DESIGN OF GROUND ANCHORS – USE OF NEW SECTION 8

DESIGN OF GROUND ANCHORS – USE OF NEW SECTION 8

Eric R FarrellAGL Consulting.

2Design of Anchors – Use of New Section 8:EC 7

Section 8 Anchors

Anchor must have a free length

This presentation will cover the design of grouted anchor

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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


= 18kN/m3 above WL = 20kN/m3 below WL ck' = 0 k' = 38o E' = 50,000 kPa 0.2 = '

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.


= 18kN/m3 above WL = 20kN/m3 below WL ck' = 0 k' = 38o E' = 50000 kPa 0.2 = '

Anchor prestressed at this stage

2m (with overdig)

1m

CONSTRUCTION STAGES


= 18kN/m3 above WL = 20kN/m3 below WL ck' = 0 k' = 38o E' = 50000 kPa 0.2 = '

6.5m with overdig

Surcharge of 20kPa



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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.


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Ultimate limit state (ULS) design force to be resisted by the anchors

EULS,d ≤ RULS;d Eq. 8.1

where EULS,d = max(FULS,d; FServ,d) Eq. 8.2 and FServ,d = ServFServ,k Eq. 8.3

RULS;d = Design value of the resistance of an anchor complying with ULS criteriaEULS,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


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Serviceability Limit State

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


WHERE REQUIRED IN THE NA

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Note:-RULS;d and, where required RSLS;d

must be determined/validated by anchor tests.


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Geotechnical ULS resistance of anchor

= min{Rm(aULS or kl;ULS) and PP} Eq. 8.5

= 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 ratekl;ULS is the load loss


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Geotechnical ULS resistance of anchor continued

= Eq. 8.6

RULS,d is the design value of the resistance of an anchor complying with ULS

a;ULS is a partial factor


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Geotechnical SLS resistance of anchor(where required)

= min{Rm(aSLS or kl;SLS) and PP } Eq. 8.8

= )min 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 ratekl;SLS is the load loss


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Geotechnical SLS resistance of anchor continued

= Eq. 8.10

RSLS,d is the design value of the resistance of an anchor complying with ULS

a;SLS is a partial factor


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Free anchor length

and where required

Free anchor/tendon free length – Requirements of EN 1537


PARAMETERS REQUIRED FROM ANCHOR DESIGN

Anchor required to satisfy the resistance values

SUMMARY


Limit equilibrium analysis

Finite element analysiswithout/with initial prestress (lock-off) of 60kN/m

Design Situation L (m) Anchor forcekN/m

Max. BMkNm/m

L (m)

Anchor forcekN/m

Max. BMkNm/m

Characteristic actions and soil parameters, no overdig

7.16 52.5/66* 66.4 7.2 67.7/78.9 72.83/67.5

Characteristic actions and soil parameters, with overdig

7.74 58.5/73* 83.0 7.8 78.6/77.4 79.6/80.1

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

Length 8.5m using char value with no overdig

8.5 64.7/75.23 55.4/39.5

*25% added for arching

FULS,d

FServ,k

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= 128.8 kN/m (386 kN)= 75.2 kN/m ( 225 kN)=128.8 kN/m ( 386 kN)

Note:- 1.35x75.2=101.52kN/mFree anchor length = 5m

and where required

Free anchor length > 5m


PARAMETERS FOR ANCHOR DESIGN – assuming 3m spacing

Anchor required to satisfy the resistance values


ANCHOR TESTING


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 22477-5When available

Number of Investigation/Suitability tests to be set in NA.


Test Proof LoadInvestigation tests

-Not on working anchor-Normally taken to failure of bonded length/ground-may require larger structural element

Eq. 8.12 PP ≥ ξULS x a;ULS x EULS,d

Suitability test -On working anchor-taken to load that verifies RULS,d-PP comes from Eqs, 8.1, 8.6 & 8.7

Eq. 8.12 PP≥ ξULS x a;ULS x EULS,d

Acceptability tests -Must be carried out on all

anchors- a,acc;ULS and a,acc;SLS from Table A.20

Eq. 8.13PP ≥ a,acc;ULS x EULS,d

orEq. 8.14PP ≥ a,acc;SLS x FServ,k


kN/m kN@3m

FServ,k Characteristic value of the maximum anchor force, including effect of lock-off load, and sufficient to prevent a SLS in the structure.

75 225

FULS;d Design value of the force required to prevent any ULS in the supported structure

128.8 386

FServ,d Eq.8.3 Serv =1.35 from Table A.18 101.3 304EULS,d Eq. 8.2 128.8 386Po Lock-off load, taken as about 80% of FServ,k. 60 180PP Proof load, Suitability tests (PP≥ ξULS x a;ULS x EULS,d ……

Eq 8.12)ξULS = 1.0 (Table A.20) a;ULS = 1.1 (Table A.19)

142 426

PP Proof load, Acceptability tests PP ≥ a,acc;ULS x EULS,d Eq. 8.13 (Table A.20)orPP ≥ a,acc;SLS x FServ,k Eq. 8.14 (Table A.20)

142

75

426

225

Taking 3m spacing of anchors

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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


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Measuring RULS,m and RSLS,m of an anchor(draft ISO/DIS 22477-5)

Method 1 Method 2 Method 3Type of loading Cycle loading Cycle loading In steps

Rest periods Maintained loads Maintained deflection Maintained 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

k1 versus anchor load Anchor head displacement vs time for each load step.

a1 versus anchor load a3 versus anchor load or bond load, if possible.

Displacement vs load for all cycles

RULS,m Currently a1=2mm Load loss ≤ 5% at PP or displacement ≤ 5% De over log cycle of time (from BS8081)

When plot of a3 vs load, is asymptote or a3 =5mm

RSLS,m Not defined Load loss ≤ 1%at PP or rate of displacement should

reduce to 1% De over log cycle of time (from BS8081)

Pc (from as vs load, end of pseudo linear portion)


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Structural Design

EULS,d ≤ Rt;d

i.e 386kN ≤ Rt;d Eq 8.11where EULS,d = Ultimate limit state design force to be

resisted by the anchor. Rt;d = Design resistance of the

structural element


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Select :- 40mm dia. GEWI bar Grade 500/600 Nominal dia. = 40mmDia. over threads = 45mmArea = 1257 mm2

Ultimate strength = 754kN (fult = 600N/mm2)Yield strength = 629kN (fy = 500N/mm2) d) Requirements of EC2-1-1:2004 (EC2Pt1.1) requirements Irish partial factorss= 1.15 - Table 2.1N Cl 2.4.2.4(1) Cl 3.3.6 (6) p44 where fpd = design value of the steel stress Fp0,1k = 0.1% proof stress for prestressing steel

s = partial factor Therefore Ftg,Rd = 629/1.15 = 547kN ≥ EULS,d therefore OK



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

STRUCTURAL REQUIREMENT FOR PROOF TESTING

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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.


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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.


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Acknowledgements

• Members of SC7/EG1 on Anchors

• Colleagues at AGL Consulting

• Brian Simpson and Georgios Katsigiannis for invitation



THANK YOU

Documents

DESIGN OF GROUND ANCHORS – USE OF NEW SECTION 8