PDHonline Course S198 (8 PDH)Residential Foundation Design Options and ConceptsInstructor: George E. Thomas, PE2012PDH Online PDH Center5272 Meadow Estates DriveFairfax, VA 22030-6658Phone & Fax: 703-988-0088www.PDHonline.orgwww.PDHcenter.comAn Approved Continuing Education Provider

www.PDHcenter.comPDH Course S198www.PDHonline.orgResidential Foundation DesignOptions and ConceptsCourse Content1.1 GeneralA foundation transfers the load of a structure to the earth and resists loadsimposed by the earth. A foundation in residential construction may consist of a footing,wall, slab, pier, pile, or a combination of these elements. This course will addresses thefollowing foundation types: crawl space; basement; slab-on-grade with stem wall; monolithic slab; piles; piers; and alternative methods.The most common residential foundation materials are concrete masonry (i.e., concreteblock) and cast-in-place concrete. Preservative-treated wood, precast concrete, and othermethods may also be used. The concrete slab on grade is the most popular foundationtype in the Southeast; basements are the most common type in the East and Midwest.Crawl spaces are common in the Northwest and Southeast. Pile foundations arecommonly used in coastal flood zones to elevate structures above flood levels, in weak orexpansive soils to reach a stable stratum, and on steeply sloped sites. Figure 1.1 depictsdifferent foundation types; a brief description follows. A crawl space is a buildingfoundation that uses a perimeter foundation wall to create an under-floor space that is nothabitable; the interior crawl space elevation may or may not be below the exterior finishgrade. A basement is typically defined as a portion of a building that is partly orcompletely below the exterior grade and that may be used as habitable or storage space.A slab on grade with an independent stem wall is a concrete floor supported by the soilindependently of the rest of the building. The stem wall supports the building loads and inturn is supported directly by the soil or a footing. A monolithic or thickened-edge slab isa ground-supported slab on grade with an integral footing (i.e., thickened edge); it isnormally used in warmer regions with little or no frost depth but is also used in colderclimates when adequate frost protection is provided. When necessary, piles are used totransmit the load to a deeper soil stratum with a higher bearing capacity, to preventfailure due to undercutting of the foundation by scour from flood water flow at highvelocities, and to elevate the building above required flood elevations. Piles are also usedto isolate the structure from expansive soil movements. Post-and-pier foundations canprovide an economical alternative to crawl space perimeter wall construction. It iscommon practice to use a brick curtain wall between piers for appearance and bracingpurposes. The design procedures and information covered in this course are George E. ThomasPage 2 of 90

www.PDHcenter.comPDH Course foundation materials and properties; soil bearing capacity and footing size; concrete or gravel footings; concrete and masonry foundation walls; preservative-treated wood walls; insulating concrete foundations; concrete slabs on grade; pile foundations; and frost protection.Concrete design procedures generally follow the strength design methodcontained in ACI-318, although certain aspects of the procedures may be consideredconservative relative to conventional residential foundation applications. For this reason,some supplemental design guidance is provided when practical and technically justified.Masonry design procedures follow the allowable stress design method of ACI-530. Wooddesign procedures are used to design the connections between the foundation system andthe structure above and follow the allowable stress design method for wood construction.In addition, the engineer is referred to the applicable design standards for symboldefinitions and additional guidance since the intent of this course is to provide onlyinformation in the efficient design of residential foundations.The LRFD load combinations of the attached table will be used in this course inlieu of those in ACI-318 for strength design of concrete. The engineer is advised of thisvariance from what may be considered accepted practice in the local building code.However, the intent is to provide engineers with an alternative consistent with currentresidential building code and construction practice. With respect to the design of concretein residential foundations, it is the intend of this course to provide reasonable safetymargins that are at least consistent with the minimums required for other more crucial(i.e., life-safety) elements of a home. If an actual design is performed in accordance withthe information provided herein, it is the responsibility of the engineer to seek any specialapproval that may be required for “alternative means and methods” of design and toidentify where and when such approval is needed. George E. ThomasPage 3 of 90

www.PDHcenter.comPDH Course S198www.PDHonline.orgFIGURE 1.1 Types of Foundations George E. ThomasPage 4 of 90

www.PDHcenter.comPDH Course S198www.PDHonline.orgTypical Load Combinations Used for the Design of Components and SystemsComponent or SystemASD Load CombinationsLRFD Load CombinationsFoundation wall (gravity and soillateral loads)D HD H L 0.3 (Lr S)D H (Lr or S) 0.3 L1.2D 1.6H1.2D 1.6H 1.6L 0.5(Lr S)1.2D 1.6H 1.6(lr or S) 0.5LHeaders, girders, joists, interior loadbearing walls and columns, footings(gravity loads)Exterior load-bearing walls andcolumns (gravity and transverselateral load)Roof rafters, trusses, and beams: roofand wall sheathing (gravity and windloads)D L 0.3 (Lr S)D (Lr or S) 0.3 L1.2D 1.6L 0.5(Lr S)1.2D 1.6(lr or S) 0.5LSame as immediately above plusD WD 0.7E 0.5L 0.2SD (Lr or S )0.6D WuD WSame as immediately above plus1.2D 1.5W1.2D 1.0E 0.5L 0.251.2D 1.6(Lr or S)0.9D 1.5Wu1.2D 1.5W0.6D (W or 0.7E)0.9D (1.5W or 1.0E)Floor diaphragms and shear walls (inplace lateral and overturning loads)Notes:The load combinations and factors are intended to apply to nominal design loads defined as follows: D estimated mean dead weightof the construction; H design lateral pressure for soil condition/type; L design floor live load; Lr maximum roof live loadanticipated from construction/maintenance; W design wind load; S design roof snow load; and E design earthquake load. Thedesign or nominal loads should be determined in accordance with this chapter.Attic loads may be included in the floor live load, but a 10 psf attic load is typically used only to size ceiling joists adequately foraccess purposes. However, if the attic is intended for storage, the attic live load (or some portion) should also be considered for thedesign of other elements in the load path.The transverse wind load for stud design is based on a localized component and cladding wind pressure; D W provides an adequateand simple design check representative of worst-case combined axial and transverse loading. Axial forces from snow loads and rooflive loads should usually not be considered simultaneously with an extreme wind load because they are mutually exclusive onresidential sloped roofs. Further, in most areas of the United States, design winds are produced by either hurricanes or thunderstorms;therefore, these wind events and snow are mutually exclusive because they occur at different times of the year.For walls supporting heavy cladding loads (such as brick veneer), an analysis of earthquake lateral loads and combined axial loadsshould be considered. However, this load combination rarely governs the design of light-frame construction.Wu is wind uplift load from negative (i.e., suction) pressures on the roof. Wind uplift loads must be resisted by continuous load pathconnections to the foundation or until offset by 0.6D.The 0.6 reduction factor on D is intended to apply to the calculation of net overturning stresses and forces. For wind, the analysis ofoverturning should also consider roof uplift forces unless a separate load path is designed to transfer those forces.1.2 Material PropertiesA residential engineer using concrete and masonry materials must have a basicunderstanding of such materials as well as an appreciation of variations in the materials’composition and structural properties. In addition, soils are considered a foundationmaterial. A brief discussion of the properties of concrete and masonry follows.1.2.1 ConcreteThe concrete compressive strength fc' used in residential construction is typicallyeither 2,500 or 3,000 psi, although other values may be specified. For example, 3,500 psiconcrete may be used for improved weathering resistance in particularly severe climatesor unusual applications. The concrete compressive strength may be verified in accordancewith ASTM C39. Given that concrete strength increases at a diminishing rate with time,the specified compressive strength is usually associated with the strength attained after 28days of curing time. At that time, concrete generally attains about 85 percent of its fullycured compressive strength. George E. ThomasPage 5 of 90

www.PDHcenter.comPDH Course S198www.PDHonline.orgConcrete is a mixture of cement, water, sand, gravel, crushed rock, or otheraggregates. Sometimes one or more admixtures are added to change certaincharacteristics of the concrete, such as workability, durability, and time of hardening. Theproportions of the components determine the concrete mix’s compressive strength anddurability.TypePortland cement is classified into several types in accordance with ASTM C150.Residential foundation walls are typically constructed with Type I cement, which is ageneral-purpose Portland cement used for the vast majority of construction projects.Other types of cement are appropriate in accommodating conditions related to heat ofhydration in massive pours and sulfate resistance. In some regions, sulfates in soils havecaused durability problems with concrete. The engineer should check into localconditions and practices.WeightThe weight of concrete varies depending on the type of aggregates used in theconcrete mix. Concrete is typically referred to as lightweight or normal weight. Thedensity of unreinforced normal weight concrete ranges between 144 and 156 pounds percubic foot (pcf) and is typically assumed to be 150 pcf. Residential foundations areconstructed with normal weight concrete.SlumpSlump is the measure of concrete consistency; the higher the slump, the wetter theconcrete and the easier it flows. Slump is measured in accordance with ASTM C143 byinverting a standard 12-inch-high metal cone, filling it with concrete, and then removingthe cone; the amount the concrete settles in units of inches is the slump. Mostfoundations, slabs, and walls consolidated by hand methods have a slump between 4 and6 inches. One problem associated with a high-slump concrete is segregation of theaggregate, which leads to cracking and scaling. Therefore, a slump of greater than 6should be avoided.AdmixturesAdmixtures are materials added to the concrete mix to improve workability anddurability and to retard or accelerate curing. Some of the most common admixtures aredescribed below. Water reducers improve the workability of concrete without reducing its strength. Retarders are used in hot weather to allow more time for placing and finishingconcrete. Retarders may also reduce the early strength of concrete. Accelerators reduce the setting time, allowing less time for placing and finishingconcrete. Accelerators may also increase the early strength of concrete. George E. ThomasPage 6 of 90

www.PDHcenter.comPDH Course Air-entrainers are used for concrete that will be exposed to freezethaw conditions anddeicing salts. Less water is needed, and desegregation of aggregate is reduced when airentrainers are added.ReinforcementConcrete has high compressive strength but low tensile strength; therefore,reinforcing steel is often embedded in the concrete to provide additional tensile strengthand ductility. In the rare event that the capacity may be exceeded, the reinforcing steelbegins to yield, eliminating an abrupt failure that may otherwise occur in plain,unreinforced concrete. For this reason, a larger safety margin is used in the design ofplain concrete construction than in reinforced concrete construction.Steel reinforcement is available in Grade 40 or Grade 60; the grade number refersto the minimum tensile yield strength fy of the steel (i.e., Grade 40 is minimum 40 ksisteel and Grade 60 is minimum 60 ksi steel). Either grade may be used for residentialconstruction; however, most reinforcement in the U.S. market today is Grade 60. It is alsoimportant that the concrete mix or slump is adjusted through the addition of anappropriate amount of water to allow the concrete to flow easily around thereinforcement bars, particularly when the bars are closely spaced or crowed at points ofoverlap. However, close spacing is rarely required in residential construction and shouldbe avoided in design.The most common steel reinforcement or rebar sizes in residential constructionare No. 3, No. 4, and No. 5, which correspond to diameters of 3/8-inch, 1/2-inch, and 5/8inch, respectively. These three sizes of rebar are easily handled at the jobsite by usingmanual bending and cutting devices. Table 1.1 provides useful relationships among therebar number, diameter, and crosssectional for reinforced concrete and masonry design.TABLE 1.1 Rebar Size, Diameter, and Cross-Sectional /81/25/83/47/81Area (square inches) Concrete Masonry UnitsConcrete masonry units (CMU) are commonly referred to as concrete blocks.They are composed of Port