Report

DESIGN OF GROUND ANCHORS – USE OF NEW SECTION 8 Eric R Farrell AGL Consulting. Section 8 Anchors Anchor must have a free length This presentation will cover the design of grouted anchor Design of Anchors – Use of New Section 8:EC 7 2 ASSOCIATED STANDARDS • EN1537:2013 Execution of special geotechnical work – Ground Anchors. Note – A ‘grouted anchor’ in EN1997-1 is termed a ‘ground anchor’ in EN 1537 • EN ISO 22477-5 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 7 3 DESIGN EXAMPLE – THE DESIGN ISSUE DESIGN ANCHOR TO SUPPORT SHEET PILED QUAY WALL Lake (water) 6m Sandy Gravel = 18kN/m3 above WL = 20kN/m3 below WL ck' = 0 k' = 38o E' = 50,000 kPa ' = 0.2 Design Approach 1 used Design is based on 3m anchor spacing. Design of Anchors – Use of New Section 8:EC 7 4 CONSTRUCTION STAGES Anchor prestressed at this stage 1m 2m (with overdig) = 18kN/m3 above WL = 20kN/m3 below WL ck' = 0 k' = 38o E' = 50000 kPa ' = 0.2 Design of Anchors – Use of New Section 8:EC 7 5 Surcharge of 20kPa 6.5m with overdig = 18kN/m3 above WL = 20kN/m3 below WL ck' = 0 k' = 38o E' = 50000 kPa ' = 0.2 Design of Anchors – Use of New Section 8:EC 7 6 Design of Anchors – Use of New Section 8:EC 7 7 Design of Anchors – Use of New Section 8:EC 7 8 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 7 9 Ultimate limit state (ULS) design force to be resisted by the anchors EULS,d ≤ RULS;d where and EULS,d = max(FULS,d; FServ,d) FServ,d = ServFServ,k Eq. 8.1 Eq. 8.2 Eq. 8.3 RULS;d = Design value of the resistance of an anchor complying with ULS criteria EULS,d = ULS design force to be resisted by the anchor FULS,d = Design value of the force required to prevent any ULS in the supported structure. FServ,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 7 10 Serviceability Limit State WHERE REQUIRED IN THE NA FServ,k ≤ RSLS;d Eq.8.4 FServ,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. RSLS;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 7 11 Note:RULS;d and, where required RSLS;d must be determined/validated by anchor tests. Design of Anchors – Use of New Section 8:EC 7 12 Geotechnical ULS resistance of anchor R ULS;m = min{Rm(aULS or kl;ULS) and PP} Eq. 8.5 R ULS;k = (RULS;m ) ULS Eq. 8.6 PP = Proof load RULS,m is the measured value of the resistance of an anchor complying with ULS (RULS,m)min is the lowest value of RULS;m measured from a number of investigation or suitability tests (n), for each distinct ground condition RULS;k is the characteristic value of the resistance of an anchor complying with ULS. aULS is the creep rate kl;ULS is the load loss Design of Anchors – Use of New Section 8:EC 7 13 Geotechnical ULS resistance of anchor continued R ULS;d = RULS;k γa;ULS Eq. 8.6 RULS,d is the design value of the resistance of an anchor complying with ULS a;ULS is a partial factor Design of Anchors – Use of New Section 8:EC 7 14 Geotechnical SLS resistance of anchor (where required) R SLS;m = min{Rm(aSLS or kl;SLS) and PP } R SLS;k = (R SLS;m )min Eq. 8.8 Eq. 8.9 RSLS,m is the measured value of the resistance of an anchor complying with SLS (RSLS,m)min is the lowest value of RSLS;m measured from a number of investigation or suitability tests (n), for each distinct ground condition RSLS;k is the characteristic value of the resistance of an anchor complying with SLS. aSLS is the creep rate kl;SLS is the load loss Design of Anchors – Use of New Section 8:EC 7 15 Geotechnical SLS resistance of anchor continued R SLS;d = RSLS;k γa;SLS Eq. 8.10 RSLS,d is the design value of the resistance of an anchor complying with ULS a;SLS is a partial factor Design of Anchors – Use of New Section 8:EC 7 16 SUMMARY PARAMETERS REQUIRED FROM ANCHOR DESIGN FULS;d FServ;k EULS;d Free anchor length Anchor required to satisfy the resistance values R ULS;d and where required R SLS;d Free anchor/tendon free length – Requirements of EN 1537 Design of Anchors – Use of New Section 8:EC 7 17 Limit equilibrium Finite element analysis without/with initial prestress analysis (lock-off) of 60kN/m Design Situation Characteristic actions parameters, no overdig L (m) Anchor force kN/m Max. BM kNm/ m L (m) Anchor force kN/m Max. BM kNm/m and soil 7.16 52.5/66* 66.4 7.2 67.7/78.9 Characteristic actions and parameters, with overdig soil 7.74 58.5/73* 83.0 7.8 78.6/77.4 DA1.C1 7.76 82.0/102.5* 115.3 8.5 101.9/123.4 88.3/87.6 DA1.C2 (Q=1.3) 8.5 90.3/112.9* 135.6 8.5 121.2/128.8 127.3/126.9 DA1.C2 (Q=1.0) 8.4 81.7/102* 125.2 8.5 107.6/115.0 114.7/113.9 8.5 64.7/75.23 55.4/39.5 Length 8.5m using char value with no overdig *25% added for arching Design of Anchors – Use of New Section 8:EC 7 72.83/67.5 FULS,d 79.6/80.1 FServ,k 18 PARAMETERS FOR ANCHOR DESIGN – assuming 3m spacing FULS;d = 128.8 kN/m (386 kN) F; = 75.2 kN/m ( 225 kN) EULS;d =128.8 kN/m ( 386 kN) Note:- 1.35x75.2=101.52kN/m Free anchor length = 5m Anchor required to satisfy the resistance values 386 kN ≤ R ULS;d and where required 225 kN ≤ R SLS;d Free anchor length > 5m Design of Anchors – Use of New Section 8:EC 7 19 ANCHOR TESTING Design of Anchors – Use of New Section 8:EC 7 20 ANCHOR DESIGN/TESTING Preliminary estimation of bonded length based on ground conditions and geotechnical parameters Not part of EC7:Sec 8 Investigation tests EN ISO 22477-5 When available Suitability tests Acceptance tests Number of Investigation/Suitability tests to be set in NA. Design of Anchors – Use of New Section 8:EC 7 21 Test Proof Load Investigation tests -Not on working anchor -Normally taken to failure of Eq. 8.12 bonded length/ground -may require larger structural PP ≥ ξULS x a;ULS x EULS,d element Suitability test -On working anchor Eq. 8.12 -taken to load that verifies RULS,d -PP comes from Eqs, 8.1, 8.6 & 8.7 PP≥ ξULS x a;ULS x EULS,d Acceptability tests Eq. 8.13 -Must be carried out on all PP ≥ a,acc;ULS x EULS,d anchors or - a,acc;ULS and a,acc;SLS from Table Eq. 8.14 A.20 PP ≥ a,acc;SLS x FServ,k Design of Anchors – Use of New Section 8:EC 7 22 Taking 3m spacing of anchors FServ,k FULS;d FServ,d EULS,d Po PP PP kN/m kN @ 3m Characteristic value of the maximum anchor force, 75 225 including effect of lock-off load, and sufficient to prevent a SLS in the structure. Design value of the force required to prevent any ULS in 128.8 386 the supported structure Eq.8.3 Serv =1.35 from Table A.18 101.3 304 Eq. 8.2 128.8 386 Lock-off load, taken as about 80% of FServ,k. 60 180 Proof load, Suitability tests (PP≥ ξULS x a;ULS x EULS,d …… 142 426 Eq 8.12) ξULS = 1.0 (Table A.20) a;ULS = 1.1 (Table A.19) Proof load, Acceptability tests PP ≥ a,acc;ULS x EULS,d Eq. 8.13 (Table A.20) 142 426 or PP ≥ a,acc;SLS x FServ,k Eq. 8.14 (Table A.20) 75 225 Design of Anchors – Use of New Section 8:EC 7 23 Summary (using default values) Anchor spacing of 3m Preliminary estimate of bond length (NOT COVERED IN EUROCODE) • • • • Hole dia = 0.25m sv'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 RULS,d and, if specified in the NA, RSLS,d Design of Anchors – Use of New Section 8:EC 7 24 Measuring RULS,m and RSLS,m of an anchor (draft ISO/DIS 22477-5) Type of loading Rest periods Method 1 Method 2 Method 3 Cycle loading Cycle loading In steps Maintained loads Maintained deflection Maintained load 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 k1 versus anchor load Anchor head displacement vs time for each load step. Measurements a1 versus anchor load RULS,m RSLS,m Displacement vs load for all cycles Load loss ≤ 5% at PP or Currently a1=2mm displacement ≤ 5% De over log cycle of time (from BS8081) Load loss ≤ 1%at PP or rate Not defined of displacement should reduce to 1% De over log cycle of time (from BS8081) Design of Anchors – Use of New Section 8:EC 7 a3 versus anchor load or bond load, if possible. When plot of a3 vs load, is asymptote or a3 =5mm Pc (from as vs load, end of pseudo linear portion) 25 Structural Design EULS,d ≤ Rt;d i.e 386kN ≤ Rt;d where EULS,d Rt;d Eq 8.11 = Ultimate limit state design force to be resisted by the anchor. = Design resistance of the structural element Design of Anchors – Use of New Section 8:EC 7 26 Select :- 40mm dia. GEWI bar Grade 500/600 Nominal dia. Dia. over threads Area Ultimate strength Yield strength d) = 40mm = 45mm = 1257 mm2 = 754kN (fult = 600N/mm2) = 629kN (fy = 500N/mm2) Requirements of EC2-1-1:2004 (EC2Pt1.1) requirements Irish partial factors s= 1.15 - Table 2.1N Cl 2.4.2.4(1) Cl 3.3.6 (6) p44 where fpd Fp0,1k s Therefore Ftg,Rd = 0,1 = design value of the steel stress = 0.1% proof stress for prestressing steel = partial factor = 629/1.15 = 547kN ≥ EULS,d therefore OK Design of Anchors – Use of New Section 8:EC 7 27 STRUCTURAL REQUIREMENT FOR PROOF TESTING Cl 8.5.4 (2)P of EN 1997-1:2004 refers to 5.10.2.1 from EN 1992-1-1:2005 Cl 5.10.2.1(2)P - overstressing to 0.95fy is permitted if the force in the jack can be measured to an accuracy of ±5% PP = 426kN (=1.1xEULS,d) Therefore PP ≤ 0.95*629 = 597 kN > 426kN therefore OK Design of Anchors – Use of New Section 8:EC 7 28 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 FServ,k is relatively novel within the Eurocode system. Design of Anchors – Use of New Section 8:EC 7 29 Comparison with present system • Current practice in UK & Ireland is to use BS8081 • Design is based on ‘working’ load Tw • No proper definition of Tw is given (Tw 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 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 7 31 THANK YOU Design of Anchors – Use of New Section 8:EC 7 32