Reasons for British choices of design approach - Geo-Engineering
Transcript of Reasons for British choices of design approach - Geo-Engineering
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
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Reasons for British choices of design approach and partial factors BP198.2
BP201.2
• British choices
• Guiding principles
• Piling
• References
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Reasons for British choices of design approach and partial factors BP198.2
BP201.2
• British choices
• Guiding principles
• 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
piles Shaft (compression) 1.6/1.4 1,3
Total/combined
(compression)
2.0/1.7 1.5
Shaft in tension 2.0/1.7 1.6
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
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Reasons for British choices of design approach and partial factors BP198.2
BP201.2
• British choices
• Guiding principles
• Piling
• References
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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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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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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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Guiding principles BP198.2 BP201.2
• Broad compatibility with previous designs • but not identity
• Cover all problems, geo and structural – SSI• Compatibility with structural design – a failing in past BS codes
• Apply the factors where the uncertainties lie
• Compatible with FE
• Proper distinction between ULS and SLS?
• No reliability calculations used
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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.
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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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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=0.8Slope
stability
Typical
foundations
Tower
foundations
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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
ααααE=-0.4, ααααR=1.0 ααααE=-1.0, ααααR=0.4
Uneconomic
Unsafe
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C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls] 14-Aug-06 21:52
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)
Uneconomic
Unsafe
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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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Apply the factors where the uncertainties lie BP198.2 BP201.2
• Where the uncertainties can be quantified
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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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Apply the factors where the uncertainties lie BP198.2 BP201.2
• Where the uncertainties can be quantified
• 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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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• DA2 + DA3 � DA1 ??
• Proper distinction between ULS and SLS?
• No reliability calculations used
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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?• 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
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Guiding principles BP198.2 BP201.2
• Broad compatibility with previous designs
• 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• 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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Reasons for British choices of design approach and partial factors BP198.2
BP201.2
• British choices
• Guiding principles
• Piling
• References
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
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
piles Shaft (compression) 1.6/1.4 1,3
Total/combined
(compression)
2.0/1.7 1.5
Shaft in tension 2.0/1.7 1.6
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
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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?
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Apply the factors where the uncertainties lie BP198.2 BP201.2
• Where the uncertainties can be quantified
• 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
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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
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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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Reasons for British choices of design approach and partial factors BP198.2
BP201.2
• British choices
• Guiding principles
• Piling
• References
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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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Reasons for British choices of design approach and partial factors BP198.2
BP201.2
• British choices
• Guiding principles
• Piling
• References
Thanks for your attention