Concrete Flat Slabs Research Articles

Experimental study on the collapse response of flat slab structures with drop panels under gravity loading condition

This paper presents the results of an experimental study on the collapse behavior of two reinforced concrete flat slab assemblies with drop panel connections subjected to uniformly distributed loads applied through 24 loading points. The two tests, named MC and PC, simulated local failures at mid-span columns and penultimate columns, respectively. Both assemblies exhibited elastic behavior until cracking values were reached, followed by degradation of stiffness due to crack development beyond the elastic zone and a long plateau near collapse. The main differences between the assemblies were the higher collapse load of the MC assembly, attributed to slab additional slab extension surrounding two test panels, and the load redistribution pattern. Both assemblies showed ductile behavior in the inelastic region, with large deformation capacity allowing tensile membrane action to mobilize. The collapse modes observed were the failure of the outer compressive ring and the tearing out of tension slab reinforcement from the concrete. The traditional yield-line method may not be suitable for quantifying the collapse resistance of this type of structure, and numerical simulations are suggested for this purpose. The experimental data from these tests, as well as previous tests, could be valuable for validating simulation models.

Proposed shear strength structures, experiment, simulation, and analytic model of reinforced concrete flat slab and concrete-filled steel tube column

Proposed shear strength structures, experiment, simulation, and analytic model of reinforced concrete flat slab and concrete-filled steel tube column

Punching Shear Response Theory (PSRT) – A two degree of freedom kinematic theory for modeling the entire punching shear vs. deformation response of RC slabs and footings

Punching failures of reinforced concrete flat slabs and footings without shear reinforcement have been responsible for a number of catastrophic collapses. Despite intense scientific efforts in recent decades that have led to failure criteria-based phenomenological models, our current mechanistic understanding is still not sufficiently advanced to fathom the physical background of punching phenomena in RC structures. That is why, this paper proposes a novel two degree of freedom mechanical theory - called Punching Shear Response Theory (PSRT) - that interconnects all relevant effects of punching shear behavior, namely strutting action within a conical concrete shell at the slab-column interface, shear transfer actions elicited along the circumferential punching shear crack, including cohesive fracture process zone behavior, aggregate interlock and dowel action of reinforcement, as well as transfer of normal forces within tangential and radial cracks between slab segments in one all-encompassing algorithm. The PSRT can thoroughly mimic and efficiently reproduce the entire punching failure process of flat slabs and squat members (footings, pile caps, column bases) including prediction of pre- and post-peak punching shear vs. deformation response, quantification of different punching shear contributions and analysis of their evolution and mutual interaction within the failure process. The paper illustrates the analytical development of the PSRT, exemplifies its ability to reproduce the punching shear response of selected benchmark problems, proves its validity by database evaluations (incl. punching strength and deformation capacity) and eventually discloses the physical causalities of punching failures in RC structures.

Flat slab response for seismic and cyclic actions prediction with numerical models

As an outcome of a recent European research project, the modelling of the response of flat slab structures for seismic and cyclic loading was the object of a blind competition. No other blind tests are available in the literature on this topic. The test results concern a full-scale, two-storey, three-by two bay reinforced concrete flat slab frame. The seismic tests were carried out using a hybrid pseudo-dynamic test up to the ULS design excitation. The cyclic test reached a drift capacity of 2.5% and 6% in two concatenated tests. A synthesis of the models of three participants is provided, with one 3D NLFE (Nonlinear finite element) models and two slab and frame models. The CSCT (Critical Shear Crack Theory) model included in the fib MC2010 was adopted in the two latter to predict punching failure. The results are presented and discussed. Conclusions are drawn on research developments and practical approaches.

Verification of deflections and cracking of RC flat slabs with numerical and analytical approaches

The Serviceability Limit States (SLS) of reinforced concrete structures, where cracking and deflection limits are verified in the design process, are directly related to time-dependent properties of concrete. In this study, the effects of creep and shrinkage of concrete, implemented in a Finite Element Model for Non-Linear Analysis, are in-deeply investigated for reinforced concrete flat slabs at SLS and used as basis for the development of an analytical framework. In particular, the paper focuses on: (i) the description of the FE model, (ii) the comparison between the numerical and experimental results of some reinforced concrete (RC) flat slab tests from literature characterized by different mechanical properties, span-to-depth ratios, boundary conditions and loading histories and (iii) the development of an analytical framework for the evaluation of long term-term deflections and crack openings in hogging regions of flat slabs near columns. Based on the findings, useful insights for the evaluation of the long term-term deflections and crack opening in the context of Serviceability Limit States are provided for practical purposes.

Two-way shear full behavior of reinforced concrete flat slabs under membrane tensile forces

Recent reinforced concrete Flat slabs (RC) failures under two-way shear have raised a question mark on the reliability of the current understanding of the two-way shear design. Moreover, the two-way shear strength of Flat slabs subjected to membrane tensile forces is a dilemma, where design codes provision varies significantly for this case. In this study, the full behavior of flat RC slabs subjected to combined two-way shear and axial tension is investigated. The behavior in terms of load versus deformation, ultimate strength, and deformation. Firstly, an extensive review of the existing literature on the experimental testing and design models for Flat slabs under combined two-way shear and axial tensile forces was conducted. Secondly, an experimental test setup designed to apply combined two-way shear and tensile forces simultaneously on flat slabs was presented. Four flat slabs (1500 × 1500 × 150 mm) were tested under two-way shear up to failure, while subjected to different levels of membrane tensile forces. Thirdly, experimentally measured strength from current and previous studies were compared with that calculated using selected design codes and mechanical models. For flat slabs under two-way shear combined with membrane tensile forces, the following was concluded: (1) a disagreement between selected design codes, while the European design code is the closer to the experimental results; (2) The stiffness and the strength were reduced by 57% and 29% respectively which is highly dependent on the level of tension.

Investigation of Impact of Span Length and Variable Live Loading Conditions on Flat Slab Material Costs

Objectives: This study investigates the optimization of reinforced concrete flat slabs and establishes a relationship between span, live load, and material costs. Methods: The study used the direct design method per Saudi Building Code limitations, iterative design, and pattern search methods to establish a mathematical model that prescribes the span-cost relationship. The study conducted variable live loading tastings and cost estimates on multiple flat slab panels with distinct dimensions. Findings: The results showed that slab material unit cost increases significantly with span while live load had little impact. Material costs included concrete, steel, and forms which compose 36%, 52%, and 12%, respectively of the total. Each increment in span by 1 meter raises slab material unit cost by 19% starting form 4 meters span as a baseline. Increasing live load by 1 kN/m2 increase cost by roughly 2% for square panels and 1% for rectangular ones. However, results proved that using 1:1 panel dimension is more economical than panels with 2:1 ratio. Novelty: The study provided a mathematical model to estimate flat slab cost more accurately in preliminary estimations before the design phase. Keywords: Material Costs; Flat Slab Cost; Live Load Impact on Cost; Span Impact on Cost; Slab Unit Cost; Cost Optimization

Using System Reliability Concepts to Derive Partial Safety Factors for Punching Shear with Shear Reinforcement: An Explorative Analysis

Punching shear-reinforced flat slabs can fail in three modes: (1) inside the shear-reinforced zone, (2) outside the shear-reinforced zone, or (3) at the maximum punching shear capacity. By using the theory of structural systems reliability, this study investigates how these failure modes combined affect the safety level of reinforced concrete flat slabs with punching shear reinforcement, and ultimately, the variation of partial factors. Based on DIN EN 1992-1-1 together with the German National Annex DIN EN 1992-1-1+NA(D), and by using distinct reliability-based methods (i.e., levels II and III), the variation in the system failure probabilities and, consequently, partial factors were analysed. The results indicated that partial factors can fluctuate significantly depending on the governing ultimate limit state function and the scatter of basic variables. Furthermore, they indicated that the chosen reliability-based method had a significant influence on the resulting safety level. The sensitivity analysis confirmed the premise that partial factors can be reduced without compromising socially accepted safety levels (e.g., recommended in DIN EN 1990). Ultimate benefits include material and cost savings and CO2 emission reductions. Finally, the approach addressed in this paper offers a new perspective on the derivation of partial factors that can be considered by engineering practitioners.

Low Velocity Impact Response of Reinforced Concrete Flat Slabs

Background Dimensions of the reference concrete slab are 1950x1950x100 mm subjected to drop-weight impact loading. Objective A comprehensive parametric study was performed to examine the influence of many parameters on the RC slabs. Method From the viewpoint of cost and time savings, a three-dimensional finite element is a very good tool to predict the real behavior of the structural elements. Result and Discussion Results showed that the use of CFRP strips enhance the impact behavior of the slab. Contrarily, the existence of opening led to a dramatic decrease in the dynamic capacity of RC slabs with stress concentration around the openings. Furthermore, changing the shape of the impactor showed a significant effect on the peak impact load as well as the ultimate deflection at impact instant. Conclusion In the scope of this paper, the response of RC slab with top and bottom reinforcements exposed to drop-weight impact loading was inspected. Time histories of impact loads and deflections were presented in detail based on ABAQUS/ Explicit analysis. The findings presented in this paper can be presented as follows: 1. The FE models show a good correlation with the experimental data. Consequently, the proposed finite element models are efficient and economical tool to explorer the effect of many parameters on the performance of RC slabs subjected to drop-weight impact load. 2. The numerical simulation confirmed that using externally bonded CFRP strips has more influence on the peak deflection of the reinforced concrete slab than the recorded impact force. 3. Comparing to the flat shape of the impactor, the hemispherical and curved shape impactor can produce large penetration depth at the impact zone with higher plastic deformations in the concrete slab. However, the flat impactor produced higher deflection at the impact instant. 4. As the radius of the impactor increases, both the duration time and the peak impact force are increased. This is because of the higher contact area was obtained when the flat impactor (infinity radius of curvature) was used as compared to other impactors. 5. Due to decreases in RC slab stiffness, the presence of openings (regardless of their shape) has considerably increased deformations in concrete especially around perimeter of the openings extended to the nearby support. 6. It has been found that the eccentric impact loading causes higher plastic deformations than the concentric one.

Numerically Evaluation of Dynamic Behavior of Post-Tensioned Concrete Flat Slabs under Free Vibration

The objective of this paper is to investigate the dynamic behaviour of post-tensioned concrete flat slabs with different geometries and damping ratios. Four groups of models with different lengths, widths, thicknesses and damping ratios designed according to the AS3600 standard. These were used to determine the influence of each parameter on the vibration serviceability by comparing the control variable method with the reference model. The vibration assessment parameters were used as natural frequency, peak acceleration, and response factor. Both the SCI/CSTR43 standard theoretical calculations method and the Strand7 finite element analysis (FEA) method are used to determine the effect of different geometries and damping ratios on vibration. The feasibility of the Strand7 FEA method for vibration analysis is also assessed by calculating the errors of the two methods. The paper concludes that the Strand7 FEA method is highly accurate and feasible. The span in both directions has a large effect on the natural frequency, and increasing both the slab thickness and the damping ratio are effective methods to improve the vibration serviceability. Based on the research in this paper, recommendations are provided for future vibration design of post-tensioned concrete slabs.

Erratum: Proposal for estimating the punching shear load-carrying capacity of steel fibers reinforced concrete flat slabs

Erratum: Proposal for estimating the punching shear load-carrying capacity of steel fibers reinforced concrete flat slabs

Proposta para estimativa de capacidade de carga na punção de lajes lisas de concreto armado reforçado com fibras de aço

abstract: This paper proposes an adjusted function for estimating the punching shear load-carrying capacity of concrete flat slabs without shear reinforcement according to the Brazilian standard code. The Adjusted function considers the influence of the steel fiber content. Data from 68 steel fiber reinforced concrete slabs (SFRC slabs) were used, simulating the connection of flat slabs with square section columns. These slabs were obtained from a study conducted by 14 different researchers. The parameters used for the adjusted function were the fiber content and the compressive strength of the concrete. The adjusted function was obtained by curve-fitting. The result from the adjusted function shows a relation in the order of 1.3 between the experimental rupture load and the calculated load, a value close to that already obtained by the normative text when the calculation is for conventional concrete. This shows that the proposal presented here keeps the same level of security with a confidence index of 96% in comparison with other authors’ models presented in this study.

Experimental analysis of concrete flat slabs with internal stud-type shear reinforcement

Abstract This study evaluates seven flat slabs made with reinforced concrete. There are three reference slabs, one of them doesn’t present any shear reinforcement. Four slabs have a new model of shear reinforcement of stud type, internally anchored to the flexural reinforcements. That reinforcement has an additional element, called on this study by the name: anti-cracking pins. The main objective of the research is to find the ideal spacing between these pins to achieve a failure mode and a failure load similar to the reference slabs that have conventional studs. For that, are evaluated: vertical displacements, rotation, shear reinforcement deformation, load capacity and failure mode. The slabs with the new stud have a load gain of 40% to 106% compared to the slab without studs LRef. The slab L-5-13 presented a load and a failure mode similar to the slab of reference, LRef-AC.

Experimental and theoretical study of the reinforced concrete flat slabs with the central support loss

The paper experimentally and theoretically considers the issues of assessing the robustness and resistance to progressive collapse of a flat slab with a sudden removal of the central support.
 The results of testing two scale models of a fragment of a flat ceiling in the case of removal of the central support under static (specimen FS-1) and dynamic (specimen FS-2) loading are presented and analyzed.
 A theoretical approach to the quantitative assessment of robustness was tested, which is based on the provisions of the energy balance of a damaged structural system in an accidental design situation.

Effect of using Engineered Cementitious Composites (ECC) on failure behavior of flat slab-column connections

Load capacity and ductility are two key characteristics of the flat slab-column connection in high seismic areas. This study specifically examined the effect of strengthening layer thickness of Engineered Cementitious Composites (ECC) on punching shear strength and post punching behavior of flat slabs. The effect of the unbalanced moment, ECC thickness of the slab and the strengthening ECC layer on flat slab punching and post punching behavior were also investigated. Thirteen reinforced concrete flat slab specimens were made and examined. It was observed that the replacement of slab concrete with ECC increased the punching capacity and improved post-punching behavior without a large and sudden drop in load. In addition, the use of ECC as a strengthening layer for the flat slabs significantly increased the punching and post-punching shear strength capacities. The change in failure mechanism was observed from a sudden and abrupt failure to a ductile failure with high-energy absorption when more ECC layer thicknesses were used.

Punching Shear Behavior of Slabs Made from Different Types of Concrete Internally Reinforced with SHCC-Filled Steel Tubes

The punching shear failure of reinforced concrete (RC) flat slabs is an undesirable type of failure, as it is sudden and brittle. This paper presents an experimental and numerical study to explore the behavior of flat slabs made of different types of concrete under the influence of punching shear. Experimental tests were carried out on four groups of flat slabs, each group representing a different type of concrete: ordinary normal concrete (NC), high-strength concrete (HSC), strain-hardening cementitious composite concrete (SHCC), and ultra-high-performance fiber concrete (UHPFC). Each group consisted of six slabs, one representing an unreinforced control slab other than the reinforcement of the bottom mesh, and the others representing slabs internally reinforced with SHCC-filled steel tubes and high-strength bolts. An analytical equation was used to predict the punching shear capacity of slabs internally reinforced using steel assemblies. A numerical model was proposed using the ABAQUS program, and was validated by comparing its results with our experimental results. Finally, a case study was performed on large-scale slabs. The results showed that using steel assemblies inside NC slabs increased the slab's punching shear capacity but does not completely prevent punching shear failure. Internally unreinforced slabs made of UHPFC and SHCC were able to avoid punching shear failure and collapse in a ductile bending pattern due to the high compressive and tensile strength of these types of concrete. The proposed analytical method succeeded in predicting the collapse load of slabs reinforced with steel assemblies with a difference not exceeding 9%.

Performance of reinforced concrete flat slab strengthened with CFRP for punching shear

Existing reinforced concrete (RC) slabs need punching shear reinforcement due to design defects, structural damage, increased loading, corroded reinforcement bars, and change in the use or function of the buildings. Strengthening methods such as using carbon fibre reinforced polymer (CFRP) sheets as external reinforcements in the tension area have proven to be useful in increasing the punching shear capacity and ductility of RC members. Therefore, this study aims to investigate the effect of using CFRP strengthening method in two-way RC slabs. Three strengthened RC solid flat slab samples in addition to unstrengthened slab samples were tested and subjected to static load to simulate the punching shear effect at the middle of the slab. Based on the test results, the samples strengthened with CFRP showed better performance in terms of delaying the appearance of the first crack, improving the resistance of shear crack propagation, and enhancing the load-displacement and ductility capacities compared to the control sample. In addition, the failure mode of the strengthened samples with CFRP was improved and the capacity for punching shear was increased. Furthermore, the analytical model to predict the ultimate punching shear strength has good agreement with the requirements of the practical codes, thus it can be used for the design and analysis of CFRP strengthened slab.

Determination of vehicle loads on bridges by acoustic emission and an improved ensemble artificial neural network

Bridges are significant hubs in the U.S. national economy, facilitating the movement of goods and vehicles. The condition of bridges in the state of South Carolina is currently under scrutiny, especially in rural areas where most of the bridges were designed using outdated standards from the 1950 s. The weight of vehicles in recent years has increased significantly compared to the past. This has created an overloading problem. In addition, bridge performance decreases during their service life due to vehicle loads, material deterioration, and environmental erosion. Therefore, it is necessary to inspect and conduct load ratings on bridges to determine whether the bridges need to be posted. Due to recent advances in sensing technology and data analysis methods, nondestructive methods such as acoustic emission (AE) have been widely utilized in monitoring damage to the bridges. The objective of this paper is to explore the possibility of using AE sensors concurrently to determine vehicle loads on the bridges while monitoring bridge damage. A load determination method leveraging an improved ensemble artificial neural network (ANN) is proposed to analyze the AE data and estimate the load of the vehicle. The significance of this vehicle load determination method is that it has the potential to be paired with an AE damage monitoring system rather than using other instrumentation such as a weigh-in-motion (WIM) system. The proposed method has been tested on an experimental bridge component. The results suggest that the proposed model has an accuracy above 70 % in estimating the vehicle loads on the precast reinforced concrete (RC) flat slabs.

Punching Shear Behavior of Fibrous High Strength Concrete Slabs Reinforced with BFRP Bars

Abstract This study offers test findings from research work conducted to investigate the punching shear behavior of fibrous high strength concrete flat slabs reinforced with Basalt fiber-reinforced polymer (BFRP) bars. Eleven two-way concrete slabs were tested until they failed under central concentrated force. BFRP bars were employed to reinforce ten of the eleven slabs for flexural reinforcement, while steel bars were used to reinforce one slab for comparison. Each slab measured 1000 × 1000 × 70 mm with a 60 × 60 mm steel column at the center of the slab. The flexural-reinforcement type, flexural-reinforcement ratio, and steel fibers content were the test parameters. According to the test findings, raising the BFRP reinforcement ratio improved the slabs’ punching shear capability and load-deflection response. In comparison to slabs with a lower reinforcing ratio, these slabs also showed fewer, narrower cracks. Steel fibers were added, which improved the failure mode from brittle, unexpected failure to ductile failure. The experimental findings are compared to a shear strength equation offered by several concrete design codes, including ACI 440.1R-15, CAN/CSA S806-12, and JSCE.

Punching Shear Characterization of Steel Fiber-Reinforced Concrete Flat Slabs

Punching shear failure in thin slabs under concentrated loads can cause shear stresses near columns. The use of steel fiber is a practical way to improve a slab-column connection's punching strength and deformation capacity. In this study, the capacity and behavior of steel fiber-reinforced concrete flat slabs are examined under punching shear force. Ten small-scale flat slabs were tested, eight of which had steel fiber and two without. Two parameters are studied in this paper, which are the fiber volume ratio (from 0% to 2%) and the stub column load shape (circle and square). The test results include the concrete compressive strength, crack patterns, punching shear, and load-defection behavior of the slabs. Based on the experimental results, it was found that the punching shear capacity of slabs with steel fiber (S5) increased by 21.8% compared to slabs without steel fiber (S1), and the slabs with steel fiber had more ductility compared to the slabs without fiber. Doi: 10.28991/HIJ-2022-03-04-08 Full Text: PDF