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
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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
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
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)
3Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
Flat slabs and punching shearFlat slabs and punching shear
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
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
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
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
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
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
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,
11Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
Punching shear strength
EN 1992‐1‐1:2004
dbVvv Ed
EdcRd0
,
12Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
EdvcRdcRd VdbvV 0,,
Punching shear design : shear‐resisting effective depth
fibMC 2010
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
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
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.
16Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
4.65 8.00 4.65
Eccentric punching
8.00/4.65 = 172 %
17Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
Shear force non uniformly distributed along the control perimeter!
Eccentric punching
18Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
Contributory loading area
Estimation of the shear force
19Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
0.60
m
Actions
According to the engineer’s drawing (1989)
1.80
m
20Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
1.00
m
Actions: excessive soil cover !
Measured after the collapse (2004)
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
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
24Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
d
Investigation of the collapse on the basis of the Critical Shear Crack Theory (CSCT)
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
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
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
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
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
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
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
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
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
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)
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
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
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
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)
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
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
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
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
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
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
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
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
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
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
49Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
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
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
51Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
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
52Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
Strengthening of flat slabs
enlargements of the support area by the addition of column
capitals
53Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
Strengthening of flat slabs
enlargements of the support area by widening of the
columns
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
55Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
Strengthening of flat slabs
Post‐installing shear reinforcement
56Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
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
57Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
Strengthening of flat slabs against punching shear using post‐installed shear reinforcement
58Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
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
59Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
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.
60Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
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.
61Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
Application of the CSCT to strengthening of slabs
Fernández Ruiz, M., Muttoni, A., Kunz, J., 2010, ACI Structural Journal
62Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
Physical model: application of the CSCTto strengthening of slabs
Fernández Ruiz, M., Muttoni, A., Kunz, J., 2010, ACI Structural Journal
63Tel Aviv, IACIE Conference, 22.5.2017, A. Muttoni
To strengthen means not only to increase the load bearing capacity, but also the deformation capacity
Faria et al., Construction and Building Materials, 2014
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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