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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Commercial Foundations Purpose Considerations Types of Foundations Shallow Foundations Spread Footings Strip Foundations Slab-on-Grade and Thickened Slabs Mat Foundation Deep Foundations Piles Cast-in-Situ Piles Why Do Foundations Fail? Project Lead The Way, Inc. Copyright 2010
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Purpose of Foundations
Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Purpose of Foundations Provide a level, stable surface to safely support a building Transfer building loads to soil Anchor the building from wind, flood, and seismic loads Project Lead The Way, Inc. Copyright 2010
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Design Considerations
Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Design Considerations Loads from the structure Allowable soil bearing pressure Frost depth Flood elevation Drainage Costs Project Lead The Way, Inc. Copyright 2010
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Loads from the Structure
Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Loads from the Structure Foundations must resist Dead Load Live Load Lateral Loads -- Wind -- Seismic activity -- Flood The foundation must resist the design load combinations that are specified by building codes. For instance, dead and live loads must be considered together. Dead and live loads will be transferred through the structural components to the earth, which will cause a soil reaction pressure. Some load combinations may actually cause the building to lift up from the foundation. For example, a load combination included in the IBC requires that only 60% of the dead load be combined with wind and lateral earth pressure. The wind and lateral earth pressure can cause a tipping effect on the building which will increase the soil reaction on one side of the building, but may actually lift up one side of the building. The foundation must be able to safely transfer both downward and upward forces into the earth. SOIL REACTIONS Project Lead The Way, Inc. Copyright 2010
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Allowable Soil Bearing Pressure
Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Allowable Soil Bearing Pressure Indicates the maximum pressure that a soil may be designed to support Includes a factor of safety Dictates the size, depth, and type of foundation Typically presented in pounds per square foot (psf) Different types of soils have different allowable soil bearing pressures Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Soil Information Local Building Department, Codes and Regulations Preliminary info: USDA Web Soil Survey Local or State Building Codes Soil testing/analysis Site inspection and simple soil testing Soil borings taken at proposed foundation locations Information on types of soils present on a site and/or the allowable soil bearing pressure are available through many sources. Project Lead The Way, Inc. Copyright 2010
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Estimated Allowable Soil Bearing Pressure
Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Estimated Allowable Soil Bearing Pressure Soil Type Allowable Bearing (lb/ft 2) Drainage BEDROCK 4,000 to 12,000 Poor GRAVELS 3,000 Good GRAVELS w/ FINES SAND 2,000 SAND W/ FINES SILT 1,500 Medium CLAYS ORGANICS 0 to 400 Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Frost Depth Freezing of soil can cause heaving of foundations Silt or clay soils with a high water table are highly susceptible to frost Defense Build base of foundation below frost depth or Provide frost protection for foundation Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Frost Heave This image shows the result of frost heave on an addition to a building. The new foundation was placed above the frost line. When the supporting wet soil froze, the ice build-up forced the foundation to lift up, causing cracking in the walls and floor. Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Flood Elevation Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Inundation by flood waters should be avoided Damage to structure Damage to contents Height of floors and/or flood proofing is dictated by building codes Courtesy Federal Emergency Management Agency. Photographer Dave Saville. Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Types of Foundations Shallow Foundation: Transfers loads to the soil very near the surface Spread footing or strip footing Mat or raft foundation Slab-on-grade Deep Foundation: Transfers loads to deeper soil layers Piles Cast-in-Situ Piles Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Shallow Foundations Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Shallow Foundation The load from the footing spreads out so that the soil bearing pressure diminishes with depth. The soil directly under the footing takes the greatest load. LOAD Critical Load Area Bearing Pressure (decreases with depth) Remember from the Residential Unit that shallow foundations are commonly used for residential structures, but are also used for light commercial structures. The next few slides will review several types of shallow foundations. Project Lead The Way, Inc. Copyright 2010
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Spread (Column) Footing
Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Spread (Column) Footing COLUMN LOAD A footing that spreads the load over a broad area which supports one (or a few) load(s) USES Usually used in low-rise buildings PIER (Concrete or Masonry) SPREAD FOOTING (Concrete) Project Lead The Way, Inc. Copyright 2010
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Continuous (Strip) Foundation
Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Continuous (Strip) Foundation A wide strip of reinforced concrete that supports loads from a bearing wall USES Light frame construction Under foundation walls FOUNDATION WALL (Concrete or Masonry) STRIP FOOTING (Concrete) LOAD Project Lead The Way, Inc. Copyright 2010
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Slab-on-Grade and Thickened Slab
Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Slab-on-Grade and Thickened Slab THICKENED SLAB WALL SLAB-ON-GRADE Slab-on-Grade – Reinforced concrete floor supported by soil Thickened Slab – A slab on grade with an integral footing created by thickening the slab USES Residential or light commercial construction Shallow frost depth or when frost protection is used (instead of strip footing) Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Mat Foundation A large, heavily reinforced concrete slab placed under the entire building to support loads from several points USES Heavy loads on weak soil MAT FOUNDATION CONCRETE PIER Mat foundations are rarely used in residential construction, but are used in commercial or industrial construction when the building loads are heavy and the soil bearing pressure is relatively low. Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Deep Foundations Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Deep Foundation Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures LOAD PILE CAP The building LOAD is transferred through friction on the sides of the piles and/or bearing on the end of the piles Top Soil PILES Friction Force (Resisting Force) Weak Soil Piles can transfer loads in two ways. Friction results when the soils cling to or rub against the sides of the pile to hold it in place and keep it from moving. Bearing results when the load is transferred through the pile to the very bottom end and pressure is applied to the supporting soil. Typically piles are designed for either friction or bearing – not both. Bearing Force (Resisting Force) Strong Soil Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Pile Foundation PIER (CONCRETE) Pile – Vertical structural member that is driven, jetted, or drilled into the ground in order to gain support from deeper soil layers LOAD PILE CAP (CONCRETE) Piles can be made of steel, concrete, or wood. The pile cap is a reinforced concrete slab that distributes the load from the structure above to the pile group. A pier may be used to transfer the column load to the pile cap. USES Weak shallow soil with deep satisfactory soils PILE Project Lead The Way, Inc. Copyright 2010
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Commercial Foundations
Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Cast-in-Situ Piles A large diameter cast-in-place concrete pile GRADE BEAM USES Weak shallow soil with satisfactory soils at intermediate depth CAST-IN-SITU PILE LOAD To place Cast-in-Place concrete piles, a hole is drilled or augered in the ground down to stable soil. Concrete can be pumped through the auger to fill the hole. A grade beam is typically a concrete member that spans across piles to transfer the load from a wall to the piles. BELL can improve bearing capacity Project Lead The Way, Inc. Copyright 2010
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Why Do Foundations Fail?
Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Why Do Foundations Fail? Foundation Failure Bending Failure Bending Failure – Foundation fractures due to bending moment Shear Failure – Foundation breaks due to excessive shear Punch Through – Structural member “punches through” concrete foundation SECTION Foundation failures are generally traced to one of two causes. The footing is essentially an inverted concrete cantilever beam. The beam can fail if the footing is not designed to resist the stresses caused by the bending moment or shear imposed by the soil pressure. The column or pier can punch through the footing if the footing is not thick enough to resist the shear forces. Punch Through PLAN Project Lead The Way, Inc. Copyright 2010
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Why Do Foundations Fail?
Commercial Foundations Civil Engineering and Architecture Unit 3– Lesson 3.2 – Structures Why Do Foundations Fail? Settlement – Foundation moves Weak or compressible soil Expansion/contraction of soil (moisture) Frost heave Soil Failure Soil can fail in many ways If the building loads apply a soil pressure that exceeds the soil bearing capacity, the soil may give way and allow excessive settlement as shown in the illustration. If the soil is expansive and the moisture content changes, the soil could swell or shrink, causing excessive movement. If the soil is susceptible to frost heave and experiences freezing temperatures, the expansion of the freezing water could cause the foundation to lift. Illustrations courtesy U. S. Marine Corp. Project Lead The Way, Inc. Copyright 2010
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