top course. Note that local codes may restrict the use of foam plastic insulation below grade in areas where the hazard of termite damage is high.

STEMWALLS FOR CRAWLSPACES

Unlike basements, crawlspaces are typically designed as unconditioned spaces, either vented or unvented. Several alternate crawlspace constructions are shown in Figures 3 and 4.

Although most building codes require operable louvered vents near each corner of a crawl space to reduce moisture buildup, research has shown that the use of a moisture retardant ground cover eliminates the need for vents in many locations (ref. 6). If the crawlspace is vented, the floor, exposed pipes and ducts are typically insulated. If unvented, either the walls or the floor above can be insulated. Unvented crawlspaces must have a floor covering to minimize moisture and, where applicable, soil gas entry. A vapor retarder (typically 6-mil (0.15 mm) polyethylene, PVC or equivalent) is good practice to minimize water migration and soil gas infiltration. A 2 1/2 in. (64 mm) concrete mud slab is generally used when a more durable surface is desired for access to utilities. A thicker concrete slab may be desirable, particularly if the crawlspace will be used for storage. A dampproof coating on the exterior crawlspace wall will also help prevent water entry into the crawlspace.

STEMWALLS FOR SLAB ON GRADE

A stemwall with slab on gradesupports the wall above and often also provides a brick (ref. 7) requires a foundation pier to have a minimum nominal thickness of 8 in. (203 mm), with a nominal height not exceeding four times its nominal thickness and a nominal length not exceeding three times its nominal thickness. Note that the International Building Code, (ref. 8) allows foundation piers to have a nominal height up to ten times the nominal thickness if the pier is solidly grouted, or four times the nominal thickness if it is not solidly grouted.

REFERENCES

  1. Annotated Design and Construction Details for Concrete Masonry, CMU-MAN-001-03, Concrete Masonry and Hardscapes Association, 2003.
  2. Concrete Masonry Basement Wall Construction, TEK 03-11, Concrete Masonry and Hardscapes Association, 2001.
  3. Preventing Water Penetration in Below-Grade Concrete Masonry Walls, 19-03B, Concrete Masonry and Hardscapes Association, 2012.
  4. Basement Manual: Design and Construction using Concrete
    Masonry, CMU-MAN-002-01, Concrete Masonry and Hardscapes
    Association, 2001.
  5. Building Code Requirements for Structural Concrete, ACI 318 -02.
    American Concrete Institute, 2002.
  6. 2001 ASHRAE Handbook, Fundamentals. American Society
    of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2001.
  7. Building Code Requirements for Masonry Structures, ACI 530-02/
    ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee, 2002.
  8. International Building Code. International Code Council, 2000.
  9. Standard Specifications for Tolerances for Concrete Construction and Materials, ACI 117-90. American Concrete Institute, 1990.

Bracing Concrete Masonry Walls Under Construction

  • August 2018

INTRODUCTION

Building codes typically place responsibility for providing a reasonable level of life safety for workers during construction on the erecting contractor. Various methods are employed to protect workers while newly constructed masonry walls are curing and/or until the roof or other structural supports are in place. This TEK provides guidelines for masonry wall stability to resist the lateral loading effects of wind during construction. It is based on principles set forth in the Council for Masonry Wall Bracing’s Standard Practice for Bracing Masonry Walls Under Construction (ref. 1), but has been updated in accordance with the design provisions of the 2011 Building Code Requirements for Masonry Structures (MSJC, ref. 2).

When other lateral loads such as impact, seismic, scaffolding, and lateral earth pressure are present, they need to be considered and evaluated separately. The Walls Subject to Backfilling section at the end of this TEK discusses bracing and support of basement walls during backfilling operations.

WALLS SUBJECT TO WIND LOADS

There are several strategies and considerations for protecting life safety on the jobsite. These include internal bracing, external bracing and evacuation zones. The combination of strategies appropriate for a particular job may depend on the type of masonry construction, masonry wall heights, the time elapsed since construction, and wind speeds at the site.

The industry term “internal bracing” is relatively new. Internal bracing refers to the stability of a masonry assembly to resist wind loads through self-weight and allowable flexural stresses within the masonry.

The use of evacuation zones recognizes that it may be impractical to prevent the collapse of a masonry wall during construction when subjected to extreme loading conditions and that life safety is the primary concern. At prescribed wind speeds (taken as three-second gusts measured at the job site), the wall and the area around it is evacuated. The critical wind speed resulting in evacuation depends on the age of the wall being constructed and involves the three terms: “restricted zone,” “initial period,” and “intermediate period.”

Restricted Zone

The restricted zone is the area on each side of a wall subject to the effect of a masonry wall collapse. It is defined by a length equal to the height of the constructed wall plus 4 ft (1.22 m) on both sides of the wall, and a width equal to the wall length plus 4 ft (1.22 m) on both ends of the wall, as shown in Figure 1. When wind speeds exceed those allowed during the initial and intermediate periods, there is a chance that the masonry wall could fail, and the restricted zone must be evacuated in order to ensure life safety.

Figure 1—Restricted Zone for Masonry Walls
Initial Period

The initial period is the period of time, not to exceed 24 hours, during which the masonry is being laid above its base or highest line of bracing, and at the end of which required bracing is installed. During this period, the mortar is assumed to have no strength and wall stability is accomplished from the masonry self-weight only. Based on this assumption and a wind speed limit of 20 mph (8.9 m/s), walls can be built to the heights shown in Table 1 without bracing during the initial period. If wind speeds exceed 20 mph (8.9 m/s) during the initial period, work on the wall must cease and the restricted zone on both sides of the wall must be evacuated. Evacuation for walls up to 8 ft (2.44 m) above grade is not necessary until wind speeds reach 35 mph (15.6 m/s) in keeping with a long-standing OSHA requirement.

Intermediate Period

The intermediate period is the period of time following the initial period but before the wall is connected to the elements that provide its final lateral support. The design wind speed is 40 mph (17.9 m/s) 3-second gust for brace design. When the wind speed exceeds 35 mph (15.6 m/s), the restricted zone must be evacuated. The difference of 5 mph (2.2 m/s) is to allow workers time to evacuate the area.

During the intermediate period, the masonry is assumed to have one-half of its design compressive strength and plain masonry allowable flexural stresses are taken as two-thirds of the design value given in the 2011 MSJC (ref. 2). The masonry structural capacity then can be determined using these reduced values in accordance with the provisions of the Code (see ref. 3 for more information).

There are several methods of providing an acceptable level of life safety for masons and others working on the construction site. They are:

  1. bracing to a design wind speed of 40 mph (17.9 m/s), 3-second gust and evacuating if the wind speed exceeds 35 mph (15.6 m/s), 3-second gust,
  2. alternative bracing designs and methods approved, sealed, and signed by a registered professional engineer if supported by data representing field conditions, and
  3. an early warning and evacuation program when the masonry is designed to resist a wind speed of 5 mph (2.2 m/s) greater than the designated evacuation wind speed. The wind speed measurement must be made by an instrument with a ± 2 mph (0.89 m/s) accuracy.

Traditionally, bracing and evacuation of the restricted zones has also been based on wind speeds lower that 35 mph (15.6 m/s). As such, Table 2 addresses evacuation wind speeds of 15 and 25 mph (6.7 and 11.2 m/s) in addition to the 35 mph (15.6 m/s) evacuation wind speed. Many jurisdictions will accept the lower wind speed criteria but users should first confirm acceptability with their local building official and/or OSHA representative before using them.

Table 2 lists maximum unbraced wall heights when early warning with an evacuation program is implemented. Design wind speeds for the heights in Table 2 are for 5 mph (2.2 m/s) greater than the evacuation speed to allow time for the masons to get off the scaffolding and evacuate the restricted zone.

Figure 2 shows a wood brace detail for support heights up to 14′-4″ (4.37 m) maximum. Proprietary pipe bracing systems and cable systems are also available for all heights shown in Table 2—see manufacturer’s recommendations for details.

Research has shown that properly designed and constructed reinforcement splices can achieve up to 75% of the specified yield stress of the reinforcing steel at 12 hours and 100% at 24 hours (ref. 1). Therefore, the full capacity of splices may be used after grout has been in place 24 hours. Alternatively, the full splice capacity can be used after only 12 hours if the design lap length is increased by one-third. Splice criteria is as follows for Grade 60 reinforcement:

  • 48 bar diameters for grout that has been in place 24 hours or more,
  • 64 bar diameters for grout that has been in place 12 hours or more but less than 24 hours.

Connections to masonry can be designed using the previously described reduced masonry strengths and design formulas. As an alternate, restricted working loads for post- drilled anchors as reported in the manufacturer’s literature may be used.

Figure 2—Wood Brace Detail
Design Example

Determine the bracing requirements for a 22 ft (6.71 m) tall wall constructed with 8 in. (203 mm) concrete masonry having a density of 110 lb/ft3 (1762 kg/m3) and reinforced with No. 5 bars at 32 in. (M#16 at 813 mm) o.c. using 30 in. (762 mm) splice lengths (i.e., 48 bar diameters). Mortar is masonry cement Type S, control joints are spaced at 24′-8″ (7.52 m), and flashing is at the base of the wall only (unbonded condition).

Initial Period

From Table 1:

Maximum unsupported height = 10′-0″ (3.05 m). (These initial period provisions apply to all of the options that follow.)

Intermediate Period—Unbraced Option

From Table 2:

Alternate 1: Evacuation wind speed of 15 mph (6.7 m/s).

NOTE: Although this type of option has historically been accepted, the designer should verify acceptance with the local building official and/or OSHA representative.

Unreinforced wall:

Maximum height above grade, unbonded = 10′-0″ (3.05 m)

Maximum height above grade or line of support, bonded = 23′-0″ (7.01 m)

Reinforced wall:

Maximum height, bonded or unbonded = 23′-4″ (7.11 m) for No. 5 at 48 in. (M#16 at 1.22 m)

This is conservative, because the wall in this example has reinforcement spaced closer than 48 in. (1.22 m).

Strategy:

Build the wall to a height of 10′-0″ (3.05 m) the first day (initial period).

The maximum height for an unbonded condition during the intermediate period is 10′-0″ (3.05 m) for this wind speed, therefore neither bracing nor grouting is required for the 10′-0″ (3.05 m) height during the intermediate period.

If the wall is reinforced and grouted, it can support a total height of 23′-4″ (7.11 m). Therefore, if the first 10′-0″ (3.05 m) is reinforced and grouted, another 10′-0″ (3.05 m) (initial period limit) could be built 24 hours after grout placement if the standard 30 in. (1,016 mm) reinforcement splice is used (or after 12 hours with a 40 in. (762 mm) splice). The 10′- 0″ (3.05 m) height is less than the 23′-0″ (7.01 m) unbraced limit for the bonded unreinforced intermediate period and the total 20′-0″ (6.10 m) of constructed wall height is less than the reinforced limit of 23′-4″ (7.11 m).

The next day, the top 2 ft (0.61 m) of masonry can be added, because the initial period limit of 10′-0″ (3.05 m) is met, the maximum unreinforced bonded limit of 23′-0″ (7.01 m) is met, and the reinforced limit of 23′-4″ (7.11 m) is met. Therefore, the wall can be built in this manner without external bracing.

NOTE: This option requires early warning and evacuation when wind speeds reach 15 mph (6.7 m/s) 3-second gust. This may not be practical in all areas.

Alternate 2: Design for an evacuation wind speed of 35 mph (15.6 m/s).

Unreinforced wall:

Maximum height above grade, unbonded = 8′-0″ (2.44 m) at ground level (see Table 2 note G), 2′-8″ (0.81 m) otherwise, Maximum height above grade or line of support, bonded = 10′-0″ (3.05 m)

Maximum vertical spacing between braces, bonded = 12′-4″ (3.75 m)

Maximum vertical height above brace, bonded = 6′-0″ (1.82 m)

Reinforced wall:

Maximum height above grade or line of support, bonded 23′-4″ (7.11 m)

Maximum vertical spacing between braces, bonded = 28′-0″ (8.53 m)

Maximum vertical height above brace, bonded = 14′-0″ (4.26 m)

Strategy:

Build the wall to a height of 10 ft (3.05 m) the first day (Table 1: initial period limit is 10′-0″ (3.05 m)). Grout that lift the same day, which after the curing period (12 or 24 hours depending on the splice length used) can support a cantilever height of 23′-4″ (7.11 m).

Then, build an additional section of wall of 6′-0″ (1.82 m) high, grout it and brace it at no lower than the 8′-0″ (2.43 m) level, because only 14′-0″ (4.26 m) of the reinforced 22 ft (6.71 m) wall can extend above the brace.

The next or following days, finish the rest of the wall and grout that portion the same day. (Note the first two sections each could have been done in 8′-0″ (2.44 m) heights as well.)

The brace will need to stay in place until the permanent support (roof or floor) is in place. Note that when counting reinforced internal bracing, the wall must be grouted the same day and the restricted zone vacated for the next 12 or 24 hours, depending on the splice length used.

NOTE: Refer to the International Masonry Institute’s Internal Bracing Design Guide for Masonry Walls Under Construction (ref. 4). That demonstrates how to effectively use low-lift grouting for internal bracing, as each lift that is grouted can be considered reinforced and able to withstand higher loadings at the bottom of the wall where stresses are highest.

WALLS SUBJECT TO BACKFILLING

Unless concrete masonry basement walls are designed and built to resist lateral earth pressure as cantilever walls, they should not be backfilled until the first floor construction is in place and anchored to the wall or until the walls are adequately braced. Figure 3 illustrates one type of temporary lateral bracing being used in the construction of concrete masonry basement walls. Heavy equipment, such as bulldozers or cranes, should not be operated over the backfill during construction unless the basement walls are appropriately designed for the higher resulting loads.

Ordinarily, earth pressures assumed in the design of basement walls are selected on the assumption that the backfill material will be in a reasonably dry condition when placed. Because lateral earth pressures increase as the moisture content of the earth increases, basement walls should not be backfilled with saturated materials nor should backfill be placed when any appreciable amount of water is standing in the excavation. Similarly, water jetting or soaking should never be used to expedite consolidation of the backfill.

Care should be taken to avoid subjecting the walls to impact loads, as would be imparted by earth sliding down a steep slope and hitting the wall. This could also damage waterproofing, dampproofing, or insulation applied to the walls. Also, if needed, a concrete masonry unit can be left out at the bottom of a wall to prevent an unbalanced accumulation of water. The unit can be replaced before backfilling.

Figure 3—Typical Temporary Bracing for Concrete Masonry Basement Walls (ref. 6)

REFERENCES

  1. Standard Practice for Bracing Masonry Walls Under Construction. Council for Masonry Wall Bracing, December 2012.
  2. Building Code Requirements for Masonry Structures, TMS 402-11/ACI 530-11/ASCE 5-11. Reported by the Masonry Standards Joint Committee, 2011.
  3. Allowable Stress Design of Concrete Masonry Based on the 2012 IBC and 2011 MSJC, TEK 14-07C. Concrete Masonry & Hardscapes Association, 2013.
  4. Internal Bracing Design Guide for Masonry Walls Under Construction. International Masonry Institute, May 2013 (available free at www.imiweb.org).
  5. Basement Manual: Design and Construction Using Concrete Masonry, CMU-MAN-002-01, Concrete Masonry & Hardscapes Association, 2001.