Overhead glazing failures are a specifier’s worst nightmare. At worst, large pieces of glass could fall on to people below causing death or serious injury. At best, such failures generate litigation and bad publicity for projects.
One high-profile case involved the Eurostar terminal at London’s Waterloo Station. Its overhead glazing suffered from spontaneous failure due to nickel sulphide inclusions. This led to litigation that was eventually settled out of court for an undisclosed sum. Moreover, safety netting fitted to catch falling glass disfigured the station for years before protective film was attached to the underside of the 2.5 acres of glazing – a difficult job that involved removing all the glazing while the station was in operation.
New guidance has just been published to help designers and specifiers avoid this type of scenario. Research body CIRIA commissioned the Centre for Cladding and Window Technology to write a 200-page guide called Guidance on Glazing at Height, which is intended to help designers specify safe overhead glazing.
Guidance on Glazing at Height covers overhead glazing including roofs, canopies, facades and glazed barriers and tackles specific safety issues. There is plenty of detail on how glass is made, the different types available (see right) and how these perform. It covers design, procurement, operation and demolition, and best practice guidance for different overhead glazing scenarios. It suggests a risk assessment approach to help designers decide whether overhead glazing is safe for a particular application, and what type of glass should be selected. The assessment is based on the likelihood of the glass breaking and whether people are likely to be under the glass when this happens.
With glazing at height you need to think every case through, as each one is different
Jonathan Sakula, glass engineer, Yolles
“With glazing at height you need to think every case through as each one is different,” says Jonathan Sakula, a glass and facade engineer at consulting engineer Yolles and member of CIRIA’s steering committee for the publication. “In some situations the specification doesn’t have to be very onerous. For example, the risk of injury due to glass breaking in a greenhouse is extremely low, as it is very unlikely anybody would be underneath it at the time.” This means that annealed glass is used for greenhouses because it is cheap. In applications where there are likely to be people around, however, specifiers demand expensive toughened or laminated glass.
Bizarrely, the Building Regulations stating what type of glass should be used at height can be ambiguous, unlike in much of Europe, USA and Canada. “Until about 10 years ago some people used toughened glass because it was considered to be relatively safe as it fragments into pieces when it breaks,” says Sakula. “But if it breaks it can clump together and potentially cause injury to people underneath. It’s curious that you can still use toughened glass overhead and not contravene Building Regulations. These should be changed to catch up.”
Details of glazing at height: Churchill Place, London Docklands
Yolles was the facade consultant for the glazing for an entrance to Churchill Place, the latest retail development at London Docklands’ Canary Wharf. It is a good example of current best practice and has been used as a case study in Guidance on Glazing at Height. Although not designed as a walk-on roof, the risk assessment indicated that the glazed roof had to be capable of taking the weight of a person, as someone could accidentally step onto the glass during maintenance work. Because people would be very likely to be underneath, the correct glass specification was crucial. Toughened glass, 8 mm thick, was specified for the upper pane of the double-glazed units as this can take the loads from someone stepping on it. This was heat-soaked to reduce the risk of spontaneous breakage caused by nickel sulphide inclusions. Laminated glass was used for the lower pane of the unit. Both leafs are 8 mm thick, annealed glass. If the upper pane breaks, the lower leaf is strong enough to retain the glass, and even if this should break, annealed glass won’t shatter into small fragments. Instead it breaks into long shards, which means that it can’t sag and fall out of the frame.
Details of glazing at height: Yorkdale Centre, Toronto, Canada
The roof of the Yorkdale Centre in Toronto, Canada, was also designed by Yolles, but is very different from Churchill Place. It is supported by bolted point fixings that are in turn suspended from the steel structure above the glazing.
Like Churchill Place, toughened glass was used for the top pane of the double-glazed units because of its resistance
to accidental breakage. The laminated lower pane differs
in that 12 mm thick toughened glass was used for the top leaf, and 6 mm heat-strengthened glass for the lower.
Stronger glass is needed for the lower leaf because of the stress concentrations around the point fixings. Very strong laminated glass, using Dupont’s SentryGlas Plus interlayer,
has been used in some areas because of the risk of snow loading. Both glass leaves are toughened. This interlayer is very stiff and will hold fragmented toughened glass together without sagging and dropping out of the frame.
This is ordinary glass that is heated to reduce internal stresses. It is very good optically but is the weakest glass type. When it breaks it forms sharp, pointed shards.
Annealed glass is made roughly twice as strong by heat treatment. Not as good optically as annealed glass, and forms the same sharp shards when it breaks.
Heat-treated annealed glass that is cooled quickly on the outside. This puts the outside of the glass under compression, and the inside under tension. The outer compressive skin creates a glass that is three to four times stronger than annealed glass. It shatters into small, blunt fragments under impact. It can also spontaneously shatter because the heat treatment process creates an unstable form of nickel sulphide crystal that is naturally present in glass. Over time these crystals expand and can cause the glass to shatter suddenly.
Heat-soaked toughened glass
Toughened glass is heated for four to eight hours to speed up the nickel sulphide transition process. It reduces the risk of subsequent spontaneous breakage. The glass has the same properties as ordinary toughened glass.
Two or more sheets of glass are joined together by a plastic interlayer. If the glass breaks the interlayer holds the glass fragments together so they don’t fall on people below. The interlayer also has some residual strength. Usually annealed or heat-strengthened glass is used for at least one leaf as the large fragments typical of these types prevent the whole pane sagging and falling out of its frame in one piece.