Anti-collapse performance of composite frame with special-shaped MCFST columns

Highlights

• A composite frame with special-shaped MCFST columns was tested in the scenario of middle-column loss.

• The fracture of the bottom beam flange caused the failure of the frame eventually.

• The designed frame exerts good robustness to resist progressive collapse.

• The length of side plate exhibits more influence on the performance of the frame than the thickness of side plate.

Abstract

A spatial steel frame with concrete slab and special-shaped multi-partition concrete filled steel tubular columns was tested in a simulated scenario of progressive collapse. The load-deformation relationship of the frame with damaged middle column under vertical load is discussed. Parametric analysis is also conducted by using a validated FE model. The results show that the failure of the specimen resulted from the fracture of the bottom beam flange eventually. The configuration of side plate connection and multi-partition concrete filled steel tubular column both could provide a reliable load-redistribution path for additional loads due to single column loss. The designed frame exerts good robustness to resist the collapse resulting from middle column loss. The simulation shows that single-column removal would cause additional lateral force and moment on the adjacent columns, besides axial force. The length of side plate exhibits more influence on the performance of the frame than the thickness of side plate.

Introduction

Recently, special-shaped multi-partition concrete filled steel tubular (MCFST) column is preferable in high-rise buildings and skyscrapers, due to its architectural flexibility and excellent mechanical behavior, such as Goldin Finance 117 tower in Tianjin, China [1] and CITIC Tower in Beijing, China [2]. As shown in Fig. 1, the columns with crisscross section or T section are beneficial for the spatial layout of buildings and increasing usable area. Up to date, the performance of special-shaped MCFST columns has been studied in many experimental and numerical researches. MCFST columns with different design parameters including cross section, slenderness ratio, steel ratio and plate thickness were tested axially, eccentrically and seismically, and compared whose results indicated that MCFST columns behaved in a ductile manner. Inner stiffeners and binding bars were proposed to improve the performance of MCFST columns. By considering the variation of the confinement effects in core concrete, strength prediction methods for MCFST columns were validated and modified based on the results of tests and simulation and the current national design codes [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. It is important to address that the behavior of the connection between the steel beam and MCFST column is complicated and insufficiently studied by test and simulation. In practice, no systematic design standard is issued for MCFST structures [13].

Progressive collapse in high-rise buildings which is triggered by the loss of vertical load-bearing members, has been widely studied in recent decades. When structural column is destroyed, the vertical loads on the damaged columns would be carried by the adjacent columns through the beams where dynamic-effect or rate-effect [14]would affect the anti-collpase performance of the remaining structures. Dynamic or load increase factors are normally used to consider the dynamic effect [15], [16], [17]. In the anti-collapse design, the beam-column joints are supposed to exert enough deformation ability and carry moment-tensional force to achieve this internal force redistribution [18], [19], as shown in Fig. 2. However these anti-collapse features of joints are not commonly covered by traditional design standards.

Since the configuration of the joint used in CFST structures is similar to that in steel structures, the relevant studies on collapse behavior prefer to use bare steel columns and joints in the tests or simulations [20], [21], [22], [23], [24], [25], [26]. Few studies were specifically conducted on the structures with CFST columns and joints [27], [28], [29]. However, it is well known that the restraint boundary condition has significant influence in resisting additional load [29], especially under the consideration of three-dimentional effect [30]. CFST column with relatively large lateral stiffness and stability is beneficial for the mobilization and acting of catenary action. Meanwhile, due to the polygonal section shape of MCFST as shown in Fig. 1, the traditional beam-column connections in CFST column, namely outer and inner ring plate, can not be used in MCFST column. Currently, side plates [31] are normally preferable to connect steel beam and MCFST column in practice as shown in Fig. 3 where the width of steel beam is supposed to be the same with that of MCFST column. If not, a transition beam section could be used to connect the small-width steel beam and large-width MCFST column whilst this transition beam section is connected to the MCFST column by side plates.

To date, the investigation on the anti-collapse behaviour of the frame with special-shaped MCFST columns is barely conducted, but indeed necessary, since MCFST column is mostly used in high-rise buildings and its connection is unconventional. In the previous experimental investigation conducted by the authors [32], the MCFST column-steel beam joint with side plates was tested in a simplified boundary condition simulating single column loss as shown in Fig. 4. The results showed that the configuration of double-side plate could provide a reliable load-redistribution path of the joint to resist additional loads resulting from single column loss.

In some studies on the anti-collapse of steel frame [30], [33], reinforce concrete (RC) slab was not considered in the specimen. Without the contribution of RC slab, the redistribution of internal force would be studied more clearly, especially regarding the effect of catenary action. However, the contribution of RC slab to the anti-collapse resistance of a frame structure resulting from composite action and membrane action is remarkable, not to mention that RC slab could restrict the local and overall buckling of steel beam [26].

In this study, a spatial steel frame with reinforced concrete slab and special-shaped MCFST columns is tested in a simulated scenario of column-removal. A double-side plate joint connecting steel beam to special-shaped MCFST columns is adopted. The experimental phenomena and results will be discussed in detail. Finite element (FE) simulation is also conducted to investigate the anti-collapse performance of steel frame with concrete slab and special-shaped MCFST columns.