design of ground anchors * use of new section 8

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
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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
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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
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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
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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
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Design of Anchors – Use of New Section 8:EC 7
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Design of Anchors – Use of New Section 8:EC 7
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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.
Design of Anchors – Use of New Section 8:EC 7
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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ANCHOR TESTING
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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
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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
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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
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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
Design of Anchors – Use of New Section 8:EC 7
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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)
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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
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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
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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
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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
Design of Anchors – Use of New Section 8:EC 7
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THANK YOU
Design of Anchors – Use of New Section 8:EC 7
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