Static behavior of planar intersecting CFST connection in diagrid structure

Abstract

Intersecting connection plays an important role in the new diagrid structural system for high-rise buildings. To investigate the static behavior of the intersecting connection of concrete-filled steel tubular (CFST) columns, a typical reduced-scale planner connection specimen is tested under monotonic axial compression. The failure modes, force mechanism and bearing capacity of intersecting CFST connections are analyzed further in the follow-up numerical simulation, considering influences of intersecting angle, elliptical plate and ring plate. Test and simulation results prove that, intersecting connection can develop fully plastic deformation and provide sufficient bearing capacity. Parametric analysis indicates that bearing capacity of planar intersecting CFST connection mainly depends on intersecting angle and thickness of elliptical plate, while the ring plate affects that little. Capacity estimation method for planar intersecting CFST connection is proposed basing on the capacity of the critical section which is located near intersecting center for a distance of steel tube radius, and the design suggestions is provided in the end of this paper.

Keywords

diagrid structure / concrete-filled steel tube / planar intersecting connection / experimental research / mechanism analysis

Introduction

Diagird structure as a new structural system for high-rise buildings provides effective lateral and torsional stiffness, with its diagonal components arranged around the building and acting both as columns and bracings. This type of structure systems are adopted in some super-high rise buildings in recent years, such as Swiss Re Building in London, the Hearst Headquarters in New York, the Ministry of Foreign Affairs Building of Qatar in Doha [1], Guangzhou West Tower in Guangzhou [2,3], Lotte Super Tower [4] in Seoul, etc. However the research on diagrid structure system is still in the preliminary stage till now, represented by the characteristics analysis and practical design guidelines conducted by Moon et al. [5-7] and Zhang et al. [8].

In this new structural system, the connection consists of two obliquely intersecting columns and commonly several beams. It is the most significant part in load transfer path for connecting diagonal components resisting both vertical and horizontal load in structure, but with rare study on the connection mechanism such as stress flow and failure mode.

In the point of view of structure material, to minimize the size of structural members and obtain sufficient capacity with excellent ductility behavior, it is advisable to employ concrete filled steel tubular (CFST) components in the structure, which takes advantage of both steel and concrete. A typical and excellent application of intersecting CFST connections is what applied in Guangzhou West Tower [2,3]. Two types of connection details (Fig. 1) are tested under symmetric or asymmetric compressive loading method and it is concluded that the connection detail of lining plate with ring reinforcing plates behaved better than the flange plate type. In the experiments, two failure patterns are observed respectively featured with tube bulging within the connection zone in specimens at intersecting angle of 20° or within column zone at angle of 35°. Numerical simulation is conducted after experiments, but stress distribution and force mechanism of intersecting CFST connections are not convictively studied.

To investigate the force mechanism, failure modes and bearing capacity, a planar intersecting CFST connection specimen is tested under monotonic axial compression and the configuration of the connection is the most typical in diagrid structure. In the follow-up numerical simulation, connection force mechanism is analyzed, and the capacity estimation method and design suggestions are proposed.

Experimental research

The purpose of the experiment is to investigate the stress distribution, failure mode and ultimate bearing capacity of planar intersecting CFST connection subjected to monotonic axial loading.

Test specimens and setup

A planar intersecting CFST connection specimen is manufactured with reduced scale 1/4 and tested under monotonic axially compressive load as its prototype. The connection specimen consisted of two intersecting CFST columns which are welded fully to an elliptical vertical plate and ring horizontal plate, as shown in Fig. 2. The acute angle between intersecting columns is 16.16°. Size of steel tube is \mathrm{\varphi }219\mathrm{m}\mathrm{m}\times 12.5\mathrm{m}\mathrm{m}, and thickness of elliptical plate and ring plate are 25 mm and 20 mm respectively. In the prototype structure, horizontal beams are linked to the connection. Since the end forces of beams are small compared with the internal force of columns, the beam components are excluded in the test specimen. Distance between column’s end and start-point of intersecting part is generally three times of steel tube diameter in order to eliminate the influence of loading end on stress distribution of connection. Pre-test on mechanical properties of steel and concrete used for the specimen are conducted and the results are listed in Table 1.

Loading procedure

In the prototype of diagrid structure, intersecting columns are compressed or tensioned due to the load conditions whether the predominant load on the structure is gravity one or horizontal one. The most important columns which are located on building’s lowest floor are compressed and should be primarily studied.

In the test, specimen is monotonically loaded in static conditions by force control manner to failure status. Two intersecting columns are axially compressed, which is simulated corresponding predominant reaction in column due to gravity loads acting on the diagrid structure.

The specimen is set in a self-balance reaction steel frame with at least 6000 kN bearing capacity, and the ends of two columns in one side are welded to the reaction frame while another two ends are directly loaded by hydraulic jacks. To realize planar deformation, two pairs of H steel beam are used near the connection by clamping the specimen to restrict its out-of-plane displacement in two points but with free sliding of specimens. The test set-up is illustrated in Fig. 3.

Measurement

Strain gauges are employed to obtain the axial strains of steel tube wall at section-1~4 in column zones by uni-strain gauges, and axial, hoop strains and strain intensities at section-L1~section-F1 in connection zone by strain rosettes. Displacement transducers are arranged properly to get axial deformation of four portions of intersecting columns (D1~D4), in-plane displacement (D5~D14) and out-of-plane displacement (D15~D17) of specimens. The arrangement of sensors is illustrated in Fig. 4.

Phenomena and failure modes

Two intersecting columns of the specimen behaved in typical mechanical characteristic of CFST component, in which steel tubes acts in axial compression and hoop tension plus filled concrete tri-axial compression.

Deformation of specimen remained linear and most strains remained elastic before load level of 3000 kN, which is equivalent 71% of column’s yield capacity by simple sum of steel tube and filled concrete F_u = F_ys + F_c referring to Table 1. After that, the connection specimen began to show nonlinearity or entered plasticity, and subsequently rust spall off the surface of steel tube wall due to the bulging of tube wall could be observed. The closer to the intersecting center the strain gauges are located, the earlier the strains grew far beyond yielding. When load exceeded 4000 kN, deformation in plane and out of plane remarkably increased, and then strains at column zone (section-1~section-4) turned plastic at load level of 4200 kN. When load reached 4400 kN, out-of-plane displacement of intersecting center is prevented by restricting H steel beams. At the end of test with maximum load of 5221 kN which is 1.03 times of CFST column’s bearing compressive capacity F_uo calculated by Chinese code [10] referring to Table 1, out-of-plane deformation of the whole specimen is obvious (Fig. 5(a)) and bulging of tubes near intersecting center (Fig. 5(b)) could be observed.

Load-deformation and load-strain curves

Load-deformation and load-steel strain curves of specimens are illustrated in Fig. 6. In these curves, label “D” refers to the column’s compressive deformation or intersecting center’s displacements measured by displacement transducers, and “S” to the strains at section-1~4 and section-L1~L4 referring to Fig. 4.

There are two stages for connection deformation during the whole test process: linear stage and nonlinear stage (Figs. 6(a)-(b)). In elastic stage, axial deformation of specimens stably grew, while in-plane and out-of-plane deflections are very small. In nonlinear stage, column compressive deformation, in-plane deflection (D6) and out-of-plane deflection (D15) increased remarkable.

On the respect of strain, steel tube walls are compressed in the axial direction and tensioned in the circular direction, which indicates that the filled concrete is tri-axial compressed. From the average axial and hoop strains curves of section-L1~L4 (Fig. 6(c)), it is obvious that axial strains are always greater than hoop strains but with the same growing trend, which accords with the mechanical characteristic of typical CFST component. Due to the variable cross-sections in the connection zone, axial strains developed plastically in the sequence from small section near ring plate to the larger sections near column zone, with the fact that the closer to the intersecting center, average axial strain and hoop strain of cross section are greater and turned plastic and nonlinear earlier. For the strains at column zone, they are apparently smaller than those at connection zone and turned plastic when load reached 4200 kN, totally later than the plastic load level for connection zone. Generally speaking, nonlinear development of strain at column zone is more sudden and sharp than connection zone. It could be concluded that under compressive load on two columns, failure of connection with configuration in this specimen would start at connection zone rather than at column zone.

Discussions of experimental results

The tests of intersecting CFST connection specimen with two columns compressed could generally reflect mechanical characteristics of the connection under the typical load condition in diagrid structure.

In the test, connection zone is weaker than column zone and will be the governing part for failure mode and bearing capacity. That is to say, this test specimen failed at the connection zone. From another viewpoint, although the gross section area is less than that of columns being connected, the connection zone and column zone can develop fully plastic deformation under axial load.

However, the conclusion above from this test specimen can not be directly applied to other intersecting CFST connections mainly because of the acute intersecting angle of 16.16°. With greater intersecting angle, CFST connections would present different failure modes such as column zone failure [2,3]. Consequently, the intersecting angle should be designed rationally to coordinate the capacity between column part and connection part.

Numerical simulation and results

3D nonlinear finite element models are created by general-purpose FEA software ABAQUS, in order to simulate the test process of connection specimen and analyze the force mechanism of intersecting CFST connection in the follow-up study.

Finite element models

For the initial defect of test specimen and eccentric loading condition to some extent, out-of-plane and in-plane deflection would take place in the test practice. However, from the test results, the asymmetrical deformations only appeared in the plastic stage and are expected not to affect much the force mechanism of intersecting connection. Consequently, the ideal models with symmetry geometric construction and loading condition could be used to simulate the test specimen.

As shown in Fig. 7, a quarter of specimen is modeled. Symmetry boundaries are imposed on corresponding symmetric planes. Linear solid element of C3D8R, C3D6 and C3D4 are used to model steel tubes, plates and concrete. The interaction between filled concrete and steel tubes or plates are simulated by the contact element defining hard contact in normal behavior and friction coefficient μ = 0.6 in tangential behavior.

Constitutive models of materials

Concrete material in CFST components, as confined concrete, behaves different from what in plain concrete components. Confined concrete shows excellent plasticity with the increased strain and more slowly descending stage. Basing on comparison of different concrete constitutive models for CFST component, a simplified expression is employed as described in Fig. 8, which generally originates from the models proposed by Liu [11] and Ding [12] and expressed in the form of what employed by Schneider [13]. Parameter f_ck is the characteristic value of compressive strength of concrete, and in the simulation model of test specimen it accords with the results of material mechanical property listed in Table 1.

Material constitutive model for steel tube and plates is described in Fig. 9, in which the tensile strength f_u is 1.6 times of yield strength f_y, strain ϵ_st at the beginning point of hardening stage is 10 times of yield strain ϵ_y and hardening modulus is 1% of elastic modulus. The ratios of f_u/f_y and ϵ_st/ϵ_y are defined with the data statistics on a certain amount of mechanical property test of steel material. While in the simulation model of test specimen, stress-strain relationship curves measured from material mechanical property is employed instead.

Simulation results of test specimen

The comparison between FE analysis and experimental results about the load-deformation and load-strain curves are shown in Fig. 10.

From the comparison, FE analysis results are in good agreement with the experimental results, and consequently could be used to analyze the force mechanism of intersecting CFST connection with two columns compressed in following work.

Stress distribution of intersecting CFST connections

The stress contours of different parts in FEA models of specimen under approximate ultimate capacity load in experiment are shown in Fig. 11, in which the red color stands for larger value and blue for smaller value.

In accordance with experimental results, FE analysis illustrates that, larger stress of steel tube wall, plates and filled concrete appears at connection zone near ring plate. That is to say, experiment and numerical simulation both conclude that, for intersecting CFST connection with small intersecting angle, center zone is the weakness of connection.

From the analysis, nonlinear deformation of the specimen is mainly due to the plastic compression of connection zone. Consequently, intersecting CFST connections could develop full plastic deformation under axial compressive load.

Force mechanism analysis

The force mechanism of planar intersecting CFST connection is investigated in this part by parametric analysis. Principle parameters include intersecting angle of two columns, thickness of elliptical plate, thickness and details of ring plate, plus the secondary parameters of steel tube’s size and material of connection. Connection model is labeled by \mathrm{\varphi }D-t-t_{\mathrm{p}}-t_{\mathrm{r}}-\alpha, in which D and t are outer diameter and thickness of steel tube respectively (unit: mm), t_p and t_r are thickness of elliptical plate and ring plate respectively (unit: mm), and α is intersecting angle (unit: degree).

Failure modes

Planar intersecting CFST connection fails under compression load in two modes: connection zone failure and column zone failure. Apparently connection specimen in previous test fails at the connection zone with maximum stress spreading from the sections near ring plate to farther sections. Connection model \mathrm{\varphi }250-12.5-25-20-60 shown in Fig. 12 is an example which fails in the second mode with the maximum stress of steel tube wall appearing at column zone. However, the largest compressive stress of filled concrete still takes place in the intersecting part near ring plate, just like what happens in those connections with connection failure mode.

The key parameters affecting significantly the failure mode of connection are intersecting angle and thickness of elliptical plate. It is more likely for connections with small intersecting angle or thinner elliptical plate to fails at connection zone. The intersecting angle affects the failure mode of connection in the way of adjusting the force component in elliptical plate’s plane of transferred load from columns to connection zone, while the elliptical plate affects by adjusting bearing capacity of connection compared to that of column.

Two connections models \mathrm{\varphi }250-12.5-25-20-\alpha, with intersecting angle α being 30° and 50° respectively, are analyzed to investigate the failure modes and bearing capacity of intersecting CFST connection. To focus on the behavior and bearing capacity of connection zone, the column zone is supposed keeping elastic and consequently would not fail in the whole analysis process. The load ratio-average strain of connection zone curves of this two models are shown in Fig. 13, in which vertical coordinates is load ratio of imposed load F divided by intersecting CFST column’s bearing capacity F_uo calculated by standard DL/T5085–1999 [10]. Two failure modes can be illustrated by different behaviors of connection zone when load reaches column’s bearing capacity with the load ratio of 1.0: connection of angle 30° displays serious stiffness degradation, while connection of angle 50° still remains good bearing behavior with only weak stiffness degradation and loading level not reaching its bearing capacity. That is to say, the connection of angle 30° with bearing capacity less than that of intersected column would fail at connection zone, and oppositely connection of angle 50° would fail at column zone since bearing capacity of connection is greater than that of intersecting column.

Mechanical behavior of elliptical plate

Elliptical plate is an essential part of intersecting CFST connection and plays an important role in force mechanism of connection zone. Elliptical plate generally resists the force component in its plane of load transferred from column zone, displaying principle axial stress and minor lateral stress. That indicates weak confining effect acted on the filled concrete from elliptical plate.

In typical CFST component, steel tube develops hoop stress with decreased axial stress when entering plastic stage. Due to the hoop stress, steel tube realizes confining effect for filled concrete and co-works with concrete as a CFST component. In design method for steel-concrete composite structure, confining effect index θ = f_yA_s/f_cA_c, in which f_y is yield strength of steel tube, f_c is compressive strength of filled concrete, A_s and A_c are section area of steel tube and concrete respectively, is used to analyze the mechanism and calculate the bearing capacity of CFST component. However elliptical plate in intersecting CFST connection doesn’t develop significant lateral stress and should not be directly taken into the confining effect index.

As shown in Fig. 14, the force distributed on elliptical plate in connection generally agrees with that under ideal compression condition which is calculated by multiplying the stress value by section area. While distributed force on steel tube is significantly less than that under ideal compression when entering plastic stage. Apparently, elliptical plate contributes to connection’s bearing capacity in a different way from steel tube. Furthermore, bearing capacity of elliptical plate could be directly appended to that of CFST-type part including steel tube and filled concrete, as the bearing capacity of intersecting CFST connection.

Influence of ring plate on force mechanism of connection

Ring plate is another essential part of intersecting CFST connection but with different forms such as outer ring plate and inner ring plate as shown in Figs. 15(a)–(b). Outer ring plate is employed in the diagrid structure of Guangzhou West Tower [2,3] which is named ring reinforcing plate instead, while inner type is used in previous test specimens of this paper.

To investigate ring plate’s mechanism and influence on bearing capacity of intersecting CFST connection, a series connection models of different details are created and analyzed, in which D_r and t_r are inner diameter and thickness of inner ring plate respectively (Fig. 15(c)). Analysis results of load-displacement of intersecting center curves shown in Fig. 16 prove that ring plate with different details does lead to the same load evolution and bearing capacity.

Although ring plate affects little on the bearing capacity of intersecting CFST connection, its existence still changes the mechanical behavior of connection and makes connection zone different from typical CFST component. For ring plate restricting the expansion of steel tube nearby and causing stronger confining effect on filled concrete, the smallest section near ring plate could not possess the lowest bearing capacity.

However, the section range affected by ring plate is limited. Stress distribution along axes of steel tube or elliptical plate in the plastic stage are picked up from connection model \mathrm{\varphi }250-12.5-25-20-\alpha in which α is 30°with connection zone failure or 50° with column zone failure, including steel tube path-1 and 2, filled concrete path and elliptical plate path (Fig. 17(a)). It could be found from stress distribution curves shown in Figs. 17(b)–(c) that maximum stress (labeled with red dots) of connection zone appears at a section with a certain distance from intersecting center rather than the smallest section no matter which failure mode happens. Consequently, this section should be the critical section governing the bearing capacity of connection zone. According to parametric analysis, distance of critical section from intersecting center is generally the steel tube radius D/2 and the section is located perpendicular to axis of steel tube.

Ultimate limit state of intersecting connection

Although the critical section of intersecting CFST connection is found and its bearing capacity could be a characteristic load for connection, ultimate limit state of connection can not be directly and simply defined as the ultimate limit state of critical section just like what generally applied in uniform-section component, due to the variable-section in connection zone.

On the other hand, with the steel tube and elliptical plate common used in practical structure, intersecting CFST connection generally behaves without the descending load stage before an unacceptable deformation or stress level is reached.

Secant stiffness degradation in the loading process of several connection models is shown in Fig. 18, in which horizontal coordinate is ratio of secant stiffness divided by initial stiffness in the load-connection zone deformation relationship and vertical coordinate is ratio of load F divided by intersecting column’s bearing capacity F_uo by code [10]. When degradation ratio is less than 0.1, curves’ turning trend is obvious with the ratio remaining at a relatively stable level, which signifies that connection behavior enters to the hardening stage with remarkable increase of deformation and slow increase of load.

According to parametric analysis, at the load level responding to degradation ratio of 0.1, Mises stress of steel tube walls in connection zone totally exceed plastic value and start hardening.

Consequently, it is more rational and logical to define the ultimate limit state according to deformation plus stress states of connection zone. Concretely speaking, degradation ratio of 0.1 could be the index to define ultimate limit state.

Design suggestions for intersecting CFST connection

According to the analysis results above, the critical section of intersecting CFST connection can be chosen at the section near intersecting center for a distance equal to the semi-diameter of steel tube D/2 and perpendicular to axis of CFST column. Bearing capacity of connection zone could be determined by that of critical section. Conservatively speaking, capacity of critical section includes two parts, one is the analytical capacity of elliptical plate and another is capacity of CFST-type part consisting of steel tube and filled concrete according to calculation method in standard DL/T5085–1999 [9]. The calculation principle is not to take the confining effect of elliptical plate into the index calculation.

Because of the important roles in load transfer path and its complicated behavior and force mechanism, intersecting CFST connections in diagrid structure should be designed stronger than the intersecting columns with greater bearing capacity. Measures listed as follow can be referred to:

1) To make the intersecting angles as large as possible but within a rational range [5] in the structural system design;

2) To increase the thickness of elliptical plate;

3) To employ thicker steel tube in connection zone;

4) To employ higher-strength steel or concrete material in the connection zone.

For the convenience of structural design and construction, the first two measures are more preferred.

Conclusions

To investigate the behavior of the intersecting connection of CFST columns in diagrid structure, a typical reduced-scale planner connection specimen is tested under monotonic compressive loading. Finite element model are created following to simulate the test results. Both of experimental results and numerical simulation conclude that the intersecting CFST connection can develop fully plastic deformation with sufficient capacities.

Numerical simulation of a series connection models is conducted to analyze mechanism of intersecting CFST connection with two columns compressed. There are two failure modes of connection zone failure and column zone failure, which are affected significantly by parameters of intersecting angle and elliptical plate thickness. Elliptical plate in connection acts as what does under ideal compression without valid confining effect on filled concrete as steel tube, and should not be directly taken into the confining effect index. Bearing capacity of elliptical plate could be appended to that of CFST-type part as the capacity of connection. Ring plate changes the resisting mechanism of connection zone by causing critical section near intersecting center for a distance of D/2, but different details of ring plate affect little on the bearing capacity of connection.

Design suggestions for intersecting CFST connection are proposed, including the estimation method of connection’s bearing capacity and measures to make connection zone stronger than intersecting columns.

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