## 1 Introduction
The definitions, which were concluded by considering the effects of connection type, wall height, framing thickness, and intersection details, were used as a guidance for a performance-based design of CFS structures. Shaking table tests of CFS shear walls were conducted by [PERSON] et al. [21] and [PERSON] et al. [22], in which inter-story drift ratio was used to represent the failure evolution process of the structures. Additionally, shaking table tests were performed on low-rise CFS shear walls [23; 24; 25]; the test results revealed that sheathing failure along with the end studs losing their axial load-bearing capacities in succession were the typical failure modes of those structures. The failure mechanism investigations were performed mainly by cyclic testing of CFS shear walls and shaking table test of CFS structures, and macro-indexes (such as inter-story drift and force) were used to reveal the failure process by analyzing the typical failure modes of the structures. However, those studies are still lack s supporting theory, and the macro-indexes cannot reveal the failure mechanism radically. With respect to structural vulnerability, the inherent weakness of a structure is identified by analyzing the relationship between its topology form and connectivity, and in doing so, the failure mechanism will be further revealed. The vulnerability of a structure was defined as the ratio of the failure consequence to its relative damage demand [26; 27], and the internal structural form and connectivity were seen as the reasons of the vulnerability of the structure. Possible failure scenarios of structures were identified by the process of clustering and unzipping [28; 29], and the inherent weakness of a structure was detected through analyzing those different failure scenarios; this theory was then extended to 3D structures. [PERSON] et al. [30] performed a vulnerability analysis on two traditional Portuguese roof timber structures, identified the vulnerability indexes for various failure scenarios, and verified the applicability of this analysis method by comparison with actual failure scenarios. A series of vulnerability analyses was performed on three single-layer reticulated K6 domes by [PERSON] et al. [31; 32; 33], and the collapse modes of three shaking table test models agreed well with the predicted results with respect to the maximum vulnerability indexes, confirming the validity of the theory in revealing the collapse mechanism for 3D space structures. Therefore, a method exists to determine the seismic failure mechanism of the traditional and RCFS shear wall systems based on structural vulnerability analyses. In this paper, such a method is improved by considering both the fracture behavior of wallboards and the hinge-forming behavior of columns. Structural vulnerability analyses on three two-story CFS shear walls were performed to verify the practicability and validity of the proposed method by comparing the analysis results and test results. Structural robustness of the traditional and RCFS shear wall systems was quantitatively represented by the vulnerability indexes. In addition, \"strong frame weak wallboard\" and \"strong column weak beam\" were proposed as the conceptional rules for the seismic design of RCFS shear wall systems on the basis of the vulnerability results.