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Punching of reinforced concrete slabs: Lessons learned from collapses and research Prof. Dr. Aurelio Muttoni Tel Aviv, IACIE Conference, May 22, 2017 École Polytechnique Fédérale de Lausanne, Switzerland
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Page 1: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

0Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Punching of reinforced concrete slabs: Lessons learned from collapses and research

Prof. Dr. Aurelio Muttoni Tel Aviv, IACIE Conference, May 22, 2017

École Polytechnique Fédérale de Lausanne, Switzerland

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1Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Load test 1909

Claude A. P. Turner (1869 –1955) 

Load test 1908

Robert Maillart (1872‐1940)

Flat slabs and punching shear

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2Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Flat slabs and punching shearFlat slabs and punching shear

Project by Robert Maillart, 1911 ‐ slab with capitals (shear‐heads) and

‐ with shear reinforcement (bent‐up bars)

Page 4: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

3Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Flat slabs and punching shearFlat slabs and punching shear

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4Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Flat slabs and punching shearFlat slabs and punching shear

Several existing slabs ‐ without capitals (shear‐heads) and

‐ without shear reinforcement (bent‐up bars)

despite the significant shear stress concentration around the column

Page 6: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

5Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Bluche, Switzerland, 1981 Cagliari, Italy, 2004

Flat slabs and punching shear

Vitoria, Brazil, 2016 Tel Aviv, Israel, 2016

Flat slabs and punching shear

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6Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Flat slabs and punching shearLessons learned from a collapse: Underground parking garage Gretzenbach, Switzerland, November 2004

Page 8: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

7Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Flat slabs and punching shear

Structure built in 1989, concrete slab on columns with capitals, no shear reinforcement, no integrity reinforcement

Covered with soil in 1990

Collapsed in 2004 after a minor fire, 7 firemen died

Lessons learned from a collapse: Underground parking garage Gretzenbach, Switzerland, November 2004

Page 9: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

8Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Flat slabs and punching shearLessons learned from a collapse: Underground parking garage Gretzenbach, Switzerland, November 2004

Page 10: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

9Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Code provisions for verifying punching shear

ACI 318‐14 EN 1992‐1‐1:2004 fibMC 2010

Page 11: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

10Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Punching shear strength

shear‐resisting effective depth

length of the control perimeter

shear strength (stress) [MPa]

EdcRdcRd VdbvV 0,, db

Vvv EdEdcRd

0,

Page 12: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

11Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Punching shear strength

EN 1992‐1‐1:2004

dbVvv Ed

EdcRd0

,

Page 13: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

12Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

EdvcRdcRd VdbvV 0,,

Punching shear design : shear‐resisting effective depth

fibMC 2010

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13Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

EdvcRdcRd VdbvV 0,,

Punching shear design : shear‐resisting effective depth

Shear‐resisting effective depth: 180 mm instead of 248

Page 15: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

14Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Eccentric punching

dbVvv Ed

EdcRd0

,

1

11Wuk

VM

Ed

Ed

EN 1992‐1‐1:2004 fibMC 2010

uEd

Ed

bVM 11

bu

Page 16: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

15Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Eccentric punching

EN 1992‐1‐1:2004

(6) For structures where the lateral stability does not depend on frame action between the slabs and the columns, and where the adjacent spans do not differ in length by more than 25%, approximate values for may be used.

Page 17: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

16Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

4.65 8.00 4.65

Eccentric punching

8.00/4.65 = 172 %

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17Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Shear force non uniformly distributed along the control perimeter!

Eccentric punching

Page 19: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

18Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Contributory loading area

Estimation of the shear force

Page 20: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

19Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

0.60

 m

Actions

According to the engineer’s drawing (1989)

1.80

 m

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20Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

1.00

 m

Actions: excessive soil cover !

Measured after the collapse (2004)

Page 22: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

21Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

The Gretzenbach collapse was not an exception !

Geneva, Switzerland (1976)

ht

Bluche, Switzerland (1981)according to the drawing = 0.60 mmeasured after collapse = 1.02 m

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22Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Influence of fire

Temperature °C

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23Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Influence of fire

MM

M

V (T)

Imposed deformation =>

1) Cracking on the upper side leading to a reduction of the punching shear strength

2) Increased column reactions

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24Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

d

Investigation of the collapse on the basis of the Critical Shear Crack Theory (CSCT)

Page 26: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

25Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Fundamentals of the punching shear design model (CSCT)

Muttoni A., 2008, Punching shear strength of reinforced concrete slabs without transverse reinforcement, ACI Structural Journal

theoretical model based on aggregate interlock

simplified failure criterion

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26Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Investigation based on the Critical Shear Crack Theory (CSCT), basis of MC2010

Rotation 

Load

V

VRc

23

Rd

sd

s

ysm m

mEf

drk

Strain based model => imposed deformations can be easily accounted for

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27Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Em

Rm

VV

Design Execution Soil cover Fire

Influence of too coarse design simplifications, execution errors and fire

1989

1989

1990

2004

Page 29: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

28Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Too coarse design simplifications (shear force and )

Influence of too coarse design simplifications, execution errors and fire

Em

Rm

VV

Design Execution Soil cover Fire

Page 30: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

29Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Execution errors

Influence of too coarse design simplifications, execution errors and fire

Em

Rm

VV

Design Execution Soil cover Fire

Page 31: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

30Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Concrete strength increase between 1989 and 1990

Influence of too coarse design simplifications, execution errors and fire

Em

Rm

VV

1989

1990

Design Execution Soil cover Fire

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31Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Excess of soil cover

Influence of too coarse design simplifications, execution errors and fire

Em

Rm

VV

Design Execution Soil cover Fire

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32Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Fire

Influence of too coarse design simplifications, execution errors and fire

Em

Rm

VV

1990

2004

VR: COV = 0.09

V (T)

Design Execution Soil cover Fire

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33Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

How to reduce the probability of a collapse?

Quality control is probably the most effective measure to reduce the probability of a collapse:

In case of underground structures, the soil cover shall be verified!

The shear resisting effective depth may be reduced by inaccurate execution. This shall be verified and corrected when necessary

vdvdvdvd

vdvdvdvd

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34Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

How to reduce the probability of a collapse?

In case of punching, due to the brittleness of the failure mode, imposed deformations may play a significant role

Potential differential settlements should be accounted for in the design

Fire is a typical case of an imposed deformation

Imposed horizontal displacements (temperature or seismic actions) may reduce the punching shear strength

Also the effect of irregular spans can be seen as an imposed deformation

=> The most efficient way to reduce these effects is to increase the deformation capacity

V (T)

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35Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

How to reduce the probability of a collapse?

Increase of the deformation capacity using transverse reinforcement

Page 37: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

36Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

How to reduce the probability of a collapse?

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

200%

0% 100% 200% 300% 400%

Deformation capacity

Forc

e

Without punching shear reinforcement

With punching shearreinforcement

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

200%

0% 100% 200% 300% 400%

Deformation capacity

Forc

e

Without punching shear reinforcement

With punching shearreinforcement

Increase of the deformation capacity using transverse reinforcement

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37Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

How to reduce the probability of a collapse?

Einpaul, Brantschen, Fernández, Muttoni, ACI, 2016

Increase of the deformation capacity using transverse reinforcement

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38Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

How to reduce the probability of a collapse?

Increase of the deformation capacity using transverse reinforcement

Jonen, Switzerland, 2007, after a severe fire

(Photo courtesy HALFEN AG, Switzerland)

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39Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Verform ung

Last

Durchstanzen

ohne Bewehrung in der Betondruckzone

mit Bewehrung nach Gl. 262.56

deflection

Load

Punching

Behaviour without integrity 

reinforcement

The risk of progressive collapse

Cagliari, Italy, 2004

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40Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

The risk of progressive collapse

Verform ung

Last

Durchstanzen

ohne Bewehrung in der Betondruckzone

mit Bewehrung nach Gl. 262.56

deflection

Load

Punching

Behaviour without integrity reinforcement

Page 42: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

41Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

The risk of progressive collapse

Page 43: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

42Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

The risk of progressive collapse

Page 44: Punching of reinforced concrete slabs: Lessons learned ... · Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni 0 Punching of reinforced concrete slabs: Lessons learned from collapses

43Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

The risk of progressive collapse

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44Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

The risk of progressive collapse

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45Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

The risk of progressive collapse

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46Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

The risk of progressive collapse

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47Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

A B C D E F G H I J K L

1

2

3

4

44.853.13 2.60 2.60 2.60 2.60 5.20 5.20 5.20 5.20 5.20 5.33

19.2

55.

887.

505.

88

0.25

0.25

0.25

0.25

0.50

0.22

0.28

0.28

1.25

The risk of progressive collapse

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48Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

How to reduce the risk of progressive collapse

Fernández Ruiz, Mirzaei, Muttoni, ACI, 2013

Integrity reinforcement

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0 30 60 90

Verformung w [mm]

Last

Durchstanzen

nicht vorgespannte Platte

vorgespannte Platte

Displacement w [mm]

Load

punching

RC slab

Post‐tensioned slab

How to reduce the risk of progressive collapse

Post‐tensioning reinforcement

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50Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

How to reduce the risk of progressive collapse

Well anchored shear reinforcement => ordinary flexural reinforcement may act as integrity reinforcement

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Assessment and retrofitting

Strengthening against punching shear :

1. increase of applied loads 

2. deficiencies during design or construction

3. to comply with more stringent code requirements

Fernández Ruiz, Muttoni, Kunz, ACI, 2010

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Strengthening of flat slabs

enlargements of the support area by the addition of column 

capitals

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Strengthening of flat slabs

enlargements of the support area by widening of the 

columns

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54Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Strengthening of flat slabs

strengthening of the flexural reinforcement by casting a concrete topping or gluing reinforcement

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Strengthening of flat slabs

Post‐installing shear reinforcement

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Those possibilities can, however, be inconvenient in many situations as they require accessing the upper face of the slab (which is usually covered by soil or floor), or enlarging the support area (which is not always possible due to architecture or space requirements).

Strengthening of flat slabs

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Strengthening of flat slabs against punching shear using post‐installed shear reinforcement

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Strengthening of flat slabs against punching shear using post‐installed shear reinforcement

holes are hammer‐drilled at 45 degrees from the soffit of an existing slab

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Strengthening of flat slabs against punching shear using post‐installed shear reinforcement

Adhesive mortar is injected into the drilled holes and special anchors are set into the mortar filled holes. 

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Strengthening of flat slabs against punching shear using post‐installed shear reinforcement

The anchor head can be installed on the concrete surface with washers inclined at 45° or be embedded in an enlarged part of the drilled hole. 

The embedded anchorage has the advantage that it can be covered with a fire protection mortar and is not visible after the installation.

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Application of the CSCT to strengthening of slabs

Fernández Ruiz, M., Muttoni, A., Kunz, J., 2010, ACI Structural Journal

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Physical model: application of the CSCTto strengthening of slabs

Fernández Ruiz, M., Muttoni, A., Kunz, J., 2010, ACI Structural Journal

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To strengthen means not only to increase the load bearing capacity, but also the deformation capacity

Faria et al.,  Construction and Building Materials, 2014

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64Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni

Conclusions

Quality control is essential in reducing the probability of a collapse

Transverse reinforcement allows increasing the punching shear resistance and the deformation capacity, thus reducing the effect of imposed deformations and the risk of a progressive collapse

Assessment and retrofitting is justified in many cases

Retrofitting may be achieved by correcting the causes (excess of soil cover for instance) and/or strengthening

Strengthening may be achieved by increasing the resistance, but also, and even more efficiently, increasing the deformation capacity and the residual strength

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© Aurelio Muttoni

EPFL,  Ecole Polytechnique Fédérale de Lausanne

Station 18,  CH‐1015 Lausanne,   Switzerland,      Tel.  ++41 21 693 28 81

http://ibeton.epfl.ch/Person/Muttoni/Default_e.asp

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