Cyclic behaviour of prefabricated connections for steel beam to concrete filled steel tube column

Highlights

• A prefabricated connection for steel beam to concrete-filled steel tube column.

• Full-scale experiments on the connections subjected to cyclic loads.

• Results on hysteretic load-displacement behaviour of full-scale connections.

• Results on energy dissipation capacity of full-scale connections.

• Seismic performances of frame structures with such prefabricated connections.

Abstract

An experimental study was conducted to investigate the cyclic behaviour of an innovative prefabricated connection system for steel beam to concrete-filled steel tube (CFST) column. In this connection system, internal stiffeners and threaded steel rods are used to stiffen the panel zone of the connection, and beams in two directions are bolted to the side plates pre-welded to the column side surfaces. Four specimens were examined under reversed cyclic loads applied by hydraulic actuators. Focus of the experimental study is given to hysteretic behaviours of the connections, including the inter-story shear force and drift ratio relationships and moment-rotation behaviours, showing superior energy dissipation than those in reinforced concrete structures in general. Furthermore, a numerical investigation is completed on the seismic performances of steel-concrete composite frame structures with integration of the moment-rotation behaviours of the beam-column connections from the experimental results. The effect of the beam-column connections on the structural behaviours, especially the yielding sequence of the structural components is clarified through this study.

Introduction

Concrete-filled steel tube (CFST) columns are often used in high-rise buildings, as they combine the advantages of compressive capacity and good flexural ductility [1]. In a CFST column, the steel and concrete mutually interact with each other, i.e. the column steel tube prevents the concrete from spalling, and the concrete prevents the column steel tube from local buckling [2]. With the confinement of the column steel tube, the compressive strength of the concrete inside the column can be significantly increased. The concrete inside the column can also increase the flexural capacity [3] and torsional capacity [4] of the column. Such columns however require specific configurations for connections to beams.

In a beam-column connection, the region of the column delimited by the upper and lower beam flanges is referred to as the panel zone [5]. The panel zone is often stiffened by steel plates to prevent the column flange from bending outward when the beam is subjected to tensile force. Typical members for stiffening the panel zone include external diaphragms (Fig. 1a) [6] and internal diaphragms [7] (Fig. 1b). External diaphragms are annular horizontal steel plates extended from the two beam flanges, surrounding the column steel tube [6]; while internal diaphragms are horizontal steel plates inside the column aligning to the heights of the beam flanges [7]. Each beam-column connection has two internal diaphragms or two external diaphragms. The column steel tube and the diaphragm plates can be in different shapes, e.g., rectangular, square or circular shapes.

To study the mechanical performances of connections between steel beam and CFST column, experiments can be performed on the specimens of beam-column connections with two different setups, i.e. loading at column ends with fixed beam ends, or loading at beam ends with fixed column ends [[8], [9], [10]]. In either way, the beam-column connection is subjected to bending moment and shear force. To achieve a high rotational stiffness, the relative rotation between the beam and the column should be limited. Such relative rotation can be measured by two inclinometers [11], i.e. one placed on the panel zone and the other at the beam end. The difference between the measuring results of the two inclinometers is the relative rotational angle of the joint. The relative rotation can also be measured by string potentiometers or linear variable differential transformers (LVDTs) installed between the beam end and the column [12,13].

Experimental investigations were performed to study the mechanical performances of beam to CFST column connections. For example, end-plate connections between steel beams and CFST columns without panel zone stiffening were experimentally investigated [14] where the connections were identified as a semi-rigid and partial strengths measure. A comparative study for steel beam to CFST column connections with or without external diaphragms was presented in [2]. It was found that a connection with external diaphragms offered about 20% higher moment-rotation stiffness and moment capacity than the one of the same sizes but without diaphragms. A study into steel beam to CFST column connections with internal diaphragms [15] indicated that this kind of connections could provide satisfactory moment capacity, allowing the beam fail prior to the joint. Both types of the connections (i.e. with internal or external diaphragms) offer high rotational stiffness and can be classified as rigid connection [16] according to Eurocode 3 [17]. Connections with internal diaphragms or external diaphragms are both suitable for two-way frames, i.e., frames that can sustain lateral loads from two horizontal directions [18]. The mechanical performances of connections with external diaphragms for two-way frames simultaneously subjected to two orthogonal bending moments in horizontal directions were numerically studied [18]. It was found that the application of bi-directional moments does not significantly affect the maximum stress of steel tubes compared to that subjected to moment in only one direction.

Because building structures may also be subjected to lateral reciprocating actions such as wind loads and/or seismic ground motions [19], the performances of beam-column connections under reversed cyclic loads are important for structural safety. One-storey frame structures with circular CFST columns, steel I-beams, and external diaphragms stiffening the beam-column connections were experimentally studied through the application of horizontal cyclic loads at the top floor [20]. It was found that the structures with connections stiffened by external diaphragms presented high ductility and energy dissipation capacity; and the energy dissipation capacity decreased with the axial compressive ratio of the column. The energy dissipation coefficients (E), as the normalized value to evaluate energy dissipation capacity of a beam-column specimen subjected to cyclic load [21], for frame structures with circular CFST columns, steel I-beams, and external diaphragms stiffening the beam-column connections were ranging from 1.46 to 2.29 [20]. Another study on the cyclic behaviours of the beam to CFST column connections with external diaphragms [22] indicated that the connection exhibited high ductility and its failure started due to the crack at the intersection between the diaphragm and beam flange. An experimental study on connections with internal diaphragms [23] revealed that the connections offered satisfactory energy dissipation capacity and their energy dissipation coefficients were between 1.15 and 2.38.

It should be noted that these two kinds of connections (i.e. with external diaphragms or internal diaphragms) require large amount of welding on construction site [24] and this may not be preferable for building prefabrication and assembly. Furthermore, the external diaphragm takes exterior space (Fig. 1a) and cannot be easily covered into the wall or ceiling. Therefore, it becomes visible inside the room and this is not aesthetically pleasing. The internal diaphragm requires a large hole in the middle (Fig. 1b) for concrete pouring and this results in a large size of the internal diaphragm plate. Consequently, the cross section of the steel column needs also be with a large area. Therefore, the column cannot be concealed within the wall and it then becomes visible in the internal space as well. To cover the column within the wall, the column may be narrow sections, i.e. with a large length-to-width ratio of the column section. The mechanical properties of such narrow rectangular columns are different from common rectangular or square tube columns, because the compressive strength of the CFST column decreases considerably with the increase of the length-to-width ratio of the column section [25,26]. This is especially the case when the length-to-width ratio is large for the columns with the same cross-sectional area, although the ductility of such narrow columns is not reduced compared to common rectangular or square columns [27].

To avoid the use of internal or external diaphragms in the conventional connection configurations, a few new connection configurations without such diaphragms have been proposed. For example, vertical steel plates in two beam directions were used to go through the panel zone of the column and were then bolted to the beam ends [28]. In this way the panel zone of the CFST column can be stiffened. It was found that the connections behaved in a semi-rigid manner and the moment capacity of the connections were about 44% of the beam plastic moment. Another connection configuration features the steel beam through the column steel tube so that the beam section can stiffen the joint panel zone [29]. The experimental investigation on this connection configuration indicated its superior ductility and the experimental results of equivalent drift ratio could reach 9.6%.

An innovative connection configuration that can be geometrically concealed into the wall or ceiling, without compromise of the convenience for on-site assembly, was proposed in [30]. Conventional horizontal diaphragms, including internal and external diaphragms, were replaced by internal vertical stiffeners and threaded steel rods in the proposed connection. Experimental study was conducted to understand the mechanical performance of such beam-column connections under monotonic loadings, where satisfactory moment resistant stiffness and capacity were found. This paper presents a further study on the hysteretic behaviours of the proposed connection system subjected to reversed cyclic load. With the implementation of such hysteretic performance of the connections into steel-concrete composite frame structures, the overall structural seismic performance are also investigated in this study, where the yielding sequence of the structural components can be clarified when seismic loads are applied.