such as a widening of the slab support with **concrete** mushrooms or steel heads (Martinez- Cruzado et al. 1994; Hassanzadeh 1996). In addition, the bending **resistance** of the slab can be increased with an external reinforcement made of steel or new materials like fiber-**reinforced** polymer (FRP) composites (Harajli and Soudki 2003; Ebead and Marzouk 2004; Chen and Li 2005; Esfahani et al. 2009; El-Enein et al. 2014). For both concepts the behavior of the slab remains brittle however. A third possibility is the installation of additional shear reinforcement, which normally increases ductility; examples are shear studs (Menétrey and Brühwiler 1997; El-Salakawy et al. 2003; Adetifa and Polak 2005; Fernández Ruiz et al. 2010; El-Shafiey and Atta 2011; Feix et al. 2012; Polak and Bu 2013), drop panels (Martinez-Cruzado et al. 1994; Ebead and Marzouk 2002), FRP shear bolts (Lawler and Polak 2011; Meisami et al. 2013) or stirrup solutions with FRP laminates (Binici and Bayrak 2005a; Sissakis and Sheikh 2007). In some cases, the elements are prestressed for immediate unloading of the slab (Menétrey and Brühwiler 1997; El-Shafiey and Atta 2011). However, in the case of prestressed bolts, due to their short length, even small deformations caused by creep for example can substantially decrease the designated prestressing force. Faria et al. (2009, 2011) strengthened square **concrete** **slabs** with a side length of 2.3 m and 100- or 120-mm thickness with prestressed steel strands. These were placed above the column on the upper **concrete** surface and anchored on both sides in the bottom part of the slab, in holes drilled at an inclination of ca. 11°, using an epoxy adhesive. The **punching** **resistance** was increased by 34–48% for **slabs** with two strands in one direction only and by 61% for one slab with strands in both directions. The prestressing method also improved the serviceability limit state and the post-collapse behavior where a second peak load at 78% of the first peak load

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Premature loading of **reinforced** **concrete** flat **slabs** during construction generally occurs because of the efforts to meet project time targets (Ding et al., 2009). Hongyan, 2015, reported that the loads applied on the partially completed structure due to the construction process could be larger than the design service load. This construction load may exceed the design loads, which in turn, led to early failure of **slabs** . Wood (2003) reported that the available strength of the immature partially completed structure is dependent upon the **concrete** strength in those members, which may be less than the specified strength, and the failure would occur if the available strength were less than that required to support the construction loads. Premature failure of such **slabs** is generally associated with a concentration of high shear forces and bending moments at the column peripheries (Rizk et al., 2011). This type of failure is catastrophic because there are no external visible signs prior to the occurrence of the failure (ACI SP-232, 2005). When a slab is loaded prematurely, its serviceability is compromised (RILEM Committee 42-CEA, 1981). Therefore, it is necessary to investigate the **effect** of premature loading on **reinforced** **concrete** **slabs** to avoid cracking and possible failure (Hongyan, 2015). Hawkins et al., 1974; Gardner, 1990; Abdel Hafez, 2005; Wood, 2003; Sagaseta et al., 2014; Rankin and Long, 2019, reported that insufficient early-age **punching** shear capacity under relatively high construction loads is one of the common reasons of failure of flat slab structures during construction. They also reported that **punching** shear failure is caused by the failure of **concrete** in tension. Figure 1 shows real case studies for the collapse of a factory building (Vetogate, 2014) and a residential building (Elshorouk City Website, 2019) in Egypt as a result of premature loading of **reinforced** **concrete** flat slab. Sudden **punching** failure took place during the **concrete** casting process of second floor. The consultant reported that the low strength of **concrete** at the time of early removal of the first-floor formwork was the main reason for the building collapse.

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or compensate the loss of **punching** **resistance**. Ashraf Mohamed (2015) carried a parametric study to look at the variables that can mainly affect the mechanical behaviours of the model such as the change of loading tyres and positions and slab with openings. Good correlation is observed between the results of the proposed model and other experimental one, resulting in its capability of capturing the fracture of flat slab under **punching** shear behaviour to an acceptable accuracy. M.Fernandez Ruiz et.al., (2015) reviewed the various potential shear transfer actions in **reinforced** **concrete** beams with rectangular cross-section and discusses on their role, governing parameters and the influences that the size and level of **deformation** may exhibit on them. This is performed by means of an analytical in integration of the stresses developed at the critical shear crack and accounting for the member kinematics. The results according to this analysis are discussed, leading to a number of conclusions. Finally, the resulting shear strength criteria are compared and related to the Critical Shear Crack Theory. This comparison show the latter to be physically consistent, accounting for the governing mechanical parameters and leading to a smooth transition between limit analysis and Linear Elastic Fracture Mechanics in agreement to the size-**effect** law provided by earlier researcher. Ahmed Ibrahim et.al., (2015) proposed a new alternative of reinforcement, the introduction of rebar mesh at the middle of flat plate thickness covering the **punching** zone and anchored outside this zone. Nevertheless, in their investigation, the proposed reinforcement system is examined for interior columns only. An experimental work consisting of eight specimens, of normal and high strength **concrete**, and an expanded analytical work using

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Test X5 without transverse reinforcement was carried out with the objective of obtaining more accurate knowledge of the **concrete** proportion of the **punching** capacity upon impact of a deformable projectile. The plastic **deformation** **resistance** of the projectile dependent on the tube wall thickness and the impact velocity were determined correctly on the basis of empirical formulas, so that in the experiment the intended formation of a **punching** cone with activation of the dowel **effect** of the bending reinforcement was achieved without a perforation of the slab. The objective of the three tests X6 to X8 was to investigate the influence of different types of shear reinforcement on the **punching** capacity. The selection of the shear reinforcement cross-section 34.9 cm²/m² has ensured that the load-bearing capacity was almost reached in all three tests, see Figure 1. The types of shear reinforcement were closed stirrups (X6), T-headed bars (X7), and C-shaped stirrups (X8). The objective of the two further tests X9 and X10 was to assess the **effect** of different bending reinforcement ratios on the interaction of bending and **punching** behaviour, which was realised by changing the bar diameter of the longitudinal reinforcement.

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A series of the elements tested by Swamy and Ali [5] were selected to gain a better understanding of the predicted **effect** of the steel ﬁbre volume on the **punching** shear strength of slab– column connections. The selected test elements had identical geometrical dimensions and ﬁbre type and the **concrete** compres- sive strength was similar for all batches. The failure criteria and the estimated load rotation relationships for the elements selected are presented in Fig. 9 a. As is shown in Fig. 9 b, the proposed model predicts well the increase of the **punching** shear strength with the increase of the ﬁbre volume. Furthermore, an increase in the quantity of similar ﬁbres provides for an increase in the deforma- tional capacity. As the **concrete** contribution to the **punching** strength decreases with the increase of the slab rotation, the weight of the ﬁbre contribution becomes more relevant to the **resistance** mechanism. The predicted **concrete** and ﬁbres contribu- tions to the **punching** shear strength for elements tested by Swamy and Ali ( [5] are shown in Fig. 9 b, with an excellent correlation ob- served between the model predictions and the test results. 5. Code-like formulation

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This paper presents an analytical model based on the Critical Shear Crack Theory which can be applied to ﬂ at **slabs** subjected to impact loading. This model is particularly useful for cases such as progressive collapse analysis and ﬂat slab-column connections subjected to an impulsive axial load in the column. The novelty of the approach is that it considers (a) the dynamic **punching** shear capacity and (b) the dynamic shear demand, both in terms of the slab **deformation** (slab rotation). The model considers in- ertial effects and material strain-rate effects although it is shown that the former has a more signiﬁcant **effect**. Moreover, the model allows a further physical understanding of the phenomena and it can be applied to different cases (**slabs** with and without transverse reinforcement) showing a good correlation with experimental data.

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Complementary numerical results are computed also with an in-house finite element program. The used Bogner-Fox-Schmit (BFS) plate element has 16 degrees of freedom (DOF), Bogner et al. (1965). The nodal degrees of freedom are:. ݓǡ ݓǡ ௫ ǡ ݓǡ ௬ ǡ ݓǡ ௫௬ (twist). The BFS element is based on the Kirchhoff plate theory, which does not consider the transverse shear **deformation**. The Reissner-Mindlin plate theory, RM-theory, includes the **effect** of transverse shear **deformation**. A four-noded 12 DOF element with nodal degrees of freedom ݓǡ ߠ ௫ ǡ ߠ ௬ (deflection and rotations) is used here. For transverse shear **deformation** special interpolation is needed for stable numerical behaviour, Bathe and Dvorkin (1985).

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dimensions. The length of fibres could vary from 25mm to 75mm and the diameter from 0.5mm to 1.3mm. Different fibre shapes are illustrated in Figure 1. The addition of fibres to **concrete** has shown improvement in **concrete** flexural strength, toughness, ductility, impact **resistance**, fatigue strength and **resistance** to cracking. In addition the **deformation** at peak stress is much greater than plain mortar. Fibres help to alter the behaviour of **concrete** after the initiation of cracking. The crack bridging behaviour of fibres is what improves the ductility of matrix. The main advantageous property of SFRC is its superior **resistance** to cracking and crack propagation. The fibres are able to hold the matrix together even after extensive cracking due to its bridging **effect**. SFRC has the ability to arrest cracks; therefore fibre composites retain increased extensibility and tensile strength, both at first crack and at ultimate stress. The net result this is the fibre composite will have a marked post-cracking behaviour and ductility which is unremarked in ordinary **concrete** in which the tension post crack is negligible. The material is therefore transformed from a brittle to a ductile type of material which would increase substantially the energy absorption characteristics of the fibre composite and its ability to withstand repeating applied load such as shock or impact loads.

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ABSTRACT: This paper aims to examine the **punching** shear **resistance** of **reinforced** **concrete** flat **slabs** with shear heads. The ACI 318-M (2005), allowed the arrangements of shear heads as one possible alternative of **punching** shear reinforcement. In this study, seven half- scale **reinforced** **concrete** flat **slabs** divided into two groups were casted and tested. The first group deals with testing three specimens of flat **slabs** connected with square columns, one specimen without any shear head and the other two specimens **reinforced** by steel shear head sections with lengths equal to 1.75h and 2.25h, respectively from column face. The second group deals with four specimens of flat **slabs** connected with rectangular columns. one specimen without any shear head and the other three specimens **reinforced** by steel shear head sections with length equal to 1.75h with two cut ends at angles 90 and 45 degrees, and the last one with length equal to 2.25h. All specimens were loaded until failure. The first **punching** crack load, ultimate load, **deformation**, **punching** perimeter, strain in steel bars, strains in steel shear head sections and failure mechanisms of each specimen were generated and analyzed. The results show that shear head reinforcement moves **punching** perimeter away from column face and accordingly increase **punching** shear capacity of flat **slabs**. Based on the experimental results, a simple process has been proposed to predict the improvement of load carrying capacity and **punching** failure load of flat **slabs** with shear heads and compared these results with different code values. The theoretical prediction values were then compared with the experimental data and good agreement was found.

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Using a seven-storey steel frame grillage model and the impact of a failed floor as an independent event, they established the pseudo-static response of the floor based on the estimated energy transfer associated with the specific characteristic of the impact event. The analytical procedure started with calculating the anticipated range of kinetic energy transferred from an impacting floor to the floor below based on the nonlinear static response approach. Static load-**deformation** curves and dynamic demand for the impacted floor was established at this stage. The least demanding impact scenario was taken as the impact from a falling floor that carried half of the impacted floor gravity load and only 20% kinetic energy being transferred. Later, the linear static load-**deformation** response was modified using the pseudo-static response to account for the **effect** of the initial deformations of the lower floor under the gravity load. The capacity of the impacted floor based on the calculated kinetic energy transfer was then established. The results from the analytical simulation showed that within all the impact scenarios being considered, the ratio of the impacted floor capacity/demand never exceeded one. They concluded that a floor system within a steel- framed composite building has limited opportunity to arrest the impact from an upper floor even in the least demanding impact scenario where the capacity only marginally exceeds half its dynamic demand.

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(FRP bars have a density ranging from 1/6 to 1/4 that of steel (ACI Committee 440 2015)), decreasing the cost of handling and transportation and high specific strength (tensile strength of FRP approximately two to three times of that of steel). In addition, FRP has good corrosion **resistance**, improved thermal insulation and low thermal expansion. However, the behaviour of FRP bars varies from that of steel in some aspects. For example, FRP bars don’t show ductile behaviour in RC structures, FRP bars have perfectly linear-elastic behaviour until failure without a yielding point. Moreover, FRP bars have a relatively lower modulus of elasticity compared with that of steel (FRP modulus of elasticity is about 1/4 or 1/3 that of steel). Furthermore, FRP bars have different bond characteristics to steel bars, for example, sand-coated GFRP bars have adhesion and friction bond which homogeneously distribute the bond stresses along the embedded length of the bar, whereas, the deformed steel bars have a mechanical bond through bearing on the **deformation** parts of the steel bars. Therefore, GFRP bar- **reinforced** **concrete** structures exhibit lower average crack spacings than those of steel bar-**reinforced** **concrete** structures.

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Ospina et al. (2003) [3] reported that the behavior of an FRP-RC slab-column connection is affected by the elastic stiffness of the reinforcing material as well as the quality of its bond characteristics with the **concrete**. However, the FRP grids may not provide the same **punching**-shear capacity as the FRP bars owing to the difference in bond behaviour and concentration of stresses in the grids. Nguyen- Minh and Rovnak [5] concluded that both the size factor and the **effect** of the span-to-effective-depth ratio (L=d) should be taken into account in computing the **punching**-shear **resistance** of the FRP-RC slab-column connections. Zhang et al. [7] reported that the reinforcement type significantly influenced the **punching** strength of **slabs** and the **concrete** strength significantly affeced the load carrying capacity and the post-**punching** capacity of **slabs**. However, it was found to be a little influence on the stiffness of the cracked **slabs**.

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The expressions for n and ϒ describing the tension-stiffening curve has so far been calibrated to only one value of flexural reinforcement ratio of 1.2% that was used in specimens 30U, 35U, 55U and 65U. The expressions in Equations (5-1) and (5-3) have to be expanded to account for the enhanced tension stiffening generated from the concentration of flexural reinforcement around the column zone. McHarg et al [19] and Lee et al [20] had demonstrated that doubling the flexural reinforcing ratio by concentrating the top mat of steel over the column zone resulted in a higher **punching** shear **resistance** and a higher post-cracking stiffness. It will be demonstrated that in an FEA model it is not sufficient to just increase the number of 2D steel reinforcing bars, but rather, it is also very important that the interaction of the steel and **concrete** be modeled as well. This interaction is simulated by adjusting the tensile stress-strain values to account for the tension-stiffening **effect**. To reinforce this point, Figure 5-14 shows the **effect** of applying the same tension stiffening properties used to model specimen 30U, in the previous section, to model slab specimen 30B [20] which has double the flexural reinforcing ratio. The resulting load-deflection curve, using n = 0.4 and ϒ= 100, did not add enough tension stiffening to the slab and as a result the load-deflection response was well below the targeted curve from the experimental data of 30B. This shows that the Equations (5-1) and (5-3) must be re-calibrated for n and ϒ to reflect the change in the flexural reinforcement ratio parameter.

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4. The collected test results show that most of the existing for- mulas gave inaccurate results with a large scatter in com- parison with the testing results, and thus, a new formula or technique should be proposed for more accurate estima- tion of **punching** shear **resistance** of FRP-**reinforced** **slabs**. This paper provides the designer with a reliable and accu- rate design tool for estimating the **punching** shear strength of two way **slabs** **reinforced** with FRP bars or grids. Two approaches are presented; the ﬁrst is the proposed equation and the second is the Neural Networks Technique. Each of them contains two new parameters, never used before; the effects of the elastic stiffness of the FRP reinforcement and the continuity **effect** of **slabs** on **punching** capacity as explained previously.

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DOI: 10.4236/ojce.2018.81001 10 Open Journal of Civil Engineering reached, the stiffness of the analyzed **slabs** deviate from each other but not sig- nificantly. However, all **slabs** act much stiffer than the one without shear rein- forcement. The highest **punching** shear capacity and effectiveness is provided by the bent bars, since these bars are perpendicular to the plane of the cracks, and they are efficiently anchored in the **concrete**. The second largest **punching** shear capacity is provided by the cages of stirrups, thanks to the efficient anchorage provided by the spatial steel bar arrangement. Despite its good general judgment (because of its effectiveness and easy mounting), shear studs provide the least **punching** shear **resistance** among the tested configurations, which is mainly caused by their small length and limited anchorage capabilities. The smallest **deformation** capability belongs to the slab with steel sections, while the largest deflections belong to **slabs** BB-3 and CS-3. Slab BB-3 contains only 40% of the amount of reinforcement applied in slab CS-3. Due to this fact bent bars are the most efficient type of shear reinforcement in the aspect of load carrying capacity and **deformation** capability as well.

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After the war in Kosovo buildings are often constructed using **reinforced** **concrete** flat **slabs** with no beams and no enlarged column heads combined with punctual supports such as columns of varying cross section and slenderness. The advantages of flat **slabs** are easy solution of architecture design that enables flexibility in the movement of non-structural walls in the desired position, easy placement of equipment, and installation underneath the slab. But these **slabs** are subjected to **punching** shear failure of slab-column connections. Load concentration around the column head generally leads to increased stresses which cannot be absorbed solely in thin slab thicknesses.

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Peeling failure often occurs at the ends of the FRP plate where there is a discontinuity as a result of the abrupt termination of the plate. It is normally associated with concentrated shear and normal stresses in the adhesive layer due to the FRP **deformation** that takes place under load. The magnitude of these stresses is influenced by various factors including the dimensions of the FRP plate, the mismatch in the modulus of elasticity of the FRP and the adhesive, and the shape of the bending moment diagram. Peeling failure usually results in ripping off the **concrete** cover along the level of the internal steel reinforcement, towards the centre of the member.

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Figure 12 shows both radial and tangential stresses ( σ rad and σ tang ) for the continuous flat slab with **concrete** shrinkage. The most interesting strain limits are marked as follows: cracking of **concrete** (corresponding to ε cr , see also Figure 12(a)); maximum crack opening with residual tensile strength (corres- ponding to w ctu , see Figure 12(a), calculated as w ctu = ε t,u a m , where a m is the distance between cracks; see Belletti et al. (2017)); and yielding strain of reinforcement ( ε sy ). It can be seen from Figures 12(b) –12(d), that the peak value of radial stresses σ rad corresponds to the achievement of the residual tensile strength of **concrete** (crack opening w ctu ) in the sagging area, which corresponds to the maximum ring **effect** for self- confined **slabs**. Depending on the intersection with the CSCT failure criterion, **punching** shear failure occurs before yielding of hogging and sagging reinforcement for high reinforcement ratio ( ρ hogg = 1·5%), after yielding of hogging reinforcement for medium reinforcement ratio ( ρ hogg = 0·75%) and after yielding of hogging and sagging reinforcement for low reinforcement ratio ( ρ hogg = 0·375%). Even if not reported in the present study, it is important to observe that the sequence of events remains the same without considering shrinkage of **concrete**.

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Hence in line with current trend of using a structural system which brings economic feasibility, speed and versatility of application, the use of waffle slab is becoming an attractive structural solution. This structural system can be defined as the constructions having a system of a flat flange plate, or deck, and equally spaced parallel beams, or grillage, that may be arranged in either an orthogonal or non-orthogonal assembly with monolithic intersections [10]. They are also known as two-way ribbed flat slab and being used increasingly in modern construction to reduce dead weight. The system exhibits higher stiffness and smaller defections. However, the most common types have large square voids or recesses between the ribs. Not only the normal **reinforced** **concrete** waffled slab has benefits over the normal **reinforced** solid flat plate, but also a pre- stressed waffle-type bridge is found to be much more efficient in carrying load than a pre-stressed bridge with constant thickness slab as presented by Kennedy [2]. Kennedy [1] studied the **effect** of orientation of rib in the load carrying capacity of waffled slab. His results indicated that the orthogonal shaped waffle slab has a superior ultimate load carrying capacity of 20% higher than the non- orthogonal (45°) waffle slab. Abdul-Wahab and Khalil [6] investigated experimentally the response of simply supported, isotropically **reinforced**, square waffle **slabs** under a midpoint patch load.

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Crack widths, calculated using the methods proposed by FIB model code and RILEM TC-162-TDF (2003), were evaluated. Spanish EHE-08 does not have any verification in serviceability limit state, regarding fibre **reinforced** **concrete** elements, and therefore, no crack width evaluation was possible. The results obtained from analysis, according to FIB model code, proved that addition of fibres has a positive **effect** on the crack width, as it decreased with increasing fibre volume. The results from analysis, according RILEM TC-162-TDF (2003), however showed that, fibres have a negative impact on the crack width as it increased with increasing fibre fractions. The reason for this unanticipated outcome is that, RILEM TC-162-TDF (2003) only considered the fibres slenderness ratios in the final crack spacing design formula, implying that the amount of fibres had no **effect**. And since the **concrete** tensile strength decreased with increasing fibre volume, such results were obtained.

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