Seismic behavior of circular tubed steel-reinforced concrete column to steel beam connections

Thin-Walled Structures xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Thin-Walled Structures journal homepage: www.elsevier.com/locate/tws

Full length article

Seismic behavior of circular tubed steel-reinforced concrete column to steel beam connections ⁎

Guozhong Chenga,b, Xuhong Zhoua,b, Jiepeng Liua,b, , Y. Frank Chenb a b

Key Laboratory of New Technology for Construction of Cities in Mountain Area (Chongqing University), Ministry of Education, Chongqing 400045, China School of Civil Engineering, Chongqing University, Chongqing 400045, China

A R T I C LE I N FO

A B S T R A C T

Keywords: TSRC column Connection Seismic behavior Design model Joint strength

The tubed steel-reinforced concrete (TSRC) column is a special type of SRC columns where the longitudinal reinforcement and reinforcement cage in the SRC column are replaced by a thin-walled steel tube. Without any traditional reinforcement, concrete pouring becomes easier in a TSRC member. Although the TSRC columns possess high load-carrying capacity and good ductility performance in seismic zones, the seismic behavior of TSRC column to beam connections has not received much attention, which limits the application of TSRC columns. This paper aims to investigate the seismic behavior of circular TSRC column to steel beam connections with cross diaphragms. Four such connection specimens were tested under cyclic loading, considering the thickness of joint tube, the height of extended tube, and the axial compression ratio of columns. Based on experimental results, the failure progression, load-displacement curves, and stresses for the circular TSRC column to steel beam connections are discussed. The favorable seismic performance for such connections is demonstrated and a design model for determining the joint strength is proposed in this paper.

1. Introduction Composite columns have been increasingly used in many modern structures. As a main type of composite columns, steel-reinforced concrete (SRC) columns can provide considerable advantages compared to steel columns. Since the steel section is encased with concrete, it becomes a solid section requiring no local buckling check on the steel section and the fire resistance of a SRC column is also enhanced [1]. However, longitudinal and transverse reinforcement is needed to prevent the concrete from spalling off [1,2]. The transverse ties cannot effectively prevent the longitudinal rebars and the flanges of the encased steel shape from buckling in a situation where the SRC column is subjected to combined high axial and cyclic lateral loads. Thus, the permitted axial compression ratio for SRC columns is the same as that for RC columns [3], which limits the use of high strength concrete. Moreover, the reinforcement in a SRC column also may hinder concrete pouring, especially at beam-column connections (Fig. 1(a)). The tubed steel-reinforced concrete (TSRC) column proposed by Zhou et al. [4] is a special type of SRC columns where the longitudinal and transverse reinforcement in the SRC column is replaced by a thinwalled steel tube (Fig. 1(b)). No direct axial load is applied on the steel tube as the steel tube is discontinued at the connection ends with the column. As such, the effectiveness of the steel tube in confining the core concrete is maximized, enabling the high strength concrete to be fully ⁎

utilized. Without any traditional reinforcement, concrete pouring becomes easier in a TSRC member. Gan et al. [5] tested 6 TSRC column specimens under the combined constant axial compression and lateral cyclic loads and investigated their seismic behavior. The test results show that the flexural strength, ductility, plastic deformation capacity, and energy dissipation capacity of circular TSRC columns are significantly higher than those of common SRC columns with the same steel ratio and axial compressive load. Liu et al. [6] developed a nonlinear three-dimensional finite element model to simulate the hysteretic behavior of TSRC columns and proposed a design formula to predict the shear strength of short square TSRC columns. Qi et al. [7] carried an experimental study on the behavior of stub TSRC columns subjected to axial compressive loads, including 14 circular and 15 square specimens. The test results indicate that height to diameter/width ratio of the discontinuous tube has little effect on the failure mode and strength of TSRC columns. Yan et al. [8] investigated the axial behavior and stability strength of circular TSRC columns by testing 8 specimens. A series of experiments have been conducted to study the behavior of TSRC columns under eccentric compression, followed by the development of axial load versus moment interaction diagrams [9–12]. Based on the literature review, previous studies mainly focused on the static and dynamic behaviors of TSRC columns. However, the seismic behavior of TSRC column to beam connections has not received much attention. To the knowledge of the authors, no work has been

Corresponding author.

https://doi.org/10.1016/j.tws.2018.10.041 Received 15 May 2018; Received in revised form 22 October 2018; Accepted 30 October 2018 0263-8231/ © 2018 Elsevier Ltd. All rights reserved.

Please cite this article as: Cheng, G., Thin-Walled Structures, https://doi.org/10.1016/j.tws.2018.10.041

Thin-Walled Structures xxx (xxxx) xxx–xxx

G. Cheng et al.

Nomenclature Ac As Av C dp dαij dσij dεij dεijp D Es fc fs fyvs fyvw he hw H k0 L N P Pu tf

tj tw Vc Vcal Vj Vt Vu Vw αij γ γ12 γj δ εij ξeq μ σ11 σ22 σij σMises τ12 υ △ △y △u

cross-sectional area of core concrete cross-sectional area of joint tube effective cross-sectional area of joint tube coefficient set as 82.5fs increment of accumulated equivalent plastic strain increment of backstress tensor increment of stress tensor increment of strain tensor increment of plastic strain tensor diameter of column elastic modulus axial compressive concrete strength yield stress of joint tube shear yield stress of joint tube shear yield stress of steel web height of extended tube sectional height of steel web story height coefficient set as 0.85 story span axial compression load applied on column lateral load applied at the top of column lateral peak load thickness of beam flange

thickness of joint tube width of steel web horizontal force resisted by the concrete compression strut calculated joint strength shear force horizontal force resisted by the joint tube maximum shear force horizontal force resisted by the steel web panel tensor of backstress coefficient set as 150 engineering shear strain shear deformation story drift tensor of strain equivalent damping coefficient ductility coefficient transverse stress longitudinal stress tensor of stress equivalent von Mises stress shear stress Poisson's ratio lateral displacement applied at the top of column lateral yield displacement lateral displacement corresponding to 0.85Pu

Fig. 1. SRC and tubed SRC column-steel beam connections in a building frame.

joints. The dimensions and details of ⅓ scaled specimens are shown in Fig. 3. As shown, the tested circular TSRC column consists of a steel tube of 3 mm (thickness) × 300 mm (diameter) × 2090 mm (length) and an encased steel shape of 200 mm (depth) × 100 mm (flange width) × 4 mm (web thickness) × 14 mm (flange thickness). The steel beam has the cross-section of 250 mm x 150 mm × 8 mm × 10 mm (Fig. 3) and is 3000 mm long. The beam flanges were made continuous with the corresponding cross diaphragms (Fig. 4), thus eliminating the uncertain effect caused by flange welds. The beam webs were welded to the joint tube. The circular steel tube was cold-formed by rolling the steel plate and the butt weld joint was enhanced by a 40 mm wide steel plate to prevent the possible premature weld failure. The investigated parameters include the thickness of joint tube (tj), the height of extended tube (he), and the axial compression ratio of TSRC columns. The specimen details are given in Table 1 and the mechanical properties of steel are listed in Table 2. The axial compressive strength of concrete fc is 35 MPa.

published on the seismic behavior of TSRC column to steel beam connections. This paper thus attempts to study the behavior of such connections under cyclic loading. A new connection system for circular TSRC column and steel beam is proposed (Fig. 2), where cross diaphragms are added to ensure the transfer of moments at the beam ends. Meanwhile, the embedded/inside portion of the cross diaphragm ensures the shear transfer at beam ends. The joint tube (Fig. 2(b)) is used to improve the joint strength. The exterior cross diaphragms prevail to improve the sectional height of steel shape. To verify the feasibility of such connection and investigate its seismic performance, four tests were carried out.

2. Experimental program 2.1. General information of specimens All specimens tested in this study were designed to have a weak link in the connections, allowing the evaluation of the inelastic response of 2

Thin-Walled Structures xxx (xxxx) xxx–xxx

G. Cheng et al.

Fig. 2. Circular TSRC column to steel beam connection.

event. The specimens were tested under the combined axial and cyclic lateral loads. The cyclic lateral load was applied at the top of column by a manual jack which was held horizontally, while a constant axial load was applied through a self-balanced system consisting of a vertical actuator, a reaction rack, and four steel rods (Fig. 5(a)). According to Chinese code JGJ/T 101–2015 [13], the applied cyclic loading history (lateral load/displacement-cycle curve) as shown in Fig. 6 was adopted. All key displacements were measured by the linear variable differential transformers (LVDT), as shown in Fig. 7, including the lateral displacements at column top (LVDT-1), the lateral displacements at beam ends (LVDT-2, LVDT-3), and the joint shear deformations (LVDT4, LVDT-5). Besides, the slip between test-up and floor was monitored by LVDT-6, which was observed to be about 2 mm during the whole experiment. The strains of steel beam, column tube, and joint tube were measured using strain gauges. A load cell was used to monitor the applied lateral loads at column top. The details of instrumentation layout are shown in Fig. 7.

Fig. 3. Connection details (unit: mm).

3. Failure progression Figs. 8–11 show the lateral load (P) versus lateral displacement (△) curves and the corresponding failure modes of the tested connections. The details of failure progression are described as follows. 3.1. Specimen 1

Fig. 4. Geometry of cross diaphragm (unit: mm).

Fig. 8(a) shows the failure modes of Specimen 1. The lateral peak load was attained at 3.1% story drift (δ), with no obvious failure phenomenon prior to that. The corners of the joint tube started to bulge at δ = 3.7% and the middle portion started to bulge at δ = 5.6%. The joint tube near the enhanced plate started to tear at δ = 6.8%. After the test, the joint tube was removed to observe the failure mode of core concrete. As a result, concrete crushing failure was noticed. Additionally, the flange of steel shape bulged slightly over a portion of the joint

2.2. Test set-up and instrumentation layout The bottom of column was pinned to the strong floor of a laboratory and the beam ends were connected to this strong floor by steel links which permit the rotations and horizontal translations of beams. The pins shown in Fig. 5 represent the inflection points that are likely to occur at the column mid-height and beam mid-span during a seismic Table 1 Details of specimens. Specimen label

tj (mm)

he (mm)

N (kN)

△y (mm)

△u (mm)

Pu (kN)

μ

ξeq

Vu (kN)

Vcal (kN)

Vcal/Vu

Specimen Specimen Specimen Specimen

2 2 2 4

120 120 0 120

1584 2375 1584 1584

50.7/− 47.3 48.9/− 44.3 44.5/− 45.9 62.7/− 62.7

126.7/− 132.1 119.8/− 124.2 117.2/− 112.9 136.8/− 136.7

168/− 170 169/− 175 157/− 150 221/− 213

2.6/2.8 2.4/2.8 2.6/2.5 2.2/2.2

0.11 0.12 0.11 0.15

1308 1331 1188 1680

1044 1044 1044 1224

0.80 0.78 0.87 0.73

1 2 3 4

3

Thin-Walled Structures xxx (xxxx) xxx–xxx

G. Cheng et al.

Table 2 Measured mechanical properties of steel. Steel type Steel Steel Steel Steel Steel Steel Steel

plate plate plate plate plate plate plate

with with with with with with with

a a a a a a a

thickness thickness thickness thickness thickness thickness thickness

of of of of of of of

2 mm 3 mm 4 mm (Web of steel shape) 4 mm (Joint tube) 8 mm 10 mm 14 mm

Thickness (mm)

Yield strength (MPa)

Ultimate strength (MPa)

1.76 2.70 3.67 3.66 8.40 9.85 14.40

273 438 367 447 435 425 385

392 557 493 563 545 533 492

Fig. 5. Test set-up.

Fig. 7. Details of instrumentation layout. Fig. 6. Cyclic loading history.

beam tore completely. After removing the joint tube, severe damage of core concrete was observed and the flange of steel shape bulged obviously over a portion of the joint region. Compared to Specimen 1, Specimen 2 showed more severe damages and worse deformability, indicating that the higher axial compression ratio resulted in more severe damages.

region. 3.2. Specimen 2 Fig. 9 shows the failure modes and P-△ curves for Specimen 2, which indicates higher axial load than Specimen 1. The lateral peak load was attained at δ = 2.7%，which is lower than Specimen 1. The corners of the joint tube started to bulge at δ = 3.4% and the joint tube near the enhanced plate started to tear at δ = 4.8%. At δ = 6.9%, the middle portion of the joint tube bulged and the joint tube near the web of steel

3.3. Specimen 3 Fig. 10(a) shows the failure modes of Specimen 3, where the height of extended tube is zero. At δ = 3.0%, the lateral peak load was 4

Thin-Walled Structures xxx (xxxx) xxx–xxx

G. Cheng et al.

Fig. 8. Failure progression of Specimen 1.

Fig. 9. Failure progression of Specimen 2.

concrete compression strut was formed by the joint core concrete. The flange of steel shape bulged over a portion of the joint region, mainly caused by the shear deformation of the steel shape web. Clearly, the joint strength was mainly contributed by the joint tube, joint core concrete and steel shape web. During to the use of the enhanced plate, the failure of Specimens 1–3 was caused by the tearing of joint tube near the enhanced plate. The failure of Specimen 4 was due to the fracturing of welds between the joint tube and the enhanced plate. Therefore, it is essential that the seamless steel tube be adopted for the joint tube.

attained and the corner portions of joint tube started to bulge. The middle portions of joint tube started to bulge at δ = 3.8% and the joint tube near the enhanced plate started to tear at δ = 4.7%. After removing the joint tube, the concrete failure by crushing was observed and the flange of steel shape bulged slightly over a portion of the joint region. Specimen 3 showed similar failure modes to Specimen 1, showing that the extended tube had little effect on the failure mode. 3.4. Specimen 4 Fig. 11 shows the failure mode of Specimen 4 in which the thickness of joint tube is 4 mm. Due to poor weld quality, the weld between the joint tube and the enhanced plate fractured completely at δ = 5.5%. After removing the joint tube, slight cracks on the core concrete were noticed. In summary, the damages of the specimens mainly concentrate on the joint tube, joint core concrete, and flange of steel shape. The middle portions of joint tube bulged, indicating a shear buckling of joint tube. The concrete failure by crushing was noted, suggesting that the

4. Analysis and discussion 4.1. Load-displacement curves 4.1.1. P-△envelope curves Fig. 12(a) shows the effect of axial load ratio on P-△ envelope curves, indicating that the axial compression ratio has little effect on the shear strength of joints within the range of parameters considered in 5

Thin-Walled Structures xxx (xxxx) xxx–xxx

G. Cheng et al.

Fig. 10. Failure progression of Specimen 3.

Fig. 11. Failure progression of Specimen 4.

apparatus and δ1 + δ2 and δ3 + δ4 are respectively the displacements measured by the LVDTs positioned diagonally over the joint panel zone (Fig. 13(b)). Fig. 14 shows the shear force versus shear deformation curves established based on the above method. These curves show significant nonlinearity and descending branch, further verifying the joint failure.

this study. Fig. 12(b) indicates that the extended tube increases the joint strength by about 6.5%. The effect of thickness of joint tube on P-△ envelope curves is shown in the Fig. 12(c), demonstrating that the joint tube thickness has a significant effect on the joint strength. 4.1.2. Shear force-shear deformation (Vj-γj) curves The shear force Vj (Fig. 13(a)) is given by

Vj = T1 + T2 − P =

PH (L − D) −P L (hb − t f )

4.2. Elastic-plastic analysis (1)

Since the strain development in the joint tube could not reveal the components behavior directly, a developed computation procedure is proposed to analyze the stresses based on the measured strains. The developed computation procedure is described as follows. In the elastic range, the stresses of joint tube under the state of plane-stress can be determined by

where P is the lateral load applied at the top of column, H and L are the story height and story span respectively (Fig. 7), D is the diameter of column, hb is the height of beam, and tf is the thickness of beam flange. In this study, the shear deformation γj was calculated by

γj =

1⎡ a2 + b2 |δ1 + δ2 |⎟⎞ + ⎜⎛|δ3 + δ4 |⎟⎞ ⎤ ⎥ 2⎢ ab ⎣ ⎠ ⎝ ⎠⎦

1 ν σ Es ⎡ ν 1 ⎡ σ11 ⎤ ⎢ = 22 ⎢ ⎥ 1 − υ2 ⎢ 0 0 ⎣ σ12 ⎦ ⎣

(2)

where a and b are respectively the width and height of the measuring 6

0 ⎤ ε11 0 ⎥ ⎡ ε22 ⎤ ⎥ 1 − ν ⎥⎢ γ 2 ⎦ ⎣ 12 ⎦

(3)

Thin-Walled Structures xxx (xxxx) xxx–xxx

G. Cheng et al.

Fig. 12. Effects of experimental parameters on the P-△ envelope curves.

Fig. 13. Shear force and shear deformation of joint.

where σ11 and σ22 are the transverse and longitudinal stresses of tube respectively, σ12 is the shear stress, ε11 and ε22 are the transverse and longitudinal strains of tube respectively, γ12 is the shear strain, Es is the elastic modulus, and υ is the Poisson's ratio (= 0.18). In the plastic range, the kinematic hardening model was used to

model the behavior of metals subjected to cyclic loading, in which the yield surface is defined by

f (σij, α ij) =

3J2 (σij − α ij) − k 0 fs = 0

(4)

where fs is the yield stress and σij and αij are the tensors of stress and 7

Thin-Walled Structures xxx (xxxx) xxx–xxx

G. Cheng et al.

Fig. 14. Shear force-shear deformation curves.

where dεijp is the increment of plastic strain tensor, dp is the increment of accumulated equivalent plastic strain, and k0, C, and γ are the material constants (= 0.85, 82.5fs, and 150 respectively) [14]. According to the Von-Misses yielding criterion, the increment of accumulated equivalent plastic strain dp is given by

dp =

2 p p dεij dεij 3

(6)

with the associated plastic flow assumed, the increment of plastic strain dεijp is given by

dεijp =

3sij (σij − α ij) ∂f dp = dp ∂σij 2k 0 fs

(7)

The consistency condition gives the following function

∂f ∂f dσij − dα ij = 0 ∂α ij ∂σij

(8)

where dσij is the increment of stress tensor determined by

1 ν ⎡ dσ11 ⎤ E ⎡ν 1 ⎢ ⎢ dσ22 ⎥ = 1 − υ2 ⎢ 0 0 ⎢ dσ ⎥ ⎣ 12 ⎦ ⎣

Fig. 15. The developed computation procedure.

2 C dεijp − γα ij dp 3

(9)

where dεij is the increment of strain tensor being known for a particular strain path. Solving Eqs. (3)–(9), the increment of stress tensor can be determined and the stresses are obtained through a numerical integration as shown in Fig. 15. The general finite element program ABAQUS [15] was utilized to validate the developed computation procedure, where the same material model (kinematic hardening

backstress respectively. The increment of backstress tensor is calculated by

dα ij =

p 0 ⎤ ⎡ dε11 − dε11 ⎤ p 0 ⎥ ⎢ dε − dε ⎥ 22 22 1 − ν ⎥⎢ p⎥ 2 ⎦ ⎣ dε12 − dε12 ⎦

(5) 8

Thin-Walled Structures xxx (xxxx) xxx–xxx

G. Cheng et al.

Fig. 16. The process of verification.

Fig. 17. Verification of the developed computation procedure.

this study. However, the deformation ability is increased by the extended tube and decreased by the axial load. The μ factor for Specimen 4 is about 2.2, which is decreased due to the premature weld fracturing. μ factors for TSRC joints without the premature weld fracturing range from 2.4 to 2.8, which is higher than the average μ of 2 for a conventional reinforced concrete (RC) joint. An equivalent damping coefficient ξeq [17] was adopted to represent the energy dissipation ability of a TSRC joint, which is defined by

model) and material properties (material constants) were assumed. Since it is difficult to directly calculate the corresponding stresses based on a particular strain path in ABAQUS, a verification process is warranted, as illustrated in Fig. 16. Fig. 17 shows the comparisons of results predicted by ABAQUS [15] and the developed computation procedure. As seen, the results predicted by the developed procedure are consistent with those predicted by the finite element analysis, thus validating the developed computation procedure. Adopting the developed calculation procedure mentioned above, the transverse, vertical, and shear stresses were obtained to evaluate the mechanical mechanism of joint tube. As a typical location, the center of joint tube was selected. Fig. 18 shows the stresses of the joint tube for all specimens tested in this study, which indicates that joint tube yielded before the peak load and the vertical stress changed from compressive to tensile gradually. The transverse stress increased first and then decreased with increasing shear forces. However, the shear stress increased with the shear force continually and became dominant when the peak load was attained. Based on the elastic-plastic analysis on the joint tube, it can be stated that the joint tube contributes to the joint strength mainly by direct shearing.

ξeq = (SABC + SADC)/[2π (SOBE + SODA )]

4.3. Ductility and dissipated energy

5. Design model for joint strength

The ductility factor μ used to characterize the ductility is defined as Δu/Δy where Δy is the lateral yield displacement (Fig. 19) and Δu is the lateral ultimate displacement taken corresponding to 85% of lateral peak load Pu [16]. The Pu, Δy, Δu, and μ values for the four test specimens are listed in Table 1. It is found that the axial load and extended tube have little effect on μ within the range of parameters considered in

The total strength of the joint is the sum of the contribution from the joint tube (Vt), joint core concrete (Vc), and steel shape web (Vw) as indicated in Eq. (11). The mechanism for each component is illustrated in Fig. 21 and the mechanisms are discussed as follows.

(10)

where SABC is the area enclosed by the curve ABC shown in Fig. 20. Similar definitions were used for SADC, SOBE, and SODA. ξeq for all specimens at the peak loads are given in Table 1, which indicates that ξeq of about 0.11 for Specimens 1–3 (joint tube thickness = 2 mm) and 0.15 for Specimen 4 (joint tube thickness = 4 mm). This implies that the joint tube thickness can significantly increase ξeq. The average ξeq for conventional RC joints is about 0.1, while it is about 0.12 for TSRC joints, indicating that the TSRC joints generally have better energy dissipation ability than the conventional RC joints.

Vcal = Vc + Vt + Vw 9

(11)

Thin-Walled Structures xxx (xxxx) xxx–xxx

G. Cheng et al.

Fig. 18. Stresses of joint tube.

Fig. 19. Method for determining Δy and Δu. Fig. 20. The calculation of equivalent damping coefficient.

5.1. Concrete compression strut

where Ac is the cross-sectional area of core concrete and taken as 0.8D2.

The concrete compression strut (Fig. 21(a)) is mobilized by the joint tube, which is similar to that used to model the shear for RC joints (Chinese code for GB 50010–2010) [18]. The horizontal shear force Vc resisted by the strut is calculated by

Vc = 0.3fc Ac

5.2. Joint tube Base on the elastic-plastic analysis on the joint tube described above, it can be concluded that the joint tube contributes to the joint strength mainly by direct shearing (Fig. 21(b)). The horizontal shear

(12) 10

Thin-Walled Structures xxx (xxxx) xxx–xxx

G. Cheng et al.

Fig. 21. The mechanism for each component.

concrete, and steel shape web. The proposed design model for the joint strength is somewhat conservative but practically acceptable.

force Vt resisted by the joint tube is calculated by [19]

Vt = fyvs Av

(13)

Acknowledgements

where fyvs is the shear yield stress of joint tube and Av is calculated by

Av = 2As / π

(14)

The authors greatly appreciate the financial supports provided by the National Natural Science Foundation of China (No. 51438001, No. 51622802). The opinions expressed in this paper are solely of the authors, however.

where As is the cross-sectional area of the joint tube. 5.3. Steel web panel The steel web was idealized as carrying the pure shear over a portion of the joint region. Since the steel section is encased with concrete, local buckling of steel web is prevented. The distribution of shear stresses (Fig. 21(c)) in a web panel proposed by Parra-Montesinos et al. [20] was adopted in this study. For interior joints, the horizontal shear force Vw resisted by the steel web panel can be expressed as

Vw = 0.9fyvw tw h w

References [1] Eurocode 4: Design of composite steel and concrete structures, Part 1.1, general rules and rules for Building. London: British Standards Institution; 2004 (BS EN 1994-1-1: 2004). [2] ANSI/AISC 341-10. Seismic provisions for structural steel buildings. Chicago, Illinois, USA: American Institute of Steel Construction, 2010. [3] JGJ 138-2016, Code for design of composite structures [in Chinese], 2016. [4] X. Zhou, J. Liu, Seismic behavior and strength of tubed steel reinforced concrete (SRC) short columns, J. Constr. Steel Res. 66 (7) (2010) 885–896. [5] D. Gan, L. Guo, J. Liu, X. Zhou, Seismic behavior and moment strength of tubed steel reinforced-concrete (SRC) beam-columns, J. Constr. Steel Res. 67 (10) (2011) 1516–1524. [6] J. Liu, J.A. Abdullah, S. Zhang, Hysteretic behavior and design of square tubed reinforced and steel reinforced concrete (STRC and/or STSRC) short columns, ThinWalled Struct. 49 (7) (2011) 874–888. [7] H. Qi, L. Guo, J. Liu, D. Gan, S. Zhang, Axial load behavior and strength of tubed steel reinforced-concrete (SRC) stub columns, Thin-walled Struct. 49 (9) (2011) 1141–1150. [8] B. Yan, J. Liu, X. Zhou, Axial load behavior and stability strength of circular tubed steel reinforced concrete (SRC) columns, Steel Compos. Struct. 25 (5) (2017) 545–556. [9] X. Zhou, B. Yan, J. Liu, Behavior of square tubed steel reinforced-concrete (SRC) columns under eccentric compression, Thin-Walled Struct. 91 (2015) 129–138. [10] J. Liu, X. Wang, H. Qi, S. Zhang, Behavior and strength of circular tubed steelreinforced-concrete short columns under eccentric loading, Adv. Struct. Eng. 18 (10) (2015) 1587–1595. [11] X. Wang, J. Liu, X. Zhou, Behavior and design method of short square tubed-steelreinforced-concrete columns under eccentric loading, J. Constr. Steel Res. 116 (2016) 193–203. [12] X. Zhou, X. Zang, X. Wang, J. Liu, Y.F. Chen, Seismic behavior of circular TSRC columns with studs on the steel section, J. Constr. Steel Res. 137 (2017) 31–36. [13] JGJ/T 101-2015, Specification for seismic test of buildings [in Chinese], 2015. [14] H. Hu, Concrete-Filled Steel Plate Composite Coupling Beam and its Application to Shear Wall Structures, Tsinghua University, 2014 (in Chinese). [15] Karlsson Hibbitt, Sorensen. ABAQUS/Standard User’s Manual. Version 6.7.1, Hibbitt, Karlsson & Sorensen, Inc., 2007. [16] Z. Guo, X. Shi, Reinforced Concrete Theory and Analyse, Tsinghua University Press, Beijing, China, 2003 (in Chinese). [17] J. Tang, Earthquake resistant design of reinforced concrete frame connection, Southeast University Press, Nanjin, China, 1989 (in Chinese). [18] GB 50010-2010, Code for design of concrete structures [in Chinese], 2010. [19] Eurocode 3: Design of steel structures, Part 1.8, design of joints. London: British Standards Institution; 2004 (EN 1993-1-8: 2005). [20] G. Parra-Montesinos, J.K. Wight, Modeling shear behavior of hybrid RCS beamcolumn connections, J. Struct. Eng. 127 (1) (2001) 3–11.

(15)

where fyvw is the shear yield stress of steel web and hw and tw are the height and width of steel web respectively. The ratio of calculated strength to experimental strength ranges from 0.73 to 0.87 as indicated in Table 1, demonstrating that the design model for joint strength is somewhat conservative but acceptable for practical designs. 6. Conclusions This article describes and discusses the seismic behavior of circular TSRC column to steel beam connections based on an experimental study involving four specimens. The following conclusions can be drawn from the study: 1) The failure of the test specimens is typically caused by the joint tube, signifying a joint failure. The joint tube thickness has a significant effect on the joint strength, while the height of extended tube and the axial compression ratio of columns have little effect on the joint strength within the range of parameters considered in this study. 2) The developed computation procedure for elastic-plastic analysis on steel tube is shown to be valid. Based on the elastic-plastic analysis on the joint tube, the joint tube contributes to the joint strength primarily by direct shearing. 3) The ductility factor μ ranges from 2.2 to 2.8 and the equivalent damping coefficient ξeq is about 0.12 for the TSRC joints, as evidenced by the tests. Both indicate that the TSRC joint generally has a better seismic performance than the conventional RC joint. 4) The joint strength is mainly contributed by the joint tube, joint core

11