## 1 Introduction  
The seismic performance of frame structures has been continually investigated by researchers, mainly through developing structures with maximum energy dissipation capacity [1; 2; 3; 4; 5; 6; 7; 8; 9]. In the 1950s, a new type of frame structure with a soft first story was designed by pioneers such as [PERSON] [10] who applied the soft-first-story idea by lifting the structure off the ground. This kind of building is functionally desirable considering it can provide parking spaces or grand entrances in hotels. The'soft story' structure is classified if it has less lateral stiffness than the story above it according to design standards such as in reference [11]. Initially, designers and researchers believed that the soft first story is like a soft spring that has a positive isolation effect on the whole structure and it thus can alleviate the seismic response of the superstructure above the soft first story [12]. However, a series of earthquake incidents showed that the soft first story was severely damaged after the earthquake, though the superstructures above the first story were only slightly damaged [13; 14]. This result could be caused as the lateral stiffness of the soft first story was abruptly decreased from that of the superstructures, resulting in a weak connection between the first story and its above stories. The low bearing capacity could then damage the boundary conditions when the weak interface moves from an elastic to a plastic state under earthquake conditions. Consequently, a large structure deformation occurred, and the energy generated by earthquakes were accumulated in the weak interface [13; 14; 15]. To illustrate the typical earthquake-caused damage in the frame structure with the soft first story, two examples of severe damage in the soft first story of two buildings in Turkey (Figure 1a) and China (Figure 1b) are shown in Figure 1. Although the structures with the soft first story are inherently vulnerable to collapse during earthquakes, it is still in demand especially in urban areas [16]. It is of great importance to improve the seismic design of this type of structures. To improve the seismic performance of soft first story structures, the lateral stiffness of the soft first story can be strengthened. For example, according to the Code for seismic design of buildings [18] the section stiffness of the column in the first soft story can be increased to avoid the building collapsing under strong earthquake excitations; this method (named as the 'hard-resistant' method), however, still fails to maintain a good structure safety of the weak interface between the first story and the above superstructure [19]. Although infill panels [4], metal shear panels [5], or curtain walls [20; 21] can be used to improve the seismic protection in frame buildings, these panels are normally constructed above the first floor to maintain general space in the first floor for the type of frame structure with a soft first story; thus, the seismic performance of a soft first story is still required for improvement. [PERSON] and [PERSON] [6; 7] proposed a new design strategy for failure mechanism control of moment resisting frames structures; this new design is under development. Alternatively, the first-story isolation system can be implemented to enhance the seismic performance of frame structures in practice. Generally, flexible laminated rubber bearings [22; 23] are inserted between the first floor and the columns that support the first floor [19; 24; 25]. To the best of our knowledge, the current studies such as reference [26] mainly focus on the steel frame buildings. The experimental study on reinforced concrete (RC) frame buildings with a first-story isolation system were ignored.