# Thin-Walled CFST Columns for Enhancing Seismic Collapse Performance of High-Rise Steel Frames
## 1 Introduction  
The 2011 Great East Japan earthquake had devastating losses on human life, buildings and infrastructures near the seismic epicenter, and long-period ground motions excited resonance with high-rise buildings [1]. Extensive high-rise steel buildings were constructed during the period 1965-2000, especially in the period of construction-boom (i.e., 1980-1995) of high-rise buildings. CFST (concrete filled steel tubular) columns have been more adopted in high-rise buildings since 1992. There is a high probability that severe earthquakes with magnitude class over the design expectation might occur in the coming years [2]. Severe damage and collapse potentials exist in existing high-rise buildings.  
Several studies on the collapse behavior of steel frame were addressed. Symmetry limit theory for steel beam-columns originally revealed the steady-state path of cantilever members [3, 4]. A symmetry limit theory has been extended to reveal the phenomenon of deformation concentration at lower stories in the process of dynamic instability for the weak-beam-type frames [5]. Seismic resistance capacity of high-rise buildings subjected to long-period ground motions was studied on the basis of E-Defense shaking table tests [6]. Sidesway collapse of deteriorating structural systems under seismic excitations was analytically and experimentally addressed [7]. However, the strength deterioration such as localbuckling has not been fully investigated in the evaluation of seismic-resistant capacity of high-rise steel buildings with thin-walled sectional members.  
The behavior of CFST columns and structural systems has been investigated. [PERSON] et al. experimentally investigated the behavior of CFST stub columns, and beam columns under various loading protocols have also been investigated [8; 9; 10; 11; 12; 13; 14; 15]. They found that CFST demonstrated ductile behavior for the applications in seismic resistant structures. A fiber element model that is able to predict the ultimate response of CFST members was also presented and agreed well with the experimental results. [PERSON] and [PERSON] (2003) investigated the seismic resistance of CFST trusses [16; 17]. [PERSON] et al. (2004) studied a method to improve the distribution of story drift angle responses in CFST moment-resistant frames under severe earthquakes [18]. [PERSON] et al. (2008) conducted a research on the bond behavior of concrete filled steel tubular frames under cyclic horizontal load [19]. [PERSON] et al. (2009) experimentally studied the seismic behavior of concrete-filled steel tubular frame to reinforced concrete (RC) shear wall high-rise mixed structures [20]. [PERSON] and [PERSON] (2012) conducted a nonlinear seismic analysis of circular concrete-filled steel tube members and frames [21]. Nevertheless, the research on deterioration behavior and collapse-resistant capacity of high-rise CFST buildings, especially considering the effect of member deterioration, is still limited. Since the existing high-rise steel moment-resisting frames (SMRFs) entail collapse potential when subjected to long-period ground motions, CFST columns are proposed to substitute the hollow steel section (HSS) columns because the composite action can delay the local buckling and improve the strength of in-filled concrete.  
In this paper, the seismic performance of existing high-rise SMRFs with compact and thin-walled members subjected to over-design long-period ground motions is numerically studied. The effect of component deterioration is taken into account to investigate the collapse capacity. Then, thin-walled CFST columns are used to replace the HSS columns in high-rise SMRFs based on flexural stiffness, in order to enhance the collapse margin of the high-rise SMRFs that demonstrated fragile collapse capacity.