Facade glass panel strength evaluation

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The glass panel strength cannot be determined by classical method of plate analysis because such analysis is valid only for deflections considerably smaller than the plate thickness.

Glass panels deflect many times their thickness and as a result develop membrane stresses which add significantly to their strength and stiffness.

Moreover, glass is a brittle material exhibiting no observable yield strength. Therefore, its behaviour under loads is best described by evaluating the strength under full-strength test.

Test results show a wide variation of strength for the same size and support conditions of glass panels. This is because the mechanism of glass failure is complex and is highly sensitive to different characteristics of flaws such as their size, orientation, and severity.

The only practical approach is therefore to evaluate the effects using an appropriate statistical model.

Check the following sites for further info and technical notes, Center for Window & Cladding Technology,
and Glass Association of North America.

The glass industry uses the normal distribution as the standard model in recommending glass thickness for various design conditions. The published charts of recommended thickness are based on an expected glass breakage probability of 8 lights per 1000, resulting in a design factor of 2.5.

From statistical point of view, it is virtually impossible to design a glass light without some probability of its failure under design load.

Therefore, in conducting a full-scale mock-up test of curtain walls, glass breakage is not considered as cause for test failure. Glass breakage may very well occur, since the test loads are usually much larger than the design load of glass.

If breakage occurs, broken glass is replaced with plywood in order to complete mock-up test. However, if the test indicates that premature failure of glass is due to inadequate stiffness of the glass supporting system, stricter deflection control should be imposed in the supporting system.

The required "glass panel strength" from an engineering point of view consists of selecting an appropriate thickness for a given area and design pressure from charts based on tests conducted by the glass industry.

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