## 6 Conclusions  
The disintegration of design and construction in the construction process makes it difficult for designers and constructors to reach a broad consensus on the sustainability indicators. This has been an outstanding issue in the construction of sustainable steel structures. Given that designers and constructors have different perceptions of sustainable steel structures, the use of the fuzzy set theory model in this study is suitable to synthesize the different views of these two entities. The following conclusions were drawn in this study: (1) The derived sustainability indicator system which is comprised of social-technical sustainability, economic sustainability, and greenness can not only fully and effectively reflects the performance dimensions of steel structure but also provides a new roadmap to integrate design and construction. (2) There is a discrepancy correlation between designers and constructors in the social-technical sustainability and greenness dimension. The social-technical indicators are divided into five categories, and the views of designers and constructors on each category are not completely consistent. Designers and constructors have a discrepancy opinion for the utilization ratio of underground space, construction land area, and construction electricity consumption in the greenness dimension. (3) Three is a correlation between designers and constructors in the economic sustainability dimension. Meanwhile, the results indicate that the top three indicators are procurement of material, investment amount, steel consumption per square meter, with the investment amount having potentially the largest impact. These similarity indicators represent the cooperation content that the designers and contractorshave approved. However, the discrepancy indicators require designers and contractors to pay more attention in practice. This research enriches and extends the existing sustainability indicator systems for steel structures by synthesizing the opinions of designers and contractors. This represents an advancement compared with historical studies that mainly explored sustainability indicators from the perspective of either designers or contractors. Further, a new approach was designed in this study to integrate the views of different professionals. Besides, the developed sustainability indicator system facilitates the sustainability performance assessment of steel structures, which supports the sustainable development of the steel structure sector. In practice, governments can better guide the application of steel structures by addressing the identified sustainability indicators in their policies. Design institutes and construction companies can better manage steel structure projects from a more comprehensive perspective to avoid the unsustainable phenomena. For instance, before the designer starts work, this set of indicators can be adopted to remind the designer to consider the construction parameters so as to ensure that the construction plan can be implemented smoothly during the construction phase. Furthermore, the indicator system can be combined with the information platform in practice to realize effective monitoring and prediction of relevant data. The study has two main limitations, and future studies are suggested to consider and address these limitations. First, the data used in this study were collected from China, and although Chinese interviewees have accumulated rich experiences in the management of steel structure construction, the data sample can be further extended to practitioners in countries with special national conditions. More extensive data will optimize the results. In addition, in the absence of experimental data, the questionnaire survey provides a feasible method to examine the proposed topic in this research. Nevertheless, as the questionnaire survey relies heavily on experts' subjective opinions, more objective data should be used to in future similar studies. Conceptualization, R.L., K.Y., K.C., X.H. and J.Z. ; methodology, R.L. and W.Z. ; software R.L.