In this paper, it is argued that conceptual design in Model Code 2010 should be considered as a design act, rather than a design phase. This creative design act can be defined as an ill-defined problem and co-exists to some extent with the 'other' design acts the engineer performs during the design phase. This is illustrated by a case study. A part of the design process for the curved roof of a sports hall is traced. While the team had already decided on the architectural form and the structural concept during the competition, afterwards the roof gradually evolved from concrete vaults to a steel truss system. Several observations are made on the design process and an alternative scheme for the flowchart in Model Code 2010 is proposed.

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