Commercial Foundations Purpose Considerations Types of Foundations Shallow

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?

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

Design Considerations • Loads from the structure • Allowable soil bearing pressure • Frost depth • Flood elevation • Drainage • Costs

Loads from the Structure Foundations must resist • Dead Load • Live Load • Lateral Loads -- Wind -- Seismic activity -- Flood SOIL REACTIONS

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

Soil Information • Local Building Department, Codes and Regulations • Preliminary info: USDA Web Soil Survey http: //websoilsurvey. nrcs. usda. gov • Local or State Building Codes • Soil testing/analysis – Site inspection and simple soil testing – Soil borings taken at proposed foundation locations

Estimated Allowable Soil Bearing Pressure Soil Type BEDROCK GRAVELS w/ FINES SAND W/ FINES SILT CLAYS ORGANICS Allowable Bearing (lb/ft 2) 4, 000 to 12, 000 3, 000 2, 000 1, 500 0 to 400 Drainage Poor Good Medium Poor

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

Frost Heave

Flood Elevation • 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.

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

Shallow Foundations

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)

Spread (Column) Footing COLUMN PIER (Concrete or Masonry) SPREAD FOOTING (Concrete) 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

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) D A LO

Slab-on-Grade and Thickened Slab WALL 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) SLAB-ONGRADE THICKENED SLAB

Mat Foundation A large, heavily reinforced concrete slab placed under the entire building to support loads from several points MAT FOUNDATION USES Heavy loads on weak soil CONCRETE PIER

Deep Foundations

Deep Foundation LOAD Top Soil PILE CAP PILES Friction Force (Resisting Force) Weak Soil Bearing Force (Resisting Force) Strong Soil The building LOAD is transferred through friction on the sides of the piles and/or bearing on the end of the piles

Pile Foundation PIER (CONCRETE) PILE CAP (CONCRETE) LOAD Pile – Vertical structural member that is driven, jetted, or drilled into the ground in order to gain support from deeper soil layers USES Weak shallow soil with deep satisfactory soils PILE

Cast-in-Situ Piles A large diameter cast-in-place concrete pile GRADE BEAM CAST-IN-SITU PILE USES Weak shallow soil with satisfactory soils at intermediate depth BELL can improve bearing capacity LOAD

Why Do Foundations Fail? Foundation 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 Bending Failure Punch Through SECTION PLAN

Why Do Foundations Fail? Soil Failure Settlement – Foundation moves • Weak or compressible soil • Expansion/contraction of soil (moisture) • Frost heave Illustrations courtesy U. S. Marine Corp.