bridge-design of shallow foundations (1)

8/13/2019 Bridge-Design of Shallow Foundations (1)

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LRFD Design of

Shallow Foundations


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Nominal Geotechnical

Resistances

ASD Failure Modes

Overal l Stabil i ty Bearing Capacity

Sett lement

Sl id ing

Overturning


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Nominal Geotechnical

Resistances LRFD Service Limit State

Overal l Stabil i ty

Vertical (Sett lement) and Horizontal

Movements

LRFD Strength Limit State

Bearing Resistance Sl id ing

Eccentr ic i ty Lim i ts (Overturning)


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Stabilize Destabilize

Service Limit State

Global Stability


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Global Stability Factor of Safety

Method of Slices

+

WT

WT

WTWT

NN

TT

T T

cc

N tan fN tan f


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ASD Factors of Safety

Soil/Rock Parameters and

Ground Water Conditions

Based On:

Slope Supports

Abutment or

Other

Structure?Yes No

In-situ or Laboratory Tests and

Measurements1.5 1.3

No Site-specific Tests 1.8 1.5

Resistance Factors

LRFD


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Stability Wrap-Up

Unfactored loads Service Lim it State

Applied stress must be limited Foot ings supported in a slope

0.65 (FS 1.5) Stress criteria for stability can control

footing design


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Service Limit State Design

Settlement

Cohesive Soils Evaluate Using Conso l idat ion Theory

Cohesionless Soils Evaluate Using Empir ical or Other Conventional

Methods

Hough Method


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Impact on Structures


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Settlement of Granular vs.

Cohesive Soils

Relative importance of settlement

components for different soil types Elast ic

Primary Consol idat ion

Secondary Settlement (Creep)


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Settlement of Granular vs.

Cohesive Soils

Structural effects of settlement

components

Include Transient Loads if Drained

Loading is Expected and for Computing

Initial Elastic Settlement

Transient Loads May Be Omitted When

Computing Consolidation Settlement of

Cohesive Soils


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Hough MethodSettlement of Cohesionless Soils


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Stress

BelowFooting

BoussinesqPressure

Isobars


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Nominal Bearing Resistance at

Service Limit State

Rn

Bf

For a constant valueof settlement


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ML M

B

LB

P

LB

P

LB

eB eL

BL

P

Eccentricity of Footings on Soil

eB= MB/ Pe

L= M

L/ P


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ML M

B

LB

eB eL

BL

P

Effective Dimensions for

Footings on Soil

B= B 2eB

L= L 2eL


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ML M

B

LB

eB eL

BL

P

q

Applied Stress Beneath Effective

Footing Area


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Stress Applied to SoilStrip Footing


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Footings on RockTrapezoidal Distribution


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Footings on RockTriangular Distribution


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Use of Eccentricity and Effective

Footing Dimensions

Service Limit State Nom inal Bearing Resistance Lim ited b y

Sett lement

Strength Limit State Nom inal Bear ing Resistance Lim ited by Bear ing

Resistance

Prevent Overturning Al l App l icable Lim it States


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Strength Limit State

Bearing Resistance


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Strength Limit State Design

Bearing Resistance

Footings on Soil Evaluate Using Conventional Bearing Theory

Footings on Rock Evaluate Using CSIR Rock Mass Rating Procedu re


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1

2 233

dade= C + s tan fSoil Shear Strength

Df

B>Df

B

GroundSurface sv= gDf

PpPp

ccb

aIb

bb

Bearing Resistance Mechanism


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Table 10.5.5.2.1-1 Resistance Factors for Geotechnical Resistance of Shallow

Foundations at the Strength Limit State

METHOD/SOIL/CONDITION RESISTANCE FACTOR

Bearing

Resistanceb

Theoretical method (Munfakh, et al. (2001), in clay 0.50

Theoretical method (Munfakh, et al. (2001), in sand,

using CPT 0.50

Theoretical method (Munfakh, et al. (2001), in sand,

using SPT0.45

Semi-empirical methods (Meyerhof), all soils 0.45

Footings on rock 0.45

Plate Load Test 0.55

Sliding

Precast concrete placed on sand 0.90

Cast-in-Place Concrete on sand 0.80

Cast-in-Place or precast Concrete on Clay 0.85

Soil on soil 0.90

epPassive earth pressure component of slidingresistance

0.50


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Footings on Rock

Service Limit State use published

presumptive bearing

Published values are al lowable

therefore settlement-limited

Procedures for computing settlement

are available


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Very little guidance available for

bearing resistance of rock

Proposed Specification revisionsprovide for evaluating the cohesion and

friction angle of rock using the CSIR

Rock Mass Rating System

Footings on RockStrength Limit State


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CSIR Rock Mass Rating System

CSIR Rock Mass Rating developed for

tunnel design

Includes life safety considerations and

therefore, margin of safety

Use of cohesion and friction angle

therefore may be conservative


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LRFD vs. ASD

All modes are expressly checked at a

limit state in LRFD

Eccentricity limits replace the

overturning Factor of Safety


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Width vs. Resistance - ASD

SettlementcontrolsShear Failurecontrols

Footing width, B (m)

0.0 1.0 2.0 3.0 4.0 5.0

800

Bear

ingPressu

re(kPa)

Allowable Bearing Capacity, FS = 3.0

Bearing Pressure for 25-mm (1in) settlement

600

400

0


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Settlement vs. Bearing

Resistance

00

1212

N=30N=30

B, ftB, ft

qqaa,

ksf

,ksf N=25N=25

N=5N=5

N=20N=20

N=15N=15

N=10N=10

22 44 66 1414101088 1212

22

00

44

66

88

1010

00

1212

N=30N=30

B, ftB, ft

qqaa,

ksf

,ksf N=25N=25

N=5N=5

N=20N=20

N=15N=15

N=10N=10

22 44 66 1414101088 1212

22

00

44

66

88

1010


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Width vs. Resistance - LRFD

Effective Footing width, B (m)

0 4 8 12 16 20

NominalB

earing

Resistanc

e(ksf)

Strength Limit State

Service Limit State

5

15

25

35


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Recommended Practice

For LRFD design of footings on soil

and rock; Size footin gs at the Service Lim it State

Check footing at al l other appl icable Lim it States

Settlement typically controls!


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Summary Comparison of ASD

and LRFD for Spread Footings

Same geotechnical theory used to

compute resistances, however

As per Limit State concepts,

presentation of design

recommendations needs to be modified


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METHOD/SOIL/CONDITION

RESISTANCE

FACTOR

Bearing

Resistance

f All methods, soil and rock 0.45

Plate Load Test 0.55

Sliding f Precast concrete placedon sand

0.90

Cast-in-Place Concrete on

sand0.80

Clay 0.85Soil on soil 0.90

fep Passive earth pressure

component of sliding

resistance

0.50

Strength Limit State Resistance Factors

bridge-design of shallow foundations (1)

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