Hybrid Steel-Glass Cell: Cold-twisting and Buckling Phenomena
The European RFCS project “S+G” has developed innovative steel-glass composite systems for high-performance building skins, meeting requirements of structural and energetic efficiency, as well as aesthetical value and versatility of applications. The aim of the project has been the design, prototype and testing of unitized cells composed by a glass panel and a contouring metallic frame, twisted by cold bending into a hyperbolic paraboloid shape, to be used in free-form curved building façades. To achieve large curvatures while maintaining the stress under the limit strength, very thin glass needs to be used, with a consequent risk of loss of stability. Preliminary investigations have been conducted to evaluate the buckling load in the glass panel and the possible stiffening effect of the contouring metallic frame. Then, accurate FE analyses, accounting for geometric and material nonlinearities, have been used to optimize the design of the unitized cell. Finally, two-steps experimental tests have been performed on full-scale samples. First, the prototypes are cold-twisted, maintaining limited the displacement of the corners, so to avoid buckling. Secondly, out-of-plane loading is applied to simulate wind, snow or man load. Thanks to the curvature, the cells showed very high strength due to the synergetic coupling of steel and glass. Here, we present the experimental and numerical investigations and the main results from full-scale tests. These have been used to refine the FE model, in order to investigate the role of both the interlayer and of the steel-to-glass bonding, and to develop a simplified model for practical design applications.