reasons for british choices of design approach and...

1

Safety Concepts and Calibration of Partial Factors

in European and North American Codes of PracticeDelft, 30 Nov – 1 Dec 2011 BP198.1

BP201.1

Reasons for British choices

of design approach and

partial factors

Brian Simpson, Arup Geotechnics

2

Reasons for British choices of design approach and partial factors BP198.2

BP201.2

• British choices

• Guiding principles

• Piling

• References

3


BP201.2

• British choices


• Piling

• References

4

Partial factors for DA1 – UK National Annex

Design approach 1 Combination 1---------------------Combination 2 -------------------------Combination 2 - piles & anchors

A1 M1 R1 A2 M2 R1 A2 M1 or …..M2 R4

Actions unfav 1,35

fav

unfav 1,5 1,3 1,3

Soil tan φ' 1,25 1,25

Effective cohesion 1,25 1,25

Undrained strength 1,4 1,4

Unconfined strength 1,4 1,4

Weight density

Spread Bearing EC7

footings Sliding values

Driven Base 1,7/1.5 1,3

piles Shaft (compression) 1.5/1.3 1,3

Total/combined

(compression)

1.7/1.5 1,3

Shaft in tension 2.0/1.7 1.6

Bored Base 2.0/1.7 1,6


Total/combined

(compression)

2.0/1.7 1.5


CFA Base As 1.45

piles Shaft (compression) for 1.3

Total/combined

(compression)

bored 1.4

Shaft in tension piles 1.6

Anchors Temporary 1,1 1,1

Permanent 1,1 1,1

Retaining Bearing capacity

walls Sliding resistance

Earth resistance

Slopes Earth resistance

indicates partial factor = 1.0

C:\BX\BX-C\EC7\[Factors.xls]

Permanent

Variable

5


BP201.2

• British choices


• Piling

• References

6

Guiding principles BP198.2 BP201.2

• Broad compatibility with previous designs • but not identity

• Cover all problems, geo and structural – SSI

• Apply the factors where the uncertainties lie

• Compatible with FE

• Proper distinction between ULS and SLS?

• No reliability calculations used

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8

Broad compatibility with previous designs, but not identity

• CP2 (1951) Earth retaining structures.• Single factors on passive resistance or sliding.

• Still used recently for gravity structures.

• CIRIA Report 104 (1984) Embedded retaining walls• Single factors on passive resistance or material factors

• BS8002 (1994) Retaining structures• Material factors – “mobilisation factors” (SLS) - γφ = 1.2

• Structural design unclear

• CIRIA C580 (2003) Embedded retaining walls• Strength factors – γφ = 1.2

• So some pressure to reduce to 1.2 in National Annex

• BS6031 (1981) Earthworks• F = 1.3 to 1.4 for slopes (first-time slides)

• BS8004 (1986) Foundations• Single factors F = 2 to 3, depending on ....

• LDSA – Piling• F = 2 to 3 depending on SI and load testing

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• Cover all problems, geo and structural – SSI• Compatibility with structural design – a failing in past BS codes





10

The slope and retaining wall are all part of the

same problem. BP87.62 BP106.33 BP111.25 BP112.46

BP119.46 BP124-F3.12 BP130.36 BP145a.11

Structure and soil must be

designed together - consistently.

11

C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls]

Ratio of ββββ achieved to ββββ required

0.6

0.7

0.8

0.9

1

1.1

1.2

0 0.2 0.4 0.6 0.8 1

SA

FE

TY

RA

TIO

.

σE/(σR+σE)

ααααE=-0.7, ααααR=0.8

Less economic

Less safe

Dominated by

strength

Dominated by

loading

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0.6

0.7

0.8

0.9

1

1.1

1.2

0 0.2 0.4 0.6 0.8 1

SA

FE

TY

RA

TIO

.

σE/(σR+σE)

ααααE=-0.7, ααααR=0.8Slope

stability

Typical

foundations

Tower

foundations

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0.6

0.7

0.8

0.9

1

1.1

1.2

0 0.2 0.4 0.6 0.8 1

SA

FE

TY

RA

TIO

.

σE/(σR+σE)

ααααE=-0.7, ααααR=0.8

ααααE=-0.4, ααααR=1.0 ααααE=-1.0, ααααR=0.4

Uneconomic

Unsafe

14

C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls] 14-Aug-06 21:52


0.6

0.7

0.8

0.9

1

1.1

1.2

0 0.2 0.4 0.6 0.8 1

SA

FE

TY

RA

TIO

.

σE/(σR+σE)

Uneconomic

Unsafe

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16

Apply the factors where the uncertainties lie BP198.2 BP201.2

• Where the uncertainties can be quantified

17

Ferrybridge power station –

actions which tend to cancel each other BP145a.27

http://www.knottingley.org/history/tales_and_events.htm

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• Generally, factor actions before combining• Possibly not for earth and water pressures – physically

unreasonable {2.4.7.3.2(2)}

• Geo engineers find it “natural” to apply factors to

soil strength – greatest uncertainty• Seen as the standard against which to compare

• Burland, Potts and Walsh (1981)

• Foye, Salgado, and Scott (2006)

• Piles are different in this respect• Uncertainty in model > in soil properties

• Design dependent on load testing of complete element

• Lot of data and experience

• Similar differences between concrete and steel design??

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• Compatible with FE• DA2 + DA3 � DA1 ??



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• Proper distinction between ULS and SLS?• Spread foundations often governed by SLS, so can accept

relatively low ULS factors

• Use larger overall factors consciously for SLS {2.4.8(4), 6.6.2(16)}

• Less clear for pile design

21


• Broad compatibility with previous designs





• No reliability calculations used• Relied on comparisons with previous codes and designs• Factoring leading variables but conscious that there are very many

secondary variables • SLS is not easily analysed or predicted, so ULS factors are not

independent of SLS• Usually data are very diverse in nature – combination of test

results, previous publications, other experience, etc• So fear of omitting important data, even though fuzzy

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BP201.2

• British choices


• Piling

• References

23

7.4 Design methods and design considerations

24

Partial factors for DA1 – UK National Annex

Design approach 1 Combination 1---------------------Combination 2 -------------------------Combination 2 - piles & anchors

A1 M1 R1 A2 M2 R1 A2 M1 or …..M2 R4

Actions unfav 1,35

fav

unfav 1,5 1,3 1,3

Soil tan φ' 1,25 1,25

Effective cohesion 1,25 1,25

Undrained strength 1,4 1,4

Unconfined strength 1,4 1,4

Weight density

Spread Bearing EC7

footings Sliding values

Driven Base 1,7/1.5 1,3


Total/combined

(compression)

1.7/1.5 1,3


Bored Base 2.0/1.7 1,6


Total/combined

(compression)

2.0/1.7 1.5


CFA Base As 1.45

piles Shaft (compression) for 1.3

Total/combined

(compression)

bored 1.4

Shaft in tension piles 1.6

Anchors Temporary 1,1 1,1

Permanent 1,1 1,1

Retaining Bearing capacity

walls Sliding resistance

Earth resistance

Slopes Earth resistance

indicates partial factor = 1.0

C:\BX\BX-C\EC7\[Factors.xls]

Permanent

Variable

These factors are applied to

characteristic (ultimate) pile

resistances.

How are characteristic resistances

obtained?

a) From load testing

b) From calculation

25

Pile design by calculation from soil tests BP168-3.23

Alternatively, calculations my be based on characteristic shaft and base resistance derived by

other means.

• What are qb;k and qs;k ?

• Where does the model factor go?

26

SLS also covered by ULS factors

27



• Generally, factor actions before combining• Possibly not for earth and water pressures – physically

unreasonable {2.4.7.3.2(2)}

• Geo engineers find it “natural” to apply factors to

soil strength – greatest uncertainty• Seen as the standard against which to compare

• Burland, Potts and Walsh (1981)

• Foye, Salgado, and Scott (2006)

• Piles are different in this respect• Uncertainty in model > in soil properties

• Design dependent on load testing of complete element

• Lot of data and experience

28

Pile design by calculation - UK National Annex BP168-3.25

Characteristic soil

strengths (cu,k, tanφφφφk, etc)

Calculated shaft and

base resistance

Characteristic shaft and

base resistance

Design shaft and base

resistance (ULS)

γγγγs and γγγγb

γγγγRd=1.4 or 1.2

Calculation model – accurate or erring on the side of safety

Value depends

on Test to ULS

Rk

Value

depends on

testing 1%

May be used

as ultimate

resistance for

SLS calcs

29

Broad compatibility with previous designs, but not identity

• CP2 (1951) Earth retaining structures.• Single factors on passive resistance or sliding.

• Still used recently for gravity structures.

• CIRIA Report 104 (1984) Embedded retaining walls• Single factors on passive resistance or material factors

• BS8002 (1994) Retaining structures• Material factors – “mobilisation factors” (SLS) - γφ = 1.2

• Structural design unclear

• CIRIA C580 (2003) Embedded retaining walls• Strength factors – γφ = 1.2

• So some pressure to reduce to 1.2 in National Annex

• BS6031 (1981) Earthworks• F = 1.3 to 1.4 for slopes (first-time slides)

• BS8004 (1986) Foundations• Single factors F = 2 to 3, depending on ....

• LDSA – Piling• F = 2 to 3 depending on SI and load testing

30

Ground Engineering, Dec 09 and Jan 10

Results similar to

previous practice

31


BP201.2

• British choices


• Piling

• References

32

References BP198.2 BP201.2

• Bond, AJ and Simpson, B (2009-10) Pile design to Eurocode 7 and the UK National Annex (2 parts). Ground engineering, Dec 2009 and Jan 2010.

• Burland, J.B., Potts, D.M. and Walsh, N.M. (1981). The overall stability of free and propped embedded cantilever retaining walls. Ground Engineering, 14 No. 5, 28-38.

• Central Electricity Generating Board. 1965. Report of the Committee of Inquiry into Collapse of Cooling Towers at Ferrybridge Monday 1 November 1965. Central Electricity Generating Board, London.

• Foye, K. C., Salgado, R., and Scott, B. (2006). Resistance factors for use in shallow foundation LRFD. J. Geotech. Geoenviron. Eng., 132(9), 1208–1218.Guiding principles

• Simpson B (2007) Approaches to ULS design - The merits of Design Approach 1 in Eurocode 7. ISGSR2007 First International Symposium on Geotechnical Safety & Risk pp 527-538. Shanghai Tongji University, China.

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BP201.2

• British choices


• Piling

• References

Thanks for your attention

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