Structural Glass Inspection and Crack Patterns
Inspecting structural glass
This article looks at how to inspect structural glass for damage, crack patterns, as well as maintenance and care requirements.
Structural glass inspection should consider the following:
Checking the manufacturer documents, each panel must be marked with:
The name, registered trademark, or code of the manufacturer or supplier
The type of safety glazing material (for example, T for toughened glass)
The standard to which the material has been tested, such as AS/NZS 2208 (see list of standards below)
The classification for impact test behaviour (for example, A for Grade A).
Check the installation with the design documents and project specification
Check the fixing location and bolt tightening (overtightening will cause local stress concertation)
Inspect the glass quality (poor edge polishing can lead to stress concentration and cracking)
Check the glass is easy to be replaced
For glass balustrade with interlinking rail, check it is connected to the structural element
Glass standards
The following are the most important glass standards for buildings in NZ:
AS/NZS 2208: 1996 – Specifications and testing for the performance of safety glazing materials
ASNZS 4668: 2000 – Glossary of terms used in the glass and glazing industry
AS/NZS 4667: 2000 – Quality requirements and tolerances for cut to size and processed glass
AS/NZ 4666: 2000 – Australia/New Zealand Insulating Glass Unit Standards
NZS 4223 Part 4: 2016 – Dead, Wind & Snow Loadings Standards
NZS 4223 Part 3: 2016 – Human Impact Safety Standard
NZS 4223 Part 1: 2008 – Selection and installation of glass in buildings
Crack patterns
The glass may break due to impact. The crack pattern often can show the cause of the breakage, and the impact location.
Laminated toughened glass breakage pattern due to edge impact (part of the origin of fracture is at the edge).
Annealed glass breakage pattern
Nickel sulphide failure
Failure due to nickel sulphide imperfections can only occur in thermally toughened glass. The centre of the crack pattern typically has a double-D or “butterfly wings” like pattern.
Maintenance and care
The edges of glass can be prone to defects such as chipping or shelling which can reduce their tensile capacity. This is a particular problem with basic annealed glass where permanent stresses of more than 7N/mm² can cause crack growth leading to failure.
Glass is a durable material and can last for years in as new condition, with very little care.
It is recommended that glass canopy or roof lights be sloped more than 5° for water runoff or else water is likely to remain on the glass. If a shallow angle is unavoidable, eliminating silicone butt joints wherever possible is recommended.
Conclusion
Glass design presents some interesting opportunities, although there are certainly challenges to overcome. Although glass is a brittle material it can be used as a structural material, and if designed properly will meet all relevant safety engineering requirements with many benefits that no other material can achieve.
View other blog posts here:
The Different Types of Glass Used in Construction
Analysing the Behaviour of Structural and Laminated Glass
Selecting the Right Glass for Structural Applications
Specifying Sealant, Fixing and Hardware for Structural Glass
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