Keywords

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Extruded glass, Structural glass, Glass floor, Sandwich structure, Glass structure.

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Project Duration

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2016 - 2018

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Initiated by

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D. Vitalis, Dr. F. Oikonomopoulou, Dr. F. Veer

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Research Team (at TU Delft)

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D. Vitalis, Dr. F. Veer, Dr. F. Oikonomopoulou, Prof. R. Nijsse 

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Collaborators

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D. Vitalis, P. Lenk (ARUP)

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About

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Glass sandwich structures are a promising solution for creating fully transparent planar elements of high stiffness and decreased weight. Such panels can reduce material consumption while sparing the necessity of a supporting substructure. Sandwich structures are common in nature, from the beak of birds to the bones of a cuttlefish. The Built Environment has already adopted the principle of a sandwich structure in a vast range of applications, from cladding elements to floor panels. Yet, until now, glass sandwich panels have been little explored and scarcely applied, making use of opaque core materials such as FRP or aluminium honeycomb. Glass had not yet been considered for the structural core elements due to its brittle nature that allows only for elastic deformation until failure. However, recent advances in the adhesive technologies have enabled us to engineer sandwich panels where both the skins and the core elements are made of glass, achieving maximum transparency. 

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The novel idea of a fully-glass sandwich panel was initially developed for a floor application by Building Technology MSc student Dimitrios Vitalis together with dr. ir. Fred Veer and ir. Faidra Oikonomopoulou from the Glass & Transparency Group of TU Delft, Faculty of Architecture.  In a floor component, a high stiffness to weight ratio is essential, as the dead load acts perpendicular to the element and adds to the general load ap¬plied to the structure.

By definition, a sandwich panel consists of two skins separated by a core which provides the shear interaction between them. As the load bearing material is held at a distance from the neutral axis, the element’s shape factor is increased. When the panel is loaded in bending the top skin is in compression, the bottom skin is in tension, while the core is loaded in shear. Glass is a material with high compressive strength; in comparison, its tensile strength is relatively low. Hence, a glass sandwich panel is expected to fail at the tension-stressed, bottom skin. Several different core topologies were initially designed and explored both numerically and by physical testing to define the parameters that influence the behaviour of a sandwich structure made fully of glass.  Seven prototypes, each measuring 1000x300 mm, were designed to have the same stiffness for providing comparable results. The top and bottom skin consisted of one pane of annealed glass each. Ready-made, standardized, objects such as glass bowls and glasses were employed for the core elements to ensure that all elements are of the same exact dimensions with very little tolerances. The core elements were bonded to the two skins using a UV-curing colourless acrylate adhesive of high bonding strength and stiffness.   All prototypes were tested until failure in four-point-bending. All panels broke at the bottom skin, indicating that they exhibit a satisfactory sandwich behaviour.

Based on the knowledge acquired through the above research, within the course of Technoledge Structural Design of MSC Building Technology, a team of seven students designed and built a large prototype out of two glass sandwich panels, each measuring 3000x1500 mm. The new concept was to create a floor out of two sandwich glass panels where a core-free zone is placed in the middle, so that an object can be exhibited below. At the same time a statement is made for the structural potential of the innovative glass construction. To increase the stiffness of the panels and prevent the generation of peak stresses close to the supports, core elements are more densely distributed close to the outline of the floor. Core elements surround as well the central core-free area and through a gradient design are distributed to the rest of the panel. The final design of the core distribution was done parametrically in Grasshopper. 

Different glass elements were considered once again for the core elements. Eventually, the students opted for extruded star-shaped glass rod profiles made by SCHOTT. This geometry can create very interesting visual results, as the star profiles can be rotated to provide a pattern direction. Moreover, they have sufficient surface to be properly bonded to the top and bottom glass panes. Two laminated annealed glass plates comprised the top and bottom skins to allow for safety and structural redundancy in case of failure. Special, custom-made 3d printed connectors, following the shape of the star profiles were made by the student team to enable the connection of the two panels without compromising the overall visual result. 

Finally, in October 2018, three glass sandwich panels of 1.5m x 6m each were made at the TU Delft Lab facilities in order to be exhibited at GlassTec 2018. Each glass sandwich panel consists of two sheets of laminated heat strengthened glass separated by glass tubes. The locations and thickness of the glass tubes were optimised by the renowned engineering firm, Arup, to evenly distribute shear forces in the panel and utilise composite action efficiently. Initial parametric studies accompanied by more detailed Finite Element Analysis (FEA) allowed for the design of the panel according to the requirements of current design standards. The numerical models were validated by a series of 4-point bending tests carried out at TU Delft before proceeding to the construction and testing of the full-scale panel. In the week before the test, the panel was constructed with the help of students from Building Technology and Building Engineering. The large 6 x 1.5 meter sheet of laminated glass was laid down first, the glass tubes were then secured to the sheet in precise locations using an extremely strong UV curing glue. Finally, the top glass sheet was secured to produce the sandwich panel. In all, 3 panels were constructed which were displayed at the Glasstec 2018 exhibition in Dusseldorf. After construction, 40 students assembled at the lab to perform a variety of tests including walking over it, loading half the panel with people, and loading the whole panel with people. The final test was marching across in sync, a condition which can be detrimental to bridges because of natural vibrations. The structure was a complete success!