THE ENGINEERING OF FOUNDATIONSA BOOK BY RODRIGO SALGADOPOWERPOINT RESOURCES FOR

CHAPTER 2

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THE DESIGN PROCESS

DESIGN GOALS

• Safety• Serviceability• Economy

The optimum foundation solution transfers the superstructure loads to the ground in a way that minimizes cost (construction and maintenance) over the life of the structures without sacrificing safety or performance

RELIABILITY-BASED DESIGN

LIMIT STATES DESIGN AND WORKING STRESS

DESIGN

TYPES OF LIMIT STATES

• ULTIMATE LIMIT STATES (ULS) : states at which a potentially unsafe/dangerous

condition is reached (typically structural failure, collapse, etc.)

• SERVICEABILITY LIMIT STATES (SLS) : states at which the structure or facility ceases to

perform as intended without loss of safety

Limit State

Nature of Limit State Consequences

IA-1 Classical bearing capacity failure (ULS)

Excessive movement/collapse of foundation causes serious damage, partial collapse or complete collapse of structure

IA-2 Structural failure of foundation element (ULS)

Column is inadequately supported by foundation element, punching through it; this causes serious damage, partial or complete collapse of superstructure

IB Excessive differential foundation settlement (ULS)

Excessive differential settlements create excessive additional loads in the structure, leading to structural damage

II Excessive settlement (total or differential) (SLS)

Excessive settlements lead to serviceability problems, such as access problems, damage to architectural finishings, etc.

III Stability failure of the whole foundation system or a subset thereof

Collapse mechanism develops that encompass the foundations for the building or structure or a part of the foundations (a classical example would be stability failure of a slope on top of which is founded a building)

Limit states for shallow foundation design

Working Stress Design

Ultimate load: the load that would lead to an ultimate limit state

Factor of Safety (FS)

d aQ Q

Ultimate load

ulta

QQ

FS

Allowable load

Design load

All load and resistance uncertainities are lumped in this single factor

Design or working load: the sum of the loads under consideration (unfactored)

ULTIMATE LIMIT STATES: LOAD AND RESISTANCE

FACTOR DESIGN

TOLERABLE FOUNDATIONMOVEMENTS

• Uniform Total Settlement

• Non-Uniform Settlement

• Tilting

Types of Settlement

Uniform Total Settlement

• No structural damage

• Damage at interface with utilities

• Access problems

• Drainage problems

Non-Uniform Settlement

• Structural– Cracking of beams/slabs/column

• Architectural– Cracking of panel walls– Door/window jamming– Uneven Floors

• Damage at interface with utility lines

• Access and drainage problems

• Excessive Tilting

Cracks Caused by Foundation Movement

• May have any direction, depending on the direction of the tensile strain (cracks are to the tensile strain)

• At a specific point of the structure, cracks have the same direction (contrast that with the web pattern observed in the case of shrinkage and drying)

• Appear on both sides of an element

Typical Building Structures

• Load Bearing Walls

• Frames