Some things Eurocode 7 doesn’t say Brian Simpson Arup Geotechnics BP188.1
UCL, 18 March 2013
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9)
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9)
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9)
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading
EN 1990
3.3 Ultimate limit states
Serious failures involving risk of injury or major cost. Must be rendered very unlikely. An “unrealistic” possibility.
EN1990
3.4 Serviceability limit states
Inconveniences, disappointments and more manageable costs. Should be rare, but it might be uneconomic to eliminate them completely.
Fundamental limit state requirement E d R d E {
F d ; X d ; ad} = E d R d = R { F d
; X d ; ad}
E { F F rep; X k/ M; ad} = E d R d = R { F F rep; X k/ M; ad}
or E { F F rep; X k/ M; ad} = E d R d = R k/ R = R nf R (LRFD) or
E E k = E d R d = R k/ R
so in total E E { F F rep; X k/ M; ad} = E d R d = R { F F rep; X k/ M; ad}/ R E = action effects
d = design (= factored)
F = actions (loads)
k = characteristic (= unfactored)
R = resistance (=capacity)
rep = representative
X = material properties a = dimensions/geometry
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9) ULS design water level must be at ground level
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading on foundations
Characteristic values in EC7
Characteristic values in EC7 – definition (2.4.5.2)
Characteristic values in EC7 2.4.3(4) also mentions:
Characteristic values in EC7 – zone of ground
Small building on estuarine beds near slope
Characteristic values in EC7 – definition (2.4.5.2)
Characteristic values in EC7 •
NOT a fractile of the results of particular, specified laboratory tests on specimens of material.
•
A cautious estimate of the value affecting the occurrence of the limit state
•
Take account of time effects, brittleness, soil fabric and structure, the effects of construction processes and the extent of the body of ground involved in a limit state
•
The designer’s expertise and understanding of the ground are all encapsulated in the characteristic value
•
Consider both project-specific information and a wider body of geotechnical knowledge and experience.
•
Characteristic = moderately conservative = representative (BS8002) = what good designers have always done.
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9)
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading on foundations
Calculations should use peak strengths of materials? Only one reference to peak strength in EC7:
Calculations should use peak strengths of materials?
f’ – peak, critical state or residual?
• • • •
•
The value that will prevent the exceedance of the limit state. Caution about progressive loss of density or strength. So caution about f’>38º. Residual Suggestion: The ULS design value of the shear strength should never be greater than a cautious (ie “characteristic”) estimate of the critical state strength of the material. (Should EC7 say this?) Interfaces between structure and ground (wall friction, sliding etc) – use critical state value as characteristic value, and apply normal factors.
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9)
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading on foundations
Partial factors for DA1 – UK National Annex Design approach 1
Actions
Permanent
Soil
Variable tan f' Effective cohesion
Combination 1---------------- Combination 2 ----------------Combination 2 - piles & anchors A1 M1 R1 A2 M2 R1 A2 R4 M1 or … M2 unfav 1,35 fav unfav
1,5
1,3
1,3 1,25
1,25
1,25
1,25
Undrained strength
1,4
1,4
Unconfined strength
1,4
1,4
Weight density Spread
Bearing
footings
Sliding
Driven
Base
piles
Shaft (compression)
1.5/1.3
1,3
Total/combined
1.7/1.5
1,3
EC7 1,7/1.5
values 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
2.0/1.7
1.5
Shaft in tension
2.0/1.7
1.6
As
1.45
CFA
Base
piles
Shaft (compression)
Anchors
for
1.3
Total/com bined
bored
1.4
Shaft in tension
piles
1.6
Temporary
1,1
1,1
Permanent
1,1
1,1
Retaining Bearing capacity Sliding resistance walls Earth resistance Slopes
Earth resistance
indicates partial factor = 1.0 C:\BX\BX-C\EC7\[Factors.xls]
B
2.4.7 Ultimate Limit States 2.4.7.1 General General 2.4.7.1 (1)P Where Where relevant, relevant, itit shall shall be be verified verified that that the the following following limit limit states states are are not not exceeded: exceeded: (1)P loss of of equilibrium equilibrium of of the the structure structure or or the the ground, ground, considered considered as as aa rigid rigid body, body, in in which which the the loss
— —
strengths of of structural structural materials materials and and the the ground ground are are iinsignificant in providing providing resistance resistance strengths nsignificant in (EQU); (EQU); internal failure failure or or excessive excessive deformation deformation of of the the structure structure or or structural structural elements, elements, including including internal
— —
e.g. footings, footings, piles piles or or basement basement walls, walls, in in which which the the strength strength of of sstructural materials is is e.g. tructural materials
significant in in providing providing resistance resistance (STR); (STR); significant failure or or excessive excessive deformation deformation of of the the ground, ground, in in which which the the strength strength of of soil soil or or rock rock is is failure
— —
significant in in providing providing resistance resistance (GEO); (GEO); significant loss of of equilibrium equilibrium of of the the structure structure or or the the ground ground due due to to uplift uplift by by water water pressure pressure (buoyancy) (buoyancy) loss
— —
or other other vertical vertical actions actions (UPL); (UPL); or — —
hydraulic heave, heave, internal internal erosion erosion and and piping piping in in the the ground ground caused caused by by hydraulic hydraulic gradients gradients hydraulic
(HYD). (HYD).
DA1 Combinations 1 and 2 correspond to STR and GEO? STR
GEO
No differentiation between STR and GEO
Eurocode case study: High speed rail station, Florence, Italy
•
454m long, 52m wide and 27 to 32m deep
•
1.2 to 1.6m thick diaphragm walls
•
Three levels of temporary strutting.
Florence Station – comparison of bending moments Prop and excavation levels Stage 1 22
45
33
40 35
30
) 30 m ( l e 25 v e L
20 15 10
5 -6,000
-4,000
-2,000 -2,000
00
2,000 2,000
4,000 4,000
6,000 6,000
8,000 8,000
Bending Moment (kNm/m) C1
C1
C2, factors all stages C2, facto rs all stages C2,factoring stages separately
C1 C1 C2, factors all stages C2, factors all stages C2, factoring stages separately
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9)
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading on foundations
G’ S
z
u
EC7 {2.4.7.5(1)P} states: “When considering a limit state of failure due to heave by seepage of water in the ground (HYD, see 10.3), it shall be verified, for every relevant soil column, that the design value of the destabilising total pore water pressure (udst;d ) at the bottom of the column, or the design value of the seepage force (S dst;d ) in the column is less than or equal to the stabilising total vertical stress ( stb;d ) at the bottom of the column, or the submerged weight (G´ stb;d ) of the same column: u (2.9a) – total stress (at the bottom of the column) dst;d stb;d S G (2.9b)” effective weight (within the column) ´
G’ S
z
u
Annex A of EC7 provides values for partial factors to be used for HYD, G;dst = 1.35 and G;stb = 0.9. But the code does not state what quantities are to be factored. EC7 {2.4.7.5(1)P} states: “When considering a limit state of failure due to heave Maybe: by seepage ofG;stb water in the (2.9a) ground (HYD, see 10.3), it shall be verified, for every G;dst u dst;k stb;k and relevant soil column, that the design value of the destabilising total pore water (2.9b) G the pressure ) at of the column, or the design value of the seepage G;dst S dst;k (udst;d G;stb stb;k bottom force (S dst;d ) in the column is less than or equal to the stabilising total vertical In this ( format, the factors are applied to different quantities in 2.9 a and b. stress stb;d ) at the bottom of the column, or the submerged weight (G´ stb;d ) of the same column: u (2.9a) – total stress (at the bottom of the column) dst;d stb;d S G (2.9b)” effective weight (within the column) ´
´
Fundamental limit state requirement E d R d E {
F d ; X d ; ad} = E d R d = R { F d
; X d ; ad}
E { F F rep; X k/ M; ad} = E d R d = R { F F rep; X k/ M; ad}
or E { F F rep; X k/ M; ad} = E d R d = R k/ R = R nf R (LRFD) or
E E k = E d R d = R k/ R
so in total E E { F F rep; X k/ M; ad} = E d R d = R { F F rep; X k/ M; ad}/ R E = action effects
d = design (= factored)
F = actions (loads)
k = characteristic (= unfactored)
R = resistance (=capacity)
rep = representative
X = material properties a = dimensions/geometry
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9) ULS design water level must be at ground level
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading on foundations
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9) ULS design water level must be at ground level
Things it doesn’t say, but I suggest it should •
ULS design values not better than worst credible - Applies to ULS factored design values.
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9) ULS design water level must be at ground level
Things it doesn’t say, but I suggest it should •
ULS design values not better than worst credible - Applies to ULS factored design values. (EN 1990)
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of •
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9) ULS design water level must be at ground level
• • • •
•
Things it doesn’t say, but I suggest it should •
ULS design values not better than worst credible - Applies to ULS factored design values. (EN 1990)
-
DA1C2 design j ≤ characteristic jcrit
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9) ULS design water level must be at ground level
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading on foundations
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9) ULS design water level must be at ground level
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading on foundations
G’ S
z
u
EC7 {2.4.7.5(1)P} states: “When considering a limit state of failure due to heave by seepage of water in the ground (HYD, see 10.3), it shall be verified, for every relevant soil column, that the design value of the destabilising total pore water pressure (udst;d ) at the bottom of the column, or the design value of the seepage force (S dst;d ) in the column is less than or equal to the stabilising total vertical stress ( stb;d ) at the bottom of the column, or the submerged weight (G´ stb;d ) of the same column: u (2.9a) – total stress (at the bottom of the column) dst;d stb;d S G (2.9b)” effective weight (within the column) ´
5 Isotropic
13 Low k layer – 5:1
Anisotropic – 5:1
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9) ULS design water level must be at ground level
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading on foundations
Some things Eurocode 7 doesn’t say •
Things it doesn’t say, but is sometimes accused of • • • • •
•
Things it doesn’t say, but I suggest it should • • •
•
Characteristic values are 5% fractiles of measured strengths Calculations should use peak strengths of materials DA1 Combinations 1 and 2 correspond to STR and GEO Design values = k-values x partial factor (HYD - Eq 2.9) ULS design water level must be at ground level
ULS design values not better than worst credible Minimum earth pressures for retaining walls Beware of permeability distributions (HYD)
Other things it doesn’t say • •
Minimum surcharge for retaining walls 25% overstress allowance for wind loading on foundations
