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Nabhajit Goswami • Joshua Schultz • Patrick Ferro • Stephen M Morse
The use of glass in the exterior facades provides greater light transmission into the building and improved views for occupants. Coastal areas in particular leverage the transparency of glass to maximize views. However, the innate characteristic of glass, due to the presence of microscopic flaws attributed to the manufacturing process and handling and weathering of glass, is to lose strength over time, a phenomenon known as static fatigue of glass. The strength of glass reduces due to the presence of water vapor and tensile stress. In the coastal areas, glass is exposed to sodium chloride (NaCl) from the sea, in addition to water vapor and tensile stress. To date, the effect of NaCl on glass has not been documented. This research presents the results from a study to observe the effect of NaCl on the strength of glass. The study loaded 39 glass specimens to failure using a three-point bending test. The glass specimens were 457 mm (18 in) long, 203 mm (8 in) wide, and of different thicknesses and were categorized into three samples based on the type of glass (i.e., annealed vs fully tempered, and 4.77 mm (0.375 in) vs 6.35 mm (0.25 in) thickness). To understand the effect of NaCl on the strength of glass, 20 specimens were loaded to failure at the time of receiving the specimen from the manufacturer, while 19 specimens were soaked in a NaCl solution for a full year before loading to failure. Surface flaw parameters for both sample sets were determined by applying the maximum likelihood estimator to the glass failure prediction model. Results for surface flaw parameters are presented for NaCl-treated and untreated specimens and sample strength was compared at 0.008 and 0.001 probability of breakage. Results did not exhibit a consistent trend; two sample sets showed a decrease in strength at both probabilities of breakage, while another sample showed an increase in strength, but further testing is recommended.
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Richard Green • Terrence McDonnell • Andrew Crosby
Other than limited special cases, there is a lack of standards providing guidance on the design of structural glass. This has resulted in an ad-hoc approach by cities (authorities having jurisdiction), architects and engineers. This paper outlines the key aspects of designing with glass in a manner that has reliability and robustness principles consistent with other structural materials while recognizing the unique aspects of glass. The Structural Glass Design Manual31 negotiates the spectrum of glass design between ‘non-structural’ applications, such as window glass and ‘structural’ applications such as glass floors by using four glass risk categories, based on occupancy and four robustness categories based on retention, redundancy and residual capacity. Developing consistent practices facilitates confident design in glass for engineers, architects and the building authorities. This paper outlines the principles guiding the Structural Glass Design Manual (Public Draft.)
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Richard Green • Terrence McDonnell • Andrew Crosby
The technology of glass now allows glass to be fabricated in sizes and structural configurations not previously conceivable. The standards and codes have not kept up. Achieving a comprehensive consensus standard is difficult, so voluntary specifications serve a useful purpose as we move forward with using glass as a structural material. This paper is a critical look at current standards, some common myths about glass design and provides a framework to design confidently and safely with structural glass. At the time of the conference it is anticipated that the Structural Glass Design Manual will be available in final public format for comment.
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Joshua Schultz • Grace Dojan
Glass structural elements have become increasingly common to the point of ubiquity; however, there currently is no universally recognized and codified Glass Standard in the United States. As a result, while there have been several efforts at developing design guides for structural glass (ISE Structural Use of Glass in Buildings, ASTM Standards, GANA Glazing manual etc.) each has somewhat different design methodologies and concomitant assumptions. With the variety of design references, there is a potential for accidental confusion of valid application of these methods.
Recently, the NCSEA Engineering Structural Glass Design Guide was published with equations for glass strength that are predicated on an “allowable” glass stress in combination with a series of adjustment factors. The resulting method is attractive in its apparent simplicity. However, sole reliance on the single largest maximum principal tensile stress (SLMPTS) may not always be representative of actual performance as random surface flaws often precipitate failure at lower stresses and different locations from the SLMPTS.
This paper analyzes the design examples from the NCSEA guide using the glass failure prediction model from ASTM E1300 to determine the probability of failure for each of the examples using various designs. Comparisons of the results show the importance of accurately modeling actual material behavior to 1) minimize material usage through efficient designs and 2) produce designs with consistently reliable levels of risk. Results show that maximal sustainability of glass (via minimal material usage) correlates to consistent probability of failure metrics. The average plate thickness found using allowable stress design methods is 16.4% greater than those calculated with strictly probabilistic-based design.
While the GFPM is potentially more computationally involved than closed-form equations in other references, the results indicate that the equations from the NCSEA guide may produce conservative designs, but they do not result in consistent probabilities of failure.
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