## 5 Lessons Learned and Limits
### Lessons Learned
Free-form designers in the design stage determine the shapes of the panels, identify the possible panel forming techniques, and estimate initial project cost, however, their concern is not cost but design. Their cost estimate is mainly to provide a preliminary estimate for selecting a suitable contractor through a competitive bidding for construction. [PERSON] et al. [10] had identified the cost overrun risks in free-form projects. Most of them were closely related to free-form panel fabrication such as lack of project-specific customization of fabrication technologies, poorly defined project scope at the time of budgeting, inappropriate subcontractor selection for free-form facades, mistakes in quantities and cost estimate, and unclear specification or scope of work. Contractors must bear in mind that those risks definitely belong to them. They should examine in the bidding stage the constructability of the panel shapes and identify suitable forming techniques in terms of technical as well as cost concern. They should perform a market study to precisely re-estimate fabrication cost of free-form facade. As well, once a contract is awarded, then must focus on the project budget limit and keep an eye on completing the project under budget. The other issue addressed is the need for preparing design change in the construction process. Design change is in general requested by project owners, designers, and sometimes contractors to resolve constructability matters. Design change in most cases is accompanied with construction cost increase. In order for contractors to protect themselves against unfavorable design change, they need to understand three approaches to panelize and optimize free-form facade surfaces: (1) shaping the surfaces into faceted planar panels; (2) shaping the surfaces into single-curved panels, i.e., smoothly bent strips; and (3) shaping the surfaces into smaller double-curved panels. In conjunction with spring-back of metallic material, larger area segments tend to have more spring-back than smaller ones. As for bend angle, the sharper the angle, the lesser the spring-back. As well, the bend curvature has a profound effect on the success of the bending operation and the attainable curvature depends majorly on the material thickness and material hardness. Mildly double-curved panels can be cut as planar panels and cold-bent on site. [PERSON] [9] commented that while the recent development of CNC shaping technology has simplified the production of double-curved metal panels, the production cost is still very offensive. In cases where only cold bending is available for free-form facade fabrication, it is better to design a facade surface having shapes and curvatures that can be divided into smoothly bent single-curved panels rather than using complex double-curved surfaces [9]. As well, he added that providing a certain range of tolerance in the panel spacing to the extent where the visual continuity of curvature among panels is maintained is necessary to obtain aesthetic products. As an example of the panel spacing and its tolerance, the DDP project, aforementioned in the Section 3.1. As-Designed Data can be referred to. The project involved a total of 45,133 panels, which consisted of 21,738 double-curved panels (48.2%), 9554 single-curved panels (21.2%), and 13,841 planar panels (30.7%). As a matter of course, each of the panels was manufactured as accurately as possible so that the panels could form a smooth surface, but 25 mm spacing between the double-curved panels were maintained and the tolerance value was 2 mm in any direction [7]. Managerial concern addressed in the discussion was the project delivery methods that would be fit to delivering free-form buildings. There are several project delivery methods commonly used in the market place such as design-bid-build (DBB), design-build (DB), and technical proposal (TP).