Panel zone behavior of diaphragm-through connection between concrete-filled steel tubular columns and steel beams

Abstract

The diaphragm-through connections are some of the most widespread connections between concrete-filled steel tubular columns and steel beams. In this work, five tee-shaped diaphragm-through connections were fabricated so that they comply with the principle of “strong members and weak panel zone.” Low-frequency cyclic loading was applied to the specimens to investigate the panel zone behavior of the connections. Based on the test results, the force transfer mechanism and the effect of different factors on the panel zone shear capacity were analyzed. In addition, different methods of calculating the panel zone shear strength were compared with the experimental data. The results show that the shear capacity of the panel zone of the connection is borne mainly by the steel tube webs and the core concrete. The steel frame composite with steel tube flanges and diaphragms strengthens the plastic deformation ability after yielding. It was found that in the existing computational methods for panel zone shear capacity of the connection, the calculated results are in agreement with the test results for the shear capacity of the steel tube webs, but the predicted values are not consistent with the concrete component capacity.

Keywords

concrete-filled square steel tubular column diaphragm-through connection experimental investigation panel zone shear capacity

Introduction

Diaphragm-through connections, as one of the typical connections between concrete-filled steel tubular columns (CFSTCs) and steel beams, are characteristic of good bearing capacity, ductility, seismic-resisting capacity, and constructional convenience and thus have been applied in steel structures widely. Over the past 30 years, much work has been done on construction shape, mechanical property, and computing methods of the diaphragm-through connections. Matsui (1985) carried out experimental study on the seismic behavior of diaphragm-through connections between concrete-filled square steel tubes and wide flange steel beams and proposed a method to calculating bending capacity. A test study of diaphragm-through connections under monotonic loading and cyclic loading has been conducted by Kanatani et al. (1987). Their test results showed that the filled concrete can delay or avoid the local buckling of the steel tube and can efficiently increase the panel zone shear strength. Morino et al. (1993) designed and tested 10 specimens of diaphragm-through connections under cyclic loading. Two failure modes were observed: a panel zone failure and a column bending failure. The analysis report showed that the deformation ability and the energy dissipation capacity in the mode of panel zone failure are better than those in the mode of column bending failure. The bearing capacity continues to improve after the shear yielding of the panel zone. Nishiyama et al. (2004) and Fukumoto and Morita (2005) carried out experimental studies on connections comprising high-strength steel and concrete. Based on the experimental results and taking into account the axial load, new computing models and calculating methods of shear capacity were proposed. The calculated results agree well with the experimental results. Walid Tizani et al. (2013) and Mahmood et al. (2014) proposed a new blind-bolted connection between concrete-filled tubular (CFT) column and steel beam. Experimental testing and numerical analysis were used to investigate the plastic behavior of this new blind-bolted connection. Experimental and analysis results show that the new connection provided stable hysteretic behavior with appropriate level of strength and stiffness, and the required performance can be achieved by controlling the tube wall thickness and concrete strength. The results also indicate that the connection can offer energy dissipation capacity and ductility appropriate for its potential use in seismic design. Rong et al. (2013a and b) used the finite element method (FEM) to analyze the tensile behavior and seismic behavior of the panel zone of diaphragm-through connections. The numerical data had a good agreement with the test results.Qin et al. (2014a, b, c, d and e) tested six through-diaphragm connections subjected to cyclic loading, in which the variables included the geometry of the diaphragm, the types of weld access holes, the presence of horizontal stiffeners, and the ways of connecting the beam web to the column. Component-based mechanical models were proposed to predict the flexural and shear strength of the connections.

Although some research has been done and methods for the calculation of the shear capacity of the panel zone of the diaphragm-through connections have been proposed, there are still some unanswered issues, mainly in the computation of the panel zone concrete. In this article, five tee-shaped diaphragm-through connections were tested under cyclic loading to investigate the panel zone behavior. The specimens were constructed according to the principle of “strong column and weak beam joint connection.” The behavior of the connections was evaluated in terms of the force–rotation response and strength skeleton curves. The shear force transfer mechanism was assessed according to the strain distribution. The test results are compared to the calculated data using different equations by various researchers and standards for predicting the shear strength of the connections.

Test program

Test specimens design

The moment distribution and an exterior joint deformation of a frame structure subjected to lateral loads are presented in Figure 1(a). According to the regulation of Specification of Testing Methods for Earthquake Resistant Building (JGJ 101-1996, 1996), the panel zone should fail before the columns and beams to ensure ductile behavior and to avoid structural instability due to large column deformation. The force and deformation configuration of an exterior joint of the frame structure is shown in Figure 1(b). The geometry of the test specimen was selected to represent an exterior joint in a frame structure subjected to lateral loads.

Figure 1.

Exterior joint in the frame structure: (a) frame subjected to lateral load and (b) idealized exterior joint.

In this article, five tee-shaped specimens of diaphragm-through connections were designed and fabricated with the “strong member and weak joint” principle. We reduced the thickness of the steel tube of panel zone to ensure that the members steel maintain elastic while the panel zone had already yielded. The joint details are shown in Figure 2. As seen in the figure, the right column is lengthened to ensure that the distance between the panel zone and two hinges is equal. The two hinges in column are on the same line and the load eccentricity will not be produced in the column.

Figure 2.

Details of the diaphragm-through connection: (a) tee-shaped specimen, (b) overhead view, (c) section view, and (d) axonometric view.

The main parameters are axial force, concrete strength, steel tube thickness, and diaphragm thickness, as shown in Table 1. Each of the tested specimens was tee-shaped and consisted of a CFSTC and a wide flange H-shaped steel beam. The ends of the beam and column of the test specimen corresponded to the inflection points. The columns and beams made use of cold-formed square steel tube and welded beam, and the steel grade of the specimens was Q235B. The columns and diaphragms were factory welded. The connection between columns and steel beams was done by welding and high-strength bolts. The connections were done according to the Code for design of Steel structures (GB 50017-2003, 2003). The type of electrode was E43 and the grade of high-strength bolt is S10.9 M16 (the bolt diameter was 16 mm). C30 and C40 concretes were filled in the steel tube.

Table 1.

Specimen series.

Name	Steel tube (mm)	Panel zone (mm)	Steel beam (mm)	Diaphragm (mm)	Concrete	Axial ratio
SJ-1	250 × 250 × 12	250 × 250 × 6	250 × 250 × 8 × 14	14	None	0.2
SJ-2	250 × 250 × 12	250 × 250 × 6		14	C30	0
SJ-3	250 × 250 × 12	250 × 250 × 6		14	C40	0.2
SJ-4	250 × 250 × 12	250 × 250 × 8		14	C40	0.2
SJ-5	250 × 250 × 12	250 × 250 × 8		16	C40	0

Material properties

The steel material of the specimens is Q235B and the concrete strength is C30 or C40. According to the regulation of Metallic materials tensile testing (GB/T 228.1-2010, 2010) and Standard for test method of mechanical properties on ordinary concrete (GB/T 50081-2002, 2002), the material test uses the same materials as that used in the diaphragm construction process. The material properties are shown in Tables 2 and 3.

Table 2.

Steel material properties.

Thickness (mm)	f_y (N/mm²)	f_u (N/mm²)	E_s (E5 MPa)
6	307.9	443.9	1.96
8 (beam web)	335.2	507.9	1.98
8 (column)	336.5	471.2	1.98
12	372.9	485.3	2.01
14 (beam flange)	262.1	437.3	2.03
14 (diaphragm)	263.3	429.9	2.04
16	259.8	425.7	2.04

Table 3.

Concrete material properties.

Grade	f_c (N/mm²)	E_c (E4 MPa)
C30	38.4	3.49
C40	49.1	3.75

Test setup

The testing methods for seismic resistance mainly include quasi-static, quasi-dynamic, and vibration. The quasi-static method with load and displacement cyclic control was used. The test setup includes self-balancing reaction frame, actuator, and jacks, as shown in Figure 3. A typical test was conducted by applying cyclic load to the beam end using a 1000 kN level hydraulic actuator whose displacement range was ±350 mm to provide lateral seismic loading effects. The jack was used to apply axial load to the column ends. The Mechanical Testing & Simulation (MTS) data collecting system was used to collect the data of the load and the horizontal displacement at the beam ends. The load–displacement hysteretic curve was drawn automatically as well.

Figure 3.

Test setup: (a) 3D diagram and (b) test setup.

Loading system

For quasi-static loading, load–displacement hybrid control was used. According to the Specification of Testing Methods for Earthquake Resistant Buildings (JGJ 101-1996, 1996), stepwise force load is applied to the beam ends and recycled one time at each step loading before yield. After yield, the specimen would be controlled by multiplying Δy (the beam tip horizontal displacement when specimens yield), Δy being the yield displacement of the specimen, and recycled three times for each step, as shown in Figure 4. Before testing, every specimen was preloaded twice to check the reaction of the test setup and the measuring devices.

Figure 4.

Regulation of loading system.

Instrumentation

The axial force of the column was applied by the jack and the control by the pressure gauge. The beam end displacement and force were recorded by displacement and force transducers. The rotation angle of the joint and the shear deformation were measured by the displacement meter and dual-axial digital protractor, as shown in Figure 5.

Figure 5.

Layout drawing of the displacement meters.

In order to investigate the force transfer mechanism and strain distribution of the diaphragm-through connections subjected to the lateral load, the development of strain in the panel zone was monitored by strain gauges. The layout of the strain gauges is presented in Figure 6.

Figure 6.

Layout of strain gauges: (a) column and beam web, (b) diaphragm and beam flange, and (c) column flange.

Test process and failure modes

Test behavior of SJ-1

SJ-1 is the only one specimen in this test program with no concrete infill in the steel tube. In specimen SJ-1, the column was subjected to a constant axial load equal to 0.2N₀, where N₀ is the compressive strength of the column. The beam was loaded cyclically until failure. At a load of 107.5 kN corresponding to 7.7-mm displacement, the panel zone yielded and had no significant deformation. Beyond this point, loading was continued with displacement controlled by applying equal multiples of yield displacement at the beam end. Local buckling initiated in the steel tube webs of the panel zone when it was loaded to the second cycle of the yield displacement. The load reached its peak value of 155.7 kN corresponding to 54.5-mm displacement. The panel zone had seriously distorted. As the load increased, the steel tube flanges started to buckle, as shown in Figure 7. The capacity drop to 84% of the peak load when it was subjected to five times the yield displacement. The specimen was then unloaded and the test was terminated.

Figure 7.

Panel zone behaviors of SJ-1: (a) slightly buckle on panel zone and (b) shear distortion on panel zone.

Test behavior of SJ-2

The axial load was not applied to specimen SJ-2. The grade of filled concrete is C30. The yield load and maximum load of the specimen are 213.3 kN corresponding to 34.8-mm displacement and 249.7 kN corresponding to 49.5-mm displacement, respectively. The shear deformation initiated in the panel zone. With the load constantly increasing, the diaphragm and beam flange started to buckle. Cracking occurred at the weld at the second cycle as presented in Figure 8. The specimen was then unloaded and the test was terminated.

Figure 8.

Test behaviors of SJ-2: (a) shear distortion and buckling and (b) weld crack.

Test behavior of SJ-3

Compared with the specimen SJ-2, the difference is that in this specimen C40 concrete was filled in the steel tube and axial load was applied to the column. The axial load ratio was 0.2. At a load of 218.8 kN corresponding to 17.1-mm displacement, there was no obvious yield deformation, but a inflection point was observed in the load–displacement curve. As the load was increasing, the local buckling initiated in the beam flanges and then in the diaphragms. The peak load of SJ-3 was 268.3 kN corresponding to 55.3-mm displacement. The weld started to crack at the connection between diaphragms and beam flanges when it was loaded to three times the yield displacement, as shown in Figure 9. The bearing capacity was rapidly falling after the weld crack and the test was stopped.

Figure 9.

Failure modes of SJ-3: (a) bending of diaphragm and (b) weld crack.

Test behavior of SJ-4

The purpose of testing specimen SJ-4 is to study, by comparison with specimen SJ-3, the effect of the thickness of the steel tube on the strength of the panel zone. The load conditions were the same as in SJ-3. The beam was loaded cyclically until failure. The yield load of the specimen was 220.7 kN corresponding to 17.1-mm displacement. Due to the poor weld quality, the weld started to crack at the connection between diaphragms and beam flanges when it was loaded to 234.6 kN corresponding to 49.5-mm displacement, as shown in Figure 10. The specimen was then unloaded and the test was terminated.

Figure 10.

Failure modes of SJ-4: (a) deformation of panel zone and (b) weld crack.

Test behavior of SJ-5

Compared with the SJ-4, the design of SJ-5 was used to investigate the influence of the thickness of the diaphragm on panel zone strength. The yield load and maximum load of the specimen are 234.1 kN corresponding to 30.2-mm displacement and 288.4 kN corresponding to 60.8-mm displacement, respectively. The shear deformation initialed appeared in the panel zone after yield. With the load constantly increasing, the local buckling initiated in beam flanges. At a load of 258.2 kN corresponding to 55.1-mm displacement, the weld which connected diaphragms and beam flanges started to crack, as shown in Figure 11. The test was terminated for safety reasons.

Figure 11.

Failure modes of SJ-5: (a) panel zone behaviors and (b) weld crack.

Slitting test of concrete

In order to study the failure mechanism of the core concrete, the steel tubes were cut and examined. Flame cutting method was used to cut the column to show concrete after testing. As shown in Figure 12(a), the concrete of the panel zone exhibits regular rhombus cracks under cyclic load. The center region is already cracked and easy to exfoliate. The corner region of the concrete still has high-strength reserves. Figure 12(b) shows the failure mode of the columns. There are some cross-cutting cracks at the middle of the section. The yield of the panel zone led to internal force redistribution. The beam and columns were subjected to a large moment and displacement. The condition of the diaphragm after the test is shown in Figure 12(c). It is apparent that the diaphragm has no obvious deformation in the panel zone. The bending deformation only occurred in the overhead diaphragms.

Figure 12.

Slitting test of panel zone: (a) core concrete, (b) column concrete, and (c) diaphragm.

Analysis of the test results

Hysteretic curve

The hysteretic curves of each specimen are shown in Figure 13. The beam tip lateral load-rotation hysteretic curves of each specimen are plump, and the deformation of the panel zone keeps stable with the lateral load increment before yield. After yielding, hysteretic curves have changed from shuttle shape to reversed S shape. A sudden drop of bearing capacity was observed in each specimen except for specimen SJ-1 at the end of the test. This was caused by the fracture of the weld which connected the beam flanges and diaphragms as previously shown in Figures 8 to 11. It can also be observed that the ultimate bearing capacity of the specimens SJ-2 to SJ-5 is higher than that of the specimen SJ-1. This indicated that the bearing capacity was well reinforced with the infill concrete. Figure 13(a) shows that the specimen SJ-1 has stable hysteretic curves and enough energy dissipation capacity. The small rigid body displacement of Figure 13(b) and (e) is probably due to some lack of perfect fit of the slotted holes. The stiffness of each specimen decreased with the increasing cyclic load, but the unloading slope was constant. Based on the above, one can infer the diaphragm with “weak panel zone” has a high seismic resistance.

Figure 13.

Load–rotation curves of (a) SJ-1, (b) SJ-2, (c) SJ-3, (d) SJ-4, and (e) SJ-5.

Skeleton curves

The effect of the parameters that were studied in this work is clear in the skeleton curves of Figure 14. The effect on the bearing capacity of the concrete strength is shown in Figure 14(a). The strength of the filled concrete can improve the shear resistance capacity and delay or avoid the local buckling of the steel tube. The bearing capacity was not increased appreciably with the increase in the concrete. The effect of the thickness of the steel tube is shown in Figure 14(b). The shear capacity of panel zone increased with the increase in the steel tube thickness. Figure 14(c) indicates that the thickness of the diaphragm has small influence on the yield strength of the panel zone, but it considerably improves the deformation capacity, ductility, and energy dissipation. The stiffness of all specimens with concrete infill in the steel tubes (SJ-2 to SJ-5) is higher than that of specimen SJ-1 in which the steel tube is not filled with concrete.

Figure 14.

Comparison of skeleton curves: (a) comparison of concrete strength, (b) comparison of tube thickness, (c) comparison of diaphragm thickness, and (d) comparison of skeleton curves.

The yield lateral load (P) and the yield rotational angle (γ) of each specimen were determined by the general yield point method (Park et al., 2010), as shown in Figure 15. The results are given in Table 4. In Figure 15, the yield point was the intersection point of the two lines of which the slopes were the initial stiffness (K) of the specimen and one-third of the specimen (K/3).

Figure 15.

General yield point method.

Table 4.

Load and rotation angle of test results.

	Specimen
	SJ-1	SJ-2	SJ-3	SJ-4	SJ-5
Yield load (kN)	107.5	212.5	218.8	220.7	234.1
Rotation (% rad)	1.22	2.84	1.40	1.49	2.47

Strain distribution

The strain gauges were used to study the stress–strain distribution of the diaphragm-through connections. During the test processing, the strain distribution of other specimens was similar to that of specimen SJ-3. Thus, for ease of presentation, only the strains of specimen SJ-3 are discussed below.

Diaphragms and beam flanges

The distribution of the strain in the diaphragm along the transverse direction is shown in Figure 16(a) and (b). The strain of the root and the end of the diaphragm are in the same level before yield. The strain of the diaphragm root increased rapidly with the increasing load after the yield of the panel zone, especially at the corner gauge station. The reason for this is the stress redistribution.

Figure 16.

Strain distribution: (a) root of diaphragm, (b) end of diaphragm, (c) beam flange, and (d) comparison of strain.

As shown in Figure 16(c), the strains in the beam flange were uniformly distributed along the width of beam flange until the failure occurred. It can be observed that the value of strain in the center of beam flange was larger than that at the side. The little out-of-plane distortion is due to the non-uniform distribution in the beam flange when the diaphragm started to bend.

Figure 16(d) shows the strain distribution in the diaphragm and beam flange along the longitudinal direction. The strain was measured at 10, 100, and 200 mm from the column face. It can be seen that the strain in the diaphragm and beam flange were uniformly distributed along the longitudinal direction of the beam before the yielding of the panel zone. The bending deformation of diaphragm led to higher strain at the diaphragm root.

Shear strain of panel zone steel tube

The shear strain distribution is shown in Figure 17. The numbers 1, 2, 4, and 5 correspond to the four corner strain gauges and 3 corresponds to the core strain gauge, as shown in Figure 17(a). It can be seen that the strain in the steel tube web was uniform before the yielding of the panel zone. The strain of the steel tube web was irregular after yielding. The comparison of shear strain between steel tube web and flange is shown in Figure 17(b). The numbers 1 and 2 correspond to steel tube web and flange, respectively. The value of the web is higher than the value of flange. This indicated that the steel tube flange has little impact on the shear capacity of the panel zone, but it improved the plastic deformation ability of the joints.

Figure 17.

Shear strain of core steel tube: (a) shear strain of tube web and (b) comparison of tube web and flange.

Outer steel tube of panel zone

The distribution of the strains in the outer steel tube of panel zone is shown in Figure 18. The strain of the column is very small before failure. The value of the strain rapidly increased with the constant increase in cyclic load.

Figure 18.

Strain of the outer steel tube: (a) steel tube web and (b) steel tube flange.

Comparison of test and theoretical results

In frame structures, the panel zone of the beam-to-column moment connection is subjected to large forces under lateral loading, as shown in Figure 19. The shear capacity of the panel zone can be calculated by equation (1)

V = \frac{M_{b}}{h_{b}} - V_{c} = \frac{P L_{b}}{H_{b} - t_{b f}} - \frac{P L}{H}

(1)

Figure 19.

Panel zone subjected to lateral force.

where, as shown in Figure 19(b), V_s is the shear capacity of the panel zone; V is the shear induced by the moment; V_c is the reaction force; P is the lateral force; L is the length from beam end to column center; L_b is the length from beam end to column surface; H is the height of the column; H_b is the depth of the beam; t_bf is the beam flange thickness; and h_b = H_b − t_bf is the distance between the beam flanges center.

The reasons that led to the differences between the predicted value and test results should be that different researchers used different mechanic modes of core concrete. Such as China Association for Engineering Construction Standardization (CECS), the calculated methods used the mechanic mode of inner diaphragm joints to calculate the shear capacity of diaphragm-through connection. Table 5 summarizes the test results and the theoretical calculated results by different methods that include CECS 159-2004 (2004), Architectural Institute of Japan (AIJ, 1987), and Fukumoto and Morita (2005). Based on the results of specimen SJ-1 in which the column was not filled with concrete, the calculated results using different models are consistent with the test results for the shear capacity of the steel tube webs. But for the specimens which were filled with concrete in the steel tube, the test data show large discreteness. The average value and standard deviation of shear-bearing capacity calculated by different methods indicated that the AIJ method is more accurate than the other two methods. Therefore, there is room for improvement in the existing computation methods.

Table 5.

Comparison of test and theoretical results.

Specimen	Axial ratio	Yield load (kN)	Shear capacity (kN)				V_C/V_T	V_A/V_T	V_F/V_T
Specimen	Axial ratio	Yield load (kN)	Test	CECS	AIJ	Fukumoto	V_C/V_T	V_A/V_T	V_F/V_T
SJ-1	0.2	107.5	506.6	507.7	520.5	510.0	1.002	1.027	1.007
SJ-2	0	212.5	1001.4	979.0	997.9	801.3	0.978	0.997	0.800
SJ-3	0.2	218.8	1031.1	1096.6	1055.7	848.3	1.064	1.024	0.823
SJ-4	0.2	220.7	1040.1	1291.9	1269.6	1176.2	1.242	1.221	1.131
SJ-5	0	234.1	1113.6	1305.9	1269.6	1114.4	1.173	1.140	1.001
Average							1.092	1.082	0.952
Standard deviation							0.101	0.085	0.124

CECS: China Engineering Construction Standardization Association; AIJ: Architectural Institute of Japan; V_T: shear capacity of test results; V_C: shear capacity which used CECS method; V_A: shear capacity which used AIJ method; V_F: shear capacity which used Fukumoto method.

Conclusion

The yield strength and strain distribution of diaphragm-through connections between CFSTC and steel beams were studied based on the results of testing five “strong members and weak panel zone” specimens with different steel tube thickness, concrete strength, diaphragm thickness, and loading conditions. The main findings can be summarized as follows:

All specimens yielded in the panel zone. Yielding of the panel zone was gradual, and no rapid loss of strength was observed. Catastrophic buckling of the steel tube did not occur except for specimen SJ-1 which was without infill concrete. The crushing occurred in the core concrete. The failure mode of each specimen was the crack of the weld connecting the beam flanges to the diaphragms.

Under the low-frequency cyclic load, the webs of steel tube of panel zone yielded first. With the increase in load, parallel inclined cracks occurred in the core concrete. At this point, the core concrete can withstand compression only. The steel frame composite constructed by steel tube flanges and diaphragms improved the plastic deformation ability after the yielding of the connections.

The presence of the axial loading has minimal impact on the shear capacity of the steel tube while it obviously increases the shear capacity of the concrete core. The infill concrete can delay or avoid the local buckling of the steel tube and thus improve the shear resistance capacity of the connection. The bearing capacity was not appreciably strengthened with the increase in the concrete strength.

The shear contribution of the steel tube observed in the test showed good agreement with the theoretical results as was verified by specimen SJ-1. However, there is appreciable difference between the tested data and the calculated ones when the connections are filled with concrete, which indicates that the theoretical models that are used to predict the shear capacity of the concrete need to be improved.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The funding for this investigation was provided by the National Natural Science Foundation of China (Nos 51268054 and 51468061) and Natural Science Foundation of Tianjin City, China (No. 13JCQNJC07300).

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