## 3 Methodology
### Identifying the Indicators  
The semi-structured interview was used to collect interviewees' opinions on a particular subject [37]. During 12-15 May 2019, five experts with rich knowledge and experience in steel structures were interviewed to judge the effectiveness of the preliminary indicators. Among these five experts, two of them are working in well-known design institutes in China. Another two are from top Chinese steel structure professional enterprises, and the remaining one is a professor working at a top university in China. As Table 5 shows, all these interviewees have over ten years of work experience in the steel construction sector. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Indicator** & **Code** & **Indicator** & **Code** & **Indicator** & **Code** \\ \hline Site Selection Conditions & x\({}_{36}\) & Utilization of Renewable Energy & x\({}_{2}\) & Construction Waste Water & x\({}_{48}\) \\ Environmental Assessment Level & x\({}_{37}\) & Construction Electricity Consumption & x\({}_{3}\) & Construction Noise Pollution & x\({}_{49}\) \\ Building Area & x\({}_{38}\) & Green Rate & x\({}_{44}\) & Construction Air Pollution & x\({}_{30}\) \\ Utilization Ratio of Underground Space & x\({}_{39}\) & Thermodynamics of Enclosure Structure & x\({}_{56}\) & Indoor and Outdoor Air & x\({}_{51}\) \\ Construction Land Area & x\({}_{40}\) & Water Consumption for Construction & x\({}_{46}\) & Quality after Completion & x\({}_{52}\) \\ Cyclic Utilization Rate of Structural & & Utilization Rate of Water-Saving & x\({}_{67}\) & Carbon Emission & x\({}_{52}\) \\ Demolition & x\({}_{41}\) & Appliances & x\({}_{67}\) & Construction Solid Waste & x\({}_{53}\) \\ \hline \hline \end{tabular} Note: details about the interviewees are omitted for privacy.  
\end{table} 
Table 4: The greenness indicators. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Respondent** & **Position** & **Type** & **Years of Work** \\ \hline A & Chief Engineer & Design Institute & 16 \\ B & Senior Engineer & Design Institute & 11 \\ C & Deputy Chief Engineer & Construction Enterprise & 21 \\ D & Chief Engineer & Construction Enterprise & 23 \\ E & Full Professor & Academic & 11 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Profiles of the interviewees. The open interview questions were designed to solicit experts' opinions on preliminary indicators of sustainable steel structure. These questions include: (1) Concerning the sustainability of steel structures, what aspects do you think designers and constructors emphasize, respectively? (2) Is it reasonable to divide the sustainability indicators into greenness, structural performance, and greenness? (3) How about the effectiveness of the indicators? The interviewees were also requested to evaluate the indicators carefully and to modify the indicators if necessary. Each interview lasted for around two hours. The interviews were conducted in three major cities of China, including Chengdu, Shenzhen, and Chongqing. The interviews were audio recorded, and the recordings were transcribed in written scripts in Chinese. The scripts were also returned to the interviewees for verification. The results of these interviews are four-faceted. First, those indicators with similar meanings (e.g., land use scheme, land master plan) were unified. Second, three indicators in the operation stage (e.g., component replaceability) were complemented because clients are sensitive to the applicability and maintainability of steel structures in the operation phase. Third, four indicators (e.g., the corrosion rate of steel and welding residual stress) were also included to reflect the features of steel materials. Another four indicators, including natural lighting coefficient and construction light pollution, were proposed from a life cycle perspective. Fourth, three additional indicators (e.g., design working life of a structure, structural demolition cycle) were included.