Experimental study on assembled truss beam-box column connections

*Cheng Xin1), Lei Honggang2) and Li Pu3)

1), 2), 3) College of Architecture and Civil Engineering, Taiyuan University of Technology,

Shanxi 030024, China 1)

xchengtyut@126.com

ABSTRACT

The assembled steel structural system has been considered as one of the most energy-saving and environmental friendly structural system, fitting the requirements of sustainable development. The experimental study, including both the monotonic and cyclic tests, on one assembled truss beam-box column connection was conducted. The mechanical properties of the specimens, such as the failure process, strength and energy dissipation capacity were carefully investigated. The influence of different loading conditions on the failure mode and fore-displacement were examined. It is observed that, the local buckling of the web and flange of the truss beam were the main failure mode of the specimens. The column and the joint zone remained undamaged during the whole loading process, satisfying the seismic design concept “strong joint weak members”. Finally, satisfactory energy dissipation capacity was observed within the designed allowable inter-story drift angle, highlighting the potential application of such connection in earthquake resistant zones. 1. INTRODUCTION

With the continuous development of urbanization in China, the assembled steel structural system has been considered as one of the most energy-saving and environmental friendly structural system (Cheng 2012, Zhang 2014). It possesses the advantages of both the steel structure (high strength, favorable seismic performance, flexible architectural space, etc.) and the assembled system (construction efficiency, economic benefits, decomposition after installation, reusability, etc.), fitting the requirements of sustainable development.

The steel structural system composed of truss beams and box columns, where the beams and columns are connected using high strength bolts, is one of the newly developed assembled steel frame systems (Zhang 2014). With the application of truss

1)

Associated Professor 2)

Professor 3)

Graduate Student

beams in housing buildings, the construction pipelines can be settled inside the truss beams, increasing the clear height of the building and improving the construction efficiency. Till now, the research on beam-column connections has been focused on H-section beams with box or H-section columns (Shanmugam 1995, Hu 2014, Erfani 2016), while the research on beam-column connection of column with truss beam is limited (Zhang 2014). In order to improve the application of such system, it is of essential value to investigate the mechanical performance of box column- truss beam connection.

In this paper, the experimental study, including both the monotonic and cyclic tests, of the assembled truss beam-box column connections was conducted. The mechanical properties of the specimens, such as the failure mechanism, strength and energy dissipation capacity were carefully investigated.

2. EXPERIMENTAL PROGRAM

2.1 Specimens

Fig. 1 Loading condition of the connection Fig. 2 Diagram of the specimen

Based on the loading condition of a middle column connection of a plane frame, a typical full scale beam-column connection subjected to constant axial force on the

N

△1

F1

1650

1650

3300

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2600

F2

△2

West East

200x200x20

Column base

truss beam C160x80x6

50x6

Flange

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975

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1380

1380

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830 250 830

1080 1080

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upper column and antisymmetry shear forces on the beam ends were conducted to investigate inelastic performance of the beam-column connection, as illustrated in Fig. 1. The effective length of the column is 1650mm, about half the column length. The effective length of the beam is 1300mm.

One full scale typical connection from practical engineering was chosen as the specimen. The detailed geometric definitions of the specimen is shown in Fig. 2. The connection specimen mainly consisted of 3 parts, including the truss beam, box column and column base. The upper and bottom flanges of the truss beam used Chanel-shaped section as C160×80×6, and the web was composed of two angel L50×6 welded on the inner surface of the flange. The whole truss beam can be prefabricated in the

factory. The column and column base adopted 口200×200×20, which can be hidden

inside the wall to prevent redundant angle of the column in the room. The column and the column based is connected with the help of two pairs of flange plates by the high strength bolts on-site. In this paper, two specimens of this exact dimensions are designed for monotonic loading and cyclic loading, and the specimens are named as J-m and J-c for monotonic and cyclic loading, respectively. In addition, both the specimens were made from Q345 steel with the axial force ratio n=0.33 according to the box parameters.

2.2 Experimental facilities

Fig. 3 Designed test setup Fig. 4 Photo of the test setup

The designed testing setup and the scene photo of the test setup is shown in Fig. 3 and Fig. 4, respectively, according to the loading condition illustrated in Fig.1. The

2000kN hydraulic jack

for vertical loading

West East

Brace

1000kN actuator 1000kN actuator

Specimen

loading system consists of two servo-controlled hydraulic actuator with 1000kN load capacity to generate the antisymmetry shear force on the beams, and a hydraulic jack with 2000kN load capacity to apply the constant vertical load to column of the specimen. It is noted that all ends of the specimen can rotate freely in the plane of bending with the help of the hinge connections respectively, thereby only the shear force and axial force were transferred to the specimen. Moreover, the out-of-plane displacement of the specimen was prevented via lateral bracings.

2.3 Loading protocol and measurement

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Fig. 5 Loading protocol for cyclic test

Fig. 6 LVDT arrangements Fig. 7 Strain gauges arrangement

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西 东

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S34S33S32

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T8

The axial load N=1635kN was applied to upper column of the connection and then kept constant. Antisymmetry shear forces with constant amplitude were applied on both the beam ends subsequently. The loading protocol for cyclic loading scenario is illustrated in Fig. 5, where the antisymmetry shear forces were applied load in terms of increasing lateral displacement amplitudes towards 10 ∆y. ∆y is the nominal yield displacement, which was roughly taken as 5 mm. Beyond that, the specimen was pushed up to complete failure, when the specimen completely loses its resistance against the lateral load.

The settlements of LVDT are illustrated in Fig.6 to measure the deformations of the test connections, including the in-plane and out-of-plane horizontal displacements. In addition, the settlements of the strain gauges are illustrated in Fig.7 to measure the stress and strain distribution development of the beam, column and the column base. 3. EXPERIMENTAL RESULTS

3.1 Failure mechanism

Fig. 8 Failure mode of the specimen J-m Fig. 9 Failure mode of the specimen J-c The final failure pictures of the specimens J-m and J-c are illustrated in Fig. 8 and

Fig. 9, respectively. It is observed that, the specimens J-m and J-c demonstrated similar failure mode. Obvious buckling and fracture were observed on the truss beams of both the specimens. Since the axial force is relatively small and thickness of the column is relatively thick, damages were concentrated in the truss beams, while the columns and the column bases maintained undamaged during the whole loading process, satisfying the seismic design concept “strong joint weak members”.

More specifically, with accumulation of plastic compressive strain, local buckling of the compressive web of the truss beam was first occurred, followed by buckling of the compressive flange of the truss beam, and the continues development of fractures started from welding area finally caused complete failure. For the specimen J-m, the compressive web buckled when Δ1=30mm, and the flange buckled when Δ1=50mm,

flange fracture

flange buckling

web buckling

Flange buckling

Web fracture

Web buckling

while the facture was observed on the flange when Δ1=100mm. For the specimen J-c, the compressive web buckled when Δ1=25mm, and the flange buckled when Δ1=40mm, while the facture was observed on the web when Δ1=65mm. It is indicated that, compared with monotonic loading condition, the cyclic loading condition would accelerate occurrence of all of the failure phenomenon with the accumulated damage.

In addition, typical strain curves at different positions of the specimens are listed in Fig. 10, including stains on the flange of truss beam (Fig. 10 a), web of truss beam (Fig. 10 b), column (Fig. 10 c) and column base (Fig. 10 d). Obvious plastic strains were observed on the flange and web zones, while the strains on the column and column base are relatively small (within elastic range). It is noted that the developments of the strains are in consistence with the above failure mode, as buckling and/or fracture concentrating on the flange and web of the truss beam and the columns maintain undamaged during the loading process.

J-m

J-c

(a) Flange (b) web (c) column (d) column base

Note: the strain on column base is effective strain derived from three direction strain gauges

Fig. 10 Typical strain curves at different positions for the specimens

3.2 Force-displacement curve The force-displacement curves of both sides of the truss beam of the specimens J-

m and J-c are illustrated in Fig. 11 and Fig. 12, respectively. ∆1 and ∆2 are the displacement generated on the truss beam, and F1 and F2 are the corresponding shear force, where the direction of these parameter are defined in Fig. 1. It is observed that the relationship between F1-∆1 is close to F2-∆2 for both of the specimens, showing good stability of the testing program.

The development of the force-displacement curves are in consistence with the failure sequence. For both of the specimens, the buckling of the web, as the first dominating failure mode, led to sharp deterioration of the stiffness. With the redistribution of plastic stress on the unbuckled zones, the strength kept increasing with decreasing stiffness. Then with the occurrence of flange buckling, the ultimate strength was achieved. The accumulated deformation of both the flange and web was accompanied with deterioration of the strength. However, the final complete failure was

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N)

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J-m (S25)

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(k

N)

ε (10-6)

J-m (S9)

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F2(k

N)

ε (10-6)

J-m (S2)

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F2(k

N)

ε (10-6)

J-m (T6)

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F2(k

N)

ε (10-6)

J-c (S21)

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F2(k

N)

ε (10-6)

J-c (S11)

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F2(k

N)

ε (10-6)

J-c (S2)

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F2(k

N)

ε (10-6)

J-c (T5)

caused by obvious fractures on the truss beam. Moreover, it can be observed from J-c, the hysteretic loops exhibits different performance by the occurrence of flange buckling. The hysteretic loops in the pre-ultimate stage was plump without deterioration, while in the post-ultimate stage, the hysteretic loop began to shrink with deteriorated energy dissipation capacity.

0 30 60 90 120 150 1800

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F1 (

kN

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J-m

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J-m

F2 (

kN

)

2 (mm)

Fig. 11 Force-displacement curve of specimen J-m

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F1 (

kN

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1 (mm)

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F2 (

kN

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Fig. 12 Force-displacement curve of specimen J-c 3.3 Effect of loading protocol

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J-c

J-m

F1 (

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J-m

F2 (

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)

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Fig. 13 Comparisons of force-displacement between J-m and J-c The skeleton curves of both sides of the specimen J-c are compared with the

corresponding force-displacement curve of J-m in Fig. 13. It is observed that, the ultimate strengths of J-m and J-c are close to each other, indicating that loading protocol has little effect on the ultimate strength. While the deterioration rate of the stiffness in the post-ultimate stage of the specimen J-c is significantly larger than that of

the specimen J-m. The reason is buckling deformation of the specimen J-c occurred on both the upper and bottom flanges and most of the webs.

4. CONCLUSIONS

The experimental study on one full scale truss beam-box column connection,

including both the monotonic and cyclic tests, was conducted. The mechanical properties of the specimens were carefully investigated. The following conclusions are obtained.

(1) For both the specimens, damages were concentrated in the truss beams, while the columns and the column bases maintained undamaged during the whole loading process, satisfying the seismic design concept “strong joint weak members”.

(2) For both of the specimens, the buckling of the web led to sharp deterioration of the stiffness; the ultimate strength was achieved with the occurrence of flange buckling; the final complete failure was caused by obvious fractures on the truss beam.

(3) For J-c, the hysteretic loops exhibits different performance by the occurrence of flange buckling.

(4) Compared with monotonic loading condition, the cyclic loading condition would accelerate occurrence of all of the failure phenomenon with the accumulated damage, and the deterioration rate of the stiffness in the post-ultimate stage of the specimen J-c is significantly larger than that of the specimen J-m.

For a more reliable treatment of truss beam-box column connection, further parameter analysis is required. Such topic is currently under investigation by the authors.

Acknowledgements

The research was supported by the National Natural Science Foundation of China (No.51408394) and Foundation research project of Shanxi Province (No.2015021122). REFERENCES Cheng, X. and X. Z. Zhao, et al. (2012). "A model study on affordable steel residential

housing in China." Frontiers of Structural and Civil Engineering 6 (3): 288-296. Erfani, S. and A. A. Asnafi, et al. (2016). "Connection of I-beam to box-column by a

short stub beam." Journal of Constructional Steel Research 127: 136-150. Hu, F. and G. Shi, et al. (2014). "Seismic performance of prefabricated steel beam-to-

column connections." Journal of Constructional Steel Research 102: 204-216. Shanmugam, N. and L. Ting (1995). "Welded Interior Box-Column to I-Beam

Connections." Journal of Structural Engineering 121 (5): 824-830. Zhang A.L. (2014). "The key issues of system innovation, drawing up standard and

industrialization for modularized prefabricated high-rise steel structures." Industrial Construction (08): 1-6. (in Chinese)

Zhang A.L. and Zhao L. et al. (2014). " Monotonic experimental study of prefabricated square tubular column to truss beam connections." China Civil Engineering Journal (S2): 169-174. (in Chinese)