Finished grade should be sloped away from the building. Finally, a higher resistance factor is used to reflect the more consistent bending strength of reinforced concrete relative to unreinforced concrete.As specified by ACI-318, a minimum of 3 inches of concrete cover over steel reinforcement is required when concrete is in contact with soil. It is used in a variety of foundation types, including basements, crawlspaces, and slabs on grade. 1) Except as required in Sentence (2), the thickness of foundation walls made of unreinforced concrete block or solid concrete and subject to lateral earth … (203 mm) thick. 4, and No. For reinforced construction (Figure 2), reinforcing bars must be properly located to be fully functional. In the rare event that the capacity may be exceeded, the reinforcing steel begins to yield, eliminating an abrupt failure that may otherwise occur in plain, unreinforced concrete. For standard pile and concrete grade beam construction, the pile is usually extended into the concrete “cap” a few inches or more. Simpson Strong-Tie Products for Concrete Foundation AT-XP ® is a fast-cure, all-weather, high-strength anchoring adhesive and may be used to anchor threaded steel rods or deformed steel reinforcing bars in unreinforced … According to ACI-318, the three modes of failure considered in reinforced concrete footing design are one-way shear, two-way shear, and flexure. Building Code Requirements for Masonry Structures (ref. Concrete Wall DeflectionACI-318 does not specifically limit wall deflection. flat: solid concrete wall of uniform thickness; post-and-beam: concrete frame constructed of vertical and horizontal concrete members with voids between the members created by the form. The net concrete cross-sectional area of most concrete masonry units ranges from 50 to 70%, depending on unit width, face-shell and web thicknesses, and core configuration. Hollow units are defined as those in which the net concrete cross-sectional area is less than 75% of the gross cross-sectional area. Plain and reinforced concrete interaction diagrams for residential applications and the methods for deriving them may be found in Structural Design of Insulating Concrete Form Walls in Residential Construction (PCA, 1998). Residential concrete walls are either plain or slightly reinforced, with one layer of reinforcement typically placed near the center of the wall. Unreinforced concrete … 5, which correspond to diameters of 3/8-inch, 1/2-inch, and 5/8-inch, respectively. A typical fraction of the specified compressive strength is 0.25, which equates to a conservative safety factor of 4. They may break apart, or be too weak to hold anchor bolts. This section discusses the issue of reinforcement and presents rational design approach for residential concrete and masonry foundation walls.In most cases, a design for concrete or concrete masonry walls can be selected from the prescriptive tables in the applicable residential building code or the International One- and Two-Family Dwelling Code (ICC, 1998). This research also indicates that the minimum reinforcement requirements in ACI-318 for beam design are conservative when a minimum #4 rebar is used as bottom reinforcement. Sway frames are not discussed in detail herein because the soil pressures surrounding a residential foundation typically provide lateral support to resist any racking and deflections associated with a sway frame. The minimum thickness for unreinforced masonry shear walls and for masonry foundation walls … The calculation of wall deflection should also use effective section properties based on EcIg for plain concrete walls and EcIe for reinforced concrete walls; refer to ACI 318•9.5.2.3 to calculate the effective moment of inertia, Ie.If unfactored load deflections prove unacceptable, the designer may increase the wall thickness or the amount of vertical wall reinforcement. When necessary, piles are used to transmit the load to a deeper soil stratum with a higher bearing capacity to prevent failure due to undercutting of the foundation by scour from floodwater flow at high velocities, and to elevate the building above required flood elevations. Most foundations, slabs, and walls consolidated by hand methods have a slump between 4 and 6 inches. Therefore, a slump of greater than 6 should be avoided.Admixtures. The concrete compressive strength may be verified in accordance with ASTM C39 (ASTM, 1996). To simplify calculations for the three failure modes, the following discussion explains the relation of the failure modes to the design of plain and reinforced concrete footings. The result is a lightly reinforced slab designed to offset the effects of temperature and shrinkage of the concrete. For square, circular or rectangular footings, shear is checked at the critical section that extends in a plane around a concrete, masonry, wood, or steel column or pier that forms the perimeter of the area described above.FIGURE 4.2 Critical Failure Planes in Continuous or Square Concrete, Flexure (Bending)The maximum moment in a footing deformed by the upward-acting soil pressures would logically occur in the middle of the footing; however, the rigidity of the wall or column above resists some of the upward-acting forces and affects the location of maximum moment. The length of the foundation walls between perpendicular masonry walls or pilasters is a maximum of 3 times the basement wall height. If greater shear capacity is required in a plain concrete wall, it may be obtained by increasing the wall thickness or increasing the concrete's compressive strength. ACI-530 makes no distinction between inspected and non-inspected masonry walls and, therefore, does not require adjustments in allowable stresses based on level of inspection.As a residential designer, keep in mind that concrete masonry units (block) are readily available in nominal 6-, 8-, 10- and 12-inch thicknesses. (64 mm) concrete mud slab is generally used when a more durable surface is desired for access to utilities. The allowable stress design procedure outlined below describes an approach by which walls are designed in accordance with ACI-530•2.3. These limits may be relaxed for residential footing design, provided that the capacity is shown to be sufficient in accordance with the ACI318 design equations. Table 4.7 summarizes some basic rules of thumb for design. Preservative-treated wood, precast concrete, and other methods may also be used. The density of unreinforced normal weight concrete ranges between 144 and 156 pounds per cubic foot (pcf) and is typically assumed to be 150 pcf. The minimum pile butt diameter should not be less than 8 inches; 10- to 12-inch diameters are common. The equations may be generalized for use with other conditions (e.g., rectangular footings and rectangular columns, round footings, etc.) In other cases, some amount of subsurface exploration (i.e., standard pertrometer test) is advisable to assist in foundation design or, alternatively, to indicate when one or more test piles may be required.It is rare for pile depth to be greater than 8 or 10 feet except in extremely soft soils, on steeply sloped sites with unstable soils, or in coastal hazard areas (beachfront property) where significant scour is possible due to storm surge velocity. For concrete foundation walls, this is generally not a concern.FIGURE 4.4 Variables Defined for Shear Calculations in Reinforced Concrete Walls. In addition, a lintel (concrete beam) is required at the top of wall openings. Depending on soil loads, analysis should confirm conventional residential foundation wall practice in typical conditions.The following checks are used to determine if a plain concrete wall has adequate strength.Shear Capacity. Slump is measured in accordance with ASTM C143 (ASTM, 1998) by inverting a standard 12-inch-high metal cone, filling it with concrete, and then removing the cone; the amount the concrete settles in units of inches is the slump. It is generally more economical if the masonry unit's compressive strength ranges between 1,500 and 3,000 psi. Grout may also be used in unreinforced concrete masonry walls for added strength.Soil-Bearing Capacity and Footing Size, When a soil-bearing investigation is desired to determine more accurate and economical footing requirements, the designer commonly turns to ASTM D1586, Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils (ASTM, 1999). It is common practice to use a brick curtain wall between piers for appearance and bracing purposes.The design procedures and information in this section covers: Concrete design procedures generally follow the strength design method contained in ACI (American Concrete Institute)-318 (ACI, 1999), although certain aspects of the procedures may be considered conservative relative to conventional residential foundation applications. The coefficient is derived from the design check that ensures that the factored moment (due to factored loads) Mu is less than the factored nominal moment strength φMn of the reinforced concrete. ... Table 5.7 indicates suitable relationships for free-standing, single-thickness, unreinforced, concrete block walls not externally … Perpendicular shear is rarely a controlling factor in the design of typical residential foundation concrete walls. Other factors, such as soil thermal conductivity, soil moisture content, and the internal temperature of a building are also important. APA-rated plywood is selected from tables based on unbalanced backfill height and stud spacing. Steel angles are the simplest shapes and are suitable for openings of moderate width typically found in residential foundation walls. Unreinforced masonry—brick, concrete block, or stone—foundations often cannot resist earthquake shaking. Combined Axial Compression and Flexural CapacityThe following equations from ACI-530 determine the relationship of the combined effects of axial load and bending moment on a masonry wall.Tension CapacityACI-530 provides allowable values for flexural tension transverse to the plane of a masonry wall. A stemwall with brick ledge is shown in Figure 6. For concrete carriageway, it is normally classified into two types: reinforced and unreinforced concrete pavement. The plywood must be preservative-treated and rated for below-ground application. Figure 4.9 illustrates a PWF.PWFs may be designed in accordance with the basic provisions provided in the International One- and Two-Family Dwelling Code (ICC, 1998). When used, horizontal reinforcement is typically specified as a ladder or truss-type wire reinforcement. One problem associated with a high-slump concrete is segregation of the aggregate, which leads to cracking and scaling. TABLE 4.4 Simplified Moment Magnification Factors, Interaction DiagramsAn interaction diagram is a graphic representation of the relationship between the axial load and bending capacity of a reinforced or plain concrete wall. air-entrainers used for concrete that will be exposed to freeze-thaw conditions and de-icing salts. The local design frost depth can vary significantly from that required by actual climate, soil, and application conditions. Though more complicated to design and construct, a fixed-end beam reduces the maximum bending moment on the lintel and allows increased spans. The American Society of Civil Engineers (ASCE) is currently developing a standard for FPSF design and construction based on the resources mentioned above. However, close spacing is rarely required in residential construction and should be avoided in design.The most common steel reinforcement or rebar sizes in residential construction are No. Values can be roughly correlated to soil-bearing values as shown in Table 4.3. This test relies on a 2-inch-diameter device driven into the ground with a 140-pound hammer dropped from a distance of 30 inches. The most efficient design is close to the interaction diagram curve. In addition, the terms 4/3, For a plain concrete footing, flexure (bending) is checked by using the equations below for footings that support walls or square columns (see Figure 4.2). Often, a No. Walls that are determined to have adequate strength to withstand shear and combined axial load and bending moment generally satisfy unspecified deflection requirements. The value of s may be taken as the inverse of the blow count for the last foot of driving. Alternatively, footing widths may be determined in accordance with Section 4.3 based on a site’s particular loading condition and presumptive soil-bearing capacity. Pile capacity is, however, difficult to predict; therefore, only rough estimates of required pile lengths and sizes can be made before installation, particularly when the designer relies only on skin friction to develop capacity in deep, soft soils. For parallel shear, the equations do not address overturning and bending action that occurs in a direction parallel to the wall, particularly for short segments of walls under significant parallel shear load. Use of a minimum safety factor of 2 (corresponding to a higher presumptive soil-bearing value) is recommended for smaller structures with continuous spread footings, such as houses. The wall resembles a thick screen made of concrete; and. In the above equation, Pa is the net allowable vertical load capacity, Wr is the hammer ram weight, h is the distance the hammer free falls, s is the pile penetration (set) per blow at the end of driving, and F is the safety factor. ACI-318 does not specifically limit lintel deflection. For homes in the Anchorage, Alaska, area, the perimeter foundation is generally classified as warm, with a required depth of 4 or 5 feet. Acceptable insulation materials include expanded and extruded polystyrenes, although adjusted insulation values are provided for below-ground use. Lateral support is provided at the top of the foundation wall before backfilling. All points of restraint need to be carefully managed to avoid these drying shrinkage stress cracks. Figure 4.6 depicts the cross-section and dimensions for analysis of concrete lintels.For additional information on concrete lintels and their design procedure, refer to the Structural Design of Insulating Concrete Form Walls in Residential Construction (PCA, 1998) and to Testing and Design of Lintels Using Insulating Concrete Forms (HUD, 2000). Grade N is typically required for general use, such as in interior and backup walls, and in above- or below-grade exterior walls that may or may not be exposed to moisture penetration or the weather. In most cases, the slab thickness is designed as an unreinforced concrete section. Often, the depths are highly conservative in accordance with frost depths experienced in applications not relevant to residential foundations. In addition, soils are considered a foundation material. Backfill material is non-expansive and is tamped no more than necessary to prevent excessive settlement. However, piles are not necessarily the most economical solution. cavity wall 300 wide carrying 80 kN/m onto 100 kN/m2 ground: b f = 800 mm a = 250 mm h f = say assuming C20/25 concrete … The instrumentation and cost of conducting the SPT test is usually not warranted for typical residential applications. In the equations given below for one- and two-way shear, the dimensions are in accordance with Figure 4.2; units of inches should be used. The nominal reinforcement approach has provided many serviceable structures. (102 to 203 mm) of backfill should be low permeability soil so rain water absorption into the backfill is minimized. The reinforcement steel may be provided to limit the crack widths resulting from shrinkage and … Flexure in a concrete footing is checked by computing the moment created by the soil-bearing forces acting over the cantilevered area of the footing that extends from the critical flexure plane to the edge of the footing (hatched area in Figure 4.2). These limits may be relaxed by the designer, provided that adequate capacity is demonstrated in the strength analysis; however, a reinforced footing thickness of significantly less than 6 inches may be considered impractical even though it may calculate acceptably. The following LRFD load combinations are suggested for the design of residential concrete foundation walls: In light-frame homes, the first load combination typically governs foundation wall design. If the foundation of your home is visibly brick, old clay block, or unreinforced concrete block this would be considered an unreinforced masonry foundation. The magnificent Pantheon in Rome, the world’s largest unreinforced concrete dome, … Engineering specifications generally require reinforcement of concrete or masonry foundation walls because of somewhat arbitrary limits on minimum steel-to-concrete ratios, even for “plain” concrete walls. A basement is typically defined as a portion of a building that is partly or completely below the exterior grade and that may be used as habitable or storage space. Building design loads, including dead and live loads, should be determined by using allowable stress design (ASD) load combinations.FootingsThe objectives of footing design are: In the next section, we'll learn about design methods for concrete and gravel footings.By far, the most common footing in residential construction is a continuous concrete spread footing. (51 mm) PVC pipe at 8 ft (2400 mm) on center, allows water on the interior to reach the foundation drain. Reinforced Concrete Footing DesignFor infrequent situations in residential construction where a plain concrete footing may not be practical, or where it is more economical to reduce the footing thickness, steel reinforcement may be considered. ... since the least concrete strength permitted by code is 2,500 psi f'c 2500 psi; AND ϕ 0.55 (ACI 318-05 provision 9.3.5) ... bearing on the foundation… Note that local codes may restrict the use of foam plastic insulation below grade in areas where the hazard of termite damage is high. Shear friction also relies on resistance from protruding portions of concrete on either side of the crack and by dowel action of the reinforcement that crosses the crack. Unreinforced Concrete footing Tdog67 (Structural) (OP) 8 May 08 13:20. Standard Specifications for Tolerances for Concrete Construction and Materials, ACI 117-90. For a conservative relationship between soil type and load-bearing value, refer to Table 4.2. Basement Manual, Design and Construction Using Concrete Masonry. Once the loads are known, the designer can perform design checks for various stresses by following ACI-318 and the recommendations contained herein.As a practical consideration, residential designers need to keep in mind that concrete foundation walls are typically 6, 8 or 10 inches thick (nominal). The larger pile diameters may be necessary for unbraced conditions with long unsupported heights.In hard material or densely compacted sand or hard clay, a typical pile meets “refusal” when the blows per foot become excessive. A stemwall with slab on gradesupports the wall above and often also provides a brick ledge to support an exterior masonry veneer. If unvented, either the walls or the floor above can be insulated. The designer should recognize that many soils may not be frost-susceptible in their natural state (e.g., sand, gravel, or other well-drained soils that are typically low in moisture content). Shear stress is a result of the lateral loads on a structure associated with wind, earthquake, or backfill forces. weak soils or non-engineered fills that require the use of piles to transfer foundation loads by skin friction or point bearing; inland floodplains and coastal flood hazard zones where buildings must be elevated; expansive soils where buildings must be isolated from soil expansion in the “active” surface layer and anchored to stable soil below. The units for s and h must be the same. Those provisions, in turn, are based on the Southern Forest Products Association’s Permanent Wood Foundations Design and Construction Guide (SPC, 1998). Axial Compression CapacityThe following equations from ACI-530•2.3 are used to design masonry walls and columns for compressive loads only. A foundation of unreinforced concrete (poured without steel reinforcement bars) is okay, but unreinforced brick, stone, or concrete block foundations are not good because they will disintegrate in an earthquake like our proverbial croquembouche. Walls determined inadequate to withstand combined axial load and bending moment may gain greater capacity through increased wall thickness or increased concrete compressive strength. water reducers to improve the workability of concrete without reducing its strength; retarders used in hot weather to allow more time for placing and finishing concrete. The connection requirements of the National Design Specification for Wood Construction (NDS, 1997) should be carefully followed for these heavy-duty connections.Frost ProtectionThe objective of frost protection in foundation design is to prevent damage to the structure from frost action (heaving and thaw weakening) in frost-susceptible soils.Conventional MethodsIn northern U.S. climates, builders and designers mitigate the effects of frost heave by constructing homes with perimeter footings that extend below a locally prescribed frost depth. A dampproof coating on the exterior crawlspace wall will also help prevent water entry into the crawlspace. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2001. Masonry homes built with reinforcement at 96 inches on-center have performed well in hurricane-prone areas, such as southern Florida.Flexural or axial stresses must be accounted for to ensure that a wall is structurally sound. One exception may be found where a nominal 4-inch-thick slab is reinforced to serve as an integral footing for an interior load-bearing wall (that is not intended to transmit uplift forces from a shear wall overturning restraint anchorage in high-hazard wind or seismic regions). American Concrete Institute, 2002. The thickened edges may or may not be reinforced in standard residential practice.Slab-on-grade foundations are often placed on 2 to 3 inches of washed gravel or sand and a 6 mil (0.006 inch) polyethylene vapor barrier. Granular (gravel or crushed rock) footings are sized accordingly. An exception may occur with the bearing points of long-spanning beams. In most cases, vertical reinforcement is positioned towards the interior face of below grade walls to provide the greatest resistance to soil pressures. Concrete piles or piers are typically cast in place in drilled holes, sometimes with “belled” bases (most common in expansive soils). Following a rule of thumb similar to that for a concrete footing, the gravel footing thickness should be no less than 1.5 times its extension beyond the edge of the foundation wall, or, in the case of a pressure-treated wood foundation, the mud sill. Because the wall is exposed to soil on both sides, waterproofing or dampproofing coatings are generally not required. To aid the designer in shortcutting these calculations, design manuals provide design tables that correlate the nominal strength coefficient of resistance Rn to the reinforcement ratio ρfor a specific concrete compressive strength and steel yield strength. Concrete Masonry Basement Wall Construction. The concrete lintel is often assumed to act as a simple span with each end pinned. Solid units are not necessarily solid but are defined as those in which the net concrete cross-sectional area is 75% of the gross cross-sectional area or greater.MortarMasonry mortar is used to join concrete masonry units into a structural wall; it also retards air and moisture infiltration.
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