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802 Fixing brackets for glazing systems

Report ID: 802

Published: Newsletter No 53 - January 2019

Report Overview

A reporter's organisation has been alerted to several cases of broken glass panels in a canopy at a transportation facility.

Report Content

Over a period of a year, a reporter's organisation has been alerted to several cases of broken glass panels in a canopy at a transportation facility.

The orientation of the panels was a mix of vertical, horizontal and sloping, with failures in all orientations.

While the original cases were attributed to vandalism, an inspection report concluded that the cause might be Nickel Sulphide (NiS) Inclusion; a defect that can cause spontaneous shattering in toughened glass. However, instances of NiS inclusion are very rare, and the reporter does not believe it accounts for the number of glass breakages at the site.

As a precaution, emergency refurbishment works were carried out that have removed high risk areas of glass, replacing the majority with polycarbonate translucent panels.

During the emergency works, the reporter's organisation carried out their own inspection to see the fixings first hand and compare to the as-built details. It was observed that at least 20 of the steel fixings had come loose, leaving a number of the large 1m x 2.5m glass panels suspended near vertically from only two of the point fixings.

In addition, the glass has countersunk holes to receive the fixing bolt which, in the event of the surrounding rubber gasket failing, would result in a very thin edge of glass sitting on a steel bolt. It is the reporter’s opinion that the glass breakages are a result of issues around fixing brackets. Accordingly, they are monitoring their assets with similar types of monolithic glazing.

Comments

This type of fixing is quite common and usually related to laminated glass where a sleeve is provided to minimise point load effects.

Structural glass elements typically fail due to either misadventure or poor detailing/construction practices. Additionally, the glazing panels should have been laminated as they are at height and a failure of one pane would have been held to the other by the laminating material.

The cause appears to be unknown, but one possibility not mentioned by the reporter could be repetitive wind flexing of the panels producing vibration which could cause screws or bolts to loosen. For near vertical panels the norm would be for the 2 bolts at the top of the unit to take the load and the lower bolts to be positioned so that they were not in contact with the glass to allow for differential thermal movement.

If a vertical panel is fixed at 4 supports it is possible for thermal ratcheting to occur. Thermal stress effects need to be catered for, either by design within the system or by allowing movements to occur.

The reporter is correct in raising concerns over the durability of the gasket to the connection, which can deteriorate in external environments and result in steel to glass contact. It is important to prevent such interfaces from occurring due to the generation of concentrated stresses within the glass that can lead to failure. Inspection regimes to periodically check safety-critical fixings are advisable.

As the report explains, Nickel Sulphide failure is a rare occurrence but can occur for a decade or so after installation depending on circumstances. It is assumed that the glass on this project was heat soaked as is the norm, but this does not guarantee all nickel sulphide was found. If the glass had not been properly heat-soaked then the whole batch may be at risk. Multiple failures on one building have been observed before.

This report shows that it is important not to jump to the obvious conclusions, and to investigate properly so as not to propose inappropriate remedial works. Failure mechanisms can be complex.

 

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View other CROSS reports published in Newsletter No. 53


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