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CROSS Safety Report

Inadequate design of cantilever glass barriers

Report ID: 735 Published: 1 April 2018 Region: CROSS-UK

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Overview

A reporter has observed a growing number of cases where cantilever glass balustrades in public buildings are, in their view, inadequately designed.

Key Learning Outcomes

For civil and structural design engineers:

  • Where specialist systems such as glass barriers are used, it is beneficial to have a close working relationship with the supplier from the earliest opportunity to ensure design requirements are met

  • The reporter highlights that the use of an interlayer may not always be the solution as it can deteriorate under UV light, or debond if water sits on top and works its way into the make-up

  • It is good practice to undertake a risk assessment to ensure that if the glass fails the risk to public safety is minimised

  • If you are involved in the design of glass panels barriers it is good practice to consult The Centre for Windows and Cladding Technology (CWCT) standards and guidelines

Full Report

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A reporter has observed a growing number of cases where cantilever glass balustrades in public buildings are, in their view, designed inadequately. They believe the underlying cause for this is contradictions both within and between current design standards - clarity is required. Monolithic toughened glass is sometimes adopted, and laminated toughened glass is often adopted - neither are appropriate.

Monolithic toughened glass

Monolithic glass shatters into both dice and large clumps of glass, which can cause serious injury to occupants below. Monolithic glass leaves a gap in the barrier when it shatters. Even where a handrail is retained to span between adjacent glass panes, small children, buggies and wheelchairs may fall through the resulting gap.

Laminated toughened glass

Toughened laminated glass is often specified, because toughened glass is the strongest form of glass and the interlayer is assumed to prevent it from falling from its support. In fact, deflection tends to govern glass barrier design meaning that achieving the highest possible strength is generally not important. Furthermore, when both plies fail, laminated toughened glass loses its out of plane stiffness and collapses. In such circumstances, a standard polyvinyl butyral (PVB) layer is at risk of tearing, which would permit the entire sheet of glass to fall en masse.

In fact, deflection tends to govern glass barrier design meaning that achieving the highest possible strength is generally not important

For occupants below, this is more dangerous than clumps of monolithic toughened glass. Such failures can occur despite the presence of a handrail. As with monolithic toughened glass, an unacceptable gap would be left in the barrier. Cantilever glass barriers should be designed with laminated glass that has at least one ply [preferably two] made of heat strengthened glass. Heat strengthened glass fractures in large pieces, which when combined with a PVB interlayer, can span vertically between the base support and handrail.

The following design criteria are proposed for cantilever glass balustrades made with laminated glass:

  • Design for strength using standard barrier loads; assume both plies contribute to resistance

  • Design for serviceability using standard barrier loads and deflection limits; assume that both plies contribute to resistance

  • Specify glass suitable for standard impact resistance

  • Assume that an accident occurs and 1 ply breaks

  • Design for strength using standard barrier loading; assume 1 ply contributes to resistance

  • Ignore the serviceability criteria

  • Assume that both plies break

  • Design the handrail to span between adjacent panes using standard barrier loading

  • Specify glass plies suitable for spanning between the handrail and base support after they have fractured

Expert Panel Comments

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This is an interesting report. There might be designers who disagree with the suggestions, but the report nevertheless highlights a key design task of looking at modes of failure first. It also highlights the need to consider the real possibility of failure but then to question what failure means and to assess an appropriate design offering least risk. The report addresses cantilever barriers, but the topic would be equally relevant to many other glass designs where different conclusions might be drawn about the best type of glass to use.

It also highlights the need to consider the real possibility of failure but then to question what failure means and to assess an appropriate design offering least risk

A useful reference is Structural use of glass in buildings (second edition) published by The Institution of Structural Engineers (IStructE) in 2014. The Centre for Window and Cladding Technology (CWCT) based at Bath University is a source of guidance in respect to the type of glass to be used and in what combination in different situations. Within this is a requirement to undertake a risk assessment to ensure that if the glass fails the risk to the public is minimised. A further source is C632 - Guidance on Glazing at Height published by the Construction Industry Research and Information Association (CIRIA).

As the reporter says the use of an interlayer is not always the solution as it can deteriorate under UV light, or debond if water sits on top and works its way into the make-up.  Glass configurations can also be subjected to impact testing for hard and soft bodies to mimic the effects of cleaning cradles or body impact.

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