1. INTRODUCTION The development of powerful CAD software has allowed contemporary architects to design geometrically complex shapes and forms that do not always correspond to the structural coherence of the building. As a consequence, the structural design of such conceptions are often carried out after the fact, by specialists who must execute the intent of the designer, which effectively disconnects the Vitruvian firmitas concept from the architectural design process [1]. In addition, this disconnect between design and analysis of the structure can lead to overly uniform and standardized structural designs, as well as a loss of the original concept in the translation of the design intent, or essence. The typology of the tall building is especially susceptible to the disconnect between architectural intent and structural necessity. The structure of tall buildings necessitates rigorous adherence to structural principles, which are not always accessible to architects. However, at the same time, by virtue of their scale, tall buildings offer a prime opportunity to fuse structural logic and efficiency with architectural expression; “[w]hen the structure is disclosed on the façade in a logical manner in the spirit of an artist, structural expression contributes to the architectural look of the building, and thus the visual experience of the city” [2]. Moreover, as tall buildings are inherently material intensive to construct, savings in the structure of tall buildings can reduce their carbon footprint. Therefore, considering the aesthetic, spatial and architectural qualities of a structural system simultaneously with the structural aspects, in the design phase, can yield a more cohesive and efficient building as a whole, than when the structure is only considered as a final step. Fazlur Khan’s structural design for the Hancock Center in Chicago was conceived in the beginning stages, in order to economically meet the height requirements. His design not only achieved new heights with greater efficiency, but also serves as a landmark for the city, in part, due to its Architectural feedback in the structural optimization process J. Felkner & E. Chatzi Institute of Structural Engineering, ETH Zurich, Zurich, Switzerland T. Kotnik Institute of Technology in Architecture, ETH Zurich, Zurich, Switzerland ABSTRACT: This paper presents an interactive framework that balances visual exploration of design variations with their structural implications, in the case of the design of truss structures with aesthetic criteria. Based on the assumption that aesthetic design goals are not easily quantifiable, key elements for an interactive optimization framework are derived and implemented. Starting with an initial design, the user can visually assess interesting solutions, save them for later assessment, actively drive the optimization towards individual goals, re- initialize the optimization with a set of available solutions, or restart the design process. In addition, a criterion is introduced to measure the similarity of the shape of candidate solutions with respect to reference designs. The framework is then applied in the design of a truss tower. The effectiveness of the similarity criteria, as well as the ability of the user to drive the optimization towards specific design goals is demonstrated. The result is a framework that is conducive to the visual exploration of architectural variation coupled with structural constraints. Structures and Architecture: Concepts, Applications and Challenges – Cruz (ed) © 2013 Taylor & Francis Group, London, ISBN 978-0-415-66195-9 676