Slab-column Connections Research Articles

Seismic behavior of RC slab-column connection with openings strengthened with CFRP

The construction of openings next to the columns may be necessary for two-way reinforced concrete slab systems. The ultimate strength of the slab is reduced by these openings, particularly when subjected to cyclic loads. Therefore, the slab is required to be strengthened in such zones. One of the most useful techniques for strengthening the slab-column connection is Carbon Fiber Reinforced polymer (CFRP). Due to its improved mechanical characteristics and high bonding strength, High-Strength Self-Compacted Concrete (HSSCC) has seen an increase in demand over the past few years. Because of these characteristics, it is preferred for use in constructions like multi-story buildings where excellent workability and strength are needed. The main objective of this research is to study experimentally and analytically the seismic behavior of HSSCC slab-column connections with openings strengthened with CFRP. Six full-scale specimens were examined under cyclic loading, including one reference specimen (without openings and without strengthening), while the five strengthened specimens have openings. The most significant factors are the size and location of the openings. Three different opening sizes were tested with a square shape of 200 mm, 228 mm, and 266 mm sides. The openings were at three different locations, next to the column face, at a distance twice the slab thickness (2d) and three times the slab thickness (3d) from the face of the column. The findings indicated that CFRP improved the ultimate load capacity of the strengthened slabs by about (8%–12%) and capacities of the energy dissipation were boosted for all the strengthened specimens. In addition, the opening size did not have a noticeable influence on the specimens’ load-carrying capacity for strengthening specimens. CFRP enhanced the maximum vertical displacement of slabs with openings by about (5%–24%), but for specimens with openings placed at the face of the column, the enhancement was not noticeable. The strengthening has decreased the ductility displacement ratio of all strengthened specimens by about (4%–20%). For additional comparison, the ABAQUS finite element simulation model was created. In comparison to the experimental results, the finite element simulation produced highly agreement results.

Finite Element Analysis of Punching Shear of Reinforced Concrete Slab–Column Connections with Shear Reinforcement

Shear reinforcement is one of the common measures used to strengthen slab–column connections. It can make the joint’s load capacity increase. However, when the connection is subjected to an unbalanced bending moment, the reduction effect of the shear reinforcement on the bending moment is worth investigating. Nonlinear finite element analysis was performed on reinforced concrete slabs with shear reinforcement for various moment-to-shear (M/V) ratios. The effects of the number and diameter of shear reinforcement and different loading methods on the slab were investigated, as along with a correction to the ACI-318 and China Concrete Structure Design code GB50010 formula for the moment transfer coefficient. The 3D finite element modeling was performed using appropriate element types and a constitutive model for concrete. The concrete damage plasticity model parameters were calibrated using the experimental results of a specimen. The amount of shear reinforcement had a certain contribution to the strength of the slab, but the diameter of the shear reinforcement had little effect. The plate’s bearing capacity decreased as the M/V increased. The corrected moment transfer coefficient formula demonstrates that M/V is positively correlated with the coefficient and that the calculated values specified in ACI-318 and GB50010 are overly conservative.

Punching shear capacity of flat slabs with recycled aggregate

PurposePunching can trigger catastrophic failures in flat slabs because of its sudden nature resulting from exceeding the shear capacity of slabs. Effect of using recycled aggregate, as an environmental-friendly alternative to traditional RC structures, on punching behavior of these slabs was not sufficiently investigated in the literature. Hence, this paper aims to experimentally study the effect of using recycled coarse aggregate (RCA) on the punching shear capacity (PSC) of RC flat slabs. The RCA is produced by crushing of waste of concrete standard cubes obtained from compression tests.Design/methodology/approachA total of 12 slab-column connection specimens with different slab thicknesses (140, 160 and 200 mm) and different RCA percentages (0%, 30% and 70%) were prepared and tested under a central point load, to test its effect on the behavior of flat slabs. The punching failure loads of the tested specimens were compared with those obtained according to the provisions of different international building codes.FindingsCompared with natural aggregate concrete, mixes with 30% and 70% RCA experienced reductions in the compressive that did not exceed 4% and 21%, while reductions of 4% and 13% were observed for the tensile strength, respectively. The increase in the amount of RCA reduced the PSC by 0%–7%, 0%–4% and 4%–10% for slabs with a thickness of 140, 160 and 200 mm, respectively. For slabs with punching shear reinforcement (PSR), ACI 318 provided the closest estimation for the PSC by 9%, whereas EURO 2 overestimated the PSC by 25% and ECP 203 underestimated the PSC by 41%.Research limitations/implicationsThe provided conclusions are obtained from the conducted experimental work where a constant W/C ratio, aggregate type and a maximum aggregate size of 19 mm for the RCA were adopted.Originality/valueEnhancement in the behavior of flat slabs with various thicknesses and amounts of RCA because of introducing PSR is experimentally evaluated. The failure loads of the tested slabs with recycled and normal coarse aggregates were compared against different code provisions.

Prediction of punching shear capacity of concrete slabs reinforced with additional middle-thickness rebar mesh

This paper aims to evaluate the influence of using additional middle-thickness rebar mesh (mid-mesh) on improving the punching shear behavior of interior slab-column connection. Sixteen two-way slabs were cast and tested including different flexural, compression, and mid-mesh reinforcement ratios at different levels of concrete strength. The performance of code-based expressions in predicting the punching shear capacity of reinforced concrete flat plates was also ascertained. The results indicated that using additional mid-mesh showed an increase in the punching shear capacity of slabs reached up to 17.7%. The results also indicated that the ACI 318-19 and CSA A23.1-19 codes underestimate the punching shear strength of tested slabs. Eurocode showed the closest predictions of the ultimate shear load compared to the other design codes investigated in this study. The authors also proposed an equation that can help the designers/engineers to accurately predict the punching shear capacity of intermediate slab–column connection with and/or without additional mid-mesh reinforcement.

Experimental and Theoretical Investigation of the Punching Shear Strength of GFRP-Reinforced Two-Way Slabs

This study aims at investigating the punching shear behavior of square RC interior slab–column connections reinforced with glass fiber reinforced polymer (GFRP) rebars. A total of eight specimens are tested under vertical shear until failure. Five of the specimens are reinforced with GFRP rebars and three specimens with conventional steel reinforcement for comparison. The investigated test parameters are flexural reinforcement type (GFRP or steel), column stub dimensions (100 mm × 100 mm or 150 mm × 150 mm), slab dimensions (900 mm × 900 mm or 1200 mm × 1200 mm), and slab thickness (100 mm or 120 mm). The slenderness effect is also taken into account by testing specimens with varying span lengths and thicknesses. The 3D nonlinear finite element method (FEM) is also implemented to validate the experimental findings. Additionally, the accuracy of existing design equations is assessed against the experimental results. The results reveal that GFRP-reinforced specimens yield higher deformation capacity, yet lower punching shear strength, as compared to steel-reinforced counterparts, owing to the lower elastic modulus of GFRP rebars. Although neglected in most studies, the net span-to-depth ratio (L net /d) is a substantial parameter and should be considered in design.

Lateral Behavior of Slab-Column Connection with Pyramid Shaped Drop Panel

Both experimental and finite element analysis (FEA) were used to study the interior slab-column connections made with pyramid-shaped drop panels subjected to vertical and horizontal loads. The dimensions of the models at

Experimental Evaluation of the Punching Shear Strength of Interior Slab-column Connections with Different Shear Reinforcement Details

This research aims to evaluate the effect of using different shear reinforcement details on the punching shear behavior of interior slab column connections. A comprehensive experimental program is conducted on sixteen specimens having the same concrete dimensions of 1100×1100×160 mm where the slab depth is chosen to be less than that stipulated by different design codes. The parameters under examination were the type of shear reinforcement arranged in a cross shape perpendicular to the column edges (single leg, multi-leg, and closed stirrups), the spacing between stirrups (25 and 50 mm), and the extended length covered by the stirrups (300 and 425 mm). Experimental results showed that slabs reinforced with multi-leg or closed stirrups, even for slabs with a thickness of 160 mm, had an increase in the shear capacity by up to 40% depending on the stirrup amount. A noticeable enhancement in ductility was also observed. Slabs reinforced with vertical single-leg stirrups did not exhibit any improvements. A finite element analysis was conducted to further assess the punching shear behavior of the tested specimens. A comparison between the test results and values obtained using design codes such as ACI 318-19 and ECP 203-2018 showed that the two design codes give a rather underestimated prediction of the punching shear capacity. Doi: 10.28991/CEJ-2022-08-09-015 Full Text: PDF

Use of Bended for Reinforcement in the Design of the Slab-Column Connection of a Flat Slab Structure

This paper presents the results of an experiment to determine the behaviour of slab-column connection of a flat slab (a column). The main objective is to compare the response of the reinforced specimens with conventional and composite reinforcement and to verify the effectiveness of the stirrups from FRP bars (FRP reinforcement used as shear reinforcement). The test specimen was a square shaped concrete slab in four different designs. The results obtained were compared with the theoretical prediction, quantifying the effect of the FRP reinforcement used on the bearing capacity. Finally, the variations in the prediction according to the different approaches are also highlighted. Comparison of the load capacities of the specimen without and with FRP shear reinforcement led to an increase in the case of stirrups with inclination of 90° by 12 %, reps. stirrups with inclination of 45° by 26 %. The results are influenced by the design of the shear reinforcement.

Seismic Performance of Strengthened Slab-Column Connections in a Full-Scale Test

ABSTRACT Strengthening of flat slab-column connections to resist imposed lateral drifts is often required for older reinforced concrete structures in which the flat slabs were typically designed to resist gravity load only or have been designed for less stringent requirements. The complex stress state around the columns means that strong experimental evidence is required for the repair and strengthening techniques to be adopted with confidence in practice. Although a significant amount of research has been devoted to strengthening of slab-column connections, a series of limitations still remain. A thorough literature review revealed that only relatively thin and isolated specimens have been tested under seismic-type loading and realistic strengthening scenarios. To overcome these limitations, a full-scale two-story flat slab building specimen was tested at the European Commission’s Joint Research Centre – ELSA Laboratory in Ispra, Italy. This paper describes and analyzes the results obtained from the last of a series of phases, on repaired and strengthened internal and external slab-column connections using post-installed bolts. Compared with previous tests, the full-scale building specimen has a thicker slab (200 mm), realistic slab continuity conditions and the connections were strengthened after they suffered damage from gravity loading and previous lateral loading phases. The results show that post-installed bolts are efficient even under the described conditions (i.e. damaged and repaired specimens with realistic thickness), and punching shear failure was prevented in the strengthened connections.

Punching shear in flat slabs with open stirrups as shear reinforcement

AbstractThe performance of open stirrups shear reinforcement not fully anchored is investigated, with a type of detail that has been used in construction sites as a support for the top flexural reinforcement to guarantee the right height position; and subsequently, it has also been used to address punching. Five internal slab–column connections subjected to symmetrical loading were tested, including a reference connection and four connections with anchorage variation of shear reinforcement, and their performance was compared with estimations by Codes and Standards and with test results from other authors. Even though the tests presented showed that an increase from 25% to 45% in the punching resistance can be achieved, and that the deformation capacity can also be improved, more tests with other detailing, ratios of flexural and shear reinforcement, and so on, are needed before it can be recommended as an effective shear reinforcement alternative.

Experimental and numerical investigation of punching shear capacity of corroded reinforced concrete slab-column connections

Rebars in reinforced concrete (RC) slab-column structures may become corroded under adverse conditions, rendering slab-column connections (SCCs) more susceptible to punching shear (PS) failure. In this study, four RC SCC specimens were designed and fabricated, and their rebars were subjected to accelerated corrosion using an electrochemical approach. Next, pseudo-static tests were performed to determine the PS capacity of the RC SCC specimens with different corrosion ratios. The test results showed that the corrosion of the rebars in an RC SCC significantly reduced the angle of its PS plane and consequently lowered its PS capacity. On this basis, a high-fidelity finite-element model was generated using LS-DYNA, and its accuracy was validated through a comparison of the simulation and test results. Based on this model, numerical simulations were performed to examine the effects of four parameters, namely, the size of the corroded area, the reinforcement ratio, the slab thickness, and the boundary conditions, on the PS capacity of corroded RC SCCs. The numerical simulation results show that increasing the size of the corroded area reduces the PS capacity of an RC SCC and increasing slab thickness weakens the effect of rebar corrosion on the PS capacity of an RC SCC, while increasing the reinforcement ratio has a limited effect on the improvement in the PS capacity of a corroded RC SCC. Additionally, increasing the horizontal boundary stiffness significantly enhances the PS capacity of a corroded RC SCC.

Investigation of the punching shear behaviors of reinforced concrete slab–steel reinforced concrete L-shaped column connections

To study the punching behaviors of reinforced concrete (RC) slab–steel reinforced concrete (SRC) L-shaped column connections, four slab–column connection tests were designed and completed, and a new punching shear resistance measure proposed. The test variables were column section shape, slab thickness and column cap. The test results show that the failure mode of L-shaped column was mainly two-way shear. Meanwhile, increasing slab thickness and installing column cap can effectively improve the ultimate bearing capacity of the slab–column connection. The punching angles formed by the punching cone and horizontal direction at the final failure of the connection were between 23.5° and 49.5°. In addition, a numerical simulation analysis was carried out using ABAQUS software, and the effects of various parameters on the performance of slab–column connections were studied. It was found that the change of column section shape will lead to more complex shear stress distribution and stress concentration phenomenon. However, the punching capacity of the special-shaped column was higher than that of the equivalent square column with the same section area. As a new punching shear resistance measure, inverted T-shaped steel can effectively improve the ultimate bearing capacity of the connections. In addition, when the ratio of slab thickness to web width of inverted T-shaped steel was less than 0.75, the failure mode of the connection changed from brittle punching failure to flexural failure.

STUDIES OF INTERIOR SLAB-COLUMN CONNECTIONS STRENGTHENED WITH HEMP FIBER REINFORCED POLYMER SHEETS

With increasing environmental concerns, using natural materials in concrete, such as bio-based fibers, is now becoming popular. Recently, hemp fibers have been utilized in the production of composite materials that have the possibility to replace synthetic fibers. The main objective of the research reported in this paper was to investigate the feasibility of using externally bonded hemp fiber-reinforced polymer (HFRP) fabric sheets as an alternative to synthetic carbon fiber-reinforced polymer (CFRP) sheets for punching shear strengthening of reinforced concrete two-way slabs. The experimental program consisted of testing small-scale, interior slab-column connections, under monotonically increased load until failure. The main test parameters included the slab thickness, type of strengthening material (HFRP or CFRP sheets), the width of HFRP sheets, number of sheets layers, and the location of the HFRP sheets relative to the column face. Test results showed that HFRP strengthening sheets enhanced the structural behavior of slab-column connections.

Punching Shear Strength of Post-Tensioned Transfer Slab-Column Connections

Punching Shear Strength of Post-Tensioned Transfer Slab-Column Connections

Seismic Response of Slab Column Connection with Pyramid Shaped Drop Panel

Both experimental and finite element analysis (FEA) was used to study the seismic response of reinforced concrete (RC) interior slab-column connection made with pyramid-shaped drop panel subjected to vertical and horizontal loads. The dimensions of the models at “¼ ” linear scale for laboratory testing and FE Analysis (FEA) are derived from rules for dimensions of column drops, given a prototype “9.60m” grid and a slab thickness of “320mm”. Lab specimens were tested with the drops (flat slab, rectangular and pyramid-shaped) facing up, with loadings (vertical down and horizontal in grid direction) applied by jacks towards the top of a central projecting “150mm” square column. One flat slab (“80mm” thick no drop), tested to failure under vertical load (80kN), provided values for setting variables used in the FEA. The remaining 5 lab specimens (1 flat and 1 each rectangular and pyramid-shaped 40mm and 30mm drop thicknesses), under a fixed vertical load (40kN), were tested to failure by increasing a moment at the column slab junction applied by a horizontal load to the column “500mm” above the slab. FEA results for the same conditions compared closely with experimental results. The pyramid-shaped drop models, with equal thickness to the rectangular drop models at the column faces (drops of “40mm” and “30mm”), exhibit similar maximum force resistances to the rectangular drop models. However, these resistances were achieved in the pyramid drops at higher maximum deflections – deflections being measured downwards at column centerline one half of slab thickness away from the face. A parametric study was conducted by FEA, at constant load, in vertical steps (10kN; 25kN 55kN), calculating deflections under increasing horizontal load. Calculations were made on the following definitions: Energy absorption is represented by the area under the deflection vs horizontal load curves; Ductility is the ratio of deflection at maximum to deflection at yield, and Stiffness and Overstrength factors are the slopes of the deflection load diagrams in the elastic and plastic zones respectively. Both drop type models exhibit significantly improved performance compared to the models without drops. The pyramid-shaped drop models exhibited improved energy absorption, ductility, and stiffness and overstrength compared to the rectangular drop models of the same column face thickness.

Distinguished punching shear behaviour of unbonded post-tensioned concrete slab – CFT column vs reinforced concrete slab – CFT column connections

This article evaluates and compares the punching shear behaviour of unbonded prestressed concrete (UPC) slab - concrete-filled steel tube (CFT) column connections/joints with that of reinforced concrete (RC) slab connection – CFT column connections. The experimental program was carried out on five large-sized samples, including three UPC slab - CFT column samples and two RC slab - CFT column samples. Steel plate connection details were utilised to increase the bond between the concrete slab and the CFT column. The experimental results showed that the prestressing tendons significantly increased the pre and post-cracking stiffness of the slab-column connections, thereby, greatly reducing the deflection of the UPC slab - CFT column samples than that of the RC slab - CFT column samples, especially during the serviceability phase (by up to 58%). However, the use of tendons resulted in a significant decrease in the ductility and energy absorption index of the UPC slab - CFT column samples compared to that of the RC slab - CFT column samples (by up to 44% and 41%, respectively). Using prestressing tendons slightly increased the punching shear capacity, and significantly reduced the strain of the connection details such as the vertical ribs and the horizontal bearing plate. At the same time, the tendons helped greatly reduce the strain of the longitudinal rebars in the UPC slabs.
 Keywords: Unbonded post-tensioned concrete (UPC); connection/joint; CFT column; flat slab; connection details; punching shear; experiment.

Performance of Mid Rised RCC Structure using Flat Slab System

Flat-slab building structures possesses major advantages over traditional slab-beam-column structures because of the free design of space, shorter construction time, architectural –functional and economical aspects. Because of the absence of deep beams and shear walls, flat-slab structural system is significantly more flexible for lateral loads then traditional RC frame system and that make the system more vulnerable under seismic events. The critical moment in design of these systems is the slab-column connection, i.e., the shear force in the slab at the connection, which should retain its bearing capacity even at maximal displacements. The behavior of flat slab building during earthquake depends critically on ‘Building Configuration’. This fact has resulted in to ensure safety against earthquake forces of tall structures hence, there is need to determine seismic responses of such building for designing earthquake resistant structures. Response Spectrum analysis is one of the important techniques for structural seismic analysis. In the present work analysis of 4 models of multi-storied RCC Flat slab structure is carried out by response spectrum analysis. The BIS guideline in IS 1893:2002 says “Regular and Irregular Configuration to perform well in an earthquake, a building should possess four main attributes, namely simple and regular configuration, and adequate lateral strength, stiffness and ductility. Buildings having simple regular geometry and uniformly distributed mass and stiffness in plan as well, suffer much less damage than buildings with irregular configurations”. Similarly, in IS 4326:1993 it is mentioned that “The building should have a simple rectangular plan and be symmetrical both with respect to mass and rigidity so that the center of mass and rigidity of the building coincide with each other.” But the limiting “plan aspect ratio” and “Slenderness ratio” for the regular structure is not prescribed. This study is concerned with the behavior of structure having same plan area but different plan aspect ratio (L/B) and slenderness ratio (H/B) under seismic condition. The structures are simulated in ETABS software and analyzed using Response Spectrum method.

STUDIES OF INTERIOR SLAB-COLUMN CONNECTIONS STRENGTHENED WITH HEMP FIBER REINFORCED POLYMER SHEETS

With increasing environmental concerns, using natural materials in concrete, such as bio-based fibers, is now becoming popular. Recently, hemp fibers have been utilized in the production of composite materials that have the possibility to replace synthetic fibers. The main objective of the research reported in this paper was to investigate the feasibility of using externally bonded hemp fiber-reinforced polymer (HFRP) fabric sheets as an alternative to synthetic carbon fiber-reinforced polymer (CFRP) sheets for punching shear strengthening of reinforced concrete two-way slabs. The experimental program consisted of testing small-scale, interior slab-column connections, under monotonically increased load until failure. The main test parameters included the slab thickness, type of strengthening material (HFRP or CFRP sheets), the width of HFRP sheets, number of sheets layers, and the location of the HFRP sheets relative to the column face. Test results showed that HFRP strengthening sheets enhanced the structural behavior of slab-column connections.

Punching shear capacity of steel-fibre recycled aggregate concrete slab

For this paper, the punching shear behaviour of 18 steel-fibre recycled aggregate concrete (SFRAC) balanced slab–column connections was experimentally investigated and analytically modelled for the following variables: the recycled concrete aggregate replacement level (0, 50 and 100%) and the volume fraction of hooked-end fibres (0, 0.4 and 0.8%). The measured punching shear capacities were practically insensitive to the recycled aggregate replacement levels, although both the inclination of the punching failure surface as well as punching capacities increased with fibre dosage. Relatively, the best performance improvement was obtained at 0.4% fibre dosage wherein the punching capacities and energy absorption increased by 23% and 70%, respectively, as compared to the non-fibrous concrete specimens. Using the experimental results, the authors propose and calibrate a simple strut-and-tie based punching shear predictive model for SFRAC, and undertake a validation using the limited results available in the literature. Among the selected current design codes under appraisal, the most conservative punching shear predictions for the SFRAC specimens were obtained from the Indian code, IS456:2000, whereas relatively more accurate predictions were obtained from the Eurocode 2.

Yield-Line solutions for flexural failure of flat slabs on columns of different shapes

There are two failure modes of slab–column connections, flexural and punching shear. However, when the tensile reinforcement ratio in the slab is low, the failure mode of the slab is difficult to distinguish, because the slab may fail in flexural before the punching shear failure occurs. To study the flexural failure of flat slabs on special-shaped columns (cruciform, L-shaped, T-shaped), the ultimate bearing capacity of slab–special-shaped column connections considering corner lever effect under vertical concentrated load was deduced and predicted by using yield-line theory. The correctness of the theoretical solution was verified by comparison with the experimental results. In addition, the effects of column section shape, column limb length to thickness ratio (c2/c1) and reinforcement ratio on flexural capacity were studied. The results show that the column section shape and the c2/c1 have little influence on the flexural capacity of the slab, but the reinforcement ratio will change the failure mode of the slab-column connections. When the reinforcement ratio was less than 0.8%, the failure mode of slab–special-shaped column connections was main flexural failure..