## 1 Introduction  
Steel-concrete hybrid structures obtained through the combination of structural components made of reinforced concrete, steel and composite steel-concrete have been investigated much in the past two decades [1; 2; 3]. Through scientific hybridization with different components, more efficient, economical and flexible seismic resistant structural systems can be created. This paper focuses on a peculiar steel-concrete hybrid structure, namely steel truss-RC column hybrid structure, which is a supporting structure to house air-cooled condenser (ACC) in the thermal power plant (TPP). It has been identified as a type of structure suitable for the application of water-saving ACC technology to TPPs. For this kind of structure, the lower parts are an array of large-scale thin-walled RC pipe columns, used to support the upper sub-structures and industrial facilities. The height of column is about \(40\sim 50\) m, the diameter is about \(4\) m, while the thickness of the column is only about \(0.4\) m. A space steel truss about \(4\sim 8\) m-height is supported by the columns, and a range of large-diameter(about 9 m) draught fans are installed inside the space steel truss. On the steel truss platform, a series of metal \(A\)-shaped brackets above 10 m-height rest to support the huge-mass industrial units (e.g., air-cooled condensers, exhaust ducts) with approximately 10,000 tons in weight. Despite more or less symmetric floor planning in both horizontal directions, the stiffness and mass are usually unevenly distributed in the vertical direction due to the peculiar characteristics of structural components and industrial requirements. Past studies mainly focused on the ACC technology including experimental investigation, numerical simulation and optimization [4; 5; 6; 7; 8; 9]. However, the supporting structure of the ACC system was addressed rarely.  
In order to apply ACC technology in China, it is necessary to investigate the seismic behavior of the supporting structure for ACC systems because most of the coal-rich but water-shortage areas in China are often located in high seismic regions. In recent years, experimental and numerical investigations on the seismic performance of this supporting structure has been conducted, and the research results showed that the steel truss-RC column hybrid structure is a seismic resistant structural system which can be applied in earthquake prone zones [10; 11]. However, with the increasing of unit capacity of TPPs, the height and span of platform will be much larger, and the load gets much heavier. The traditional structural type barely satisfy the seismic demand in high intensity seismic regions. It is urgent to propose a new supporting structure for the ACC system which should have more ductile seismic capacity.  
Steel braces have been identified as one kind of effective components to improve the seismic capacity of steel, RC and some other structures [12; 13; 14; 15]. In these references, a new supporting structure consisting of steel truss-RC column with steel diagonal braces referred to be as steel braced truss-RC column hybrid structure is proposed, as shown in Figure 1. In this study, a range of pseudo-dynamic tests (PDTs) and quasi-static tests (QSTs) on a 1/8 scaled sub-structure were conducted to evaluate the seismic performance of the steel braced truss-RC column hybrid structural system. The influences of steel diagonal braces on failure mechanisms were also investigated. This study is expected to explicitly support the seismic design of the steel truss-RC column hybrid structure with steel diagonal braces.