A multi-objective optimization approach for novel shell/frame systems under seismic load

Large -span buildings are used for a wide range of architectural purposes, such as commercial spaces, railway stations, ferry terminals, and airports. Yet, the construction of these structures in seismically active regions requires a large quantity of construction materials, leading to high resource consumption and carbon emissions. A solution to this challenging problem is the development of more efficient, optimized structures, the strategy adopted in this paper. In particular, a novel hybrid structural system is proposed, consisting of a multi-storey steel frame suspended from a long span reinforced concrete shell. The aim of the structural design of this hybrid system is to harness the beneficial seismic behaviour of shells and couple it to a deformable steel frame such that long -period vibration modes of the hybrid system are obtained, allowing for reasonably low stresses and displacements even under severe seismic actions. The innovative design procedure of the hybrid system is based on the preliminary form finding of the reinforced concrete shell geometry under gravitational loads; in this phase the curvature of the constrained edges of the shell is varied, and also openings are introduced on these edges such that the form finding yields to the formation of corrugations, improving the stiffness of the structure. A further refinement of the reinforced concrete shell is obtained by a thickness optimization under gravitational loads. The obtained innovative hybrid structural system, optimized for gravitational loads, is then analysed under high -intensity seismic actions through a nonlinear transient dynamic analysis, showing good results. Indeed, the beneficial dynamic interaction between the reinforced concrete shell and the suspended steel frame allows for a high overall seismic efficiency of the hybrid structure. Moreover, it is observed that in the hybrid structure the suspended steel frame yields a significant reduction of stresses and displacements of the reinforced concrete shell under earthquake actions with respect to the reinforced concrete shell alone. This phenomenon indicates that the suspended steel frame, as part of the whole construction, acts like a tuned mass damper for the reinforced concrete shell.