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.
Finite element analysis of a structural silicone shear bead used in skylight applications
Finite element analysis (FEA) was used to predict stresses and strains in a 6 mm x 6 mm structural silicone joint on the edge of an overhead piece of glass. The project was undertaken because of a marketplace report that this particular type of joint was showing field leaks after 5--10 years of service. FEA was used to show the stresses and strains in the nominal joint design under negative wind uplifts. After a three dimensional FEA model of the skylight system was completed, the deformations in the model were used to load a series of two dimensional FEA models of the silicone bead. The two dimensional bead models were completed at repeated intervals down the span, providing a finer mesh for recovering stresses and strains. All stresses and strains in this model were shown to be well within the working range of the silicone sealant properties. It was concluded that the field leaks were not due to excessive strains and could possibly be due to installation issues, mechanical damage or improper joints resulting from construction tolerances.
Development of a Practical Method to Evaluate the Fatigue Properties of Structural Silicone Glazing Adhesives
Structural silicone sealants have been used since the mid 1960s in the construction industry to attach glass to curtainwall framing systems. Design standards for these sealants have been in practice since the early 1970s and continue to be developed today. Most of these standards require tests that are based on a one-time destructive test of the joint material in tension. The industry established structural silicone design stress is intended to be reached only during the highest windload for the specified return wind period. The existing practice has had great success because the industry guidelines minimize stress on the silicone, and dictate quality assurance procedures that are instituted by curtainwall producers. In real life situations the sealant is additionally fatigued through a cyclic shear mechanism caused by daily thermal movement differences between the glass pane and the aluminum frame. This paper discusses a method for testing the effect of fatigue in sealants used in structural designs and comparing the data to control samples.