## 5 Lessons Learned and Limits
### Lessons Learned  
The engineers emphasized in the first place contractors should understand the distinctive features of materials contracted to use. They pointed that the in case of metallic materials including aluminum, the biggest drawback would be spring-back. Spring-back is defined as 'the amount of elastic distortion a material has to go through before it becomes permanently deformed, or formed [18; 29].' Three factors were investigated through the literature review as critically influencing the amount of spring-back of metal, i.e., size of segment, bend angle, and bend curvature. The contractors need to reflect the attributes in planning to fabricate metal free-form panels lest they should meet unexpected as-built quality. The next issue deliberated was using proper forming methods to the materials. As stated earlier, bending techniques including roller bending, press bending, and stretch bending are conventional methods to economically bend metal panels. According to previous research, those are not suitable for producing double-curved panels but are for a uniformly bent profile, that is, asing-curved panel [7]. In the mock-up tests, however, the engineers fabricated the aluminum double-curved panels by forceful cold bending with roller and press as well as hammering on site. The numbers in Table 4 and Figure 11 show unfavorable results and empirically demonstrated that the previous research is reasonable to refer to. In choosing a suitable forming method to the materials, one of the issues significantly highlighted in the discussion was the panel production cost by the method. The cost of forming free-form panels obviously differ depending on the shapes of the panels and the forming techniques to produce the panels. The author performed a market research as a part of the study to investigate the unit cost of fabricating the mock-up panels according to the four panel types [17]. Four companies with a different technique in Seoul, Korea were contacted to request their cost estimation. The first and second companies were Korea-based and had worked with metallic materials and GFRC respectively. The third and fourth companies were Korean branches of international enterprises specializing explosive forming and hydroforming techniques respectively. The cost the first company suggested \begin{table}
\begin{tabular}{c c c c} \hline
**Dimension Range (mm)** & **Length (L), Width (W) (mm)** & **Diagonal (mm)** & **Bending** \\ \hline W, L \(<\) 1500 & \(\pm\)1.5 & \(\pm\)2.0 & 1 mm per 1 m \\
1500 \(\leq\) W, L \(<\) 4000 & \(\pm\)2.0 & \(\pm\)3.0 & \\ \hline \end{tabular}
\end{table} 
Table 5: Korean Industry Standard of aluminum production allowance. was $2250/m\({}^{2}\) for a planar and single-curved aluminum panels by roller and press bending and the second company $190/m for GFRC panels. The third company offered $12,000/m\({}^{2}\) for a simple double-curved panel and $18,000/m\({}^{2}\) for a complex double-curved panel by explosive forming, and the fourth firm $18,000/m\({}^{2}\) for a simple double-curved panel by hydroforming and no estimation for a complex double-curved panel. The estimation seemed not to be reasonable and could not be bearable so that the author decided to form all the aluminum mock-up panels with the first company by means of forceful cold bending with roller and press. However, the following comment from previous research paradoxically advocates the wide cost range investigated: \"Processing unit costs for curved panels are classified into five grades, grade A to E according to the production difficulty. For grade A panels, which are the easiest to fabricate, the processing cost is approximately five times that of planar panels. Grade E curved panels, which are the most difficult to fabricate, cost three to four times more than Grade A panels, which is approximately 20 times more than planar panels [7]\".