Fixed Edge Supports
Impacts on Load Resistance and Glass Deflection
Presented on October 13, 2022 at Facade Tectonics 2022 World Congress
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Model building codes and standards in the United States use a probabilistic model to define glass load resistance (LR). In general, these model building codes and standards address LR of rectangular glass lites continuously supported along one, two, three, or four sides. In the case of a rectangular glass lite supported along all four edges, the probabilistic model treats the glass lite as a simply supported flat rectangular plate. Originally, glass lites were typically supported by relatively rigid window frames. Neoprene or similar gasket materials held the glass in the frame. This type of attachment approximated simply supported edges that prevented out-of-plane deflection while allowing the edges to rotate and deform in the plane of the glass lite. Advances in structural silicone make it possible to produce rectangular glass lites with almost fully fixed edges. The authors developed a nonlinear finite element model and applied the glass failure prediction model (GFPM) to the nonlinear finite element model output to determine the probability of breakage for selected glass lite geometries with fully fixed edges under uniform lateral loads. They compared analytical results for selected glass lite geometries with simply supported and fixed edges. The comparison indicated that not all glass lite geometries benefit from the use of fully fixed edges, but the benefits of fixed edges appeared to increase as lite aspect ratio (AR) increases.
Architectural glazing designers in the United States and many other parts of the world use ASTM International Standard E1300 (ASTM, 2016), Standard Practice for Determining Load Resistance of Glass in Buildings as their primary source to determine LR of glazing and glazing constructions in buildings. ASTM International published the current edition of ASTM E1300 in 2016. The practice described in ASTM (2016) is restricted to rectangular glass lites continuously supported along one, two, three, or four sides. The stress analysis underlying the GFPM ASTM E1300 (ASTM 2016) relies on a finite difference method advanced by Vallabhan (1983). In the specific case of a rectangular glass lite supported on all four edges, the glass lite is treated as a flat simply supported rectangular plate. Originally, glass lites were typically supported in rigid window frames held in place by neoprene or similar gasket material. This approximated simply supported edges that are prevented from deflecting out-of-plane, are allowed to rotate in the lateral direction, and are allowed to slip in the plane of the glass lite. Soules, Morse, and Norville (2020) developed a procedure for modeling a flat rectangular glass lite with simply supported edges using commercially available finite element software followed by a procedure for applying the glass failure prediction model (GFPM) developed by Beason and Morgan (1984) to the output of the analysis based on the Analytical Method of ASTM (2016). Advances in structural silicone make it possible to install rectangular glass lites with almost fully fixed edges.
The analysis below compares load resistances and maximum center deflections obtained from modelling glass lites with simply supported edges to those obtained from modelling glass lites with fixed edges. The analysis was performed using a model developed by Soules, Morse, and Norville (2020; in review) that applies the GFPM (Beason and Morgan, 1984) with stresses obtained from a commercial finite element program. The comparison of analyses results indicates as AR increases, LR increases when supporting glass lites with fixed edges instead of simply supported edges. Similarly, center deflections and maximum stresses tend to decrease for a given load. These findings are significant in that they indicate that glass lites with fixed edges have, in general, greater LR and smaller center deflections that glass lites with simply supported edges, as ASTM E 1300 (2016) assumes.
Soules, Morse, and Norville (2020; 2021; in review) presented the model used in this research. Their model combines the GFPM that serves as the basis of ASTM E1300 (ASTM, 2016)
To facilitate evaluation of the effect of fully fixed edges on flat rectangular glass lites, the authors used the finite element model to compute in-plane maximum principal stresses and maximum
Discussion and Conclusions
The comparisons made in this paper of LRs and center deflections from analyses based on both simply supported edges and fully fixed edges indicate the benefits of glazing lites with
Rights and Permissions
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