PUNCHING OF FLAT SLABS WITH SHEAR REINFORCEMENT · PDF filedoctoral program in structures 1 plan de recherche research plan punching of flat slabs with shear reinforcement stefan lips

DOCTORAL PROGRAM IN STRUCTURES

1

PLAN DE RECHERCHE RESEARCH PLAN

PUNCHING OF FLAT SLABS WITH SHEAR REINFORCEMENT

Stefan Lips 18.03.2009

Online Version


2

Summary The objective of this research is to introduce a new theoretical model for the calculation of the

punching shear resistance of flat slabs with shear reinforcement. In the past, several investigations took

place but no theory so far could describe the punching shear behavior of flat slabs with shear

reinforcement satisfactorily. Consequently, the design codes still rely on either empirical formulations or

use coarse simplifications. Thus, a new comprehensive theory should be established to improve the current

approach of punching shear calculations. Besides the theoretical model, new design provisions should be

proposed based on a simplified model. In addition, the theoretical work will be validated by experimental

tests from the literature and by an experimental test series at the EPFL. In fact, several slab specimens with

varying parameter such as longitudinal reinforcement ratio, transversal reinforcement ratio, and slab

thickness will be investigated. The results from these tests will contribute significantly to the achievement

of the previous mentioned objectives.

Keywords: flat slab, slab‐column connection, punching shear, shear reinforcement, critical shear crack


3

Table of Contents

1. Introduction ............................................................................................................................................... 4

2. State of Research ....................................................................................................................................... 5

Performed research ....................................................................................................................................... 5

Critical shear crack theory (CSCT) .................................................................................................................. 6

Code Provisions ............................................................................................................................................. 7

3. Research Plan ............................................................................................................................................ 8

Objectives ...................................................................................................................................................... 8

Experimental Program ................................................................................................................................... 8

Experimental Test Set‐up .............................................................................................................................. 8

Measurements............................................................................................................................................. 10

4. Research Schedule ................................................................................................................................... 12

5. Bibliography ............................................................................................................................................. 13


4

1. Introduction During the last century, the application of flat slabs in buildings and especially in parking garages

has prevailed. Flat slabs are easy to build and have through their narrow height an economical and

architectural advantage compared to ripped slabs. The limited height of these slabs leads to the main

design criteria, which are typically the deflection at the service state and the punching shear at the limit

state. The main drawback of the latter design criteria is that punching is an extremely brittle failure mode.

In fact, a column can suddenly punch through the slab without nearly any warning sign that could

eventually lead to an evacuation of the building in case of a collapse. One promising approach to prevent

brittle failure is the use of punching shear reinforcement. In the last decades, several studies investigated

the influence of shear reinforcement at the slab column connections. The main performed research of

punching shear with transverse reinforcement is summarized in Chapter 2. Currently, none of the resulting

theories explains the behavior of the strength of punching shear of flat slabs with transverse reinforcement

physically satisfactorily. Consequently, they have no general acceptance so that a new approach has to be

developed which not only predicts the resistance accurately but also bases on a consistent theory. This

would allow the use of one comprehensive model for different shear reinforcement systems and could

finally improve the current design specifications.

Figure 1. Examples of different shear reinforcement systems


5

2. State of Research

Performed research

At the beginning of the 20th century, the first flat slabs were designed in Europe [1] and North

America [2]. Until the 1950s, they were typically designed with mushroom‐headed columns to prevent

punching. At the early 60s in Sweden, the development of the first rational approach for punching shear

without transverse reinforcement began [3]. Nevertheless, most codes did not implement this approach

due to the complicated design expressions. Nearly at the same time, the investigation of punching shear

reinforcement began. One of the first studies of punching of flat slabs with shear reinforcement was done

by Andersson in Sweden [4]. In North America, Hawkins investigated the influence of shear reinforcement

on the punching behavior at the University of Washington [5], [6], [7], [8], [9] and Ghali and Dilger at the

University of Calgary [10], [11], [12], [13], [14], [15] [16], [17]. Hawkins studied the shear strength of slabs

with moment transferred to the column due to eccentric loadings. For the experimental investigation, he

used slabs without and with shear reinforcement, for which he used stirrups. Gahli and Dilger investigated

different shear reinforcement types and their arrangements. In 2001, Dechta introduced the shear friction

model for flat slabs with shear reinforcement [18] and Birkle applied and further improved the model with

respect to the shear reinforcement [19]. Although, the approach is promising, it was not implemented to

the Canadian code so far. Further research concerning punching shear reinforcement have been done in

the United Kingdom and later on in Brazil by Regan [20], [21], [22], [23], [24] [25]. Primarily, he varied the

type of the shear reinforcement such as bent‐up bars, stirrups, and studs. Likewise, Broms investigated in

Sweden a special combination of bent‐up bars and a stirrup cage to enhance a ductile behavior at the slab‐

column connection [26], [27], [28], [29], [30]. In Germany at the University of Aachen, Hegger has done

research on different shear reinforcement types, mostly in respect to the anchorage of the different

systems [31], [32], [33], [34], [35]. Additionally, they investigated the use of finite element analysis

programs to model the punching phenomena. In conclusion, it can be said that although a lot of research

has been done in this area, no physical consistent approach has been proposed for punching of flat slabs

with shear reinforcement so far. Therefore, a new approach has been proposed by Muttoni and Fernandez

based on the critical shear crack theory [36].


6

Critical shear crack theory (CSCT)

The critical shear crack theory was developed during the 1980s and later on further developed for

the application of punching of flat slabs without transverse reinforcement [37], [38], [39], [40] [41], [42],

[43], [44], [45]. Finally, the CSCT was also implemented in the current Swiss code SIA 262:2003 [46] for the

design of flat slabs without transverse reinforcement. The CSCT bases on the assumption that the shear

strength in members without transverse reinforcement correlates to the product of the square root of the

concrete compressive strength and a function of the crack width and the aggregate size. This relationship

can be described as

·

· , , ( )

where VR is the shear strength, b0 is a control perimeter, d is the effective depth of the member, fc is the

compressive strength of the concrete, w is the width of the critical shear crack, and dg is the maximum

aggregate size.

This approach shows an excellent agreement to experimental test results. Figure 2 shows the comparison

of results of 99 punching tests on slabs without shear reinforcement and the theoretical approach by

displaying the shear resistance as a function of the rotation.

Figure 2. Comparison of the failure criterion to the strength of 99 slabs without shear reinforcement failing in punching shear

[44]

However, so far only flat slabs without shear reinforcement have been considered by the CSCT. Although

Fernandez and Muttoni proposed a promising approach based on the CSCT for flat slabs with transverse

reinforcement [36], it is still associated with an uncertainty of influential parameters such as the activation

of the shear reinforcement due to the rotation or the positioning and detailing of the shear reinforcement.


7

Code Provisions

Currently, most codes such as the Euro Code 2 (EC‐2) [47] and the American Concrete Institute

Building Code (ACI 318‐05) [48] base on empirical formulation for the calculation of the punching shear

resistance with shear reinforcement. As Figure 3 shows, these formulations lead, in comparison to

experimental test result, to scattered results. The Swiss Code (SIA 262:2003) [46] uses an approach based

on the theory of plasticity but with coarse simplification. In fact, it neglects the contribution of the concrete

and considers only the shear reinforcement for the overall shear resistance. This fact leads to conservative

calculation of the amount of shear reinforcement. Therefore, all these code design specifications result due

to the scattering in either conservative thus costly design (ACI 318‐05 and SIA 262:2003) or in even unsafe

design (EC‐2). Hence, a new approach could improve the design guidelines significantly.

a) b)

c)

Figure 3. Comparison of a) ACI 318‐05, b) EC 2, and c) SIA 262:2003 with available test results [36]


8

3. Research Plan

Objectives

The main objective of this research is to develop a new, comprehensive theoretical model for the

calculation of the punching shear resistance of flat slabs with shear reinforcement. The formulated

theoretical model will be compared to existing experimental data. Additionally, a test series containing

fifteen specimens will be performed to validate and improve the model. Finally, new design guidelines

should be proposed.

Experimental Program

Specimen Slab thickness [mm]

Support plate [mm]

Flexural reinforcement ratio[%]

Shear reinforcement ratio [%]

PL‐075‐012‐250 250 260 x 260 0.75 0.12PL‐075‐050‐250 250 260 x 260 0.75 0.50PL‐075‐100‐250 250 260 x 260 0.75 1.00PL‐150‐012‐250 250 260 x 260 1.50 0.12PL‐150‐050‐250 250 260 x 260 1.50 0.50PL‐150‐100‐250 250 260 x 260 1.50 1.00PL‐075‐056‐320 320 340 x 340 0.75 0.56PL‐150‐056‐320 320 340 x 340 1.50 0.56PL‐075‐000‐320 320 340 x 340 0.75 0.00PL‐150‐000‐320 320 340 x 340 1.50 0.00PL‐150‐100‐320‐2 320 260 x 260 1.50 1.00PL‐150‐000‐320‐2 320 260 x 260 1.50 0.00

Table 1. Mechanical properties of the first series

Specimen Distance of the first stud to the support plate [mm]

PL‐150‐100‐250‐02 42 (0.2d)PL‐150‐100‐250‐06 126 (0.6d)PL‐150‐100‐250‐08 168 (0.8d)

Table 2. Geometrical properties for tests of the second series

Experimental Test Setup

Figure 4 and Figure 5 show the details of the experimental set‐up. This experimental test set‐up

has been used for several previous punching shear tests. The applied force is provided by four hydraulic

cylinders. The force is distributed by four steel beams and afterwards transferred to eight tension bars.

These bars go through openings in the concrete slab and apply the forces on the top surface of the slab. To

resist the load, the slab is supported by a quadratic steel plate (260 x 260 mm / 340 x 340 mm). Below the

steel plate load cells measure the support forces. Finally, a steel beam directs the load to a concrete cubic

(500 x 500 x 500 mm), which carries the load to the floor.


9

Figure 4. Drawings of the experimental test set‐up


10

Figure 5. Pictures of the experimental test set‐up

Measurements

The measurements correspond to the measurement of former punching shear tests, which can be

seen in Figure 7. Additionally, the strain of the shear reinforcement will be measured. The following list

concludes the measurements that will be taken:

o Deflection at the top and bottom surface of the slab (Figure 7 b, c)

o Rotations of the slab in three directions (Figure 7 a)

o Strains at the concrete surface using omega‐shaped gauges (Figure 7 d, e)

o Strains in the shear reinforcement close to the support area region (using strain gauges)

o In‐plane dilatancy of the slab (using mechanical devices)

o Observe the shape and the position of the critical shear crack by saw‐cut of the slabs after testing

(Figure 6)

These measurements allow validating and further developing the established theoretical model.

a) b)

c) d)

Figure 6. Possible failure modes


11

a) b)

c) d)

e) f)

Figure 7. Type and placing of the measurement devices


12

4. Research Schedule The research project is planned to be performed according the fallowing schedule.

2008 2009 2010 2011 2012

Literature review

Development of a theoretical model

Development of test campaign

Testing

Validation and further development of the theoretical model

Establish design rules based on the theoretical model

Documentation / Writing thesis

Table 3. Research schedule


13

5. Bibliography [1] Fürst, A., & Marti, P. (1997). Robert Maillart's Design Approach for Flat Slabs. ASCE Journal of Structural Engineering , 1102‐1110.

[2] Gasparini, D. A. (2002). Contributions of C. A. P. Turner to Development of Reinforced Concrete Flat Slabs 1905‐1909. ASCE Journal of Structural Engineering (Vol. 128), 1243‐1252.

[3] Kinnunen , S., & Nylander , H. (1960). Punching of Concrete Slabs Without Shear Reinforcement. Stockholm: Transactions of the Royal Institute of Technology.

[4] Andersson , J. (1963). Punching of Concrete Slabs with Shear Reinforcement. Stockholm: Transactions of the Royal Institute of Technology.

[5] Hawkins , N., & Corley , W. (1971). Influence of Column Rectangularity on the Behavior of Flat Plate Structures. Detroit: ACI SP‐30‐6.

[6] Hawkins , N. (1974). Shear Strength of Slabs with Moments Transferred to Columns. ACI SP‐42‐35.

[7] Hawkins , N. (1974). Shear Strength of Slabs with Shear Reinforcement. Detroit: ACI SP‐42‐34.

[8] Hawkins , N., & Corley , W. (1974). Moment Transfer to Columns in Slabs with Shearhead Reinforcement. ACI SP‐42‐36.

[9] Hawkins, N. M., Bao, A., & Yamazaki, J. (1989). Moment Transfer from Concrete Slabs to Columns. ACI Structural Journal (6), 705‐716.

[10] Langohr, P. H., Ghali, A., & Dilger, W. H. (1976). Special Shear Reinforcement for Concrete Flat Plates. ACI Journal (Vol. 73), 141‐146.

[11] Dilger, W. H., Mahmoud, Z., & Ghali, A. (1978). Flat Plates with Special Shear Reinforcement Subjected to Static Dynamic Moment Transfer. ACI Journal (75‐56), 543‐549.

[12] Seible, F., Ghali, A., & Dilger, W. H. (1980). Preassembled shear reinforcing units for flat plates. ACI Journal, Proceedings 77 (1), 28‐35.

[13] Mokhtar, A. S., Ghali, A., & Dilger, W. H. (1985). Stud Shear Reinforcement for Flat Concrete Plates. ACI Journal (82‐60), 676‐683.

[14] Ghali , A., & Hammill , N. (1992). Effectiveness of Shear Reinforcement in Slabs. Concrete International.

[15] Megally, S. H., & Ghali, A. (1994). Design Considerations for Slab‐Column Connections in Seismic Zones. ACI Structural Journal , 91 (3), 303‐314.

[16] Ghali, A., & Megally, S. H. (2000). Stud Shear Reinforcement for Punching: North American and European Practices. International Workshop on Punching Shear Capacity of RC Slabs, (pp. 201 ‐ 209). Stockholm.

[17] Megally, S. H., & Ghali, A. (2000). Punching Shear Design of Earthquake‐Resistant Slab‐Column Connections. ACI Structural Journal (Vol. 97), 720‐730.

[18] Dechka, D. C. (2001). Response of Shear‐Stud‐Reinforcement Continous Slab‐Column Frames to Seismic Loads. Calgary, Canada: PhD Thesis.

[19] Birkle , G. (2004). Punching of Flat Slabs: The Influence of Slab Thickness and Stud Layout. Calgary: Dissertation, University of Calgary.

[20] Regan , P., & Khan , M. (1974). Bent‐Up Bars as Shear Reinforcement. ACI Special Publication SP42‐11.

[21] Regan , P. (1985). Shear Combs, Reinforcement against Punching (Vol. 63B). The Structural Engineering.

[22] Regan , P. (1986). Symmetric Punching of Reinforced Concrete Slabs. Magazine of Concrete Research.


14

[23] Oliveira, D. R., Melo, G. S., & Regan, P. E. (2000). Punching Strengths of Flat Plates with Vertical or Inclined Stirrups. ACI Structural Journal (Vol. 97), 485‐491.

[24] Regan , P., & Samadian , F. (2001). Shear Reinforcement against punching in reinforced concrete flat slabs. London: The Structural Engineer.

[25] Oliveira , D., Regan , P., & Melo , G. (2004). Punching resistance of RC slabs with rectangular columns. Magazine of Concrete Research.

[26] Broms, C. E. (1990). Punching of Flat Plates ‐ A Question of Concrete Properties in Biaxial Compression and Size Effect. ACI Structural Journal (V. 87), 292‐304.

[27] Broms, C. E. (1990). Shear Reinforcement for Deflection Ductility of Flat Plates. ACI Structural Journal , 87 (6), 696 ‐ 705.

[28] Broms, C. E. (2000). A method to avoid the punching failure mode. International Workshop on Punching Shear Capacity of RC Slabs, (pp. 117‐124). Stockholm.

[29] Broms , C. (2006). Concrete flat slabs and footings : Design method for punching and detailing for ductility. Stockholm: KTH, Civil and Architectural Engineering.

[30] Broms, C. E. (2007). Ductility of Flat Plates: Comparison of Shear Reinforcement Systems. ACI Structural Journal , 104, 703‐711.

[31] Hegger , J., & Beutel , R. (1998). Sicherheit gegen Durchstanzen von schubbewehrten Flachdecken. Aachen: RWTH Report, AiF Forschungsvorhaben 10644 N (DBV 185).

[32] Hegger , J., & Beutel , R. (1999). Durchstanzen ‐ Versuche und Bemessung. Der Prüfingenieur, Zeitschrift der Bundesvereinigung der Prüfingenieure für Bautechnik.

[33] Beutel , R., & Hegger , J. (2000). Punching behaviour of Shear Reinforced Flat Slabs at Interior columns ‐ Effective and Economic Shear Systems. (pp. 171‐179). Stockholm: International Workshop on Punching Shear Capacity of RC Slabs.

[34] Beutel , R., & Hegger , J. (2002). The effect of anchorage on the effectiveness of the shear reinforcement in the punching zone. Cement and Concrete Composites.

[35] Hegger , J., Beutel , R., & Kerkeni , N. (2004). Einfluß der Deckenschlankheit auf den Durchstanzwiderstand nach DIN 1045‐1, SIA 262, Ö‐Norm B 4700(01) und Eurocode prEN 1992‐1‐1. Beton‐ und Stahlbetonbau.

[36] Fernández, R. M., & Muttoni, A. (2008). Applications of the critical shear crack theory to punching of R/C slabs with transverse reinforcement. ACI Structural Journal (Sumbitted for publication).

[37] Muttoni, A., & Schwartz, J. (1991). Behavior of Beams and Punching in Slabs without Shear Reinforcement. IABSE Colloquium, (pp. 703‐708). Stuttgart.

[38] Muttoni , A. (2003). Schubfestigkeit und Durchstanzen von Platten ohne Querkarftbewehrung (Vol. 98). Berlin: Beton‐ und Stahlbetonbau.

[39] Guandalini , S., & Muttoni , A. (2004). Essais de poinçonnement symétrique des dalles en béton armé sans armature à l'effort tranchant. Lausanne: Rapport d'essai.

[40] Guandalini , S. (2005). Poinçonnement symétrique des dalles en béton armé. Lausanne: Thèse de doctorat.

[41] Muttoni , A., & Guandalini , S. (2006). Kommentar zum Durchstanzen nach SIA 262. Lausanne.

[42] Muttoni, A., & Fernández, R. M. (2007). Shear strength predictions according to the critical shear crack theory and the Swiss code SIA 262 (2003). Workshop on assessment methods for determining the shear strength of existing structures, (p. 14). Rotterdam.


15

[43] Muttoni, A., & Fernández, R. M. (2008). Shear strength of members without transverse reinforcement as function of critical shear crack width. ACI Structural Journal , 105 (2), 163‐172.

[44] Muttoni, A. (2008). Punching shear strength of reinforced concrete slabs without transverse reinforcement. ACI Structural Journal , 105 (4), 440‐450.

[45] Guandalini, S., Burdet, O., & Muttoni, A. (2008). Punching tests of slabs with low reinforcement ratios. ACI Structural Journal (Accepted for publication).

[46] SIA. (2003). Betonbau. Zurich.

[47] Eurocode , 2. (2002). Design of concrete structures, part 1 : General rules and rules for buildings.

[48] ACI. (1999). Building Code Requirements for Structural Concrete. Detroit: ACI 318‐99, American Concrete Institute.

[49] Yamazaki , J., & Hawkins , N. (1980). Finite Element Predictions of the Behavior of Slab‐Column Connections Transferring Moment. Detroit: ACI SP‐63.

[50] Vaz , R. (2007). Shear Strength of Reinforced Concrete Bridge Deck Slabs. Lausanne: EPFL, PhD thesis.

[51] Shehata , I. (1985). Theory of punching in RC slabs (Vol. Ph.D Thesis).

[52] Nölting , D. (1984). Das Durchstanzen von Platten aus Stahlbeton ‐ Tragverhalten, Berechnung, Bemessung ‐. Braunschweig: Thèse de doctorat.

[53] Negele , A. (2006). Durchstanzverhalten von Flachdecken mit neuer Schubbewehrung und Drucklagern aus UHPC. Beton‐ und Stahlbetonbau.

[54] Muttoni , A., Fernández , R., & Guandalini , S. (2007). Poinçonnement des ponts‐dalles. Bern: Documentation SIA, 4. FBH/ASTRA Studientagung 'Neues aus der Brückenforschung'.

[55] Hassanzadeh , G., & Sundquist , H. (2000). Strengthening Of Bridge Slabs On Columns. Stockholm: Nordic concrete research.

[56] Guidotti , R., Fernández , R., & Muttoni , A. (2007). Essais de poinçonnement. Rapport d'essais, Essais de poinçonnement.

[57] Broms , C. (2006). Ductility reinforcement for flat slabs in sismic areas. London: Magazine of Concrete Research, Thomas Telford.

[58] Andrä , H.‐P. (1981). Zum Tragverhalten von Flachdecken mit Dübelleisten, Bewehrung in Auflagerbereich (The strength of the support zones of flat slabs reinforced with shear combs). Beton‐ und Stahlbetonbau.

[59] Andrä , H.‐P., Dilger , W., & Ghali , A. (1979). Durchstanzbewehrung für Flachdecken (Vol. 74). Berlin: Beton‐ und Stahlbetonbau.

[60] Andrä , H.‐P. (1979). Dübelleisten zur Verhinderung des Durchstanzens bei hochbelasteten Flachdecken (Shear combs as reinforcement against punching in heavily loaded flat slabs). Bautechnik.

[61] Alexander , S., & Hawkins , N. (2005). A Design Perspective on Punching Shear. ACI.

[62] Corley, W. G., & Hawkins, N. M. (1968). Shearhead Reinforcement for Slabs. ACI Journal (65‐59), 811‐824.

[63] Broms, C. E. (2000). Elimination of Flat Plate Punching Failure Mode. ACI Structural Journal , 97 (1), 94 ‐ 101.

[64] Broms, C. E. (2007). Flat Plates in Seismic Areas: Comparison of Shear Reinforcement Systems. ACI Structural Journal , 104, 712‐721.

[65] Elgabry, A., & Ghali, A. (1990). Design of Stud‐Shear Reinforcement for Slabs. ACI Structural Journal (Vol. 87‐3), 351‐361.


16

[66] Elgabry, A., & Ghali, A. (1987). Tests on Concrete Slab‐Column Connections with Stud‐Shear Reinforcement Subjected to Shear‐Moment Transfer. ACI Structural Journal , 433 ‐ 442.

[67] Gardner, N. J. (1990). Relation of the Punching Shear Capacity of Reinforced Concrete Slabs with Concrete Strength. ACI Structural Journal (Vol. 87), 66‐71.

[68] Gomes, R. B., & Regan, P. E. (1999). Punching Resistance of RC Flat Slabs with Shear Reinforcement. ASCE Journal of Structural Engineering , 125 (ue 6), 684‐692.

[69] Hammill, N., & Ghali, A. (1994). Punching Shear Resistance of Corner Slab‐Column Connections. ACI Structural Journal , 91 (6), 697‐707.

[70] Megally, S. H., & Ghali, A. (1993). Punching Shear of Slabs in Seismic Zones. Annual Conference ‐ Canadian Society for Civil Engineering CSCE/SCGC ‐ June 8‐11, (pp. 145‐154). Fredericton.

[71] Menétrey, P. (1999). Punching in Slabs with Shear Reinforcements: a Tensile Failure. fib Symposium. Prague.

[72] Menétrey, P., Walther, R., Zimmermann, T., Willam, K. J., & Regan, P. E. (1997). Simulation of Punching Failure in Reinforced‐Concrete Structures. ASCE Journal of Structural Engineering (Vol 123), 652‐659.

[73] Pilakoutas, K., & Li, X. (2003). Alternative Shear Reinforcement for Reinforced Concrete Flat Slabs. ASCE Journal of Structural Engineering (Vol. 129), 1164‐1172.

[74] Rosenthal, I. (1959). Experimental Investigation of Flat Plate Floors. ACI Journal , Proceedings 59 (56‐12), 153‐166.

[75] Van, d. V., Dilger, W. H., & Ghali, A. (1982). Concrete Flat Plates with Well‐Anchored Shear Reinforcement Elements. Canadian Journal of Civil Engineering , 9 (1), 107‐114.

Documents

PUNCHING OF FLAT SLABS WITH SHEAR REINFORCEMENT · PDF filedoctoral program in structures 1 plan de recherche research plan punching of flat slabs with shear reinforcement stefan lips