I read an article recently about structural glass that was quite interesting. Many of you may know that glass, despite its high strength, is very breakable. Glass has a strength of 140 ksi (kips per square inch) and a modulus of elasticity of 10,000 psi (pounds per square inch). In comparison steel has a strength of 35-75 ksi (up 290 for steel cables used in bridges and prestressed concrete, but that doesn’t exhibit the typical steel stress-strain relationship) and a modulus of elasticity of ~29,000 psi. Concrete has a strength of 3-20 ksi and a modulus of 57,000 *SQRT(f’c) psi (f’c is the 28 day strength of concrete). So with glass being so strong why don’t we have any scy scraper with a glass structural system?
Part of the reason is that glass is a brittle material. This means it has a purely elastic response to stress until it’s limit and then has a sudden failure. Steel has a yielding point then experiences strain hardening, in which it gains strength. Through this experience it can endure a certain amount of deformation. This allows it to absorb more energy, referred to as toughness, that’s why steel is so tough. Glass however has a very small area under the stress strain curve, meaning that it has a small amount of toughness, or energy absorbed. Another reason is because of its tendency to fracture. It is very dependent on any surface fractures or defects.
Hopefully architects and engineers will continue to expand the use of structural glass and experiment with the material, and maybe we will have an all glass tower in the future!
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