CROSS Safety Report
Dangerously defective glass smoke screens
This report is over 2 years old
A reporter was asked to investigate the spontaneous failure of glass smoke control screens at a major retail store.
They are concerned that standard glass smoke barriers are essentially designed for a specific fire rating, but there may be little consideration of structural factors such as internal wind pressures.
Key Learning Outcomes
For the construction team:
Quality control and competent supervision on site can ensure safety critical fixings for glass smoke screens are installed in accordance with the design
Glazing should be installed as per the manufacturer’s instructions. If there are any uncertainties seek advice from the supplier’s technical support.
For designers and specifiers:
Be aware that fixings between glass and other materials can be critical and often flexibility between them is required to allow for different expansion rates and other minor movements
Connections and fixings can often be the weak link in systems and attention to detail is required
The Construction Industry Research and Information Association (CIRIA) guidance provides general advice on glazing at height – Guidance on glazing at height (C632F)
The Institution of Structural Engineers (IStructE) publication Structural use of glass in buildings (second edition) also provides guidance
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A reporter was asked to investigate the spontaneous failure of glass smoke control screens at a major retail store. The screens were made of fire rated toughened glass suspended from the roof structure by aluminium cleats. There were 3 bolts per glass pane.
It became evident, based on examination of the remaining glass panes, that there were no gaskets or bushes to separate the glass panes from their connections. There was also no tolerance within the bolt holes.
In addition, bolts were loose, and washers were missing, indicating they were working in shear rather than acting as tension clamps.
It was also discovered that a smoke extract vent was locked open adjacent to the failed panes. This permitted a differential temperature to arise between the ceiling void and occupied space adjacent to the failure. In this case the external air temperature was as low as -6oC.
The trigger for failure was thus differential expansion and contraction between the aluminium cleats and glass screen. However, the defective connections were the underlying cause. They were inherently vulnerable to a trigger mechanism - an accident waiting to happen. Had the building been occupied at the time of failure, a serious accident could have resulted.
The trigger for failure was thus differential expansion and contraction between the aluminium cleats and glass screen. However, the defective connections were the underlying cause
Structural factors for glazed screens
In preparing remedial details, it has become apparent that standard glass smoke barriers are essentially designed for a specific fire rating, but there may be little consideration of structural factors. For example, they may not be designed for internal wind pressures and toughened glass is used irrespective of the height at which the barrier is installed.
Toughened glass fails as a mixture of dice and heavy clumps of glass, which can cause serious injury, particularly as height increases. Glass suppliers do not necessarily make the specifier (normally an architect) or client aware of the inherent risks that may be associated with their product - it is not even clear that they appreciate risks exist.
It is the reporter's view that a risk assessment should always be carried out when designing overhead glass components.
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Further to Report Item 734 - toughened glass can also suffer from spontaneous failure due to nickle sulfide (NiS) inclusions in the glass. This can be reduced, but not eliminated, by heat soaking the toughened glass panel before leaving the factory. This was identified as the most likely cause of a similar failure of a smoke baffle to the edge of an atrium in a building that I am aware of. The replacement glass was proposed to be heat-soaked toughened laminated glass - the toughened glass being retained for performance at elevated temperatures, the heat soaking to reduce the risk of NiS failure and laminated to hold the toughened glass together if it were to fail. The whole panel was trialled to demonstrate that it would not detach from the fixings if both sheets of glass were to fail (perhaps due to accidental impact) at the same time. It is always useful to understand how a particular glass make up will perform post breakage.
Expert Panel Comments
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Glass should never be directly supported by a material like steel as it does not yield and will cause concentrated stresses to form in the glass, causing it to fail. This is a well-known behaviour of glass so to hear of support details not following this principle is somewhat concerning.
Glass should never be directly supported by a material like steel as it does not yield and will cause concentrated stresses to form in the glass, causing it to fail
Separating glass from supporting metalwork
Normal practice would be to separate the glass from the supporting metalwork by nylon bushes, neoprene gaskets, poured resin or applied mastic with plastic spacer bars, appropriate to the situation.
Failure modes of glass panels
As to the adoption of toughened glass at height, there is some merit in its use as its mode of failure is small pieces known as 'dice'. Once the tension and compression pre-stress becomes unbalanced within the glass pane it causes it to shatter.
The report's author is right to point out toughened glass can fail with clumps of 'dice' falling from height that can cause harm. However, this is not as dangerous as large shards of glass falling following the failure of annealed and heat strengthened glass.
The use of laminated glass is advisable for overhead glazing to reduce the risk of falling glass. However, the support detailing must be robust enough to restrain the glass in the first place following a failure of one of the plies within the laminated pane. For further guidance on this see the CIRIA Guidance on glazing at height (C632F).
Design of structural glass
The report is not about ‘structural glass’ as such. If there is any doubt, then it should be borne in mind that the design and installation of structural glass is a specialised business and appropriate competence and expertise is needed. The specifics of each location need to be considered including thermal and external loads and the consequences of failure understood and mitigated against.
Guidance is given in the Institution of Structural Engineers (IStructE) guidance on Structural use of glass in buildings (second edition).