# Progressive Collapse Analysis of SRC Frame-RC Core Tube Hybrid Structure
## 1 Introduction  
Compared with traditional structural systems composed of steel or reinforced concrete members, steel reinforced concrete (SRC) frame-reinforced concrete (RC) core tube hybrid structure has a better combination of small sectional dimensions, higher strength, higher rigidity and resistance to corrosion, abrasion and fire. The SRC columns and RC core tube are rigidly connected by steel beams and composite floors. Benefiting from this connection and high stiffness of the core tube, most of the shear force caused by the horizontal load is assumed to be resisted by the core tube, and the lateral deformation can be restricted to an acceptable level. The vertical load of the building and partial overturning moment aroused by horizontal load is undertaken by the frame. Moreover, with the stiffness and resistance degeneration of the core tube under a strong earthquake, the frame will play the role of the second line of defence to resist shear force and avoid collapse. Hence, as a suitable and economical structure form of high-rise buildings, it is widely applied in the US, China, and Japan, especially in earthquake prone regions. Due to the advantages of concrete-encased composite structure [1], there are extensive studies [2; 3; 4; 5; 6] focusing on its seismic performance. The research on progressive collapse has become of increasing interest in recent years, especially since the World Trade Center towers collapse following the terrorist attacks of 11 September 2001. The progressive collapse of a structure can be caused by the failure of structural components under unexpected loads including, car accidents, earthquakes, or explosions and so on. [7]. To study the response of structures under extreme conditions such as explosions and fire, some research has been carried out. A new analysis method for the progressive collapse analysis of a structure with consideration of both the non-zero initial condition and existing damage in structural members [8], and a new finite element model was proposed as a feasible tool to evaluate the fire response of composite floor systems [9]. Recently, the alternate load path method (APM) recommended by the current codes and manuals of practice [10; 11] for anti-collapse design and analysis has been popular. The alternate load path method is easy to implement. In this method, the robustness of a structure is evaluated through removal of the key vertical components to determine whether the local damage may be absorbed by the remaining structural members and whether the structural system can bridge over the removed components. In the literature, there have been extensive experimental studies focusing on progressive collapse behavior of reinforced concrete frame and steel frame structures subjected to the loss of key components [12; 13; 14; 15; 16; 17]. The compressive arch action and catenary action were clearly observed in the experiments. Due to exorbitant cost and safety issues, numerical simulations are preferred for studying the progressive collapse resistance of structures [18; 19; 20; 21; 22; 23], in which nonlinear static and dynamic analyses had been conducted [24; 25; 26; 27; 28; 29]. As the two main components resisting the horizontal force aroused by earthquake, the frame and the core tube operate cooperatively with different stiffness. An unequal distribution of shear in the two components is produced and will affect the behavior of building structures against progressive collapse. To effectively prevent earthquake-induced structural collapse it is necessary to study a widely used structural system in high-rise buildings, SRC frame-RC core tube hybrid structures, focusing on progressive collapse behavior. Nevertheless, to date, limited related experimental and analysis work has been carried out on this particular structural system.