Next generation structural silicone glazing
This paper presents an advanced engineering evaluation, using nonlinear analysis of hyper elastic material that provides significant improvement to structural silicone glazing (SSG) design in high performance curtain wall systems. Very high cladding wind pressures required in hurricane zones often result in bulky SSG profile dimensions. Architectural desire for aesthetically slender curtain wall framing sight-lines in combination with a desire to reduce aluminium usage led to optimization of silicone material geometry for better stress distribution. To accomplish accurate simulation of predicted behaviour under structural load, robust stress-strain curves of the silicone material are essential. The silicone manufacturer provided physical property testing via a specialized laboratory protocol. A series of rigorous curve fit techniques were then made to closely model test data in the finite element computer analysis that accounts for nonlinear strain of hyper elastic silicone. Comparison of this advanced design technique to traditional SSG design highlights differences in stress distribution contours in the silicone material. Simplified structural engineering per the traditional SSG design method does not provide accurate forecasting of material and stress optimization as shown in the advanced design. Full-scale specimens subject to structural load testing were performed to verify the design capacity, not only for high wind pressure values, but also for debris impact per ASTM E1886 and ASTM E1996. Also, construction of the test specimens allowed development of SSG installation techniques necessitated by the unique geometry of the silicone profile. Finally, correlation of physical test results with theoretical simulations is made, so evaluation of design confidence is possible. This design technique will introduce significant engineering advancement to the curtain wall industry.
Nonlinear elastic finite fracture mechanics: Modeling mixed-mode crack nucleation in structural glazing silicone sealants
Requiring both stress and energy conditions to be met simultaneously proved key to modeling brittle crackformation at singular and nonsingular stress concentrations in linear elastic materials. The present workextends this so-called coupled stress and energy criterion to brittle crack nucleation in hyperelastic mediausing the example of silicone adhesives. For this purpose, we provide a comprehensive constitutive as wellas fracture mechanical characterization of the structural silicone adhesive DOWSIL™ 993 using a large setof experiments and propose a mixed-mode failure model for crack initiation in nonlinear elastic materials.Characterized in independent experiments, the model is used to determine critical loads of hyperelasticadhesive bonds in both shear and tension dominated configurations. For any of the examined adhesive jointconfigurations the model predicts and explains size effects and agrees well with experimental findings. Westudy stable and unstable crack propagation observed in video recordings of our experiments. It is shownthat crack initiation, crack growth and crack arrest are caused by nonmonotonic energy release rates andcan be predicted. Effects of excess energy available after crack nucleation and initial unstable crack growthare discussed.
Durability by Design: Load Carrying Silicone Bonding, Herz Jesu Church, Munich
The design of conventional glass facades in Germany is typically based on the extensive use of point supports that keep the glass panels in position and which provide the load transfer of the facade. Besides the aesthetic disadvantage of discrete elements being visible in the glass facade, the application of point supports leads to highly concentrated stress regions substantially affecting sizing and durability of the facade components. In order to overcome these limitations, a new approach has been chosen for the design of the glass facade of the Herz Jesu Church, Munich. One peculiarity of the overall glass facade of this box-shaped building consists in horizontal and vertical glass beams used for the support of the facade. Silicone adhesives bond steel stringers along the main edges of the horizontal and vertical glass beams in order to establish load paths between beams and facade. The hereby-realized bonding design offers special features being favorable for the durability of the building. Load carrying capacities have been provided by line-type connections, substantially reducing stress concentrations. Major attention has been given to the geometric layout of the bonding. Thus, detailed finite element (FE) analysis has guided the careful selection of an adequate channel cross section for the stringers. The design philosophy of the bonding is characterized by a small exposed surface of the silicone adhesive, offering only a very small area of attack for environmental degradation. Additionally, the exposed surface is only stressed at a low level avoiding the critical combination of highly loaded areas exposed to aggressive environmental conditions. Furthermore, two principal load paths (tension and shear) are established, leading to a fail-safe design principle of the bonding. These key considerations provide major contributions to the high durability design of the glass facade bonding.