Reinforced Concrete Flat Plates Research Articles

Vertical Punching Capacity of Reinforced Concrete Flat Plates without Shear Reinforcement

Vertical Punching Capacity of Reinforced Concrete Flat Plates without Shear Reinforcement

Assessment of the embodied carbon performance of post-tensioned voided concrete plates as a sustainable floor solution in multi-story buildings

With growing concerns for the building industry’s contribution to climate change, designers are considering the global warming potential and corresponding embodied carbon (EC) emissions of building structures. However, conventional reinforced concrete (RC) floors are one of the highest contributors to the EC in a multi-story building. A solution to reduce EC that combines proven construction technologies is post-tensioned (PT) voided plates, where void formers are placed in areas where less material is required, therefore increasing the effectiveness of the pre-stressing, and reducing substantial structural material. This paper assesses the EC of PT voided plates for three different tendon layouts and compares them to RC flat plates, RC voided plates, and PT flat plates. 3280 different design conditions are assessed to replicate the range of common applications of each concrete floor system. The minimum required material quantities were determined for each design condition, which were used to calculate and compare the corresponding EC. The EC results for the floor systems were also compared to other sustainable floor designs. This paper found that PT voided plates have an EC savings up to 51 % compared to conventional floors and are a sustainable floor system that can be built worldwide using current construction practices.

Punching Shear Strength of Reinforced Concrete Flat Plates with Openings

Test results of six half-scale reinforced concrete flat plates connections with an opening in the vicinity of the column are reported. The test specimens represent a portion of a slab bounded by the lines of contraflexure around the column. The tests were designed to study the effect of openings on the punching shear behavior of the slab-column connections. The test parameters were the location and the size of the openings. One specimen had no opening and the remaining five had various arrangements of openings around the column. All specimens were cast with normal density concrete of approximately 30 MPa compressive strength. The openings in the specimens were square, with the sides parallel to the sides of the column. Three sizes of openings were used: the same size as the column (150 x 150 mm), 67 percent of the column size (100 x 100mm), and 150 percent of the column size (225×225mm). Due to the presence of the openings, the specimens showed a decrease in punching shear capacity ranged between 11.43% and 29.25% with respect to the control solid slab. Also, the stiffness decreased between 0.31% and 83.00%, depending on the size and location of these openings with respect to the column.

Dynamic response and collapse resistance of RC flat plate structures subjected to instantaneous removal of an interior column

Through quasi-static loading regimes, notable research efforts have been made in recent years towards understanding the structural behaviour and resistance mechanism of reinforced concrete (RC) flat plate structures against progressive collapse events. Nevertheless, dynamic responses, being an inherent nature of any progressive collapse, were unable to be adequately studied in these quasi-static studies. Moreover, fewer studies were performed on flat plate structures under dynamic loading conditions, especially their behaviours under large deformations, i.e., the post-punching stages. To fill this research gap, this paper presents three sequential dynamic tests performed on a 1/3-scaled 2 × 2-bay RC flat plate substructure specimen by instantaneously removing the interior column under three levels of additional gravity loads. In these tests, the substructure specimen was able to withstand the gravity loads up to 2.08 times the load capacities recommended by the DoD and GSA guidelines for nonlinear dynamic analyses. The dynamic responses of the specimen in the flexural, punching and post-punching stages were examined in detail. In addition, a high-fidelity finite element model was developed and validated by the experimental test results. The validated numerical model was subsequently used to quantify the dynamic load redistribution patterns, and the influences of two loading considerations (i.e., loading on the slab overhangs and load release time) on the dynamic responses were also investigated. Finally, the dynamic load increase effects were evaluated based on the analysis model results. The obtained load-based dynamic increase factors (DIFs) of the specimen under different gravity loads were between 1.11 and 1.46 in the pre-punching (flexural) stage, whereas they were no more than 1.06 in the post-punching stage.

Punching Shear Strength of Reinforced Concrete Flat Plates with GFRP Vertical Grids

In this study, the structural behavior of reinforced concrete flat plates shear reinforced with vertical grids made of a glass fiber reinforced polymer (GFRP) was experimentally evaluated. To examine the shear strength, experiments were performed on nine concrete slabs with different amounts and spacings of shear reinforcement. The test results indicated that the shear strength increased as the amount of shear reinforcement increased and as the spacing of the shear reinforcement decreased. The GFRP shear reinforcement changed the cracks and failure mode of the specimens from a brittle punching to flexure one. In addition, the experimental results are compared with a shear strength equation provided by different concrete design codes. This comparison demonstrates that all of the equations underestimate the shear strength of reinforced concrete flat plates shear reinforced with GFRP vertical grids. The shear strength of the equation by BS 8110 is able to calculate the punching shear strength reasonably for a concrete flat plate shear reinforced with GFRP vertical grids.

Shear-Resisting Performance of Reinforced Concrete Flat Plates with Different Headed Stud Layouts

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Punching Shear Strength Model for Reinforced Concrete Flat Plate Slab–Column Connection without Shear Reinforcement

AbstractMany existing studies on punching shear in a RC flat plate slab without shear reinforcement consider uncracked concrete in the compression zone or the aggregate interlock mechanism in the c...

Effect of Shear Reinforcement on Punching Shear Capacity of Reinforced Concrete Flat Plates

Punching or two-way shear is a critical failure mode that must be accounted for in reinforced concrete (RC) slabs that are not supported by beams, known as flat plates. The often gravity-driven failure occurs around supporting columns may also be influenced by lateral forces. This paper presents the findings of a numerical study that evaluated the effect on punching shear strength of shear reinforcement bar diameter, and stirrups distribution length (L), measured from the face of the supporting column. The study was conducted on seven 3D nonlinear finite element models representing RC flat plates subjected to concentric vertical loads. Each of the seven models simulates the punching shear behaviour of interior slab-column connections with different shear reinforcement bar diameters and/or reinforcement distance from the face of the supporting column. Numerical investigation showed that increasing the stirrup diameter causes a significant increase in the ultimate two-way shear capacity. On the other hand, increasing the stirrups distribution length provides a moderate increase in the ultimate punching shear capacity. Furthermore, punching shear assessment using 3D finite element analyses were in good agreement with the mathematical models of the ACI-318-2019 building code.

Survival of Reinforced Concrete Flat Plate System against Column Loss

Column loss in a flat plate building, due to punching shear, explosion, impact load or any acci-dental event, can lead to what is termed progressive collapse. Progressive collapse is inherently a dynamic process, which makes it difficult to experimentally explore structures with real scale. Therefore, this paper aims to numerically investigate the behavior of flat plate systems due to column loss utilizing nonlinear finite element analysis with the aid of the computer software (ABAQUS). In this investigation, the nonlinear dynamic re-sponse of both an old flat plate building, designed according to the ACI 318-71, and a similar modern building, designed according the ACI 318-14, subjected to an instant removal of a column is examined. The obtained results clearly reveal that the old flat plate building without continuous slab bottom reinforcement at columns is highly vulnerable to progressive collapse and the efficiency of the continuous bottom reinforcement within the column strip, as recommended by the ACI 318-14, in preventing the disproportion or progressive collapse of a reinforced concrete flat plate building. Such reinforcement is able to produce alternate load path through the tensile membrane action, thus providing ductility and robustness in the system.

Experimental study on the progressive collapse behaviour of RC flat plate substructures subjected to edge-column and edge-interior-column removal scenarios

Existing studies on progressive collapse of reinforced concrete (RC) flat plate structures have mainly focused on single column loss scenarios. However, accidental events, such as earthquakes, blasts or vehicle collisions, may cause a more severe initial damage beyond the loss of only one column. To fill this gap in knowledge, this study reports two quasi-static large-displacement experimental tests on two nominally identical 1/3-scale, 2 × 2-bay RC flat plate substructures under an edge-column (S-E) and an edge-interior-column (S-EI) removal scenarios. Two types of uniformly distributed loads (UDLs) were applied to the slab in the two tests: (1) an increased UDL during the tests on the bays adjacent to the removed column(s) and (2) a constant 5 kPa UDL elsewhere to simulate the design live load. In both tests, punching shear failures were observed and led to the first peak loads (FPLs) which were all followed by a sharp load drop. Subsequently, the load increased again to reach the post-failure peak loads (PPLs). The experimental results showed that the PPL was 9.6% higher than the FPL in S-E and 81.9% higher in S-EI. Relative to S-E at the PPL, S-EI presented a more ductile structural behaviour, in which the applied load at the PPL was 26.9% lower, but with a 112% larger displacement, than S-E. In both tests, tensile membrane actions were observed at large deformations and found to be essential in developing post-failure capacities. Such resisting mechanism cannot be neglected when investigating progressive collapse of RC flat plate structures. The yield line theory was found to overestimate the flexural capacity of the two tests.

Punching shear strength of UHPFRC-RC composite flat plates

Reinforced concrete (RC) flat plates are vulnerable to punching shear failure due to small effective area for shear resistance. To strengthen the behavior of existing flat plates, use of ultra high performance fiber reinforced concrete (UHPFRC) as a laminate has been pursued. This study focuses on punching shear behavior of RC flat plates retrofitted with UHPFRC overlay (i.e., UHPFRC-RC composite flat plates). To model the axisymmetric UHPFRC-RC composite flat plate, use of the unified strength theory-based punching shear strength model was proposed. Predictions using the proposed model, when compared to existing test results of UHPFRC-RC composite flat plates under concentric load and nonlinear analysis results using ABAQUS, agreed well with the results. Additionally, from the parametric analysis results using the proposed model and nonlinear analysis, the effects of UHPFRC overlay thickness and tensile strength on the punching shear strength were identified.

Experimental study on the progressive collapse behaviour of RC flat plate substructures subjected to corner column removal scenarios

Reinforced concrete (RC) flat plate structures are broadly used in car parks, residential and office buildings due to their economic and architectural advantages. However, this structural system is inherently prone to punching shear failure, which may propagate horizontally and vertically, ultimately leading to the progressive collapse of the entire structure or of a large portion of it. This paper presents the experimental results from two quasi-static large-displacement tests performed on a 1/3 scale, 2 × 2-bay, RC flat plate substructure subjected to corner column removal scenarios. The specimen was tested twice with different corner reinforcement configurations: (i) firstly, one corner column, with torsional strips, was removed and the Uniformly Distributed Load (UDL) on the bay adjacent to the removed column was increased to failure (Test T1), and (ii) secondly, as the damage was concentrated in the vicinity of removed corner column in (i), the corner column diagonally opposite to the first removed one, without torsional strips, was removed. The UDL on the bay adjacent to the second removed column was also increased to failure (Test T2). Different failure and post-failure behaviours, failure modes, and collapse resisting mechanisms between the two tests were witnessed, presented and analysed. Results show that 80% to 110% of the applied load is transferred to the two edge columns adjacent to the removed corner column throughout the entire two tests. The ultimate load carrying capacity for T1 is found to be 1.7 times smaller than the one for T2.

Numerical Investigation of the Punching Resistance of Reinforced Concrete Flat Plates

AbstractNonlinear finite-element analyses are used to numerically investigate the punching shear resistance of reinforced concrete flat plate structures. Material nonlinearity is based principally ...

Unified equivalent frame method for post-tensioned flat plate slab structures

The post-tensioned (PT) flat plate slab system is commonly used in practice, and this simple and fast construction method is also considered to be a very efficient method because it can provide excellent deflection and crack control performance under a service load condition and consequently can be advantageous when applying to long-span structures. However, a detailed design guideline for evaluating the lateral behavior of the PT flat plate slab system is not available in current design codes. Thus, typical design methods used for conventional reinforced concrete (RC) flat plate slab structures have inevitably been adopted in practice for the lateral load design of PT flat plate structures. In the authors\' previous studies, the unified equivalent frame method (UEFM) was proposed, which considers the combined effect of gravity and lateral loads for the lateral behavior analysis of RC flat plate slab structures. The aim of this study is to extend the concept of the UEFM to the lateral analysis of PT flat plate slab structures. In addition, the stiffness reduction factors of torsional members on interior and exterior equivalent frames were newly introduced considering the effect of post-tensioning. Test results of various PT flat plate slab-column connection specimens were collected from literature, and compared to the analysis results estimated by the extended UEFM.

Simulating Post Punching Behaviour of RC Slab-Column Connections Using a Micro Model

Punching shear is a common failure mode occurring at the slab-column connection region of a reinforced concrete (RC) flat plate. Progressive collapse of RC flat plates poses a significant scientific question on the post punching behaviour of such a structural system. The challenge lies in the complex interactions amongst various internal actions including large unbalanced moments and shear forces. Existing numerical models are unable to differentiate the influence of each individual action within the connection region after punching occurs. Therefore, a new numerical model is required to model these actions individually as well as to evaluate their interrelationships. This paper thus aims to propose a numerical method to investigate the structural response of RC slab-column connections by using a micro model, based on a representative post punching failure experiment. In the micro model, concrete is simulated using solid elements whilst the reinforcement is modelled with truss elements. In this micro model, the constitutive laws and failure criteria of materials play a crucial role in describing the model’s structural behaviour. A typical structural response is discussed and a calibration method is established. Ultimately this study is expected to facilitate the development of an effective, yet simplified numerical model for future progressive collapse simulation of slab-column connections.

Punching of slab–column connections strengthened using external steel shear bolts

This paper describes the use and effectiveness of external steel shear bolts as shear reinforcement for retrofitting of slab–column connections in reinforced concrete (RC) flat plates and also compares the behaviour of slab–column connections with finite-element analysis. Four full-scale test specimens, representing interior slab–column connections in RC flat plates, were cast. Two concrete grades – low strength (14 MPa) and normal strength (28 MPa) – were used. One specimen for each concrete grade was used as a control and the other was strengthened using external steel shear bolts as the transverse shear reinforcement. The slab–column specimens were tested experimentally and their numerical models were analysed using Ansys. The results obtained through static non-linear analysis compared well with the experimental data. Shear strengthening of slabs changed the failure mode of the test specimens from punching shear to flexural mode and increased the ultimate load capacity of the slab–column connection by 64–73%. The shear strengthening also resulted in increased deflections and ductility. Analytical predictions of ultimate load were in good agreement with the experiments.

Behavior of Reinforced Concrete Flat Plate with Embedded Bearing Plate

This study presents an experimental investigation on the influence of embedded shearhead reinforcement (steel plate) on the punching shear strength of reinforced concrete slabs. This work includes the investigation and testing of one control slab and five scale models of slab-column connection simply supported along the four edges. A shearhead was made by using a steel plate with T-section stiffeners, the stiffeners were fabricated to be with two lines welded on the tension face of the plates. The main variables studied were the dimensions and thickness of the steel plate (used as punching resistance) .The results show that ultimate load capacity at failure increases by (16.48%) over the reference slab. Also, punching shear stress decreases about (29.24%) below the reference slab.

Preliminary Collapse Simulation of a Reinforced Concrete Flat Plate Substructure Using Spring Connection Modelling

To investigate progressive collapse behavior of reinforced concrete (RC) flat plate structures, a reliable and efficient numerical approach is developed in this study using spring connection modelling. This connection unit aims to simulate complicate punching shear behavior at critical regions surrounding the columns. Five springs are used as the connection elements: two for flexural and integrity steel bars and three for concrete contributions. The flexural and integrity steel bars embedded in the columns are modeled explicitly, which enables the model to present the structural behavior post punching shear failure. Bending and shear actions are represented by two concrete springs. The third concrete spring is assigned axial action property to restrain two end nodes of the connection on the model. In particular, the punching shear spring controls the connection unit when punching shear failure occurs. To apply the connection unit, the regions of slab-column connections are partitioned from the slab regions according to the critical shear surfaces. Then the connection unit links two corresponding nodes on the two edges formed from the partition. A physical experiment of a RC flat plate substructure under progressive collapse is simulated. Result comparison demonstrates that the numerical model has the capability to capture the structural behavior in progressive collapse. However, further improvement of the modelling technique is necessary to enhance numerical accuracy.

Seismic performance evaluation for gravity-designed flat plate frames

Most existing reinforced concrete (RC) buildings in regions of low and moderate seismicity have been designed only considering gravity loads. Gravity-designed RC flat plate frames may be more vulnerable than RC frames with beams and columns owing to the potential for abrupt and brittle punching shear failure at slab–column connections. This study evaluates the seismic performance of gravity-designed three- and seven-storey flat plate frames, which were assumed to be located at sites having low, moderate and high seismic risks. An analytic model for simulating the hysteretic behaviour of slab–column connections obtained from experiments was proposed. It was found that the level of gravity load as measured by the gravity shear ratio significantly affects the inelastic behaviour of flat plate frames. The frames satisfied the basic safety objective (BSO) specified in AISC 41 subjected to ground motions at the Boston site, but not at the Seattle and LA sites.

Punching Shear of Reinforced Concrete Flat Plates with Openings

Punching shear tests were conducted on 13 reinforced concrete flat plates with and without openings or/and shear reinforcement. The openings (one or two) were adjacent to the shorter sides of rectangular supports and had widths equal to those of the supports. The methods of calculating punching shear strengths given in ACI 318-11 and MC90/EC2 are reviewed along with some proposed formulations, and their predictions are compared with the test results. Conclusions are drawn on the influence of plate depth on the unit shear resistance from the concrete; the possibility of using straight projections of openings onto control perimeters, rather than radial ones, to evaluate the effect of openings; and the case for considering eccentricity in a pattern of openings as an influencing factor and the detailing of shear reinforcement. It is shown that for the relatively small openings considered, the provision of continuous bars adjacent to openings--to replace the areas of reinforcement--seems to be an adequate approach to flexural design. It is suggested that the shear stress used for the concrete in ACI 318-11 could be increased for the assessment of strengths at the edges of zones with shear reinforcement (outer control perimeter).