IABSE Conference – Engineering the Developing World April 25-27 2018, Kuala Lumpur, Malaysia

Experimental Investigation of the Punching Shear Behaviour of RC Flat Slabs Connections Under Seismic Loading Amr Abdelkhalik*

Assistant Lecturer, Structural Engineering Department, Badr University, Cairo, EGYPT

Tamer Elafandy

Associate Professor, RC Institute, Housing and Building National Research Center, Giza, EGYPT

Amr Abdelrahman

Professor, Structural Engineering Department, Ain Shams University, Cairo, EGYPT

Alaa Sherif

Professor, Civil Engineering Department, Mataria, Helwan University, Cairo, EGYPT

Contact*:amr_civileng@yahoo.com Tel*: +2-01008444069

Abstract

Reinforced concrete flat slab-column structures are widely used because of their practicality. However, this type of structures can be subjected to punching-shear failure with in the slab-column connections. Without shear reinforcement, the slab-column connection can undergo brittle punching failure, especially when the structure is subjected to lateral loading in seismic zones. This research is a part of an extensive investigation about the punching shear behavior of interior RC slab-column connections under seismic loading. The current paper represents only the results of the first two tested specimens. The main objective is to discuss the nature and mechanism of effect seismic loading on punching shear behaviour. Finally, the experimental results are analyzed and compared to international codes such as American Code ACI318-14[1] and Euro Code EC2-2004[2]. In light of these results, some preliminary conclusions are presented.

Keywords: Punching Shear; Shear Studs; Seismic Loading; Interior slab-column connections.

1 Experimental Program

Full scale specimens were tested. The specimen can be regarded as part a prototype structure of which the flat concrete slab spans 4.5 m between columns. The slab thickness is 200 mm.

The specimens represent interior slab-column Connections, which are isolated specimens with dimensions corresponding to the lines of contra flexure under gravity loads [3,4].

ABSE Conference – Engineering the Developing World April 25-27 2018, Kuala Lumpur, Malaysia

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The control specimen FP-GR-CTRL, was subjected to a monotonic vertical load up to punching shear failure. Specimen FP-VR-0.4 was subjected to a constant vertical load and a reversed cyclic loading

up to punching shear failure. Figure 1 shows the concrete dimensions of specimens (FP-GR-CTRL) and (FP-VR-0.4).

(a). Specimen (FP-GR-CTRL)

(b). Specimen (FP-VR-0.4)

Figure 1. Concrete dimensions of tested specimens FP-GR-CTRL and FP-VR-0.4

1.1 Description of Specimens and Flexural Reinforcement

In the tension surface of the concrete slab, the flexural reinforcement ratio is 1.62% within a width of 824 mm from the center of the slab as shown in Figure 2. The reinforcing ratio on the compression surface of the slab is 0.6%. The reinforcement is designed to ensure punching shear failure of these connections not failure due to flexure. [5,6]. The reinforcing ratio of the columns is 4.86% and closed

ties are used in order to make the column strong enough to transfer the axial load and unbalanced moment to the slab. Figure 2 shows the reinforcement details of the tested specimens (FP-GR-CTRL) and (FP-VR-0.4). For control purposes, standard concrete cubes were cast alongside the specimens and were tested at the same day as the specimen. The compressive concrete cube strength

ABSE Conference – Engineering the Developing World April 25-27 2018, Kuala Lumpur, Malaysia

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For specimens FP-GR-CTRL and FP-VR-0.4 equal 25 and 35 N/mm2 respectively. The main reinforcement was made of deformed steel bars (Grade 40/60) of actual yield stress (Fy) of 400 MPa, and actual ultimate tensile strength

(Fu) of 600 MPa. All the previous reinforcement had a modulus of elasticity (Es) of 200 GPA.

(a). Flexure reinforcement mesh (b). Compression reinforcement mesh

(c). FP-GR-CTRL (d). FP-VR-0.4 (e). Column reinforcement

Figure 2. Reinforcement details of tested specimens FP-GR-CTRL and FP-VR-0.4

1.2 Test Setup, Boundary Conditions and Loading Scheme

The specimen FP-GR-CTRL was tested under a monotonic vertical load up to punching shear failure. Figure 3 shows the test setup of the specimen FP-GR-CTRL. A single concentrated load of applied on the column using 1000 KN hydraulic jack. A stiff steel I-beam is continuously supporting the edges of the specimen. At the end of each load

step, the load was held constant for a period of two minutes, during which measurements and marking of cracks took place. A schematic of test set up of specimen FP-VR-0.4 is shown in Fig.4. Figure 5 show the protocol cyclic loading of specimen FP-VR-0.4.

ABSE Conference – Engineering the Developing World April 25-27 2018, Kuala Lumpur, Malaysia

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Figure 3. Test Setup, Boundary Conditions and Loading Mechanism of specimen FP-GR-CTRL

Figure 5. Loading path of horizontal cyclic load

Figure 4. Schematic of test setup of specimen FP-VR-0.4

Figure 6. Instrumentation Scheme for Specimen FP-GR-CTRL

2 Instrumentation

Measurements were made thoroughly for displacements and steel strains at key locations of the tested specimens as shown as in Figures 6 and 7. All LVDTs and strain gauges were connected to a computer controlled data acquisition system. The

crack pattern was monitored and marked on the specimen with the associated load level indicated next to it.

(a). LVDT to measure of Lateral drift of top and bottom column

(b). Vertical Displacement at bottom surface of the slab

(c). Strain gauges' positions in flexure and compression mesh reinforcement

Figure 7. Instrumentation Scheme for Specimen FP-VR-0.4

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3 Experimental Results 3.1 Observation and Crack Pattern

Cracks on the slab surfaces started from the corners of the column at the tension side. In the specimen FP-GR-CTRL (concrete strength: 25MPa) the First crack at the compression surface of the slab was observed at about a vertical gravity load equal 290 KN (about 43.5% of the failure load). For specimen FP-VR-0.4 (concrete strength: 35MPa) cracks on slab surface started from corners of the column at the tension side, first on the top slab

surface (which was subjected to tension from gravity load equal 230 KN) and then on bottom surface. First crack at the bottom of the slab was observed at about 0.6~0.8% drift ratio. The final crack patterns of top and bottom slab surfaces for both specimens FP-GR-CTRL and FP-VR-0.4 are shown in Figure 8. Both specimens failed in a pure punching mode.

(a). Crack pattern of specimen FP-GR-CTRL

(b). Crack pattern of specimen FP-GR-CTRL

Figure 8. Crack pattern and punching shear failure cone of each specimen

3.2 Load-deflection Curves

In specimen FP-GR-CTRL, the punching shear failure occurred at a vertical load of 667 KN and the maximum displacement measured under the bottom surface equals 13.36 mm. Figure 9 shows

the load-deflection response of slab as well as shear stress calculated at a distance d/2 from column face versus the measured vertical displacement.

ABSE Conference – Engineering the Developing World April 25-27 2018, Kuala Lumpur, Malaysia

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(a) Load-deflection response (b) Shear stress-deflection response

Figure 9. Ductility response of specimen FP-GR-CTRL

In specimen FP-VR-0.4, the horizontal lateral load applied at the top column end versus its horizontal lateral drift ratio is shown in Figure 10(a). The specimen is subjected to a vertical load of 230KN which corresponds to 34% of the failure load of specimen FP-GR-CTRL and which corresponds to 40% of the nominal punching shear capacity according to ACI318-14. As shown in Table 1, the peak lateral negative load for FP-VR-0.4 was 148KN

at 4% drift ratio. For positive peak load was 132KN at 3.05% drift ratio.

Figure 10 (b) shows the backbone curve of the hysteresis curve of the lateral load versus lateral drift. The backbone curve is formed by connecting peak points at the first cycle of each same-drift cycle’s group.

Table 1. Peak load and drift ductility of specimen FP-VR-0.4

Note : Nominal punching shear capacity of concrete Vo= 0.333√𝑓𝑐 bo d (ACI 318-14, in metric units) While The

actual punching shear capacity of concrete =2.26x1696x174/1000=667KN from experimental results of controlled specimen FP-GR-CTRL at d/2 from column face.

(a). Horizontal load versus horizontal drift ratio at top column end

(b). Backbone curves of horizontal load versus horizontal drift ratio at top column end

Figure 10. Horizontal load versus horizontal drift measured for specimen FP-VR-0.4

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3.3 Comparison Between Shear Stress at Failure from Experimental Results and Theoretical Calculations from International Codes such as ACI318-14 and EC2-2004. Tables 2 and 3 compare the experimental results with the punching shear stress resistance of the slab according to ACI 318-14 (Eq.1) and EC2-2004 (Eq.2). Both codes accurately estimate the failure stresses for FP-VR-0.4. ACI underestimates the failure shear stress for specimen FP-GR-CTRL (𝒗EX / 𝒗c =1.36), while EC2 accurately estimate the test

result (𝒗EX / 𝒗c =0.99) for specimen FP-GR-CTRL and ( 𝒗EX / 𝒗c =1.054) for specimen FP-VR-0.4.

VC = 0.333√𝑓𝑐 (1)

VC = 0.18𝑘(100𝜌𝑓𝑐𝑘)1

3 (2)

Table 2. Failure shear stress (N/mm2) according to ACI318-14 (At d/2 from column face)

Specimen 𝒇′𝒄 (𝑵/𝒎𝒎𝟐) b0 (mm) d 𝒗c (N/mm2) 𝒗EXP (N/mm2)

Ratio 𝒗EX / 𝒗c

FP-GR-CTRL 25 1696 174 1.665 2.26 1.36

FP-VR-0.4 35 1696 174 1.97 1.802 0.915

Table 3. Failure shear stress (N/mm2) according to EC2-2004 (At 2d from column face)

Specimen 𝒇′𝒄𝒌 (𝑵/𝒎𝒎𝟐) K 𝝆 % b0 (mm) d 𝒗c (N/mm2)

𝒗EXP

(N/mm2)

Ratio 𝒗EX / 𝒗c

FP-GR-CTRL 25 2 1.62 3185 174 1.221 1.204 0.99

FP-VR-0.4 35 2 1.62 3185 174 1.364 1.437 1.054

Figure 11 presents the relation between the displacement and the shear stress calculated at a distance d/2 from column face according to ACI318-14. Punching shear failure occurred at shear stress equal 2.26 MPa for specimen FP-GR-CTRL. While for the specimen FP-VR-0.4 punching shear failure occurred at shear stress equal 1.802 MPa. Equation 3 of ACI318-14 determines the zone in which shear reinforcement is required as a

relation between the ultimate design drift ratio DRu and the gravity shear ratio V/V0 as shown in Figure 12[7,8]. Fig. 12 includes test result of FP-VR-0.4 as well as some test results without shear reinforcement from the literature. The results indicate that the draft ratio was achieved without shear reinforcement in contrary to the ACI requirements

.

Figure 11. Displacement response versus shear stress at distance d/2 from column face for

specimens FP-GR-CTRL and FP-VR-0.4

Figure 12. Maximum drift ratio DRu which can

be achieved in interior slab-column connections without shear reinforcement

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DRu = 0.035 - 0.05*(V/Vo)

for (V/Vo ≤ 0.6) (3)

DRu = 0.005 for (V/Vo >0.6)

4 Conclusions

This paper presents the first test results of an experimental investigation of the punching shear response of interior slab-column connections. The following are the main conclusions

1- The Experimental results in the test of slab-column connections showed a clear difference in the shape and distribution of cracks around the RC column section due to punching shear failure subjected to vertical load only or vertical load plus seismic loading.

2- The Experimental results showed that the punching shear resistance of the slab-column connections are reduced if subjected to lateral cyclic loading in addition to the vertical load.

3-The code provisions of ACI318-14 and EC2-2004 predicted accurately the failure loads for the tested specimens.

4- Tests subjected to cyclic moments (own test and from the literature) indicate that the anticipated drift ratio can be achieved without shear reinforcement indicating conservative provisions of ACI 318-14 in this regard.

5 References

[1]. ACI 318-14, “Building Code Requirements for Structural Concrete and Commentary”, American Concrete Institute, Farmington hills, 2014.

[2]. European Standard, “Eurocode 2: Design of concrete structures”, CEN, European Standard, 2004.

[3]. Megally, S., and Ghali, A., “Punching Shear Design of Earthquake-Resistant Slab-Column Connections”, ACI Structural Journal, 97, 5, Sept.-Oct. 2002, pp. 720-730.

[4]. Dilger, W.H. Flat Slab-Column Connections. Progress in Structural Engineering and Materials. John Wiley & Sons. Ltd. 2000; Volume 2, Issue 3, p. 386-399.

[5]. Choi, K. K., Taha, M. M. R., Sherif, A. G. “Simplified Punching Shear Design Method for Slab-Column Connections Using Fuzzy learning”, ACI Structural Journal Vol. 104, No.4, p. 438-447, July-August 2007.

[6]. ACI-ASCE Committee 421.Shear Reinforcement for Slabs (ACI 421.1 R-99). American Concrete Institute, Farmington Hills, Michigan, 2005.

[7]. Robertson, I.N., Kawai, T., Lee, J., and Enomoto, B.” Cyclic Testing of Slab-Column Connections with Shear Reinforcement,” ACI Structural Journal, V.99, No.5, September-October, pp.605-6013, 2008.

[8]. Robertson, I., and Johnson, G.” Cyclic Lateral Loading of Nonductile Slab-Column Connections,” ACI Structural Journal, V. 103, No. 3, May-June, pp. 356-364, 2006.