Some things Eurocode 7 doesn t say

Transcription

1 BP188.1 Some things Eurocode 7 doesn t say Brian Simpson Arup Geotechnics 1 UCL, 18 March 2013

2 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) Things it doesn t say, but I suggest it should 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 2

3 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) Things it doesn t say, but I suggest it should 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 3

4 EN Ultimate limit states Serious failures involving risk of injury or major cost. Must be rendered very unlikely. An unrealistic possibility. 4

5 EN Serviceability limit states Inconveniences, disappointments and more manageable costs. Should be rare, but it might be uneconomic to eliminate them completely. 5

6 Fundamental limit state requirement E d R d E{ F d ; X d ; a d } = E d R d = R{ F d ; X d ; a d } E{ F F rep ; X k / M ; a d } = E d R d = R{ F F rep ; X k / M ; a d } or E{ F F rep ; X k / M ; a d } = E d R d = R k / R = R n f R (LRFD) or so in total E E k = E d R d = R k / R E E{ F F rep ; X k / M ; a d } = E d R d = R{ F F rep ; X k / M ; a d }/ R E = action effects F = actions (loads) R = resistance (=capacity) X = material properties a = dimensions/geometry d = design (= factored) k = characteristic (= unfactored) rep = representative 6

7 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 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 7

8 8 Characteristic values in EC7

9 9 Characteristic values in EC7 definition ( )

10 Characteristic values in EC (4) also mentions: 10

11 Characteristic values in EC7 zone of ground Small building on estuarine beds near slope 11

12 12 Characteristic values in EC7 definition ( )

13 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. 13

14 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) Things it doesn t say, but I suggest it should 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 14

15 Calculations should use peak strengths of materials? Only one reference to peak strength in EC7: 15

16 16 Calculations should use peak strengths of materials?

17 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.

18 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) Things it doesn t say, but I suggest it should 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 18

19 Partial factors for DA1 UK National Annex Values of partial factors recommended in EN Annex A B Design approach 1 Combination Combination Combination 2 - piles & anchors A1 M1 R1 A2 M2 R1 A2 M1 or M2 R4 Actions Permanent unfav 1,35 fav Variable unfav 1,5 1,3 1,3 Soil tan f' 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 1.7/1.5 1,3 (compression) Shaft in tension 2.0/ Bored Base 2.0/1.7 1,6 piles Shaft (compression) 1.6/1.4 1,3 Total/combined 2.0/ (compression) Shaft in tension 2.0/ CFA Base As 1.45 piles Shaft (compression) for 1.3 Total/combined bored 1.4 (compression) 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] 19

20 2.4.7 Ultimate Limit States General (1)P Where relevant, it shall be verified that the following limit states are not exceeded: loss of equilibrium of the structure or the ground, considered as a rigid body, in which the strengths of structural materials and the ground are insignificant in providing resistance (EQU); internal failure or excessive deformation of the structure or structural elements, including e.g. footings, piles or basement walls, in which the strength of structural materials is significant in providing resistance (STR); failure or excessive deformation of the ground, in which the strength of soil or rock is significant in providing resistance (GEO); loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy) or other vertical actions (UPL); hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients (HYD). 20 2

21 DA1 Combinations 1 and 2 correspond to STR and GEO? STR GEO No differentiation between STR and GEO 21

22 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. 22

23 Level (m) ULS design Florence Station comparison bending moments of bending moments FREW analyses 45 Prop and excavation levels Stage ,000-4,000-2, ,000 4,000 6,000 8,000 Bending Moment (knm/m) C1 C1 C1 C2, factors all stages C2, factors all stages C2, factors all stages C2,factoring stages separately C2, factors all stages C2, factoring stages separately 23

24 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) Things it doesn t say, but I suggest it should 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 24

25 z u G S EC7 { (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 (u dst;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 dst;d stb;d (2.9a) total stress (at the bottom of the column) S dst;d G stb;d (2.9b) effective weight (within the column) 25

26 z u G S 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. Maybe: EC7 { (1)P} states: When considering a limit state of failure due to heave by G;dst seepage u dst;k of G;stb water stb;k in the (2.9a) ground (HYD, see 10.3), it shall be verified, for every and relevant soil column, that the design value of the destabilising total pore water pressure G;dst S dst;k (u dst;d G;stb ) at G stb;k the bottom (2.9b) 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 In stress this format, ( stb;d ) at the the factors bottom are of applied the column, to different or the quantities submerged in weight 2.9 a and b. (G stb;d ) of the same column: u dst;d stb;d (2.9a) total stress (at the bottom of the column) S dst;d G stb;d (2.9b) effective weight (within the column) 26

27 Fundamental limit state requirement E d R d E{ F d ; X d ; a d } = E d R d = R{ F d ; X d ; a d } E{ F F rep ; X k / M ; a d } = E d R d = R{ F F rep ; X k / M ; a d } or E{ F F rep ; X k / M ; a d } = E d R d = R k / R = R n f R (LRFD) or so in total E E k = E d R d = R k / R E E{ F F rep ; X k / M ; a d } = E d R d = R{ F F rep ; X k / M ; a d }/ R E = action effects F = actions (loads) R = resistance (=capacity) X = material properties a = dimensions/geometry d = design (= factored) k = characteristic (= unfactored) rep = representative 27

28 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 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 28

29 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. 29

30 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) 30

31 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 j crit 31

32 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 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 32

33 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 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 33

34 z u G S EC7 { (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 (u dst;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 dst;d stb;d (2.9a) total stress (at the bottom of the column) S dst;d G stb;d (2.9b) effective weight (within the column) 34

35 5 Isotropic 13 Low k layer 5:1 Anisotropic 5:1 35

36 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 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 36

37 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 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 37

38 BP188.1 Some things Eurocode 7 doesn t say Brian Simpson Arup Geotechnics 38 UCL, 18 March 2013