Thin Glass As Cold Bent Laminated Panels in Architectural Applications by N.I
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Thin Glass as Cold Bent Laminated Panels in Architectural Applications by N.I. Schlösser Thin Glass as Cold Bent Laminated Panels in Architectural Applications in partial fulfilment of the requirements for the degree of Master of Science in Building Engineering at Delft University of Technology to be defended publicly on Friday the 27th of July 2018 by Naomi Schlösser 4154266 An electronic version of this thesis is available at http://repository.tudelft.nl/ Preface Before you lies the final report of my graduation research, written to meet the requirements for a Master of Science degree in Building Engineering with a specialization in Structural Design at Delft University of Technology. The topic of this research concerns thin glass in architectural applications, focussing on cold bent laminated panels. By choosing this field of expertise, I had the opportunity to combine my love for architecture and engineering. I was able to expand my theoretical knowledge by conducting real life experiments. An opportunity I’m very grateful for with many people supporting me. I therefore would like to acknowledge those who have helped me complete this research. First of all, I would like to thank my graduation committee and all the people involved from Delft University of Technology. Rob Nijsse, the chairman of the committee, who always had the patience to share his broad knowledge on structural glass and inspired me to push the boundaries. Telesilla Bristogianni, who introduced me to the field of glass, continuously remained positive and guided me throughout my entire research. Christian Louter, who shared his knowledge on thin glass and introduced me to everyone needed to complete this thesis. Special thanks to Gertjan Mulder and Johan Boender at Aerospace Engineering for providing equipment to manufacture the panels and helping me to obtain the necessary data during different stages of the analyses. I would also like to thank Kees Baardolf and Fred Schilperoort at Civil Engineering for preparing and supporting the experiments. Secondly, I would like to express my gratitude to all the companies who have assisted me during my research. Special thanks to Eckersley O’Callaghan, and in particular James O’Callaghan, for giving me the opportunity to study glass as a structural material in the first two months of my graduation at their office in London. By introducing me to their expertise, I could gather crucial information I wouldn’t be able to get by carrying out a regular desk study. I would also like to thank AGC and Anton Peters for providing Leoflex sheets and sharing their knowledge. Many thanks to Qdel and Trosifol for providing Saflex DG41 and SentryGlas respectively. Finally, I would like to thank my family and friends for their unconditional support and patience. Especially my housemates and dear friends Silke and Suman, who had to listen to my endless stories on thin glass. Robbie, who was willing to help me with great patience to understand the quirky features of Strand7. Michiel, who was always there when I needed him. My parents and sister, for their love and support, and who even helped me manufacture the supports needed to conduct experiments. Thanks to everyone who made my masters an unforgettable journey! N.I. Schlösser Delft, July 2018 i Abstract Glass has been used for centuries. Originally to create weapons, jewellery and decorative ceramics. Nowadays as windows, reagent bottles and electronic devices. The Egyptians seem to have been the first to realise what could be done with glass by trailing molten glass around a shaped core. The idea of inserting glass in windows was conceived by the Romans, which laid the foundation fundamental to today's processes. By innovating its production process the market could be extended to larger window panes and eventually structural glass. It’s only in the last two centuries that there has been substantial development of flat glass. A breakthrough came when the development of float glass was announced in 1959 by sir Alastair Pilkington. A continuous pane of glass moves out of the melting furnace and floats along the surface of an enclosed bath of molten tin, allowing excellent surface quality. A process that is still widely used today due to its high productivity and quality without the need for expensive machinery. The resulting products provide comfort, protection, energy reduction, security and transparency (Nascimento, 2014). It is for this reason that glass in modern architecture grew very rapidly in the past decade. Glass panels increased in size to push the boundaries for engineers and for architects to show what they are capable of and create even greater transparency. With that growth, the thickness of glass panels has increased from a modest 8 mm to a currently more regular thickness of 32 mm. On top of this, the required insulation values of glass constructions also increased. Future glass panels can include triple or more layers to provide for the required insulation, creating even heavier panels. According to Hundevad (2014), at a time where we strive to make ever larger and heavier glass panels, shouldn’t we investigate new ways of pushing technological boundaries in order to save material rather than using more of it? Where regular float glass is a rather thick, hard and brittle substance that can break easily when subjected to large stresses, thin glass can be found on the other side of the spectrum with its flexibility, clarity and higher strength. Thin glass has rarely been used for architectural applications, mainly because technologies for manufacturing thin glass in construction element sizes were not available or were often too expensive (Spitzhüttl, Nehring, & Maniatis, 2014). This product however shows large potential provided sufficient interest is shown from different sectors to lower the price and stimulate innovation. This research will therefore focus on how to implement thin glass on a larger architectural scale and embrace its features to create thinner and stronger load-bearing glazing elements in areas where regular float glass doesn’t work. The main drawbacks of float glass are that it is hard to curve and tension, it doesn’t allow for flexibility and it is becoming a rather heavy building material due to the build-up. Thin aluminosilicate glass is lightweight, flexible, able to curve and a more promising material to tension due to its higher tensile bending stress. However, limited geometrical bending stiffness in its flat form prevents thin glass to be used for structural purposes. Therefore, three main approaches can be distinguished to stiffen a thin glass panel. Firstly, to laminate the glass to a substrate providing increased out-of- plane stiffness. Secondly, to embrace its flexibility by cold forming the glass into curved developable surfaces. Thirdly, to treat the material as a fabric glass and design it for tensile membrane structures (Lambert & O’Callaghan, 2013). From these approaches, several design configurations are introduced. A rating system has been introduced to give an indication of the potential of every design configuration. Boundaries and demands from different industries (e.g. building and marine industry) are translated into requirements concerning optical quality, geometry, mechanical properties, sustainability and economical properties. According to the system, a composite panel combining float glass with thin glass, a composite panel combining polymers with thin glass, a curved panel with only thin glass and a curved panel combining polymers with thin glass are still in the running to become the chosen concept. It must be noted that this list is not binding and every principle could benefit from further research. The most important demands concern larger transparent panels, minimal weight, more free formed architecture and high thermal insulation. Cold bent laminated thin glass panels tick most of these boxes, since they encourages transparency, lightweight structures and free formed architecture. Furthermore, the flexible nature of thin glass is an important and promising characteristic and shouldn’t be compromised by introducing stiffening techniques. However, applying thin glass as load-bearing structures, deflections become governing and should be minimised to prevent causing alarm to users. Cold bent laminated thin glass panels use this flexibility to create a stiffer structure and therefore seems to be the perfect configuration to further explore. Subsequently, the main goal of this research is to gain insight in the structural and post-breakage behaviour of cold bent laminated thin glass panels. To obtain a curvature, glass panes have to be cold bent into a certain shape. Although introducing higher bending stresses at the top, the curvature is chosen to be sinusoidal, because it provides the smoothest distribution of shear and the lowest risk of delamination. Four two layered thin glass panels with the same curvature and different interlayers are produced. These panels consist of Leoflex glass provided by AGC, where two panels include an interlayer of Saflex DG41 (SAF) provided by Qdel and two panels include an interlayer of SentryGlas (SG) provided by Trosifol. The panels are cold bent to a value well below the recommended tensile bending strength of 260 MPa to allow for an increase in stress at spring back and loading. From the moment the panels are released from their mould, a certain spring back can be observed causing higher stresses at the top of the lower ply. Afterwards the relaxation phase is initiated. During this stage, the panels are under a constant load due to thin glass wanting to go back to its original flat shape. From the experiments, it can be concluded that the panels with a SAF interlayer relax significantly, but ii seem to come to a standstill after a certain period. The panels with a SG interlayer do not seem to relax after spring back. The measured geometries of this series are more or less the same at spring back and after two weeks of relaxation.