Alternatives To Enhance Flat Slab Ductility: Mohamed Husain, Ahmed S. Eisa, and Ramy Roshdy
Alternatives To Enhance Flat Slab Ductility: Mohamed Husain, Ahmed S. Eisa, and Ramy Roshdy
Alternatives To Enhance Flat Slab Ductility: Mohamed Husain, Ahmed S. Eisa, and Ramy Roshdy
(Received January 23, 2016, Accepted November 17, 2016, Published online February 28, 2017)
Abstract: Flat slab systems are vastly used in multi-story buildings because of their savings in story height and construction time,
as well as for their flexibility in architectural remodeling. However, they frequently suffer brittle punching-shear failure around
columns, especially when subjected to lateral loads. Therefore, seismic codes labeled flat slabs as non-ductile systems. This
research goal is investigating some construction alternatives to enhance flat slab ductility and deformability. The alternatives are:
adding different types of punching-shear reinforcement, using discreet fibers in concrete mixes, and increasing thickness of slab
around columns. The experimental study included preparation and testing of seven half-scale interior slab-column connections up
to failure. The first specimen is considered a reference, the second two specimens made of concrete mixes with different volumetric
ratios of polymer fibers. Another three specimens reinforced with different types of punching-shear reinforcement, and the last
specimen constructed with drop panel of inverted pyramidal shape. It is found that using the inverted pyramid-shape drop panel of
specimen, increases the punching-shear capacity, and the initial and the post-cracking stiffnesses. The initial elastic stiffnesses are
different for all specimens especially for the slab with closed stirrups where it is experienced the highest initial stiffness compared
to the reference slab.
Keywords: flat slab, punching-shear, stud-rails, ductility, punching reinforcement, fibers.
161
2. Experimental Program
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Table 1 Characteristics and dimensions of the specimens.
Specimen S1 S2 S3 S4 S5 S6 S7
Effect Control Fiber 0.2% Fiber 0.3% Stud-rails Multi-leg stirrups Closed stirrups Drop panel
Thickness (mm) 120 120 120 120 120 120 180
Effective depth 95 95 95 95 95 95 155
(mm)
Top 200
reinforcement
spacing (mm)
Bottom 167
reinforcement
spacing (mm)
Steel studs N/A N/A N/A 60 N/A N/A N/A
spacing (mm)
Closed stirrups N/A N/A N/A N/A N/A 60 N/A
spacing (mm)
Ingredient Cement (kg) Fine aggregate (m3) Coarse aggregate (m3) Water (kg)
350 0.4 0.8 250
International Journal of Concrete Structures and Materials (Vol.11, No.1, March 2017) | 163
Fig. 6 Different specimen configurations (unit: m).
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Table 3 Specimen’s cracking load, failure load, and deflection.
Specimens Cracking load (kN) Failure load (kN) Deflection at failure (mm) Failure mode
S1 20 130 13 Brittle
S2 22 135 15.5 Brittle
S3 23 141 17 Brittle
S4 25 200 22 Ductile
S5 27 220 28 Ductile
S6 30 244 35 Ductile
S7 35 300 – N/A
International Journal of Concrete Structures and Materials (Vol.11, No.1, March 2017) | 165
ultimate load by 230% compared to the control specimens polar moment of inertia. Y is the location where the maxi-
(Fig. 8). mum shear stress is calculated. More details could be found
in the ACI 421.1R.
3.2 Crack Pattern While the nominal shear strength in slabs S1, and S7,
The cracking load Pcr and the mid-span deflection D where shear reinforcement is not provided, were taken the
corresponding to the cracking load of the specimens are smallest value of Eqs. 4–7b, 4–8b, and 4–9b in ACI421.1R.
given in Table 4. In regard to the Pcr values reported in The shear strength of slabs with stirrups (S5 and S6) is
Table 4, it is concluded that the cracking load of the control calculated based on Eq. 4–11 in ACI 421.1R, and finally the
slab without fibers, S1 is around 9–10% less than the
cracking load of the slabs with fibers. This can be attributed
partly to shrinkage induced cracks in the non-symmetrically
reinforced sections; however, the uniform distribution of
fibers in slabs S2 and S3 can lessen the shrinkage induced
cracks.
The initiation and development of first cracking at mid-
point of the control slab and the slabs with fibers of 0.2 and
0.3% by volume (S2 and S3) took place at deflection Dcr of
0.5–0.6 mm, respectively (Table 4). The development of
cracks at mid-span is associated with a small reduction in the
load and a load drop followed the onset of first cracking is a
characteristic of reinforced concrete members tested. The
cracking load increased with the addition of the studs, stirrup
legs and the closed stirrups by 25, 35, and 50%, respectively
compared to the control specimen. It was seen that the
maximum cracking load recorded from all the specimens
tested was related to the addition if the pyramidal drop panel
shape, where the cracking load reached 35 kN by an increase
equal 75% compared to the control slab with and enhance-
ment of the cracking deflection of 60%. The crack patterns
of specimens S1, S4, and S5 are shown in Fig. 9. Specimen
S7 did not reach failure due to lab limitations (available load
capacity = 300 kN).
For measured shear stress (at maximum applied load Pu
recorded during the tests). The shear stress for all concrete
slabs was predicted by the shear equation provided by ACI
421.1R as in the following:
V u cvx Mux y
vu ¼ þ : ð1Þ
Ac Jc
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Table 5 Ductility ratio of all specimens.
Slab Dy (mm) Du (mm) lD = Du/Dy
S1 10.5 13 1.238
S2 7.5 15.5 2.067
S3 8 17 2.125
S4 7.2 22 3.056
S5 6.4 28 4.375
S6 9 35 3.888
S7 14.5 – N/A
3.2.1 Example Slab S4 (Shear Studs Provided) specimen at yield point, Pu and Du: load and deflection of the
specimen at yield point at failure. The following equations
200;000 cos 20
vu ðEq:1Þ ¼ are used to define the cracking and yield points from the
2ð295 95Þ þ 2ð295 þ 95Þ steel strain readings.
0:4 200;000 sin 20 600 295=2
þ 953 295 3
þ 295 695 þ 295295
2 95
d n fctr
6 2 eConcrete ¼ eSteel ¼ ¼
¼ 4:61 N=mm2 ESteel ESteel
pffiffiffiffiffiffiffiffi
10 1:9 320
Column dimensions (c) = 200 9 200 mm2. eSteel ¼ ¼ 150 106 ¼ 150 le
The critical section perimeter = c ? d = 200 ? 95 = 2 106
295 mm. dy 3600
Column height = 600 mm. ey ¼ ¼ ¼ 1800 106 ¼ 1800 le
ESteel 2 106
Pu = 200,000 N.
The nominal shear strength is calculated based on the Table 4 shows the shear strength resisted by concrete for
following: all the slabs except S2, and S3. The predicted values based
pffiffiffi0ffi on the ACI 421-1R (2008) and it is noticed that the per-
fc centages of actual/predicted shear strength are varied. The
vc ¼ ¼ 1:4 N/mm2
4 closest prediction was for slab S7 where the actual-to-
Av fy 8 78:5 360 predicted shear strength was 0.96 and the worst prediction
vs ¼ ¼ was for slab S4 where the percentage of difference was
S b0 60 ð2 295 þ 2 295Þ
¼ 2:79 N/mm2 0.65.
International Journal of Concrete Structures and Materials (Vol.11, No.1, March 2017) | 167
Table 6 EAI and stiffness of all slab specimens.
Slab A1 A2 A1 ? A2 EAI Ki Ks
S1 5.88 3.25 9.13 1.55 40.0 10.7
S2 4.5 10.8 15.3 3.40 44.0 16.0
S3 4.8 12.69 17.49 3.64 38.3 15.0
S4 4.97 29.6 34.57 6.96 35.7 19.2
S5 4.48 47.52 52 11.61 45.0 21.9
S6 6.53 63.44 69.97 10.72 50.0 16.1
S7 13.41 NA NA NA 43.8 12.8
A1 þ A2
EAI ¼ ð2Þ
A1
168 | International Journal of Concrete Structures and Materials (Vol.11, No.1, March 2017)
control specimen S1. Specimens S4 and S6 exhibited stirrups reinforcements. ACI Structural Journal, 107(5),
energy absorption about 3.8, and 7 times of the value of S1. 597–606.
6. Finally, the following conclusions on ductility shall be Dovich, L., & Wight, J. K. (1996). Lateral response of older flat
emphasized: The use of fiber concrete has increased the slab frames and the economic effect on retrofit. Earthquake
post-crack stiffness only with no ductility, deformability, Spectra, 12(4), 667–691.
or energy dissipation enhancements. Eurocode 8—EC8. (2004). Design provision for earthquake
7. Good ductility enhancements obtained by using multi- resistance of structures. EN 1998-1-1.
leg and closed stirrups as punching-shear reinforce- Graf, W. P., & Mehrain, M. (1992). Analysis and testing of a flat
ments, even better than the ductility of the famous stud- slab concrete building. In World conference on earthquake
rail reinforcement. engineering, Rotter Dam, ISBN90 54100605.
Hawkins, N. M. (1974). Shear strength of slabs with shear
reinforcement. ACI Publication, 42(34), 785–815.
ICC, International Building Code—IBC. (2009). Birmingham,
Open Access
AL: International Code Council.
Matzke, E. M., Lequesne, R. D., Parra-Montesinos, G. J., &
This article is distributed under the terms of the Creative
Shield, C. K. (2015). Behavior of biaxially loaded slab-
Commons Attribution 4.0 International License (http:/
column connections with shear studs. ACI Structural
/creativecommons.org/licenses/by/4.0/), which permits un-
Journal, 112(3), 335.
restricted use, distribution, and reproduction in any medium,
Meisami, M., Mostofinejad, D., & Nakamura, H. (2013).
provided you give appropriate credit to the original author(s)
Punching shear strengthening of two-way flat slabs with
and the source, provide a link to the Creative Commons
CFRP grids. Journal of Composites for Construction,
license, and indicate if changes were made.
18(2). doi:10.1061/(ASCE)CC.1943-5614.0000443.
Oliveira, D. R., Melo, G. S., & Regan, P. E. (2000). Punching
strengths of flat plates with vertical or inclined stirrups. ACI
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