CYCLIC TESTING OF BOLTED CONTINUOUS I-BEAM-TO-HOLLOW SECTION COLUMN CONNECTIONS

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1 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska CYCLIC TESTING OF BOLTED CONTINUOUS I-BEAM-TO-HOLLOW SECTION COLUMN CONNECTIONS Wei Wang 1, 2, Liu Haojin 2, Yiyi Chen 1, 2, Chao Zou 2 ABSTRACT Beam through connections were improved and experimentally studied in order to be adopted in the floor-by-floor assembled steel braced structures for prefabricated buildings. The effects of connection details and the axial compression in the column on the rigidity and strength of the connections were investigated. Test results show that the failure modes of the connections significantly depend on the connection details and axial compression in the column, demonstrating excessive plastic deformation of beam flanges or plastic local buckling of tubular column. All the connections are found to have certain bending rigidity and moment resistance with favorable ductility, which can ensure that the columns participate in resisting horizontal seismic force under large story drift. 1 State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai , China 2 Department of Building Engineering, Tongji University, Shanghai , China Wei Wang, Liu Haojin, Yiyi Chen, Chao Zou. Cyclic testing and analysis of bolted continuous I-beam-to-hollow section column connections. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

2 Cyclic testing of bolted continuous I-beam-to-hollow section column connections Wei Wang 1, 2, Liu Haojin 2, Yiyi Chen 1, 2, Chao Zou 2 ABSTRACT Bolted continuous I-beam-to-hollow section column connections were improved and experimentally studied in order to be adopted in the floor-by-floor assembled steel braced structures for prefabricated buildings. The effects of connection details and the axial compression in the column on the rigidity and strength of the connections were investigated. Test results show that the failure modes of the connections significantly depend on the connection details and axial compression in the column, demonstrating excessive plastic deformation of beam flanges or plastic local buckling of tubular column. All the connections are found to have certain bending rigidity and moment resistance with favorable ductility, which can ensure that the columns participate in resisting horizontal seismic force under large story drift. Introduction Recently, a new system of floor-by-floor assembled steel braced structure, commonly used in Japan is introduced to China [1]. This structure is composed of hollow structural section (HSS) columns and H-section beams. In this system, the steel beam runs through the column, breaking the column into two parts. The column is then connected to the beam with high-strength bearing type bolts [2-4]. In Japan, the beam through connection of this system is a hinge-connection as illustrated in Fig. 1(a). As a result, the lateral force resistance of the structure is completely provided by the brace, which means the whole structure will lose load carrying capacity once the brace fails. However, the seismic philosophy adopted in the Code for seismic design of buildings in China [5] recommends that structures should possess multiple aseismic defenses, which this structural system currently lacks of. To be adopted in China, the system s aseismic performance should be improved by modifying connection details as shown in Fig. 1, making it to possess a certain degree of bending rigidity and ductility in order to guarantee the column can bear part of horizontal seismic force under large deflection. In an effort to prove the system s aseismic performance, behavior testing of bolted continuous I-beam-to-hollow section column connections of floor-by-floor assembled steel 1 State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai , China 2 Department of Building Engineering, Tongji University, Shanghai , China Wei Wang, Liu Haojin, Yiyi Chen, Chao Zou. Cyclic testing of bolted continuous I-beam-to-hollow section column connections. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

3 braced structure was carried out in this study. Specimens (a) Figure 1. Beam through connections: (a) before improvements; after improvements. Test Program A total of four full-scale specimens were tested and all the specimens were named according to their connection details and loading condition as summarized in Table 1: the first letter S represents the connections whose end-plate s size was (with four M12 high-strength bearing type bolts); the middle 4 letters and numbers denote the connection details, namely, B stands for beam, the first number is the number of stiffeners under the flange of beam, while C stands for column, the second number is the number of stiffeners at the column side; the last number represents the column axial compression ratio, with C standing for cyclic lateral loading. Table 1. Specimens. Specimen No. Column stiffeners Beam stiffeners Axial compression ratio S-B1C2-0C Yes 1 0 S-B1C2-0.3C Yes S-B1C0-0.3C No S-B2C2-0.3C Yes The section size of the beams in all specimens was H , while that of the columns was RHS 80 4, and the thickness of the stiffeners was 8mm. The typical connection detail is shown in Fig. 2.

4 80x4 column -8 column stiffener H300x150x4.5 x6 beam 200 mm High strength bolts Graded column stiffener x z z x -8 beam stiffener -10 endplate x y y x Figure 2. Typical beam through connections. Test Setup Fig. 3 shows the schematic test setup. Assuming the beam inflection point was at the mid-span, the beam length on each side of the column was half the bay width. The horizontal loading was imposed by a servo actuator (with stroke ±150mm and installed with a high precision force sensor at the end), with its end connected with the reaction frame by fixed support. The vertical loading was imposed by a hydraulic actuator, with a universal ball joint installed between its end and reaction frame to allow the follow-up movement of the actuator. base plate follow-up device vertical jack support of horizontal actuator actuator and pressure sensor two pressing beams up and down H-shaped steel beam 1500 square steel tube column Figure 3. Test setup.

5 Specimens in S-B1C2 Series Test Observations and Failure modes Specimens in S-B1C2 series included two specimens: S-B1C2-0C and S-B1C2-0.3C. During the loading process of specimen S-B1C2-0C, the column showed plasticity at the upper section of the column stiffener because of local stress concentration caused by stiffeners. Then part of the beam flange entered into plastic phase as a result of the out-of-plane tension action. The plastic zone continuously developed and the beam flange entered into plasticity in succession. Later the part of the end-plate entered into plasticity too. The final failure mode was characterized by excessive plastic deformation at the beam flange as shown in Fig. 4. Figure 4. Failure mode of S-B1C2-0C: Excessive plastic deformation of the beam flange. Compared with specimen S-B1C2-0C, specimen S-B1C2-0.3C was subject to a constant axial force applied at the end of the column (axial compression ratio of 0.3). The aim was to study the influence of axial load on the performance of the connections. As a result of the axial loads, flanges of both sides of the upper section of the column stiffener yielded first, and then the plastic zone of the column expanded to the middle section. All strain gauge at the beam flange entered into plasticity in succession. Following that, the strain gauges at the end-plate and the web of the upper section of the column stiffeners went into plastic phase. Finally significant deformation appeared at the beam flange of specimen S-B1C2-0.3C (Fig. 5(a)), accompanied by slight local buckling of the column (Fig. 5). However, compared with specimen S-B1C2-0C, S-B1C2-0.3C showed significant smaller deformation of the beam flange in the end. (a) Figure 5. Failure mode of specimen S-B1C2-0.3C: (a) Excessive plastic deformation of the beam flange; Slight buckling of the column.

6 Specimen S-B1C0-0.3C Compared with specimens in S-B1C2 series, specimen S-B1C0-0.3C was not equipped with column side stiffeners, and was exerted by column axial force with axial compression ratio of 0.3. During the loading process, the bottom section of the column and part of the beam web yielded first, and then the strain gauge at the beam flange entered into plasticity in succession. At the late loading period, the middle section of the column and the beam-column connecting endplate reached plastic phase. The final failure mode was marked by excessive plastic deformation of the beam flange, as shown in Fig. 6(a). Specimen S-B2C2-0.3C In comparison with specimens in S-B1C2 series, specimen S-B2C2-0.3C was equipped with two stiffeners at the beam, so the out-of-plane stiffness and bearing capacity of the beam flange had obvious enhancements. In the loading process, the failure of the specimen occurred at the upper section of the column stiffeners, and later the plastic zone developed into the middle section of the column, then the strain gauge at the beam-column connecting end-plate entered into plasticity. The specimen failed at last because of the buckling of the column section, but the buckling was significantly greater than specimen S-B1C2-0.3C. The final failure mode is shown in Fig. 6. (a) Figure 6. Test observations: (a) Failure mode of specimen S-B1C0-0.3C:Excessive plastic deformation of the beam flange; Failure mode of specimen S-B2C2-0.3C:Column buckling. Cyclic Responses In this section, the experimental results of each specimen are discussed in terms of cyclic responses, including: (1) Curve of P - δ, of which the vertical ordinate is expressed by horizontal force P, while the horizontal ordinate is indicated by column top deformation; (2) Curve of M - θj, of which the vertical ordinate is expressed by the actual bending moment of the connection M, which is calculated to the interface of the beam flange and the beam-column connecting endplate, including the additional bending moment caused by vertical axial force; while the horizontal ordinate is indicated by turning angle of the connection θj, which is measured by the

7 two displacement meter at the column side; (3) Curve of M - θ t, of which the vertical ordinate is expressed by the actual bending moment M of the connection, including the additional bending moment caused by vertical axial force, while the horizontal ordinate is the inter-story displacement angle θ t, that is, the ratio of the displacement δ at the top of column and the column height H (H=1500mm). Specimens in S-B1C2 series As shown in the P-δ plot (Fig. 7), unlike specimen S-B1C2-0.3C, the horizontal bearing capacity of specimen S-B1C2-0C doesn t show peak load because no second-order effect was generated. But even for specimen S-B1C2-0.3C, the horizontal force hasn t yet reached the peak value when the inter-story displacement angle reaches 0.02rad. Both specimens demonstrate good ductility. It can be seen from the M - θ j plot (Fig. 8) that a certain amount of axial compression can effectively restrict the relative rotation between beam axis and column axis and decrease the rotation angle of the connection, thus enhancing the initial bending rigidity of the connection. In terms of specimen S-B1C2-0C, a larger deformation gap between beam-column connecting endplate and beam flange is generated during the experiment, which makes gap closing in the reverse loading process. During this process, the lateral rigidity of the column top and the bending rigidity of the connection are very small, and later they get increased again because of gap closure. For specimen S-B1C2-0.3C, however, the above phenomenon is not obvious, because the axial compression greatly reduces the deformation gap. It is shown in the M - θ t plot (Fig. 9) that neither of the flexural capacity of connections of S-B1C2-0C and S-B1C2-0.3C shows descending path when inter-story displacement angle exceeds 0.06rad, which means they have good ductility. (a) Figure 7. P - δ curves for S-B1C2 series: (a) S-B1C2-0C; S-B1C2-0C

8 (a) Figure 8. M - θ j curves for S-B1C2 series: (a) S-B1C2-0C; S-B1C2-0C (a) Figure 9. M - θ t curves for S-B1C2 series: (a) S-B1C2-0C; S-B1C2-0C Specimen S-B1C0-0.3C Compared with specimen S-B1C2-0.3C, the beam-column connecting end-plate of S-B1C0-0.3C is not equipped with stiffeners at the connection, so its lateral force resistance significantly reduces under the same axial compression. As is shown in the P δ plot (Fig. 10), the P δ plot doesn t enter into descending stage when the inter-story displacement angle reaches 1/50 (displacement at the top of column being 30mm), showing great ductility of the specimen. Clearly seen from the M - θ j plot (Fig. 11), the connection bending rigidity and bearing capacity of S-B1C0-0.3C is smaller than S-B1C2-0.3C. From the M - θ t plot shown in Fig. 12, it can be seen that the connection flexural capacity of specimen S-B1C0-0.3C doesn t show descending path when the inter-story displacement angle exceeds 0.06rad, which means it has good ductility.

9 Figure 10. P - δ curve for S-B1C0-0.3C Figure 11. M - θ j curve for S-B1C0-0.3C Figure 12. M - θ t curve for S-B1C0-0.3C Specimen S-B2C2-0.3C The failure mode of S-B2C2-0.3C is governed by the cross-section failure of the RHS column. It can be seen from the P - δ plot shown in Fig. 13 that for this failure mode, the hysteretic loop of the curves is plump, but it fails with sudden, once the plastic local buckling of the tube column takes place, the bearing capacity drops rapidly. However, when the plastic local buckling of the tubular column takes place, the inter-story displacement angle has already reached 1/37.5 (displacement at the top of column being 40mm), which shows good ductility of the specimen. As is shown in the M - θ j plot (Fig. 14), the turning angle of the connection of S-B2C2-0.3C is

10 obviously smaller than that of specimen S-B1C2-0.3C because of two stiffeners at the beam flanges, namely, the connection bending rigidity of specimen S-B2C2-0.3C is relatively larger. It can be drawn from the M - θ t plot (Fig. 15) that the plastic local buckling of the tube column doesn t take place until the inter-story displacement angle of S-B2C2-0.3C reaches 0.047rad, which means the specimen has good ductility. Figure 13. P - δ curve for S-B2C2-0.3C. Figure 14. M - θ j curve for S-B2C2-0.3C. Figure 15. M - θ t curve for S-B2C2-0.3C Conclusions In this paper, bolted continuous I-beam-to-hollow section column connections were improved and experimentally studied in order to be adopted in the floor-by-floor assembled steel braced structures, and the following conclusions can be made: (1) The beam through connections after improvements could bear certain bending moment, and the flexural capacity of the joints even exceeded the sectional bearing capacity of HSS column on some occasions. The column end joints that possessed certain flexural capacity made the beam-column frame able to bear certain horizontal force, therefor serving as the second aseismic defence by proper arrangement of the columns. (2)The main failure modes of the joints were column local buckling, excessive plastic deformation of the beam flanges and excessive plastic deformation of the beam-column connecting end-plate. The failure modes were closely related to connection details and axial compression of the column. When equipped with two stiffeners at the lower beam flanges or the axial compression was greater, the joints were more easily to fail in the column section. Certain

11 axial compression could effectively improve the flexural rigidity and bearing capacity of the endplate and the beam flanges, but it reduced the ultimate flexural capacity of the column section, so it was necessary to limit the column axial compression ratio. (3) All the joints showed good ductility with the bearing capacity no deterioration before local buckling of the RHS column. Acknowledgements The work presented in this paper was supported by the Key Projects of Natural Science Foundation of China (NSFC) through Grant No References 1. Wang W, Zhou Q, Chen YY, Tong LW. Experimental and numerical investigation on full-scale tension-only concentrically braced steel beam-through frames. Journal of Constructional Steel Research, 2013; 80: Azizinamini A, Yerrapalli S, Saadeghvaziri MA. Design of through beam connection detail for circular composite columns. Engineering Structures 1995; 17(3): Elr y A, Azizinamini A. Design provisions for connections between steel beams and concrete filled tube columns. Journal of Constructional Steel Research 2001, 57: Elr y A. Connections between steel beams and concrete-filled steel tube columns. Ph.D. Dissertation, Department of Civil Engineering, University of Nebraska-Lincoln, May, Ministry of Housing and Urban-Rural Development of China. Code for Seismic Design of Buildings, GB , Beijing; 2010.