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DESIGN OF GROUND ANCHORS – USE OF NEW SECTION 8 Eric R Farrell AGL Consulting.

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Presentation on theme: "DESIGN OF GROUND ANCHORS – USE OF NEW SECTION 8 Eric R Farrell AGL Consulting."— Presentation transcript:

1 DESIGN OF GROUND ANCHORS – USE OF NEW SECTION 8 Eric R Farrell AGL Consulting.

2 Design of Anchors – Use of New Section 8:EC 72 Section 8 Anchors  Anchor must have a free length  This presentation will cover the design of grouted anchor

3 ASSOCIATED STANDARDS EN1537:2013 Execution of special geotechnical work – Ground Anchors. Note – A ‘grouted anchor’ in EN is termed a ‘ground anchor’ in EN 1537 EN ISO Geotechnical investigation and testing-Testing of geotechnical structures-Part 5:Testing of pre-stressed ground anchors (only in draft form at present) Design of Anchors – Use of New Section 8:EC 73

4 4 6m Lake (water) DESIGN EXAMPLE – THE DESIGN ISSUE DESIGN ANCHOR TO SUPPORT SHEET PILED QUAY WALL Sandy Gravel Design Approach 1 used Design is based on 3m anchor spacing.

5 Design of Anchors – Use of New Section 8:EC 75 Anchor prestressed at this stage 2m (with overdig) 1m CONSTRUCTION STAGES

6 Design of Anchors – Use of New Section 8:EC m with overdig Surcharge of 20kPa

7 Design of Anchors – Use of New Section 8:EC 77

8 8

9 FUNDAMENTAL DESIGN REQUIREMENTS The design of the anchors must consider:-  Ultimate Limit States (ULS) and Serviceability Limit States (SLS) of the anchor,  ULS and SLS of the supported structure.  Prestress forces and the effect of prestress forces, where relevant. Design of Anchors – Use of New Section 8:EC 79

10 Ultimate limit state (ULS) design force to be resisted by the anchors E ULS,d ≤ R ULS;d Eq. 8.1 where E ULS,d = max(F ULS,d ; F Serv,d ) Eq. 8.2 and F Serv,d =  Serv F Serv,k Eq. 8.3 R ULS;d = Design value of the resistance of an anchor complying with ULS criteria E ULS,d = ULS design force to be resisted by the anchor F ULS,d = Design value of the force required to prevent any ULS in the supported structure. F Serv,d = design value of the maximum anchor force (expected within the design life of the anchor), including effect of lock off load, and sufficient to prevent a SLS in the supported structure.  Serv = partial factor Design of Anchors – Use of New Section 8:EC 710

11 Serviceability Limit State F Serv,k ≤ R SLS;d Eq.8.4 F Serv,k = characteristic value of the maximum anchor force (expected within the design life of the anchor), including effect of lock off load, and sufficient to prevent a SLS in the supported structure. R SLS;d = design value of the resistance of an anchor complying with SLS criteria Note, assumes partial factor of unity Design of Anchors – Use of New Section 8:EC 711 WHERE REQUIRED IN THE NA

12 Note:- R ULS;d and, where required R SLS;d must be determined/validated by anchor tests. Design of Anchors – Use of New Section 8:EC 712

13 Geotechnical ULS resistance of anchor Design of Anchors – Use of New Section 8:EC 7 13

14 Geotechnical ULS resistance of anchor continued Design of Anchors – Use of New Section 8:EC 7 14

15 Geotechnical SLS resistance of anchor (where required) Design of Anchors – Use of New Section 8:EC 715

16 Geotechnical SLS resistance of anchor continued Design of Anchors – Use of New Section 8:EC 716

17 Design of Anchors – Use of New Section 8:EC 717 PARAMETERS REQUIRED FROM ANCHOR DESIGN Anchor required to satisfy the resistance values SUMMARY

18 Design of Anchors – Use of New Section 8:EC 718 Limit equilibrium analysis Finite element analysis without/with initial prestress (lock-off) of 60kN/m Design SituationL (m) Anchor force kN/m Max. BM kNm/ m L (m) Anchor force kN/m Max. BM kNm/m Characteristic actions and soil parameters, no overdig /66* / /67.5 Characteristic actions and soil parameters, with overdig /73* / /80.1 DA1.C /102.5* / /87.6 DA1.C2 (  Q =1.3) /112.9* / /126.9 DA1.C2 (  Q =1.0) /102* / /113.9 Length 8.5m using char value with no overdig / /39.5 *25% added for arching F ULS,d F Serv,k

19 Design of Anchors – Use of New Section 8:EC 7 19 PARAMETERS FOR ANCHOR DESIGN – assuming 3m spacing Anchor required to satisfy the resistance values

20 Design of Anchors – Use of New Section 8:EC 7 20 ANCHOR TESTING

21 Design of Anchors – Use of New Section 8:EC 721 ANCHOR DESIGN/TESTING  Preliminary estimation of bonded length based on ground conditions and geotechnical parameters  Investigation tests  Suitability tests  Acceptance tests Not part of EC7:Sec 8 EN ISO When available Number of Investigation/Suitability tests to be set in NA.

22 Design of Anchors – Use of New Section 8:EC 722 Test Proof Load Investigation tests -Not on working anchor -Normally taken to failure of bonded length/ground -may require larger structural element Eq P P ≥ ξ ULS x  a;ULS x E ULS,d Suitability test-On working anchor -taken to load that verifies R ULS,d -P P comes from Eqs, 8.1, 8.6 & 8.7 Eq P P ≥ ξ ULS x  a;ULS x E ULS,d Acceptability tests-Must be carried out on all anchors -  a,acc;ULS and  a,acc;SLS from Table A.20 Eq P P ≥  a,acc;ULS x E ULS,d or Eq P P ≥  a,acc;SLS x F Serv,k

23 Design of Anchors – Use of New Section 8:EC 723 kN/m 3m F Serv,k Characteristic value of the maximum anchor force, including effect of lock-off load, and sufficient to prevent a SLS in the structure F ULS;d Design value of the force required to prevent any ULS in the supported structure F Serv,d Eq.8.3  Serv =1.35 from Table A E ULS,d Eq PoPo Lock-off load, taken as about 80% of F Serv,k P Proof load, Suitability tests (P P ≥ ξ ULS x  a;ULS x E ULS,d …… Eq 8.12) ξ ULS = 1.0 (Table A.20)  a;ULS = 1.1 (Table A.19) P Proof load, Acceptability tests P P ≥  a,acc;ULS x E ULS,d Eq (Table A.20) or P P ≥  a,acc;SLS x F Serv,k Eq (Table A.20) Taking 3m spacing of anchors

24 Summary (using default values) Anchor spacing of 3m Preliminary estimate of bond length (NOT COVERED IN EUROCODE) Hole dia = 0.25m  v'ave = 3*18+7*10 = 74 kPa and taking K value =2 L = 426/(2x74xTan 38x3.1416x0.25)= 4.7m Investigation tests – Load to be taken to at least 426kN. Suitability tests on selected anchors, load to be taken to 426kN Acceptability tests (on all anchors), load to be taken to 426kN ???? These tests are required to validate that the anchors have the required R ULS,d and, if specified in the NA, R SLS,d Design of Anchors – Use of New Section 8:EC 724

25 Measuring R ULS,m and R SLS,m of an anchor (draft ISO/DIS ) Method 1Method 2Method 3 Type of loading Cycle loading In steps Rest periods Maintained loadsMaintained deflectionMaintained load Measurements Tendon head displacement vs applied load at end of each cycle Load-loss vs time at the highest load of each cycle Anchor head displacement vs anchor load at the beginning and end of each load step., Tendon head displacement vs time k 1 versus anchor loadAnchor head displacement vs time for each load step.  1 versus anchor load  3 versus anchor load or bond load, if possible. Displacement vs load for all cycles R ULS,m Currently  1 =2mm Load loss ≤ 5% at P P or displacement ≤ 5%  e over log cycle of time (from BS8081) When plot of  3 vs load, is asymptote or  3 =5mm R SLS,m Not definedLoad loss ≤ 1%at P P or rate of displacement should reduce to 1%  e over log cycle of time (from BS8081) Pc (from a s vs load, end of pseudo linear portion) Design of Anchors – Use of New Section 8:EC 725

26 Structural Design E ULS,d ≤ R t;d i.e 386kN ≤ R t;d Eq 8.11 where E ULS,d = Ultimate limit state design force to be resisted by the anchor. R t;d = Design resistance of the structural element Design of Anchors – Use of New Section 8:EC 726

27 Design of Anchors – Use of New Section 8:EC 727

28 Design of Anchors – Use of New Section 8:EC 728 Cl (2)P of EN :2004 refers to from EN :2005 Cl (2)P- overstressing to 0.95f y is permitted if the force in the jack can be measured to an accuracy of ±5% P P = 426kN (=1.1xE ULS,d ) Therefore P P ≤ 0.95*629 = 597 kN > 426kN therefore OK STRUCTURAL REQUIREMENT FOR PROOF TESTING

29 Significant of changes The new proposals presents a rational design method for anchors The current anchor testing practices in many countries will required modification. The introduction of F Serv,k is relatively novel within the Eurocode system. Design of Anchors – Use of New Section 8:EC 729

30 Comparison with present system Current practice in UK & Ireland is to use BS8081 Design is based on ‘working’ load T w No proper definition of T w is given (T w can be zero in some uplift situation, for example) Design of anchors to BS8081 does not consider ULS explicitly. Design of Anchors – Use of New Section 8:EC 7 30

31 Acknowledgements Members of SC7/EG1 on Anchors Colleagues at AGL Consulting Brian Simpson and Georgios Katsigiannis for invitation Design of Anchors – Use of New Section 8:EC 731

32 Design of Anchors – Use of New Section 8:EC 7 32 THANK YOU


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