T.TASSIOSA.KOUTSIASome comparisonsbetween retrofitting provisions of EC8-P3 and other Codesfor RC elements

EUROPEAN CENTER OF PREVENTION AND FORECASTING OF EARTHQUAKES – ECPFEEARTHQUAKE PLANNING AND PROTECTION ORGANIZATION – EPPO

Athens WorkshopApril 12/2013

IMPLEMENTATIONOF THE EC8-P3:2005

ASSESSMENT AND INTERVENTIONS ON BUILDINGSIN EARTHQUAKE PRONE AREAS

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1. Scope…………………………………………………3 3.1.2 Conclusion……………………………………………202. Strengthening against shear……………….5 3.2 Steel jacketing………………………………… 21

2.1 FRP jacketing……………………………………….5 i. EC8-P3……………………………………………….. 21i. EC8-P3……………………………………………….. 5 ii. EC8-P1……………………………………………….. 22

α. Design formulae……………………………………5 α. Design formulae……………………………………..22b. Notation……………………………………………..6 b. Notation…………………………………………….. 22ii. KANEPE……………………………………..…….. 7 iii. KANEPE…………………………………………….. 23

Design formulae………………………………….7 Design formulae…………………………………..232.1.1 Numerical example…………………………… 8 3.2.1 Numerical example…………………………… 242.1.2 Conclusion……………………………………………10 3.2.2 Conclusion……………………………………………26

2.2 Steel jacketing……………………………………..11 4. Clamping of lap-splices………………………………………27i. EC8-P3…………………………………………………..11 4.1 FRP jacketing………………………………………27

α. Design formulae……………………………………..11 i. EC8-P3……………………………………………….. 27b. Notation………………………………………………..11 α. Design formulae……………………………………27ii. KANEPE/TH.P.T……………………………………….12 b. Notation…………………………………………….. 27

Design formulae……………………………………..12 ii. KANEPE……………………………………………. 292.2.1 Numerical example……………………………13 Design formulae……………………………………292.2.2 Conclusion……………………………………………15 4.1.1 Numerical example…………………………… 30

3. Confinement versus local ductility…….16 4.1.2 Conclusion……………………………………………323.1 FRP jacketing………………………………………16 4.2 Steel jacketing……………………………………..33

i. EC8-P3………………………………………………..16 i. EC8-P3……………………………………………….. 33α. Design formulae……………………………………..16 ii. KANEPE……………………………………………. 34b. Notation……………………………………………..16 Design formulae……………………………………34ii. KANEPE……………………………………………. 17 4.2.1 Numerical example…………………………… 35

Design formulae…………………………………17 4.2.2 Conclusion……………………………………………373.1.1 Numerical example……………………………18

Table of Contents

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1. Scope (a)

EC8-P3 will need, as all Codes do,to be improved and modified on a regular basis,to intergrade: the ongoing work, the feedback from Code-users and continuing developments in “repair technology”;as well as to eliminate: possible mistakes and internal inconsistencies.

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1. Scope (b)

This study attempts to make some comparisonsbetween retrofitting provisionsof EC8-P3 and KANEPE (GCSI)for RC elements;regarding: Strengthening against shear, Confinement action versus local ductility and Clamping of lap-splices.

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a. Design formulae

2. Strengthening against shear2.1 FRP jacketing

i. EC8-P3

NOTE: γfd=1,5 is the partial factor for FRP debonding

For fully wrapped (i.e. closed) or properly anchored (in the compression zone) jackets:

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θ: strut inclination angle, β: angle between the (strong) fibre direction in the

FRP sheet and the axis of the member, wf: width of the FRP sheet measured orthogonally to the

(strong) direction of the fibres (for sheets: wf=min(0,9d,hw)sin(θ+β)/sinθ),

sf: spacing of FRP sheet measured orthogonally to the (strong) fibre direction (=wf),

Le: effective bond length, z =0,9d; internal lever arm, ffdd: design debonding strength, ffu,W(R): ultimate strength of the FRP sheet wrapped around the

corner with a radius R and fdd,e,W: design FRP effective debonding strength

2. Strengthening against shear2.1 FRP jacketing

i. EC8-P3

b. Notation

7 ii. KANEPE2. Strengthening against shear2.1 FRP jacketing

Design formulae

Fully wrapped (i.e., closed) or properly anchored (in the compression zone) jackets

U-shaped (i.e., open) jackets

For beams:

For columns:

NOTE: γRd=1,5

82.1.1 Numerical example (a)

2. Strengthening against shear2.1 FRP jacketing

Column: b=250 mmConcrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250FRP: ffu=2900 MPa, Ef=260000 MPa, tl=0,12 mmAdditional shear load: ΔV=70 kN

NOTE: the partial factor of the FRP is taken equal

to 1,1EC8-P3

for θ=π/4 in favor of safetyand β=π/2,

4 layers of FRP with Σtf=0,48 mm account for:

ffdd =467,88 MPa <ηRffu-ffdd>=903,03 MPa>0

ffu,w=1370,91 MPa τmax=2,73 MPa

ffdde,w=392,70 MPa VRd,f=74,64 kN

KANEPE σj0=1757,58 MPa z0=176 mm tj,req=0,23 mm2 layers of FRP with Σtj=0,24 mm are required

92.1.1 Numerical example (b)

2. Strengthening against shear2.1 FRP jacketing

Column: b=250 mmConcrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250FRP: ffu=2900 MPa, Ef=260000 MPa, tl=0,12 mm

(VRd,f=74,64 kN)

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The two documents lead in averageto the same values of required FRP thickness.

However, we confess thatwe were unable to understandwhy, following EC8-P3A.4.4.2 (5), such a simplestrengthening, in such asmall column, fails bydebonding in spite of theequilibrium offered near thecurved corner (forces Fc).

2.1.2 Conclusion2. Strengthening against shear2.1 FRP jacketing

Fully wrapped (i.e., closed) or properly anchored (in the compression zone) jackets

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θ: as previously stated, β: as previously stated, b: width of the steel

straps and s: spacing of the steel

straps

Evidently b/s=1 in case of continuous steel plates.

i. EC8-P32. Strengthening against shear2.2 Steel jacketing

a. Design formulae

b. Notation

122. Strengthening against shear2.2 Steel jacketing

ii. KANEPE/TH.P.T.

Fully wrapped (i.e., closed) or properly anchored (in the compression zone) jackets

U-shaped (i.e., open) jackets

For beams:

For columns:

Design formulae

NOTE:

γRd=1,5

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EC8-P3for θ=π/4 in favor of safety

and β=π/2,tj,req=1,17 mm

KANEPE σj0=159,42 Mpa z0=176 mmtj,req=2,50 mm

2. Strengthening against shear2.2 Steel jacketing

2.2.1 Numerical example (a)

Column: b=250 mmConcrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250Steel plate: f 'sy=275 MPaAdditional shear load: ΔV=70 kN NOTE: the partial factor

of the steel plate is taken equal to 1,15

142. Strengthening against shear2.2 Steel jacketing

2.2.1 Numerical example (b)

Column: b=250 mmConcrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250Steel plate: f 'sy=275 MPa

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EC8-P3 leads to much lower values of required steel-plate thickness because:

it considers zo~h and

it does not leave stress-margins (~ 30%) for possible local overstress along the diagonal crack.

2.2.2 Conclusion2. Strengthening against shear2.2 Steel jacketing

16 i. EC8-P33. Confinement action versus local ductility3.1 FRP jacketing

bw: larger section width, εcu =0,0035, εju =ffu/Ef; adopted FRP

jacket ultimate strain, lower than the

ultimate strain εfu=0,015 for CFRP and

f’1: confinement pressure f1 applied by the FRP

sheet after its corners have been rounded to allow wrapping around them

a. Design formulae

b. Notation

Effectively confined area in an FRP-wrapped section

17 ii. KANEPE3. Confinement action versus local ductility3.1 FRP jacketing

Design formulae

NOTE: If the number of required FRP layers k is higher than 3,

the effectiveness of the additional FRP layers is reduced;

183. Confinement action versus local ductility3.1 FRP jacketing

3.1.1 Numerical example (a)

EC8-P3 Ix=9,000

εju=0,0112 f’1=6,40 MPa

ks=0,400 f1=16,00 MPatf,req=0,69 mm

KANEPE fjd=2636,36 MPa v=0,674 εsy=0,0020 ano=0,333 an=0,733tj,req=0,81 mm for k=7>3, ψ=0,615 f’jd=ψfjd=1620,81 MPat’j,req=1,32 mm

Column: b=250 mm, μφ,av=1,00Concrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250FRP: ffu=2900 MPa, Ef=260000 MPaAdditional axial load: N=800 kNIn order to achieve μφ,tar=9,00:

NOTE: the partial factor of the FRP is taken equal

to 1,1

193. Confinement action versus local ductility3.1 FRP jacketing

3.1.1 Numerical example (b)

Column: b=250 mm, μφ,av=1,00Concrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250FRP: ffu=2900 MPa, Ef=260000 MPaAdditional axial load: N=800 kN

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A question may be raised here about the first approach of EC8-P3 (A.34) on confinement action versus local ductility; the cross sectional effectiveness factor a is not included in the design formulae – as opposed to the second EC8-P3 approach A.4.4.3 (6) - which was examined by Prof. Dritsos in the previous presentation - and KANEPE provisions.

It is also noted that EC8-P3, as opposed to KANEPE provisions, does not consider the reduced effectiveness of additional FRP layers after a certain number (say 5).

As a result, it is rather a numerical coincidence that the values of required FRP thickness for this first approach (A.34) are quite similar in the case of EC8-P3 and KANEPE provisions for targeted local ductility values around 10,00 – as opposed for the case of lower (<9) and higher (>11) μ1/r -values when EC8-P3 values become disproportionally low and high respectively.

3.1.2 Conclusion3. Confinement action versus local ductility3.1 FRP jacketing

21 i. EC8-P33. Confinement action versus local ductility3.2 Steel jacketing

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bc: gross cross-sectionalwidth and

bo: width of confined core

Evidently bc/bo=1 in case ofcontinuous steel plates.

NOTE: According to Table 5.1 and assuming au/a1=1,0,qo,DCM=3,0 and qo,DCH=4,5.

Furthermore, assuming Tc=2T1 in accordance with Equation 5.5μφ=1+2(qo-1)Tc/T1, μφ,DCM=9 and μφ,DCH=15.

ii. EC8-P13. Confinement action versus local ductility3.2 Steel jacketing

a. Design formulae

b. Notation

23 iii. KANEPE3. Confinement action versus local ductility3.2 Steel jacketing

Design formulae

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EC8-P1 KANEPE

vd=0,876εsyd=0,0019

an=0,333as=1,000a=0,333

ωwd,min=1,246 tj,req=3,38 mm tj,req=4,76 mm

3. Confinement action versus local ductility3.2 Steel jacketing

3.2.1 Numerical example (a)

Column: b=250 mm, μφ,av=1,00Concrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250Steel plate: f 'sy=275 MPaAdditional design axial load: Nd=800 kN

In order to achieve μφ,tar=9,00:NOTE: the partial factor of the steel plate is taken equal to 1,15,while the reduced partial factors of concrete and reinforcing steel

are taken equal to 1,3 and 1,05 respectively

253. Confinement action versus local ductility3.2 Steel jacketing

3.2.1 Numerical example (b)

Column: b=250 mm, μφ,av=1,00Concrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250Steel plate: f 'sy=275 MPaAdditional design axial load: Nd=800 kN

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EC8-P3 does not offer any quantitative guidance regarding the use of external steel confinement in the form of steel jacketing as a strengthening method for increasing the local ductility of a member.

Following EC8-P1 on this subject, however, with the appropriate modifications regarding the partial factors of existing materials, the values of required steel-plate thickness are relatively high for higher targeted local ductility values compared to the values resulting from KANEPE provisions.

3.2.2 Conclusion3. Confinement action versus local ductility3.2 Steel jacketing

27 i. EC8-P3 (a)4. Clamping of lap-splices4.1 FRP jacketing

bw: as previously stated, p: perimeter line in the column

cross-section along the inside of longitudinal steel,

n: number of spliced bars along p,

fyL =fy/CFKL3; yield strength of longitudinal steel

reinforcement; σ1: clamping stress over the lap-

splice length Ls and σ’1: active pressure from the

groutingbetween the FRP and the column at a strain of 0,001

a. Design formulae

b. Notation

NOTE: CFKL3 = 1,00 isthe confidence factor

for full knowledge

28 i. EC8-P3 (b)4. Clamping of lap-splices4.1 FRP jacketing

“For members of rectangular section with longitudinal bars lapped over a length Ls starting from the end section of the member, an alternative to the previous for the calculation of the effect of FRP wrapping over a length exceeding by no less than 25% the length of the lapping, is:”

29 ii. KANEPE4. Clamping of lap-splices4.1 FRP jacketing

Design formulae

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EC8-P3 (a) σ1=1,52 MPa

ks=0,4 σ’1=0,61 MPatf,req=0,29 mm

EC8-P3 (b) a=al,f=0,760

ρf=0,0022 ff,e=2081,21 Mpa

tf,req=0,27 mm

KANEPE su=2,0 mm sd=0,4 mm wd=0,33 mmtj,req=0,36 mm

4. Clamping of lap-splices4.1 FRP jacketing

4.1.1 Numerical example (a)

Column: b=250 mmConcrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250FRP: ffu=2900 MPa, Ef=260000 MPa

Assuming ls/db=25:NOTE: the partial factor of the FRP is taken equal

to 1,1

314. Clamping of lap-splices4.1 FRP jacketing

4.1.1 Numerical example (b)

Column: b=250 mmConcrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250FRP: ffu=2900 MPa, Ef=260000 MPa

324.1.2 Conclusion4. Clamping of lap-splices4.1 FRP jacketing Several opinions exist about the meaning of the

symbol εf,u being used in the alternative second approach A.4.4.4 (3) of EC8-P3 regarding the clamping of lap-splices along with FRP jacketing.

Our application is literally in accordance with the provisions of the text of the Code putting εf,u equal to the actual ultimate elongation of the FRP that however should not be taken higher than 0,015.

On the other hand, if εf,u were the real strain of the FRP under the given conditions, the resulting values of required FRP thickness would then be approximately 100% higher than the previous ones.

33 i. EC8-P34. Clamping of lap-splices4.2 Steel jacketing

34 ii. KANEPE4. Clamping of lap-splices4.2 Steel jacketing

Design formulae

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KANEPE su=2,0 mm sd=0,4 mm

(AB)=99 mm

provided that: wy=0,13 mm<wd=0,33 mmtj,req=2,32 mm

4. Clamping of lap-splices4.2 Steel jacketing

4.2.1 Numerical example (a)

Column: b=250 mmConcrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250Steel plate: f 'sy=275 Mpa

Assuming ls/db=25:NOTE: the partial factor of the steel plate is taken

equal to 1,15

364. Clamping of lap-splices4.2 Steel jacketing

4.2.1 Numerical example (b)

Column: b=250 mmConcrete: fc=19 MPa, c=20 mmReinforcement: S400, 4Φ20, Φ6/250Steel plate: f 'sy=275 Mpa

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It is hoped that in the near futureEC8-P3 will also offer quantitative guidanceregarding the use of external steel confinementin the form of steel jacketingas a strengthening method of inadequate splices.

4.2.2 Conclusion4. Clamping of lap-splices4.2 Steel jacketing

Thank you!