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 Allowable Bearing(lb/ft 2)

Drainage

BEDROCK 4,000 to 12,000 PoorGRAVELS 3,000 GoodGRAVELS w/ FINES 3,000 Good SAND 2,000 GoodSAND W/ FINES 2,000 GoodSILT 1,500 MediumCLAYS 1,500 MediumORGANICS 0 to 400 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

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)

COLUMNLOAD

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

THICKENED SLAB

WALL

SLAB-ON-GRADE

Slab-on-Grade and Thickened Slab

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)

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

Deep Foundations

Deep Foundation

PILES

LOAD

Friction Force (Resisting Force)

Bearing Force (Resisting Force)

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

Weak Soil

Strong Soil

Pile Foundation

Pile – Vertical structural member that is driven, jetted, or drilled into the ground in order to gain support from deeper soil layers

PILE

PILE CAP (CONCRETE)

USESWeak shallow soil withdeep satisfactory soils

PIER (CONCRETE) LOAD

Cast-in-Situ Piles

A large diameter cast-in-place concrete pile

USES

Weak shallow soil with satisfactory soils at intermediate depth

CAST-IN-SITU PILE

BELL can improve bearing capacity

GRADE BEAM

LOAD

Why Do Foundations Fail?

Bending Failure – Foundation fractures due to bending moment

Shear Failure – Foundation breaks due to excessive shear

Punch Through – Structural member “punches through” concrete foundation

Foundation Failure

SECTION

PLAN

Bending Failure

Punch Through

Why Do Foundations Fail?

Settlement – Foundation moves

• Weak or compressible soil• Expansion/contraction of soil

(moisture)• Frost heave

Soil Failure

Illustrations courtesy U. S. Marine Corp.