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Enhancement of punching shear strength of flat slabs using shear-band reinforcement
HBRC Journal (2017) xxx, xxx–xxx
Housing and Building National Research Center
HBRC Journal http://ees.elsevier.com/hbrcj
FULL LENGTH ARTICLE
Enhancement of punching shear strength of flat slabs using shear-band reinforcement Nasr Z. Hassan *, Mostafa A. Osman, Awad M. El-Hashimy, Heba K. Tantawy Civil Eng. Dept., Helwan University, Cairo, Egypt Received 23 September 2016; accepted 26 November 2017
KEYWORDS Flat slab; Shear band; Punching shear; Failure load
Abstract Flat-slab system is widely used nowadays. Major and critical problem of this system is its sudden brittle failure is called punching shear failure. To overcome the punching failure problem, there are many ways to increase the punching shear strength of concrete slabs, increasing slab thickness in the area adjacent to the column, increasing column thickness which is against the architectural desire, and finally providing slab with shear reinforcement. Shear reinforcement is more advanced from both the structural and economical point of view. An experimental program includes seven full scale square flat slab interior column specimens tested under gravity loads. All slabs have same dimensions of 1700 mm 1700 mm with thickness 160 mm and reinforcement ratio of 1.2%. Column was square of 200 mm length and 250 mm height. Elongated steel strips of 25 mm width and 1.5 mm thickness undulated into the slab in different ways to investigate punching shear resistance. The program is divided into five groups. First group investigates the effect of installing the shearband reinforcement (hanged up on top mesh, knit the top and bottom mesh together). The second group investigates the effect of inclination of shear reinforcement (shear band with vertical leg, with bended leg 45°). The third group investigates the effect of concentrating the shear reinforcement by increasing the quantity around the column. The fourth group investigates effect of radial distribution of shear-band system around the column. Finally, the fifth group investigates the effect of box distribution of shear-band system around the column. Ó 2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ncnd/4.0/).
Introduction
* Corresponding author. E-mail addresses: [email protected], [email protected] (N.Z. Hassan). Peer review under responsibility of Housing and Building National Research Center.
There are many methods of enhancing punching shear resistance of flat slabs by providing shear reinforcement [1–3] which is placed in the slab around the column. The importance of shear reinforcement that can easily be installed will be emphasized. Shear-band system differs from all other existing systems [4,5] It is made of steel strips of high ductility. The strip can be bent to variety shape which is undulated into
https://doi.org/10.1016/j.hbrcj.2017.11.003 1687-4048 Ó 2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
2
N.Z. Hassan et al.
the slab from top surface after all flexural reinforcement is placed with minimum loss of cover. The advantages of using shear-band system can be concluded in the following [6]: – Prevents brittle punching shear failure and greatly improves the ductility of flat slabs. – Increases the punching shear capacity [6] of flat slabs without increasing the slab flexural capacity. – Easily fabricated, light weight, simple and efficient placement of reinforcement in addition to it is easy to store and transport.
Experimental program The experimental program of this study includes seven full scale square flat slab interior column connections specimens tested under gravity loads. All slabs have same dimensions of 1700 mm 1700 mm with thickness 160 mm and reinforcement ratio 1.2% (Fig. 2). Elongated steel strips [2] (Fig. 1) of 40 mm width and 1.5 mm thickness are undulated into the slab in different ways to investigate punching shear resistance. No holes induced in the band strips to achieve maximum capacity of punching shear strength, while it does not achieve gave
Fig. 1
bond with concrete. Table 1 indicates details of specimens of the experimental program. The program divided into five groups: First group investigates the effect of install the shear-band reinforcement (hanged up on top mesh, knit the top and bottom mesh together), The second group investigates the effect of inclination of shear reinforcement (shear band with vertical leg, with bended leg 45°). The third group investigates the effect of concentrate the shear reinforcement by increasing the quantity around the column. The fourth group investigates the effect of radial distribution of shear-band system around the column. Finally, the fifth group investigates the effect of box distribution of shear-band system around the column (Table 2). Fig. 2 indicates details of specimen with shear band of orthogonal distribution. Fig. 3 indicates details of specimen with shear band of or radial distribution, while Fig. 4 represents of specimen with shear band of box distribution. Material characteristics The materials used in fabricating the test specimens mixtures were local available materials and the process of manufacturing was closely similar to the common way of the concrete manufacture in Egypt The concrete mix used in the tested
Stages of shear-bands manufacture for specimens.
t = 1.5 mm
Fig. 2
Details of tested specimens (S2) orthogonal distribution.
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
Enhancement of punching shear strength of flat slabs using shear-band reinforcement Table 1
3
Test program and specimen details.
Symbol
Shear band no
Inclination degree
Layout distribution
Installation method
S1 S2 S3 S4 S5 S6 S7
– 8 8 16 8 12 16
– 90 90 90 45 90 90
– Orthogonal Orthogonal Radial Orthogonal Box shape Orthogonal
Control Specimen Knit Tension and Compression Mesh Suspend on Tension Mesh Only Placed Bet. Tens. and Comp. Mesh Suspend on Tension Mesh Only Suspend on Tension Mesh Only Suspend on Tension Mesh Only
Table 2 Group no
(1) (2) (3) (4) (5)
Test specimen groups. Factor to be investigated
Effect Effect Effect Effect Effect
of of of of of
shear band installation method shear band inclination increasing shear band quantity radial distribution of shear band box distribution of shear band
Containing specimens Control specimen
Variable specimen (1)
Variable specimen (2)
S1 S1 S1 S1 S1
S2 S3 S3 S4 S5
S3 S5 S7 – –
beams is consisted of ordinary portland cement, sand, crushed dolomite, and water with ratios of 1:1.80:3.80:0.50 by weight respectively. All used materials are matches with ECP 203 limits. The characteristic strength of concrete assigned by testing standard cubes of dimensions 150 mm length after 28 days of casting. The average grade was 36 MPa. Steel band strips of 25 mm width, and 1.5 mm thickness are used. The yield stress of the shear bans strips and main reinforcement is 360 MPa.
I-sections was used as a base to support a beam specimen. The load was applied vertically using a hydraulic jack with maximum capacity 1000 kN in centric of the column of slab. A system of rigid steel I beams used to transmit the applied single concentrated load coming from load cell directly to the column using rigid steel plate Fig. 5.
Test setup
To record vertical deflection of specimens, two dial gauges were used. One located along the center line of the specimen and fixed to calculate the deflection at column face and the other at distance of 1/4 the length of distance between column face and slab edge. Applied load was measured incrementally during loading till failure.
The tests were carried out in the reinforced concrete laboratory of the Faculty of Engineering, El-Mataria, Helwan University. A very rigid steel frame consisting of horizontal and vertical
Fig. 3
Radial distribution layout of tested specimen (S4).
Measurements
Fig. 4
Box distribution layout of tested specimen (S6).
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
4
N.Z. Hassan et al. Crack patterns
Fig. 5
Test setup and loading system.
Before casting the slabs, electric resistance gauge (5 mm length, 120 ohms resistance with gauge factor 2.10) were mounted and glued to the reinforcement using epoxy. Installation of strain gauges at both flexural and shear reinforcement shown in Fig. 6. The steel strain was measured and recorded using a digital strain meter (TC-31K) device connected to the strain gauges by wires and the reading were taken at each increment of loading.
Fig. 6
Top View Fig. 7
The initial crack development in all seven specimens followed almost a similar pattern. The first crack opened up on the tension surface in form of flexural Micro cracks starts parallel to steel mesh near the column at about 100 kN, As the applied load increased the cracks propagated from middle to outwards. With increasing loads, more cracks developed that advanced radials along the four axes of symmetry of the slab from column faces towards the slab edges. Cracks parallel to X and Y axes opened up at load less than 200 kN, while cracks parallel to the diagonal axes opened up at a load greater than 200 kN. By the time when the applied load reached 300 kN, only a few new diagonal cracks developed. After that, with increasing load, the number and width of these cracks close to the column increased substantially. Fig. 7 indicates the recorded crack pattern from bottom and top view of slab specimen S2. Test results Load deflection relationship The load deflection curves for all specimens have almost the same profile. Fig. 8 indicates load-Deflection curve of group
Stain gauge installation.
Bottom View Crack pattern for specimen S2.
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
Enhancement of punching shear strength of flat slabs using shear-band reinforcement Stiffness, measured ductility and absorbed energy
600 500
Load (kN)
5
The stiffness degradation are calculated as the difference between stiffness at plastic stage and initial stiffness divided by the initial stiffness, Ductility measured as 0.7 the value of deflection at ultimate. The absorbed energy is the area under the curve of load deflection. Each of stiffness degradation, ductility and energy absorption are calculated for each slab specimen and listed in Table 4 for all specimens.
S1
400
S2
300
S3
200 100 Group 1
0
Comparison between test results and codes estimation [6–11] 0
5
10
15
20
25
30
35
40
Deflection (mm)
Fig. 8
Even today, almost codes take into consideration the effect of main steel reinforcement and shear reinforcements in the calculating of punching shear capacity, except Egyptian code [7]. Table 5, indicates the considered and unconsidered parameters (C) and (N) in the different codes. Vexp is the ratio between failure punching shear force, which VCode was obtained from the experimental tests and the nominal
Load deflection curve of group 1.
1, where the first part of the curves are steep, and after cracks, most of profiles started to be more curved till the failure occurred. Table 3 indicates the cracking and ultimate loads and the deflections at these loads for all specimens. Table 3
Test results of loaded specimens.
Specimen ID
S1 S2 S3 S4 S5 S6 S7
Table 4
Crack/Ultimate (%)
Enhancement ratio (%)
Pcr (KN)
Dcr (mm)
Failure stage Pf (KN)
Df (mm)
Load (%)
Deflection (%)
ðPf ÞSpecimen ðPf ÞControl S1
92 95 80 80 90 130 75
4.76 7.77 4.68 4.13 6.11 5 4.02
378 585 475 436 440 505 500
20.33 35.5 28.15 24.4 25.24 24.96 29.29
24.34 14.53 16.84 18.35 20.45 25.74 15.00
23.41 21.19 16.62 16.93 24.21 20.03 13.72
100% 155% 126% 115% 116% 134% 132%
Stiffness degradation, ductility and energy absorption.
Specimen ID
S1 S2 S3 S4 S5 S6 S7
Table 5
Cracking stage
Stiffness Ki KN/mm
Ku KN/mm
Ku/Ki Stiffness degradation
19.33 12.23 17.09 19.37 14.73 26.00 18.66
18.56 7.18 14.00 17.55 13.30 23.01 12.95
0.96 0.59 0.82 0.91 0.90 0.89 0.69
Measured ductility 0.7 Du (mm) (%)
Energy absorption (KN mm)
14.23 25.52 19.71 17.37 17.67 18.17 20.50
4278.00 10623.00 7069.40 5220 5365.00 8307.00 7984.60
Comparison between different codes parameters.
Code/Parameter
ACI
ECCS
CSA
Euro Code 2
BS-8110
CEB-FIB
Concrete strength Main flexural reinforcements Reinforcements concentration Punching shear reinforcements Column aspect ratio Perimeter to depth ratio Location of critical section Angle between shear reinforcement and main steel
C N N C C C d/2 N
C N N N C N d/2 N
C N N C C C d/2 N
C C C C C C 2d C
C C C C N N 1.5d C
C C C C N C 2d C
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6
N.Z. Hassan et al. Table 6
Experimental results compared with ACI [6], CSA [8] and BS-8110 code [10].
Specimen ID
S1 S2 S3 S4 S5 S6 S7
Vexp VCode
Vexp VCode
Limited
Unlimited
ACI
CSA
BS-8110
ACI
CSA
BS-8110
1.13 1.15 0.94 0.93 0.94 1.07 0.99
0.98 1.05 0.852 0.84 0.85 0.98 0.897
1.25 – – – – – –
– 1.09 0.89 0.49 0.84 0.31 0.55
– 1.04 0.845 0.475 0.803 0.305 0.537
– 1.11 0.90 0.59 0.99 0.53 0.67
values that were calculated by the code equations [6,8,9] after omitting the material safety factor, Noteworthy that used codes limited the nominal shear strength of concrete and steel individually and for shear reinforced section also. Two comparisons were taken into consideration for the limited value Vexp and for actual unlimited calculated value. For ratios VCode , limited is greater than unity, the code becomes conservative. For a ratio approximately equal to unity, the code is in agreement with the test results. However, for ratios less than unity, the code becomes non-conservative. The ratio between Vexp Vexp Limited and VCode Unlimited for ACI 318-05, CSA VCode
4.
and BS-8110 codes are presented in Table 6 [6,8,10]. Conclusions 5. Through the number of specimens and variables taken into consideration in the present study, the obtained experimental results and the conducted comparisons, the main conclusions can be summarized as follows: 1. Control slab which have left without shear reinforcement failed abruptly in a brittle manner. However the columns did not penetrate the slab completely, whilst specimens which are provided with shear bands failed eventually in typical punching shear failure after reaching the maximum flexural load with less brittle manner outside the shear reinforcement zone. The failure reflects the huge potential of strip reinforcement in preventing failure in the shear reinforcement zone. Furthermore, indicating that the slab reached its most of full flexure potential before failure is supported by strain readings (from high elongation strain gauges) on the flexure reinforcement is excess of 2000 le. 2. Shear bands distributed over the critical punching shear zone provide a very good economic solution regarding increasing the punching shear capacity, ductility and energy absorption. Their efficiency depends on their reinforcement installation method with flexural reinforcement, layout of distribution, concentration and inclination with slab plane. 3. Load capacity is directly proportional with increasing in measure of ductility as an indication for ductility enhancement. Also crack propagation area is proportional with ductility i.e. the higher measure of ductility the larger propagated area of cracks appears over all
6.
7.
8.
9.
specimens outside the shear reinforcement zone. Load capacity enhancement range was between (15–55%), while improvement in measure of ductility range was between (22–79%). It was observed that the failure capacity load increases obviously when shear bands are installed in woven way with the flexure steel mats at specimen S2 than when hanging on tension flexure mat only at Specimen S3. The knitting method provides additional anchorage to reinforcement with the compression zone. All characteristics are improved strongly, load capacity increased by 55%, energy absorption increased by 148%, ductility increased by 79% while the stiffness degradation decreased by 38.5%, S2 is considered the most improved specimen of all specimens absolutely. A slight increase in the failure load capacity of specimen which is reinforced by bended shear bands 45° placed parallel to the potential shear crack compared to specimen reinforced by shear bands with vertical leg., while shear bands with vertical legs easier in detailing, placement and guaranteed to fail in more ductile mode. So, using shear band parallel to the potential shear crack is less useful comparing with vertical leg or perpendicularly to the potential shear crack as proven at other research. Stiffness degradation is used to assess the ductility of the specimen in a way that the lower the stiffness degradation ratio, the higher is the ductility. Also the essential demand of energy absorption is to make the concrete structures have enough ability to dissipate energy during cyclic loading especially earthquake loads. The high energy absorption means that the specimen exhibits more plastic deformation, as the energy absorption proportional to the measure of ductility. We can conclude that all specimens are enhanced compared with the control sample specimen. It reflects significant decrease in stiffness degradation while ranges between (5.2–38.5%) in addition to increase in energy absorption which ranges between (22–148.3%) and measure of ductility ranges between (22–79%). Concentration of shear bands (orthogonal distribution) by doubling quantity of shear bands over the punching zone area leads to efficient system. The slightly enhancement in failure load capacity ranges is 6%. Radial layout distribution exhibits inconsiderable improved punching shear resistance although the large quantity of shear bands which was not beneficial as
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
Enhancement of punching shear strength of flat slabs using shear-band reinforcement
10.
11.
12.
13.
14.
15.
expected. All characteristics improved slightly, load capacity increased by 15%, energy absorption increased by 22%, measure of ductility increased by 22% while the stiffness degradation decreased by 5.2%. S4 was considered the least improved specimen of all specimens absolutely. The arrangement of shear bands around the column circumference (Box layout distribution) provides awesome enhancement in the failure load capacity which increased by 34% compared with control specimen. Box distribution layout of specimen S6 is more adequate than orthogonal distribution of specimen S3. Specimen S6 allows more shear bands located at the punching perimeter than S3, thus the failure load capacity increased by about 5%. Also, the ductility and energy absorption were increased because of wide propagation of cracks outside the shear reinforcement area. S6 is considered the second-best improved specimen of all specimens absolutely. S6 comes after S2 in descending order of improvement ratio in all characteristics. Regardless the specimen is supplied with or without shear reinforcement, ACI code gives the most appropriate assumptions for calculating the punching shear strength, while BS-8110 code is the most nonconservative code with specimens and need to reconsider its limitations with this specific technique. ACI code sets hard limitations through minimizing the expected shear stress provided by shear reinforcement whatever the used quantity of shear reinforcement. CSA code follow the same concept but with more freely, where nominal shear stress of shear reinforcement calculated using CSA code are more that calculated using ACI code by about 9%. BS-8110 code characterized as the most freely limitation, probably due to the higher critical shear perimeter on distance of 1.5d from column face. It was considered most non-conservative code especially comparing with S4, S6 and S7 where VCode is higher than Vexp. by 68.4%, 88.6% and 49.3% respectively, while the ratios of ACI codes were 7.5%, 6.5% and 1% respectively. And finally CSA code ratios were 19%, 2% and 12.4% respectively. BS-8110 code limitation neither matches the experimental results nor other codes estimations, so it should be revaluated with more experimental studies for bridging the gap between test results and code overestimations.
7
16. Comparisons of the ACI 318, CSA and BS-8110 code predictions, and experimental results, which confirm that the band strips reduces punching shear cracks which enabled the slabs to avoid punching shear failure and achieve their flexural potential.
Conflict of interest The authors declare no conflicts of interest. References [1] Deˆnio R. Oliveira, Guilherme S. Melo, Paul E. Regan, Punching strengths of flat plates with vertical or inclined stirrups, ACI Struct. J. (2000), Title no: 97-S52. [2] Thomas H.-K. Kang, Hong-Gun Park, Performance of shearbands in concrete slab-column connections, ACI Struct. J. 287 (2012) 1–16. [3] Lips Stefan, Ferna´ndez Ruiz Miguel, Muttoni Aurelio, Experimental investigation on punching strength and deformation capacity of shear-reinforced slabs, ACI Struct. J. 109 (2012) 889–900. [4] Centre for Cement and Concrete, CCC Shear band Background Information. University of Sheffield; 2004. Presentation on ‘‘Shear bands verification of a novel punching shear reinforcement system for flat slabs” by Professor Kypors pilakoutas, center for cement and concrete – University of Sheffield. [5] Maurı´ cio P. Ferreira, Guilherme S. Melo, Paul E. Regan, Robert L. Vollum, Punching of reinforced concrete flat slabs with double- headed shear reinforcement, ACI Struct. J. 111 (2014) 363–374. [6] ACI Committee 318. Building Code Requirement for Structural Concrete (ACI-318M-14) and Commentary (ACI 318RM-14). American Concrete Institute, 2014. [7] Egyptian Code Committee ECP 203. Egyptian Code of Practice for Design and Construction of Reinforced Concrete Structures. Building Research Centre, Cairo, Egypt, 2007. [8] CSA-A23.3. Canadian Standard Association. Design of Concrete Structure for Building, 2004. [9] Eurocode2. Design of Concrete Structures. DD ENV-1992-2, 2001. [10] BS-8110. British Standards Institution, London. Structural use of Concrete. Part 1. Code of Practice for Design and Construction, 1997. [11] CEB-FIP Model Code. Model Code for Concrete Structures. Committee Euro-international du Federation International de la Precontrainte, Switzerland, 1990.
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
Housing and Building National Research Center
HBRC Journal http://ees.elsevier.com/hbrcj
FULL LENGTH ARTICLE
Enhancement of punching shear strength of flat slabs using shear-band reinforcement Nasr Z. Hassan *, Mostafa A. Osman, Awad M. El-Hashimy, Heba K. Tantawy Civil Eng. Dept., Helwan University, Cairo, Egypt Received 23 September 2016; accepted 26 November 2017
KEYWORDS Flat slab; Shear band; Punching shear; Failure load
Abstract Flat-slab system is widely used nowadays. Major and critical problem of this system is its sudden brittle failure is called punching shear failure. To overcome the punching failure problem, there are many ways to increase the punching shear strength of concrete slabs, increasing slab thickness in the area adjacent to the column, increasing column thickness which is against the architectural desire, and finally providing slab with shear reinforcement. Shear reinforcement is more advanced from both the structural and economical point of view. An experimental program includes seven full scale square flat slab interior column specimens tested under gravity loads. All slabs have same dimensions of 1700 mm 1700 mm with thickness 160 mm and reinforcement ratio of 1.2%. Column was square of 200 mm length and 250 mm height. Elongated steel strips of 25 mm width and 1.5 mm thickness undulated into the slab in different ways to investigate punching shear resistance. The program is divided into five groups. First group investigates the effect of installing the shearband reinforcement (hanged up on top mesh, knit the top and bottom mesh together). The second group investigates the effect of inclination of shear reinforcement (shear band with vertical leg, with bended leg 45°). The third group investigates the effect of concentrating the shear reinforcement by increasing the quantity around the column. The fourth group investigates effect of radial distribution of shear-band system around the column. Finally, the fifth group investigates the effect of box distribution of shear-band system around the column. Ó 2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ncnd/4.0/).
Introduction
* Corresponding author. E-mail addresses: [email protected], [email protected] (N.Z. Hassan). Peer review under responsibility of Housing and Building National Research Center.
There are many methods of enhancing punching shear resistance of flat slabs by providing shear reinforcement [1–3] which is placed in the slab around the column. The importance of shear reinforcement that can easily be installed will be emphasized. Shear-band system differs from all other existing systems [4,5] It is made of steel strips of high ductility. The strip can be bent to variety shape which is undulated into
https://doi.org/10.1016/j.hbrcj.2017.11.003 1687-4048 Ó 2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
2
N.Z. Hassan et al.
the slab from top surface after all flexural reinforcement is placed with minimum loss of cover. The advantages of using shear-band system can be concluded in the following [6]: – Prevents brittle punching shear failure and greatly improves the ductility of flat slabs. – Increases the punching shear capacity [6] of flat slabs without increasing the slab flexural capacity. – Easily fabricated, light weight, simple and efficient placement of reinforcement in addition to it is easy to store and transport.
Experimental program The experimental program of this study includes seven full scale square flat slab interior column connections specimens tested under gravity loads. All slabs have same dimensions of 1700 mm 1700 mm with thickness 160 mm and reinforcement ratio 1.2% (Fig. 2). Elongated steel strips [2] (Fig. 1) of 40 mm width and 1.5 mm thickness are undulated into the slab in different ways to investigate punching shear resistance. No holes induced in the band strips to achieve maximum capacity of punching shear strength, while it does not achieve gave
Fig. 1
bond with concrete. Table 1 indicates details of specimens of the experimental program. The program divided into five groups: First group investigates the effect of install the shear-band reinforcement (hanged up on top mesh, knit the top and bottom mesh together), The second group investigates the effect of inclination of shear reinforcement (shear band with vertical leg, with bended leg 45°). The third group investigates the effect of concentrate the shear reinforcement by increasing the quantity around the column. The fourth group investigates the effect of radial distribution of shear-band system around the column. Finally, the fifth group investigates the effect of box distribution of shear-band system around the column (Table 2). Fig. 2 indicates details of specimen with shear band of orthogonal distribution. Fig. 3 indicates details of specimen with shear band of or radial distribution, while Fig. 4 represents of specimen with shear band of box distribution. Material characteristics The materials used in fabricating the test specimens mixtures were local available materials and the process of manufacturing was closely similar to the common way of the concrete manufacture in Egypt The concrete mix used in the tested
Stages of shear-bands manufacture for specimens.
t = 1.5 mm
Fig. 2
Details of tested specimens (S2) orthogonal distribution.
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
Enhancement of punching shear strength of flat slabs using shear-band reinforcement Table 1
3
Test program and specimen details.
Symbol
Shear band no
Inclination degree
Layout distribution
Installation method
S1 S2 S3 S4 S5 S6 S7
– 8 8 16 8 12 16
– 90 90 90 45 90 90
– Orthogonal Orthogonal Radial Orthogonal Box shape Orthogonal
Control Specimen Knit Tension and Compression Mesh Suspend on Tension Mesh Only Placed Bet. Tens. and Comp. Mesh Suspend on Tension Mesh Only Suspend on Tension Mesh Only Suspend on Tension Mesh Only
Table 2 Group no
(1) (2) (3) (4) (5)
Test specimen groups. Factor to be investigated
Effect Effect Effect Effect Effect
of of of of of
shear band installation method shear band inclination increasing shear band quantity radial distribution of shear band box distribution of shear band
Containing specimens Control specimen
Variable specimen (1)
Variable specimen (2)
S1 S1 S1 S1 S1
S2 S3 S3 S4 S5
S3 S5 S7 – –
beams is consisted of ordinary portland cement, sand, crushed dolomite, and water with ratios of 1:1.80:3.80:0.50 by weight respectively. All used materials are matches with ECP 203 limits. The characteristic strength of concrete assigned by testing standard cubes of dimensions 150 mm length after 28 days of casting. The average grade was 36 MPa. Steel band strips of 25 mm width, and 1.5 mm thickness are used. The yield stress of the shear bans strips and main reinforcement is 360 MPa.
I-sections was used as a base to support a beam specimen. The load was applied vertically using a hydraulic jack with maximum capacity 1000 kN in centric of the column of slab. A system of rigid steel I beams used to transmit the applied single concentrated load coming from load cell directly to the column using rigid steel plate Fig. 5.
Test setup
To record vertical deflection of specimens, two dial gauges were used. One located along the center line of the specimen and fixed to calculate the deflection at column face and the other at distance of 1/4 the length of distance between column face and slab edge. Applied load was measured incrementally during loading till failure.
The tests were carried out in the reinforced concrete laboratory of the Faculty of Engineering, El-Mataria, Helwan University. A very rigid steel frame consisting of horizontal and vertical
Fig. 3
Radial distribution layout of tested specimen (S4).
Measurements
Fig. 4
Box distribution layout of tested specimen (S6).
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
4
N.Z. Hassan et al. Crack patterns
Fig. 5
Test setup and loading system.
Before casting the slabs, electric resistance gauge (5 mm length, 120 ohms resistance with gauge factor 2.10) were mounted and glued to the reinforcement using epoxy. Installation of strain gauges at both flexural and shear reinforcement shown in Fig. 6. The steel strain was measured and recorded using a digital strain meter (TC-31K) device connected to the strain gauges by wires and the reading were taken at each increment of loading.
Fig. 6
Top View Fig. 7
The initial crack development in all seven specimens followed almost a similar pattern. The first crack opened up on the tension surface in form of flexural Micro cracks starts parallel to steel mesh near the column at about 100 kN, As the applied load increased the cracks propagated from middle to outwards. With increasing loads, more cracks developed that advanced radials along the four axes of symmetry of the slab from column faces towards the slab edges. Cracks parallel to X and Y axes opened up at load less than 200 kN, while cracks parallel to the diagonal axes opened up at a load greater than 200 kN. By the time when the applied load reached 300 kN, only a few new diagonal cracks developed. After that, with increasing load, the number and width of these cracks close to the column increased substantially. Fig. 7 indicates the recorded crack pattern from bottom and top view of slab specimen S2. Test results Load deflection relationship The load deflection curves for all specimens have almost the same profile. Fig. 8 indicates load-Deflection curve of group
Stain gauge installation.
Bottom View Crack pattern for specimen S2.
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
Enhancement of punching shear strength of flat slabs using shear-band reinforcement Stiffness, measured ductility and absorbed energy
600 500
Load (kN)
5
The stiffness degradation are calculated as the difference between stiffness at plastic stage and initial stiffness divided by the initial stiffness, Ductility measured as 0.7 the value of deflection at ultimate. The absorbed energy is the area under the curve of load deflection. Each of stiffness degradation, ductility and energy absorption are calculated for each slab specimen and listed in Table 4 for all specimens.
S1
400
S2
300
S3
200 100 Group 1
0
Comparison between test results and codes estimation [6–11] 0
5
10
15
20
25
30
35
40
Deflection (mm)
Fig. 8
Even today, almost codes take into consideration the effect of main steel reinforcement and shear reinforcements in the calculating of punching shear capacity, except Egyptian code [7]. Table 5, indicates the considered and unconsidered parameters (C) and (N) in the different codes. Vexp is the ratio between failure punching shear force, which VCode was obtained from the experimental tests and the nominal
Load deflection curve of group 1.
1, where the first part of the curves are steep, and after cracks, most of profiles started to be more curved till the failure occurred. Table 3 indicates the cracking and ultimate loads and the deflections at these loads for all specimens. Table 3
Test results of loaded specimens.
Specimen ID
S1 S2 S3 S4 S5 S6 S7
Table 4
Crack/Ultimate (%)
Enhancement ratio (%)
Pcr (KN)
Dcr (mm)
Failure stage Pf (KN)
Df (mm)
Load (%)
Deflection (%)
ðPf ÞSpecimen ðPf ÞControl S1
92 95 80 80 90 130 75
4.76 7.77 4.68 4.13 6.11 5 4.02
378 585 475 436 440 505 500
20.33 35.5 28.15 24.4 25.24 24.96 29.29
24.34 14.53 16.84 18.35 20.45 25.74 15.00
23.41 21.19 16.62 16.93 24.21 20.03 13.72
100% 155% 126% 115% 116% 134% 132%
Stiffness degradation, ductility and energy absorption.
Specimen ID
S1 S2 S3 S4 S5 S6 S7
Table 5
Cracking stage
Stiffness Ki KN/mm
Ku KN/mm
Ku/Ki Stiffness degradation
19.33 12.23 17.09 19.37 14.73 26.00 18.66
18.56 7.18 14.00 17.55 13.30 23.01 12.95
0.96 0.59 0.82 0.91 0.90 0.89 0.69
Measured ductility 0.7 Du (mm) (%)
Energy absorption (KN mm)
14.23 25.52 19.71 17.37 17.67 18.17 20.50
4278.00 10623.00 7069.40 5220 5365.00 8307.00 7984.60
Comparison between different codes parameters.
Code/Parameter
ACI
ECCS
CSA
Euro Code 2
BS-8110
CEB-FIB
Concrete strength Main flexural reinforcements Reinforcements concentration Punching shear reinforcements Column aspect ratio Perimeter to depth ratio Location of critical section Angle between shear reinforcement and main steel
C N N C C C d/2 N
C N N N C N d/2 N
C N N C C C d/2 N
C C C C C C 2d C
C C C C N N 1.5d C
C C C C N C 2d C
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
6
N.Z. Hassan et al. Table 6
Experimental results compared with ACI [6], CSA [8] and BS-8110 code [10].
Specimen ID
S1 S2 S3 S4 S5 S6 S7
Vexp VCode
Vexp VCode
Limited
Unlimited
ACI
CSA
BS-8110
ACI
CSA
BS-8110
1.13 1.15 0.94 0.93 0.94 1.07 0.99
0.98 1.05 0.852 0.84 0.85 0.98 0.897
1.25 – – – – – –
– 1.09 0.89 0.49 0.84 0.31 0.55
– 1.04 0.845 0.475 0.803 0.305 0.537
– 1.11 0.90 0.59 0.99 0.53 0.67
values that were calculated by the code equations [6,8,9] after omitting the material safety factor, Noteworthy that used codes limited the nominal shear strength of concrete and steel individually and for shear reinforced section also. Two comparisons were taken into consideration for the limited value Vexp and for actual unlimited calculated value. For ratios VCode , limited is greater than unity, the code becomes conservative. For a ratio approximately equal to unity, the code is in agreement with the test results. However, for ratios less than unity, the code becomes non-conservative. The ratio between Vexp Vexp Limited and VCode Unlimited for ACI 318-05, CSA VCode
4.
and BS-8110 codes are presented in Table 6 [6,8,10]. Conclusions 5. Through the number of specimens and variables taken into consideration in the present study, the obtained experimental results and the conducted comparisons, the main conclusions can be summarized as follows: 1. Control slab which have left without shear reinforcement failed abruptly in a brittle manner. However the columns did not penetrate the slab completely, whilst specimens which are provided with shear bands failed eventually in typical punching shear failure after reaching the maximum flexural load with less brittle manner outside the shear reinforcement zone. The failure reflects the huge potential of strip reinforcement in preventing failure in the shear reinforcement zone. Furthermore, indicating that the slab reached its most of full flexure potential before failure is supported by strain readings (from high elongation strain gauges) on the flexure reinforcement is excess of 2000 le. 2. Shear bands distributed over the critical punching shear zone provide a very good economic solution regarding increasing the punching shear capacity, ductility and energy absorption. Their efficiency depends on their reinforcement installation method with flexural reinforcement, layout of distribution, concentration and inclination with slab plane. 3. Load capacity is directly proportional with increasing in measure of ductility as an indication for ductility enhancement. Also crack propagation area is proportional with ductility i.e. the higher measure of ductility the larger propagated area of cracks appears over all
6.
7.
8.
9.
specimens outside the shear reinforcement zone. Load capacity enhancement range was between (15–55%), while improvement in measure of ductility range was between (22–79%). It was observed that the failure capacity load increases obviously when shear bands are installed in woven way with the flexure steel mats at specimen S2 than when hanging on tension flexure mat only at Specimen S3. The knitting method provides additional anchorage to reinforcement with the compression zone. All characteristics are improved strongly, load capacity increased by 55%, energy absorption increased by 148%, ductility increased by 79% while the stiffness degradation decreased by 38.5%, S2 is considered the most improved specimen of all specimens absolutely. A slight increase in the failure load capacity of specimen which is reinforced by bended shear bands 45° placed parallel to the potential shear crack compared to specimen reinforced by shear bands with vertical leg., while shear bands with vertical legs easier in detailing, placement and guaranteed to fail in more ductile mode. So, using shear band parallel to the potential shear crack is less useful comparing with vertical leg or perpendicularly to the potential shear crack as proven at other research. Stiffness degradation is used to assess the ductility of the specimen in a way that the lower the stiffness degradation ratio, the higher is the ductility. Also the essential demand of energy absorption is to make the concrete structures have enough ability to dissipate energy during cyclic loading especially earthquake loads. The high energy absorption means that the specimen exhibits more plastic deformation, as the energy absorption proportional to the measure of ductility. We can conclude that all specimens are enhanced compared with the control sample specimen. It reflects significant decrease in stiffness degradation while ranges between (5.2–38.5%) in addition to increase in energy absorption which ranges between (22–148.3%) and measure of ductility ranges between (22–79%). Concentration of shear bands (orthogonal distribution) by doubling quantity of shear bands over the punching zone area leads to efficient system. The slightly enhancement in failure load capacity ranges is 6%. Radial layout distribution exhibits inconsiderable improved punching shear resistance although the large quantity of shear bands which was not beneficial as
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003
Enhancement of punching shear strength of flat slabs using shear-band reinforcement
10.
11.
12.
13.
14.
15.
expected. All characteristics improved slightly, load capacity increased by 15%, energy absorption increased by 22%, measure of ductility increased by 22% while the stiffness degradation decreased by 5.2%. S4 was considered the least improved specimen of all specimens absolutely. The arrangement of shear bands around the column circumference (Box layout distribution) provides awesome enhancement in the failure load capacity which increased by 34% compared with control specimen. Box distribution layout of specimen S6 is more adequate than orthogonal distribution of specimen S3. Specimen S6 allows more shear bands located at the punching perimeter than S3, thus the failure load capacity increased by about 5%. Also, the ductility and energy absorption were increased because of wide propagation of cracks outside the shear reinforcement area. S6 is considered the second-best improved specimen of all specimens absolutely. S6 comes after S2 in descending order of improvement ratio in all characteristics. Regardless the specimen is supplied with or without shear reinforcement, ACI code gives the most appropriate assumptions for calculating the punching shear strength, while BS-8110 code is the most nonconservative code with specimens and need to reconsider its limitations with this specific technique. ACI code sets hard limitations through minimizing the expected shear stress provided by shear reinforcement whatever the used quantity of shear reinforcement. CSA code follow the same concept but with more freely, where nominal shear stress of shear reinforcement calculated using CSA code are more that calculated using ACI code by about 9%. BS-8110 code characterized as the most freely limitation, probably due to the higher critical shear perimeter on distance of 1.5d from column face. It was considered most non-conservative code especially comparing with S4, S6 and S7 where VCode is higher than Vexp. by 68.4%, 88.6% and 49.3% respectively, while the ratios of ACI codes were 7.5%, 6.5% and 1% respectively. And finally CSA code ratios were 19%, 2% and 12.4% respectively. BS-8110 code limitation neither matches the experimental results nor other codes estimations, so it should be revaluated with more experimental studies for bridging the gap between test results and code overestimations.
7
16. Comparisons of the ACI 318, CSA and BS-8110 code predictions, and experimental results, which confirm that the band strips reduces punching shear cracks which enabled the slabs to avoid punching shear failure and achieve their flexural potential.
Conflict of interest The authors declare no conflicts of interest. References [1] Deˆnio R. Oliveira, Guilherme S. Melo, Paul E. Regan, Punching strengths of flat plates with vertical or inclined stirrups, ACI Struct. J. (2000), Title no: 97-S52. [2] Thomas H.-K. Kang, Hong-Gun Park, Performance of shearbands in concrete slab-column connections, ACI Struct. J. 287 (2012) 1–16. [3] Lips Stefan, Ferna´ndez Ruiz Miguel, Muttoni Aurelio, Experimental investigation on punching strength and deformation capacity of shear-reinforced slabs, ACI Struct. J. 109 (2012) 889–900. [4] Centre for Cement and Concrete, CCC Shear band Background Information. University of Sheffield; 2004. Presentation on ‘‘Shear bands verification of a novel punching shear reinforcement system for flat slabs” by Professor Kypors pilakoutas, center for cement and concrete – University of Sheffield. [5] Maurı´ cio P. Ferreira, Guilherme S. Melo, Paul E. Regan, Robert L. Vollum, Punching of reinforced concrete flat slabs with double- headed shear reinforcement, ACI Struct. J. 111 (2014) 363–374. [6] ACI Committee 318. Building Code Requirement for Structural Concrete (ACI-318M-14) and Commentary (ACI 318RM-14). American Concrete Institute, 2014. [7] Egyptian Code Committee ECP 203. Egyptian Code of Practice for Design and Construction of Reinforced Concrete Structures. Building Research Centre, Cairo, Egypt, 2007. [8] CSA-A23.3. Canadian Standard Association. Design of Concrete Structure for Building, 2004. [9] Eurocode2. Design of Concrete Structures. DD ENV-1992-2, 2001. [10] BS-8110. British Standards Institution, London. Structural use of Concrete. Part 1. Code of Practice for Design and Construction, 1997. [11] CEB-FIP Model Code. Model Code for Concrete Structures. Committee Euro-international du Federation International de la Precontrainte, Switzerland, 1990.
Please cite this article in press as: N.Z. Hassan et al., Enhancement of punching shear strength of flat slabs using shear-band reinforcement, HBRC Journal (2017), https://doi.org/10.1016/j.hbrcj.2017.11.003