Punching Shear in Flatslab

ABSTRACT Despite more than a century of continuous effort, shear still remains one of the few areas of research into fundamentals of the behavior of concrete structures where dispute remains amongst researchers about the mechanisms that enable the force flow through a concrete member and across cracks. Due to our incomplete understanding and the brittle failure modes associated with shear, current codes of practice tend to propose highly conservative shear design provisions. This paper investigates the shear capacity and mechanical behavior of prestressed concrete beams. The results of 9 full-scale I-shaped prestressed concrete beams subjected to a four-point bending test until failure are presented. Two stereo-vision digital image correlation (DIC) systems were used to discretely measure three-dimensional displacements and in-plane deformations in both zones where a shear force exists. A numerical technique has been adopted to generate optimized patterns for DIC and the resulting speckle pattern was applied onto each specimen using a stencil printing technique. Using the sectional shear design procedure found in Eurocode 2, a severe underestimation of the experimentally observed shear capacity was found. Direct strut action was the main bearing mechanism of the reported prestressed concrete members with shear reinforcement whereas the presented specimens without shear reinforcement failed when the principal tensile stress reached the tensile strength limit.

This paper presents results of tests on eight reinforced concrete slab-column connections without and with shear reinforcement under
monotonic concentric and eccentric loading. The slabs were square, with sides of 2.5 m, 180 mm thick, and were centrally supported on square column stubs with sides of 300 mm. The slabs were cast with concrete with a mean compressive strength of 49 MPa, with constant flexural reinforcement ratio (1.5%) and with different amounts of double-headed studs as shear reinforcement. All the slabs failed by punching shear and their flexural and shear responses, as well as their ultimate punching strengths, are described and discussed by comparing the experimental observations with theoretical results obtained following the recommendations presented by ACI 318, Eurocode 2 and fib Model Code 2010.

The long-standing problem of shear in structural concrete elements is typically studied as one-way shear in beams, or as
two-way shear in slab–column connections. The problem of one-way shear in slabs, as encountered by bridge engineers
when considering the concentrated live loads on slab bridges, is not described by codes. This paper reviews the literature
regarding one-way slabs and wide beams failing in shear. The mechanisms of shear transfer and the existing models and
code methods for one-way and two-way shear are reviewed. Subsequently, the similarities, differences and the
transition zone between one-way and two-way shear, and the models representing these failure modes, are studied.
This overview of the literature highlights that knowledge of one-way shear is limited to the comparison with small,
heavily reinforced slender beams and of two-way shear to the comparison with slab–column connections. The transition
zone between these two failure modes, which often occurs in structural concrete elements used in practice, is typically
not studied. Possible solutions for the transition zone between the two failure modes are listed.

Twelve slabs, 11 of which contained double-headed studs as shear reinforcement, were tested supported by central column and loaded concentrically. Their behavior is described in terms of deflections, rotations, strains of the concrete close to the column, strains of the flexural reinforcement across the slab width, and strains of the studs. All failures were by punching, in most cases within the shear reinforced region. The treatments of punching resistance in ACI 318, Eurocode 2 (EC2), and the critical shear crack theory (CSCT) are described, and their predictions are compared with the results of the present tests and 39 others from the literature. The accuracy of predictions improves from ACI 318 to EC2 to CSCT—that is, with increasing complexity. However, the CSCT assumptions about behavior are not well supported by the experimental observations.

Twelve slabs, 11 of which contained double-headed studs as shear reinforcement, were tested supported by central column and loaded concentrically. Their behavior is described in terms of deflections, rotations, strains of the concrete close to the column, strains of the flexural reinforcement across the slab width, and strains of the studs. All failures were by punching, in most cases within the shear reinforced region. The treatments of punching resistance in ACI 318, Eurocode 2 (EC2), and the critical shear crack theory (CSCT) are described, and their predictions are compared with the results of the present tests and 39 others from the literature. The accuracy of predictions improves from ACI 318 to EC2 to CSCT—that is, with increasing complexity. However, the CSCT assumptions about behavior are not well supported by the experimental observations.

It is known that the Greek codes allow flat-slab structural systems, however they provide specific compliance criteria. In the present study an extended parametric investigation was carried out in order to identify the seismic response of structural systems consisting of α) slabs-columns b) columns-perimetric beams c) columnsshear walls-slabs d) columns-shear walls-slabs and perimetric beams. The aforementioned systems were studied for all possible storey heights in Greece by means of F.E.M. Code SAP2000 ver.9. The compliance criteria provided by the Greek Code for earthquake resistance are related to second order effects, torsional flexibility, capacity design and the sensitivity of masonry infill. Conclusions were extracted concerning the number of storeys which can be applied to each case.

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It is known that the Greek codes allow flat-slab structural systems, however they provide specific compliance criteria. In the present study an extended parametric investigation was carried out in order to identify the seismic response of structural systems consisting of α) slabs-columns b) columns-perimetric beams c) columns-shear walls-slabs d) columns-shear walls-slabs and perimetric beams. The aforementioned systems were studied for all possible storey heights in Greece by means of F.E.M. Code SAP2000 ver.9. The compliance criteria provided by the Greek Code for earthquake resistance are related to second order effects, torsional flexibility, capacity design and the sensitivity of masonry infill. Conclusions were extracted concerning the number of storeys which can be applied to each case.

It is known that the Greek codes allow flat-slab structural systems, however they provide specific compliance criteria. In the present study an extended parametric investigation was carried out in order to identify the seismic response of structural systems consisting of a) slabs-columns b) columns-perimetric beams c) columns-shear walls-slabs d) columns-shear walls-slabs and perimetric beams. The afore-mentioned systems were studied for all possible storey heights in Greece by means of F.E.M. Code SAP2000 ver.9. The compliance criteria provided by the Greek Code for earthquake resistance are related to second order effects, torsional flexibility , capacity design and the sensitivity of masonry infill. Conclusions were extracted concerning the number of storeys which can be applied to each case.

Flat slab is reinforced concrete plate without beam. The benefits are the height per floor can be reduced, lowering structural load, less ceiling, simpler reinforcement, simple and economical scaffolding and formwork installation when constructing a building. This paper presents redesign study of the structure of Awana Condotel Yogyakarta using flat slab method in moderate earthquake zone. The Special Moment Resisting Frame (SRPMK) was obtained by manual calculation and ETABS 2016. Parameters to be found were the dimension and reinforcement of floor plate, roof plate, column, and drop panel, as well as deflection of the plates. The results of analysis and calculation of structural dimension using flat slab method showed that the thickness of the floor plate is 210 mm, the thickness of the roof plate 150 mm, the thickness of drop panel 300 mm and the width of drop panel 2000 mm both for x and y, and the dimension of column 1 is 800 mm x 800 mm and column 2 400 x 400 mm. The deflections of the plates are 0.17 mm for floor plate and 0.30 mm for roof plate. Analysis by ETABS 2016 showed that the building structure with (T) period was 1.04 seconds.

This paper presents the results of tests on slabs with punching shear reinforcement formed by prefabricated truss assemblies disconnected from the flexural reinforcement, developed to optimize the construction process. General guidelines for design, detailing, and assembly of this reinforcement are presented, based on the recommendations presented by ACI 318, Eurocode 2, and fib Model Code 2010. One slab without shear reinforcement and one slab with stud rails were tested as a reference for tests on two slabs with the prefabricated truss modules as punching shear reinforcement. The load-rotation response and the failure loads are presented and discussed in comparison with theoretical estimates obtained following the design codes. The profile of stress and strain distribution on the flexural and shear reinforcement are also presented and discussed. The slab with inclined prefabricated truss bars registered similar response and resistance compared to the one with stud rails. These tests confirm the effectiveness of this kind of punching shear reinforcement, as one of the slabs had almost twice the strength and more than three times the deformation capacity of the reference slab without shear reinforcement.

Strut and tie semi analytical model has been developed to reproduce flow of the forces at the failure in the symmetrically loaded test specimen like flat circular slabs, without shear reinforcement, supported at the central column. Conditions of all types of failure, bending, shear and shear-bending, are determined for all ratios of influencing parameters. Pure punching failure happens when ultimate combination of biaxial compression–tension stress is reached at the upper point of punching cone. Shear-bending failure is characterized by tri-axial concrete compression failure at the point where bottom surface of the slab meets the column, where compression stresses are caused by both web and compression chord internal forces. And finally, bending failure occurs due to achievement of ultimate strain in tensile chord representing reinforcement. For punching failure, failure cone is initiated from the top surface of the slab and it at failure load opens suddenly downward toward the point where bottom surface of the slab meets the column. Punching cone forms along surface of highest principal stresses in the web of the slab and it is not extension of other flexural-shear cracks, which start to develop from earlier load stages.

Reinforced concrete (RC) slabs are characterised by reduced construction time, versatility, and easier space partitioning. Their structural behaviour is not straightforward and, specifically, punching shear strength is a current research topic. In this study an experimental database of 113 RC slabs without shear reinforcement under punching loads was compiled using data available in the literature. A sensitivity analysis of the parameters involved in the punching shear strength assessment was conducted, which highlighted the importance of the flexural reinforcement that are not typically considered for punching shear strength. After a discussion of the current international standards, a new proposed model for punching shear strength and rotation of RC slabs without shear reinforcement was discussed. It was based on a simplified load-rotation curve and new failure criteria that takes into account the flexural reinforcement effects. This experimental database was used to validate the approaches of the current international standards as well as the new proposed model. The latter proved to be a potentially useful design tool.

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The experimental program of this study consisted of 12 slab specimens. The parameters investigated include ratio of reinforcement in compression and tension, amount of shear reinforcement and arrangement of shear reinforcement. Flexural reinforcement ratio especially in tension had a noticeable effect on the mode of failure and ultimate punching capacity of slabs. The enhancement in the ultimate loads due to increasing tensile reinforcement ratio was ranging between 26.0 and 42.0%. Slightly enhancement (up to 12%) in ultimate loads was observed as a result of increasing compressive steel ratio. Provision of shear reinforcement was shown to be increased the perimeter of the failure. The ultimate loads were increased with the addition of single leg stirrups as shear reinforcement particularly in case of radial arrangement of shear reinforcement. The ECCS shows the most conservative prediction for punching shear capacity specially in case of using shear reinforcement and the mean predicted-to-experimental ultimate load is shown to be 0.7. The predictions following the ACI and CSA are closet to the experimental results. The mean predicted-to-experimental ultimate load is shown to be 0.8 for ACI and 0.96 for CSA. The BS provisions for punching shear analysis were shown to be overestimated in some cases, where the mean predicted-to-experimental ultimate load is shown to be 1.19.

To reduce the surface crack width and to optimize the ultimate punching load of warehouse concrete floors, fibre reinforcement and special reinforcement mats above piles are used. Due to the special reinforcement mats, current design rules cannot be used to correctly predict the surface crack width and the ultimate punching load. Therefore, full-scale experiments have been carried out for six different reinforcement types. A fibre-reinforced floor with circular pile mat is the best solution, both for reducing the surface crack width and for optimizing the ultimate punching load.

Disproportionate collapse of structures can lead to catastrophic economic loss and casualties, and thus, it is of utmost concern to
mitigate the risks of such events. Flat-slab structures are much more vulnerable to disproportionate collapse than moment-frame structures as
there are no beams to redistribute the loads initially resisted by the lost column. Moreover, the propagation of punching-shear failure at slab–
column connections due to load redistribution may result in the collapse of the entire slab. Thus, increased attentions should be paid toward
assessing the disproportionate collapse performance of flat-slab structures. However, to date, limited knowledge exists regarding the risks of
disproportionate collapse in flat-slab structures, especially in terms of dynamic tests. For this purpose, a series of one-third-scaled flat-slab
substructures were subjected to the simulated sudden-column-removal scenario and the failure mode, acceleration, and displacement responses
were presented and discussed. To attain a deeper understanding of the dynamic load–redistribution capacity of flat-slab structures,
numerical and parametric analyses were also carried out. The possible load-resisting mechanism (membrane action, postpunching behavior,
and flexural strength) of flat-slab structures to mitigate disproportionate collapse is also discussed

It is known that the Greek codes allow flat-slab structural systems, however they provide specific compliance criteria. In the present study an extended parametric investigation was carried out in order to identify the seismic response of structural systems consisting of α) slabs-columns b) columns-perimetric beams c) columns-shear walls-slabs d) columns-shear walls-slabs and perimetric beams. The aforementioned systems were studied for all possible storey heights in Greece by means of F.E.M. Code SAP2000 ver.9. The compliance criteria provided by the Greek Code for earthquake resistance are related to second order effects, torsional flexibility, capacity design and the sensitivity of masonry infill. Conclusions were extracted concerning the number of storeys which can be applied to each case.

This paper investigates the strengthening techniques of lightweight concrete (LWC) flat slabs vulnerable to punching shear failure. Thirteen reinforced LWC flat slab specimens were experimentally tested. The main investigated parameters were the type of shear reinforcement (steel bars, high strength bolts, or glass fiber reinforced polymers (GFRP) rods), spacing, shear reinforcement configuration around the column and the fixation method of shear reinforcement. The test results showed that using radial shear reinforcement with spacing of (d/2) resulted in the most efficient strengthening technique. The punching shear capacities were improved by 77%, 61% and 54% by using steel bars, glass fiber rods and high strength bolts, respectively, compared to the reference specimen. Moreover, using high strength steel bolts fixed to the slabs with steel plates showed the highest ductility compared with the other strengthening techniques Also, the failure mode converted from being brittle shear failure to flexural-shear mode. Non-linear finite element analysis (NLFEA) was carried out for the numerical verification. The analysis adequately reflected the trend of experimental results. Finally, the experimental test results were evaluated with code equations. The evaluation showed that the ACI 318 code underestimated the punching shear capacity of the tested specimens. On the other hand, Euro Code showed good prediction of the punching shear strength of the tested slabs.

Metallic powder and binder mixing mechanism plays a vital role in the quality of molded parts in metal injection molding. The present study is intended for experimental and theoretical investigation of powder-binder mixing mechanism to investigate the functional correlation among mixing parameters and performance characteristics for different composition of feedstocks. Powder loading and shear rate are considered as input parameter. Fuzzy expert system is adopted to test the validity of the experimental results by analyzing different numerical error criteria using the viscosity as output with respect to input parameters. The mean relative error and correlation coefficient for type A and type B were found to be 6.09% and 8.51% (<10%) and 0.990 and 0.998, respectively. Hence the result indicates a reliable acceptability of the proposed amount of powder loading for feedstock preparation.

http://www.tandfonline.com/doi/abs/10.1080/10426914.2014.930955#.VRIhCo7fV5k 
http://www.ingentaconnect.com/content/tandf/amp/2015/00000030/00000001/art00004