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Permissible Stress

approach

Limit Stateapproach

EuroCodes have now replaced British Standards fordesigning buildings and civil engineering structures

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Limit States

Ultimate

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Limit States

Ultimate

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Limit States

Serviceability

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Load / Resistance

Variation in strength betweennotionally identical elements

Variation ofmaximum

lifetime load

Load used in calculations Strength used incalculations

Probabilityof Failure

Statistical meaning of Safety

Risk = (Probability of Failure) × (Consequence of Failure)

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• Conceptual Design– Structural Loadings and Functionality– Foundation Conditions– Explore possibilities– Evaluate constructability– Economic considerations– Plant availability

• Detailed design

Conceptual Design Considerations

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Foundation Types

1. Pad foundations (3.2.4*)

2. Strip foundations (3.2.5*)

3. Raft (3.2.6*)

4. Buoyant raft or compensated foundation (4.3.3.2*)

5. Piled foundation – end bearing and/or friction (Chapter 7*)

These foundation types comply with Geotechnical Category2 in EuroCode 1997-1: 2004 (2.1(19)).

See also Chapter 2 in Tomlinson

* Refers to section is BS8004

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Pad FoundationsVarious types

singlesteppedslopedpedestal

Construction and Design• blinding layer at base level (50-75mm)• uniform base pressure can be assumed• reinforced in both directions• check moment at face of column• check punching shear – critical shear perimeter• check no tension on base (middle third rule)• consider economics with strip

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Strip Foundations

• used under walls• used for closely spaced columns (consider

economics with pad)

• helps reduce differential settlement• longitudinal and transverse steel

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Raft Foundations• useful in soils of low-bearing capacity (large plan

area)• economic reasons (closely spaced columns)• variations include compensated and partially

compensated rafts

• can be stiffened by beams cast (monolithically) intothe foundation (under columns)

RaftBuoyant Raft (partially / fully

compensated