Experimental behavior of connection between composite columns and reinforced concrete flat slab

Recently, the construction industry has embraced CFST columns for high-rise buildings due to their advantages: significantly higher axial load capacity, greater stiffness, improved energy absorption under seismic activity, and faster on-site construction. In addition, the shortcomings of the two sections (steel tube and concrete infill) may be compensated, and the advantages can be combined to form an effective structural element, owing to the composite effects. Steel and concrete composites, which make use of the interaction between steel tube and concrete sections, are an effective solution for bearing a large axial load, where the steel tube confines the concrete, resulting in improving their properties and increasing their strength, and other stresses and the concrete in the CFST column prevents the steel component from local buckling and resists compressive loading. Moreover, this type of structural element is favored in practice as Abstract The combination of concrete-filled steel tube (CFST) columns and reinforced concrete flat slabs provides a suitable structural solution that can be used to replace traditional structures in high-rise buildings. The connection between the CFST column and the RC flat slab is an important factor for this construction system to operate efficiently. In this study, a new form for the connection between the CFST column and the RC flat slab has been proposed. This connection has more constructability, where the steel tube is cut at the level of the slab to prevent the separation between the slab and the concrete inside the steel tube as the slab reinforcement is allowed to continue in the connection zone. The joint zone is strengthened with rebar rings to compensate for the loss in confinement caused by cutting the steel tube at the slab level. In addition to using longitudinal rebar in the joint zone to bear part of the vertical load and to ensure bending rigidity. The behavior of the proposed connection was investigated by testing three sets of small-sized samples. In addition, an equation based on the outcomes of the second set of tests has been carried out to determine the joint bearing capacity for the vertical load.

a suitable replacement for traditional columns, where the external steel t`ube works as formwork, reducing building costs and time.RC flat slab structure offers many advantages including reduced story height, where the use of beams to support slabs in high-rise buildings reduces the clear height of the floor, which affects the layout of the structure and the effective utilization of the building.In addition, the flat slab system provides ease of construction and flexibility in space and mechanical equipment systems.As a result, the combination of a CFST column and an RC flat slab offers a great structural system for high-rise structures.CFST Column-RC slab connection is the most critical factor for the proper performance of this type of structure.Therefore this connection has been studied by some authors, such as.
H Satoh, and K Shimazaki (2004) [1] suggest a shape for the connection between (CFST) columns and the flat slab by using a steel diaphragm around the CFST column and connecting plate, welded to it, and the web reinforcement of H-shape steel embedded in a flat slab is connected by high-strength bolts, as shown in Fig. 1.
CH Lee et al. (2008) [2] in this study, some forms of connection schemes of CFT columns to reinforced concrete flat plates were developed and evaluated.In addition, a semi-analytical model was provided to describe the behavior of this connection.M.A. Eder et al. (2012) [3] proposed a shear head mechanism for attaching steel columns to reinforced concrete flat slabs.The connection's favorable inelastic performance under combined gravity and cyclic lateral loads is demonstrated using experimental and computational data.
YK Ju et al. (2013) [4] proposed initially three different forms of (CFT) columns with reinforced concrete flat slab connections and chose one of them based on the constructability of the connections and the economy of construction.P Yan, YC Wang (2014) [5] suggested a two-shear head system between a steel tubular column and RC flat slab.These systems can provide extremely high punching shear resistance, but they are difficult and costly to manufacture.2016) [6] have studied the structural behavior of hybrid elements composed of steel tube columns connected to RC flat slabs by shear heads, with and without shear reinforcement.
Zhang et al. (2018) [7] proposed a novel connection between a prefabricated RC flat slab and a square steel tube column, and the seismic behavior of this connection was investigated in an experiment.
Dao Ngoc Luc, et al. ( 2019) [8] (https:// www.ijeat.org/ wp-conte nt/ uploa ds/ papers/ v9i2/ B2974 129219.pdf ) proposed an improved connection of RC flat slabs and square (CFST) columns using a shear head.Two large-scale samples were used in trials to determine the capacity and dependability of the proposed connection.The test findings are additionally validated using ABAQUS numerical simulation.An analytical prediction model for estimating the punching shear resistance of the flat slab was provided.
CH Lee et al. (2019) [9] presented full-scale cyclic test results for three CFT column-RC slab connections and one traditional RC slab-column connection.Shear heads welded on structural steel sections were used as keys for vertical interface shear transfer and as partial punching shear reinforcement for slab column connections.Shear bands were used to further strengthen one CFT column-RC slab connection sample.The overall seismic performance of the tested samples was evaluated experimentally.
JL Yu and YC Wang (2020) [10] presented the findings of an experimental and theoretical study of the punching shear behavior of a simple and practical way of connecting a steel tubular column to a concrete flat slab using welded shear studs, as shown in Fig. 2.An analytical approach appropriate for use in design is modified using experimental and numerical simulations.BT Luu, et al. (2022) [11] have proposed innovative connections between (CFST) columns and unbounded post-tensioned slabs.The punching shear behavior of the proposed connection was studied experimentally by testing six large-sized samples.
RH Ahmed et al. (2024) [12] presented a special shear-head design for RC flat slabs and steel tube columns connection.Two thorough tests were carried out to ensure the durability and strength of this connection.
Finally, proposed connections between the CFST columns and the RC flat slab in previous studies have limits in terms of construction and reliability, where these This study proposes a new form of connection, which is characterized by ease of construction on-site, in addition to a large bearing capacity for stresses.
Three series of small-sized samples are presented below to study the behavior of the proposed connection.Finally, an equation was proposed to calculate the bearing capacity of the joint for the vertical load, taking into consideration the confinement provided by the rebar rings.The validity of the proposed formula was confirmed by comparing its calculated values with experimental results.

Methods
This study proposes a new method for connecting concrete-filled steel tube (CFST) columns to reinforced concrete (RC) flat slabs.The focus of this technique was to ensure both ease of construction on-site and sufficient strength in the final connection.The steel tube is cut at the level of the slab and rebar rings are used to strengthen the joint area to compensate for the loss in confinement as a result of cutting the steel tube.Additionally, the vertical longitudinal reinforcement bars in the joint region are used to bear part of the vertical load in addition to resisting the bending moments on the joint region as illustrated in Fig. 3.
Small-scaled models of the connection were built, with a scale down of one to six compared to the actual size.These models were then subjected to a series of tests in three groups.
The first series of tests consisted of six samples.Two samples represented the standard CFST column design, while the remaining four incorporated the proposed connection region.This group aimed to compare the axial load capacity of the standard CFST column to the connected column samples.
The second series of tests involved six samples, each containing the connection zone and two short CFST columns.This group's purpose was to determine the exact axial The third series focused on punching shear behavior.It included four samples: slabs connected to CFST columns, both with and without the proposed stirrups (reinforcing rings).This group assessed the effectiveness of the connection in preventing this type of failure.
Finally, based on the test results, a formula was developed to predict the load capacity of the joint, taking into account the additional strength provided by the reinforcing rings used in the connection.The tests confirmed that this formula accurately predicts the joint's capacity.

Concrete properties
To minimize the effect of using small-sized samples, the mortar was used instead of conventional concrete, for each mortar mix.As the samples were being cast, three mortar cylinder samples were prepared.These mortar samples were cured in the same way as the samples and tested on the same day as the samples.Only commercially available ingredients and standard mixing and curing processes were used to achieve compressive strengths of 20 and 25 MPa in mortar.Before the testing began, two experimental mix designs were developed.Following the European Code, cylinder sample tests were carried out with a diameter of 80 mm and height of 160 mm to measure the compressive strength of the mortar.The proportions of the mortar mixtures used in that study are presented in Table 1 by weight.

Steel properties
Tests on coupon steel for steel tube and each diameter of rebar were conducted to assess the rebar strength and steel tube strength, steel parameters such as yield, and fracture stresses were obtained from coupon samples from used steel tubes for further investigation and computation.
The reinforcing rebar used in the samples was tested to calculate the yield stress and maximum tensile strength, in addition to testing two coupon samples extracted from the steel tube and tested to failure with a constant rate of loading 5 mm/min, as well to obtain the stress-strain curve for the steel tube used in the CFST columns and to deduce the yield stress and the maximum tensile strength from the curve.The results of the tested bar with a diameter of 2.7 mm showed that the yield stress was 355 MPa approximately, as shown by the curve in Fig. 4, and the results of the bar tested with a diameter of 2.0 mm showed that the yield stress is not significantly different from that in the case of a diameter of 2.7 mm and is approximately equal to 350 MPa as shown in Fig. 5, while the yield stress of the steel tube was 300 MPa, as shown in Fig. 6.One of the coupon samples was tested under a low strain rate.As may be predicted, the reduction in the cross-sectional area caused the failure of all samples to occur in the gauge length segment of the sample.Additionally, as shown in Fig. 7, the coupon sample showed a suitable level of necking gradually before the rupture.

Experimental program
To assess the performance of the new connection design, a total of sixteen smallscale models were built and tested, each one-sixth the size of an actual connection.The experimental program was divided into three separate series, each with its own unique aim, samples, and parameters.The following sections will detail the specific setup, tested samples, and parameters for each test series.

Test setup
All samples were tested under axial compression loading using the testing machine at the Concrete Research Laboratory at Cairo University.The samples were loaded to failure, with displacement control and a constant rate of loading.Linear variable displacement transducers (LVDT) were placed at the upper surface of samples to measure the total shortening of the sample with loading as shown in Fig. 8.
It is also worth mentioning that in the first set of samples, the goal was to compare the behavior of CFST columns in the case of the presence of a connection in the column and the case of the absence of a connection to estimate the effect of the presence of the connection on the bearing capacity and ductility of the column.Therefore, the total shortening resulting from the impact with a vertical load on the column was measured to compare the presence and absence of connection.The shortening of columns was measured by installing the LVDT in the upper part of the testing machine (the moving part), considering that the lower part is fixed, and therefore the resulting shortening of the column is the reading of the LVDT.

Tested samples
Six small-sized samples were tested in the first series of samples; two samples of them were CFST columns (control samples) and the other four samples consisted of (CFST) columns with the convection zone, which strengthened with rebar rings.The percentage of vertical reinforcement in the joint zone and the difference in concrete compressive strength between the joint zone and the column were identified as two variables in the first group of tested samples as illustrated in Table 2.For samples that contain the connection zone, the ratio between the diameter of the column and the thickness of the slab d/ts is equal to 2, as shown in the samples' geometry in Fig. 9a.The connection zone was strengthened with 6 rebar rings divided into two layers, thus the reinforcement ratio for the strengthened ring was 1%.The rebar rings were fixed with 6 square stirrups, and the longitudinal vertical rebar of the column went through the joint zone and extended with the overlapping length in the upper and lower columns as shown in Fig. 9b.Moreover, in the first and second group of samples, no reinforcement steel was used for the slab for several reasons.Firstly, slab reinforcement can enhance the bearing capacity of the proposed connection.By excluding it, the effects of  rebar rings on the connection's behavior can be more readily assessed.Secondly, the presence of slab reinforcement would complicate the isolation and evaluation of the specific benefits provided by rebar rings.Finally, the primary function of slab reinforcement is to resist bending moments, which were not present in the first and second test sets.

Test setup
The second series of samples were tested under axial compression loading using the testing machine at the Concrete Research Laboratory at Cairo University.The samples were loaded to failure at a constant rate of loading.One linear variable displacement transducer (LVDT) was placed at the upper surface of the samples to measure the total shortening of the sample with loading as shown in Fig. 10.

Tested samples
The second series of tests were carried out to further study and assess the joint zone's axial load capacity.Six small-sized samples for the connection zone were tested, which included two parts: the connection zone and two short CFST columns acting as compression parts.In the second group of samples, the percentage of vertical reinforcement in the connection area was constant, which is 1% of the cross-sectional area of the (CFST) column, and the percentage of transverse reinforcement with rebar rings in connection region was variable, unlike the first group of specimens, in order to study the effect of the percentage of rebar rings reinforcement ratio on the bearing capacity of the proposed connection for the vertical load, as shown in the reinforcement details of the second series of specimens in Fig. 11.The thickness of the steel tube in the second group of samples was also increased to 2 mm instead of 1 mm as in the first group, in order to avoid a failure in the CFST column or a local buckling in the steel tube.Whereas the CFST columns in the second group of samples act as a loading element on the joint region, the rigidity of these parts must be ensured and The slab thickness factor (d/t s ) and the rebar ring area ratio in the strengthened ring (A/t 2 ) were specified as two non-dimensional parameters.These two factors were used to classify the tested samples, with S1 and S2 referring to ρ = 0.5% and 1.0%, and, A1, A2, and A3 referring to β = 1.25, 1.5, and 2.0, respectively as illustrated in Table 3.The concrete cylinder compressive strength of the second group of samples at the time of testing was 22 MPa.

The third series of tested samples
In the third series of tests, the punching shear behavior of the proposed connection was evaluated by testing four small-sized samples.The vertical stirrups are used to stabilize rebar rings in addition to increasing punching shear capacity, the behavior of samples with these stirrups was compared with those without reinforcement.A 45° inclined branch has been added to the stirrups distributed in the radial direction around the column as shown in Fig. 13.Hence, the additional branch is perpendicular to the crack direction caused by the shear stress.

Test setup
All samples were tested with the settings of the punching shear experiment, where the sample is placed on a steel frame to prevent the perimeter of the slab from moving vertically, and the column in the middle of the slab is affected by a downward pressure load using the testing machine in the Concrete Research Laboratory at Cairo University.The samples were loaded to failure at a constant rate.Two (LVDTs) were placed on the bottom surface of the samples to measure the net deflection at the slab's center with loading as shown in Fig. 14.

Tested samples
The test samples were classified based on two variables, namely the concrete's compressive strength and the usage of shear reinforcement stirrups, as shown in Table 4.
To verify the possibility of increasing the punching shear resistance of the joint as a result of using shear reinforcement, four small-sized samples were tested.The  percentage of rebar rings used is 1%, as is the case in the first series of tested samples, in addition to using six vertical stirrups equipped with an inclined branch to improve the punching shear resistance.The distance between the stirrups from the inside is approximately equal to the slab's effective depth (d).Whereas the effective depth is the thickness of the concrete resistant to punching shear and is equal to the thickness of the slab minus the concrete cover (ts-cover).However, in the third group of samples, the flat slab was reinforced as shown in Fig. 15, where in this group the shear punching resistance of the samples was tested, so there were bending moments on the slab during the test, so the slab was reinforced, unlike the first and second groups.The dimensions of the slab were (400* 400) mm and its thickness was 40 mm, while the CFST column's outer diameter was 60 mm, so the ratio of the column's diameter to the slab's thickness was approximately 1.5.

Results of testing the first series of samples
In the first set of tests, six samples were tested.Two of them were a CFST column, and the other four consisted of a column with a connection zone to compare the column's axial capacity to the joint's bearing capacity.Initially, two samples of CFST  columns were tested as shown in Fig. 16, to compare their bearing capacity to samples with the connection.From the experimental results, apparently, the axial capacities of the CFST column with connection samples (CFST-C1 to CFST-C4) were not less than the axial capacity of the two CFST columns as shown in Fig. 17.This proved the effectiveness of confining through the rebar rings to strengthen the joint region.The increase in the longitudinal reinforcement ratio in samples CFST-C2 and CFST-C4 did not affect much, because the failure occurred in the column, not the joint for all samples, including the two samples with the lower longitudinal reinforcement ratio.In addition, when testing a sample with a connection without reinforcement with rebar rings, to verify the effectiveness of the rebar rings in strengthening the joint region, failure occurred in the connection region, as shown in Fig. 18.This confirms the effectiveness of reinforcing the connection region with rebar rings.
Samples with connection (CFST-C1 to CFST-C4) acted similarly during the tests.The first radial crack always appeared on the slab's top surface.As the load increased, more radial cracks appeared.Until reaching the maximum load of the experiment, no failure was observed on the tested samples except for narrow radial cracks on the surface of the slab, while no failure was observed on the CFST column.
After the load reached Nu, the CFST columns appeared to fail, while the joint did not reach its maximum capacity.Visual inspection following the test revealed no significant degradation in the integrity of the concrete directly below the steel tube.Finally, the thin steel tube buckled locally, followed by crushing the concrete core.This resulted in an increase in the transverse strain of the steel tube gradually, which led to a longitudinal fracture in the steel tube for some samples such as the CFST-C3 sample, as a result of the steel tube reaching fracture stress, as shown in Fig. 19.
The four samples with connection failed in the column zone with different failure positions, where the failure occurred in the two samples (CFST-C1 and CFST-C2) in the upper third of the column height, while the two samples (CFST-C3 and CFST-C4) the failure occurred in the middle third of the column height.The reason for the different failure locations in columns is that in this type of column, the section properties are almost constant over the entire length of the column and hence the failure location is variable and occurs approximately in the part where local buckling occurs in the steel tube.

Results of testing the second series of samples
In the second series of tests, only the connection zone was evaluated.Six small-sized samples were tested to calculate the bearing capacity of the proposed connection for the vertical load and to know the effect of the identified variables.The slab thickness and the ratio of transverse rebar rings in the connection region are the variables that were identified in this set of samples.
As it seems from the load versus shortening curves shown in Fig. 20 the ratio of rebar rings has a significant effect on the axial capacity of the joint, while the effect of the change in the thickness of the slab is small due to the effect of the confinement.
Unlike unconfined concrete where increasing the t s /d ratio leads to a decrease in axial capacity due to lateral expansion.
All samples behaved identically during the tests, as narrow cracks in the concrete of the connection region appear gradually in the diagonal direction.As the load increased, more radial cracks were observed on both the top and bottom surfaces of the connection region and the initial cracks were widening until the outer rings fractured then a sudden decrease in the load occurred.Finally, the failure shapes were similar for almost all samples as illustrated in Fig. 21.

Results of testing the third series of samples
The results of the tests showed that the increase in the concrete compressive strength led to an increase in the punching shear resistance of the samples with the highest concrete strength, namely samples (B2-C1 and B2-C2).As well as using rings rebar and stirrups to strengthen the connection region increased the punching shear capacity of those samples (B1-C2 and B2-C2), in addition to the ductility of those samples and improved their punching shear behavior.The failure of the samples with no shear reinforcement was brittle, especially the sample with the lowest concrete resistance (B1-C1) as shown in Fig. 22.
Increasing concrete strength by 25% increased the connection punching shear capacity by approximately 20% for the sample without shear reinforcement, and the punching shear resistance increased by approximately the same value for strengthening the sample due to increasing the concrete strength.The shear reinforcement increased the punching shear resistance of the connection up to 28.7% for the sample with a concrete compressive strength of 20 MPa, and the punching resistance of the sample with a concrete compressive strength of 25 MPa increased by 31%.
The increase in the punching shear resistance of the strengthened samples using shear reinforcement due to using stirrups distributed in the radial direction around the column increased the punching shear circumference, as shown in Fig. 23.Thus, the enhancement of the punching shear capacity of the connection as a result of increasing the surface area resisting shear due to the enlargement of the failure perimeter.

Capacity calculations
Based on the experimental results, an equation is deduced to calculate the bearing capacity of the proposed connection.The confinement pressures resulting from the rebar rings used to strengthen the joint region were taken into consideration through the increase of the confinement pressure on this region and thus increases the concrete compressive strength.The computational values deduced from this equation were investigated by comparing them with the results of the second group of the tests.Moreover, the European code EC4 [13] equation was used to calculate the axial capacitance of a CFST column.

The axial compressive capacity of the CFST column
There are many equations for calculating the axial capacity of the CFST column based on many researches that have been carried out to study the behavior of this type of  2004) [14], it was concluded that the most recent international standard code for composite construction is Europe code (EC4).Concrete-filled tube sections with or without reinforcement, as well as concreteencased and partly-encased steel sections, are all covered by EC4.By applying partial safety factors to loads and material properties, EC4 achieves the objectives of safety and serviceability.Accordingly, the European code equation was chosen to calculate the axial capacity of the CFST column, and the validity of this equation was verified by comparing the bearing capacity of the CFST column calculated by this equation with the results of experimental tests of CFST samples in this research as shown in Table 5.
According to EC4 code, the CFST column's ultimate axial force is the following: (1)

Mechanical behavior of joint zone
According to the observation of experimental tests, the joint zone resisted the axial compressive stress, with the entire region compressed in the vertical direction while extending in the lateral direction as illustrated in Fig. 24.
Due to the expansion of the concrete part allocated between the upper and lower columns, the rebar rings gradually yielded outward from the inside, and radial cracks gradually appeared on the top and bottom faces of the slab around the column.As a result of the confinement of the exterior concrete slab and rebar rings, the concrete in this region is subjected to tri-axial compression stresses.

Rebar rings confinement effect
For this connection, the concrete in the joint region is confined by rebar rings.Therefore, this study used the Mander et al. (1988) [15] equations to calculate the increase in where f cc = confined concrete compressive strength; f co = unconfined concrete compressive strength; and.fl = the rebar ring effective lateral confining pressure, which is given by Where As = ring rebar cross-sectional area; f y = rebar ring yield strength; ds = rebar ring diameter; S = rings rebar vertical spacing (center-to-center); and.ρ l = the longitudinal reinforcing rebar area to the total area of the ratio of the column.Since the concrete in the joint region is surrounded by two layers of rebar rings, an inner layer with a diameter equal to the diameter of the column, and an outer layer at a distance from the column face equal to the effective thickness of the slab (d).Therefore, to take the effect of the two layers of rebar rings on the concrete in the joint region, the superposition theory proposed by Chen, Q. et al. (2015) [16] is used to estimate the increase in concrete compressive strength of the joint zone, in which not only the inner rebar rings confinement effect but also the outer rebar rings confinement effect, are considered as illustrated in Fig. 25.
Thus, after applying the superposition theory between the outer and inner rings, Eq. 2 becomes as follows: Where fl1 = the outer rebar rings' effective lateral confining pressure.fl2 = the inner rebar rings' effective lateral confining pressure.Since the connection region has a larger cross-section area rather than the bottom and top columns, the vertical loads are distributed over a larger area in the joint region with a slope of 2 horizontal to 1 vertical, so the area loaded becomes at the middle of the thickness of slab A2. (2) According to ACI (318-08) [17], the bearing capacity of the joint region under compression is Where A1 = cross section area of the column; and.A2 = area of the truncated cone's lower base at half the slab thickness, with the column area for its upper base, and a side slope of 2 horizontal to 1 vertical, as illustrated in Fig. 26.
Thus, due to the difference in concrete strength between parts A1 and A2 as a result of the difference in confining strength, where A2 is confined by the outer rings layer only, while A1 is confined by the inner and outer layers together as shown in Fig. 27.
Finally, in this study, after considering the confinement effect induced by the rebar rings to strengthen the joint region, in addition to taking into account the effect of increasing the loaded area in the joint zone as a result of the load distribution when load transfer from the column to the slab, it is possible to deduce an equation for predicting the proposed connection bearing.
Where f cc1 = the concrete compressive strength confined by the outer rebar rings.f cc2 = the concrete compressive strength confined by both outer and inner rebar rings.This equation can be used to determine the proposed connection-bearing capacity, after verifying the accuracy of the proposed equation by comparing the calculated values using it with the results of experimental tests, as illustrated in Table 6.The average value of the ratios between the bearing capacity values of the connection from the experimental tests to the calculated values is approximately equal to 0.99, while the standard deviation of the ratios is approximately equal to 0.054.
Finally, the (CEN 2005) equation was used to determine the CFST column axial capacity of the first set of tested samples, in addition to calculating the bearing capacity of connections by Eq. ( 5) proposed in this study to predict the location of the failure, whether it occurred in the joint or the CFST column, accordingly the failure occurs in the weakest element as shown in Table 7.
Although the maximum bearing capacity of the proposed connection is greater than the CFST column's bearing capacity in some cases, the behavior of the connection after reaching the maximum load differs from the behavior of the CFST column, as is evident from the experimental test curves.Therefore, when using the proposed equation to calculate the bearing capacity of the connection, one must use safety factors to ensure that the connection does not reach its maximum axial load in order to ensure that the connection region remains in the elastic stage and its behavior does not differ greatly from the behavior of the CFST column.

Conclusion
A series of conclusions were drawn from the results of laboratory tests conducted to examine the behavior of the suggested connection and compare it with the proposed equation for determining the joint's axial bearing capacity: • According to the first set of tests, the joint zone's axial capacity may be greater than the CFST columns after strengthening the connection with rebar rings to compensatefor the loss in confinement caused by cutting the steel tube at the flat slab level.• The results of the second set of tests indicated that the area ratio of the rebar rings used to confine the concrete in the connecting zone had a significant effect on the joint's axial compressive bearing capacity.• Rebar rings can completely compensate for the confinement loss caused by the steel tube interruption through the flat slab.• The rebar rings produce a confining force that strengthens the joint and reduces shear deformation of the core region, as well as increasing the core concrete strength.• The joint zone's compressive strength rises with increasing confining pressure.
Meanwhile, the effect of the t s /d ratio lessens because of the development of lateral confinement from slab and rebar rings around the joint zone.• The accuracy of the proposed equation for predicting the joint's maximum axial capacity is validated by comparing the results of the second experimental series with the equation's predictions.• The equation of EC4 code's for calculating the axial capacity of CFST columns demonstrates high accuracy when compared to experimental data.• Tests from the third group investigated punching shear behavior in the connection between the slab and CFST column.The results showed a 20% increase in punching shear resistance for a 25% increase in concrete strength.Additionally, strengthening the connection area with rebar increased punching shear resistance by 30%.

Fig. 1
Fig. 1 Flat plate-CFT column connection proposed by H Satoh, K Shimazaki

Fig. 2
Fig. 2 Shear connection system using welded steel studs proposed by JL Yu, YC Wang

Fig. 3
Fig. 3 Details of the proposed connection

Fig. 4 Fig. 5
Fig. 4 Engineering stress-strain curve of reinforcement bar with diameter 2.7 mm

Fig. 8
Fig. 8 Test setup of the first series of tests

Fig. 9
Fig. 9 Details for the first series of samples.a The geometry for the first series of samples.b Reinforcement details of the first series of samples

Fig. 10
Fig. 10 Test setup of the second series of samples

Fig. 11
Fig. 11 Geometry and reinforcement details of the second series of samples

Fig. 15
Fig. 15 Details of reinforcement of the third set of samples.a Without punching a shear reinforcement.b With punching shear reinforcement

Fig. 18
Fig. 18 Failure in connection region for the sample without rebar rings

Fig. 22
Fig. 22 Load-deflection curves for the third series of samples

Fig. 25
Fig. 25 Superposition theory to sum the confinement effect

Fig. 26
Fig. 26 Load distribution in the slab zone

Table 2
Parameters of the first series of samples t and d are the steel tube thickness and diameter, respectively; ts is the slab thickness; fc' is the concrete cylinder compressive strength of the column/connection zone, respectively; and µ% is the column's longitudinal rebar ratio Sample d × t (mm × mm) ts(mm) d/ts fc'(MPa)

Table 3
Parameters of the second series of samples d is the diameter of the steel tube, ts is the slab thickness, and ρ% the ratio of the area of the rebar rings to the area of the RC strengthened ring

Table 4
Parameters of the third series of samples B1 stands for samples with concrete compressive strength of 20 MPa, B2 for sample's with 25 MPa concrete compressive strength, C1 stands for samples without shear reinforcement, and C2 for samples with shear reinforcement

Table 5
Comparing the axial capacity values of the first set of samples calculated with the EC4 code equation and experimental tests Joint behavior due to axial compression concrete compressive strength due to the confinement effect, which is presented as the following:

Table 6
Comparing the bearing capacity values of the second set of samples calculated with the proposed equation and experimental tests

Table 7
Prediction of failure location for the first series of samples