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The considerations to be made about fire resistant glazing are not limited to whether or not to specify it. Colin Bennett sheds some light on a topic that is less transparent than it seems.
Glass has always been an important building material. It allows natural light into the building and there appears to be a general trend in modern buildings to introduce more and more glazing. This is most noticeable in hospitals and schools, where designs are moving away from the more institutionalised ones of the past, which consisted of a series of dark uninspiring corridors and rooms. As well as being required to satisfy design criteria – ranging from environmental/energy efficiency to impact safety and acoustic requirements – there can also be important fire safety implications the fire safety engineer needs to take onboard when developing the fire strategy, as the glazed elements can form part of the compartment walls and floors, protected enclosures and atria.
One of the most important aspects of a fire safety strategy is to develop compartmentation for a building. This needs to be coordinated with the available means of escape and fire service access, together with the type of use and level of management within a building. This is particularly important in hospitals, where progressive horizontal evacuation is usually adopted and also in schools, where compartment sizes can be relatively small, i.e. Building Bulletin 100 recommends that compartments should be a maximum of 800m 2 in a non-sprinklered school.
Early start
It is therefore critical for the fire safety engineer to be involved in the design at a very early stage, and identify any glazing application that needs to be fire resistant, either integrity only or integrity with insulation. In the early stages of a design, it is generally easier to reconfigure where the fire compartmentation is placed and this can reduce the amount of fire resisting glazing required. This is an important aspect in the value engineering phase of the project, as fire resisting glass can be an expensive option compared to standard glass and other forms of fire resisting construction, such as stud partitions and block-work walls. This additional cost is understandable given the amount of technology behind the development of fire resisting glass, and it is no different to other high performance glazing systems such as heated glazing, triple glazing with low E and solar control, and structural all-glass facades. It is with these types of glazing systems that fire resisting glass should be compared against. But a breakdown of the costs should generally show that the majority of the final cost of a fire resisting glazed system will be the supply and installation, with only a small part being attributed to that of the actual material.
It is important that fire safety engineers do not produce fire strategies which focus solely on achieving the required level of safety; they also need to consider the cost implications of the proposed solutions. It is essential that the rest of the design team is aware of any cost implications, as this may determine if the fire strategy is feasible or not. Cost should not, however, be viewed in isolation but assessed against the wider implications and benefits the glazing can bring to the overall design, such as building function, security, space utilisation, crowd movement and management.
If the glass is required to provide protection during a fire it should be specified as having an appropriate fire resistance classification suitable for the application. Failure characteristics for all types of standard glass are well defined through testing, and glass manufacturer Pilkington has identified the conditions that lead to failure. In all cases though, non-fire resisting glazing will give insignificant levels of fire resistance when tested under developed fire conditions.
Fire stress
The primary failure mode for conventional glass is the onset of cracking caused by temperature differentials on the exposed surface. Since the glass is a relatively poor conductor, the edge remains unheated while the fire raises the temperature of the central portion by radiant heat and hot gas convection. This can cause large, thermally induced tensile strains to be generated between the exposed surface and the surface protected by the framing system. The initial crack will normally occur at the edges shielded by the frame due to stress concentration, typically at points with flaws, some of which can originate from cutting and sizing the pane during manufacture and installation.
To add to the confusion, BS 5588-7, which deals with the fire safety design of atria, recommends that in some instances where it can be demonstrated that the temperature is limited, non-fire rated glazing may be used as a separating element if toughened glass is specified, or if a laminated glass with a polyvinyl butyral (pvb) interlayer is used and the gas temperature will not exceed 400 degrees C. It is interesting to note that this section has been significantly revised in the proposed BS 9999, which was recently out for public comment and is a result of a much greater understanding of the limits of glass behaviour, based on testing carried out over the last two years. BS 9999 should eventually replace the majority of the BS 5588 series, and will provide a more flexible approach to fire safety design which takes into account varying human and physical factors.
The recommendations above should be used with caution and evidence suggests that the physical deterioration of laminated glass starts from below 100 degrees C, while chemical deterioration, with smoking and ultimately flash burning, advances progressively from 180 degrees C onwards. Toughened glass has a wide range of failure probability determined by the glass condition and temperature exposure. Standard commercial, volume produced toughened glass may fail catastrophically from developed temperature differentials on the glass, even as low as 120 degrees C. However, given the uncertainties associated with fire and the potential consequences a fire can have with regards to the occupants, the building’s structure and its contents, it would be unwise to rely on the performance of a fire separating element which has not been sufficiently tested and which may add an unknown element of risk, which cannot be quantified.
When specifying fire resistant glazing it is important that a specialist – such as a manufacturer or accredited installer – is consulted. The glazing system must be installed as tested since the performance of the system can be affected by the:
– glass type
– fire resistance classification
– area of the glass pane
– glass pane aspect ratio, i.e. height and width
– orientation, i.e. vertical, horizontal, inclined
– overall screen size and fenestration layout within the screen
– glazed system components, i.e. frame material, seal, beads and fixings
– framing system design
– amount of edge cover and edge clearance
– quality of the installation and the workmanship
Evidence
As part of the approvals process, authorities should be asking for sufficient test evidence when glazing is used as a fire separating element. When installed, all fire rated glazing should have a permanent stamp indicating at least the product name, manufacturer/supplier and the fire performance rating. It is important that the stamp should remain be visible and readable after installation – not covered by the frame, for example – otherwise it may be difficult for occupiers and building owners to comply with their statutory duties under the Regulatory Reform (Fire Safety) Order. The Glass and Glazing Federation (GGF) produces a best practice guidance document on fire resisting glazing, and this is referenced in Approved Document B. This document provides a wealth of information for both designers and approving authorities.
When specifying fire resistant glazing there are many different manufacturers and types of fire resisting glass available on the market, but they can essentially be divided into two major categories:
– Integrity-only (un-insulating) glass: this type of glass has the ability to prevent the passage of flames and hot gases for a specified time
– Insulating glass (integrity and insulation): this has the same ability as integrity glazing but it also restricts the temperature rise on the unexposed face by all heat transfer mechanisms.
It is also important to note that integrity glass types based on an intumescent interlayer offer reliable integrity performance with a measure of insulation for a shorter time period – 15 minutes, for example. Such products can therefore provide an additional dimension in performance above standard integrity, which can provide benefits for the designer when considering the means of escape strategy.
There is also a third category which has been introduced in the European Standard, BS EN 13501-2, which details radiation performance. This category has been introduced as some European countries require this performance as part of their national regulations, and is defined as follows: ‘The ability of the element of construction to withstand fire exposure on one side only, so as to reduce the probability of the transmission of fire as a result of significant radiated heat either through the element, or from its unexposed surface to adjacent materials.’
Insulation and radiation
This category should not be confused with insulated glazing, given that there can still be a significant amount of radiant heat emitted from/though the element, up to a maximum of 15kW/m 2 measured at a 1m distance. This can be sufficient to ignite any nearby combustibles on the non-fire side and cause serious burns in a few seconds. As a comparison, insulated glazing typically has an emission level of less than 2kW/m 2, a level of heat exposure that constitutes little risk to people and materials. Any glazing specified in a fire strategy which is required to achieve this radiant heat category should be viewed with caution in the UK, as our guidance documents do not support this category. Fire resistance in the UK is measured in terms of load-bearing capacity, integrity and insulation only.
Generally, if the glazing in a fire separating element is to satisfy Requirement B3 of the Building Regulations in terms of compartmentation, it should have both integrity and insulation criteria; although there are some exceptions listed in Approved Document B where integrity-only glazing can be specified. Additionally, there may be alternative designs as part of a fire engineered solution such as smoke/temperature control systems, fire suppression systems and other passive systems (e.g. shutters and curtains). In combination with any of the above it may be possible to reduce any glazing that would normally be required to have both integrity and insulation qualities, and replace it with integrity-only glazing. Nonetheless, this would need to be supported by a detailed fire strategy. When determining the acceptability of integrity-only glazing it is important to remember that the insulation criterion allows a maximum temperature rise of 180 degrees C anywhere on the surface, and a mean temperature rise of 140 degrees C.
The use of fire resisting glazing is becoming a more and more common way of satisfying the fire safety requirements of a building, without compromising on the architectural and aesthetic qualities of the design. However it is important that any glazing specified as part of the fire safety strategy is suitably specified and installed, otherwise the consequences could be severe.
Colin Bennett is senior fire engineer with Cundall Fire Engineering www.cundall.com. He would like to acknowledge Mike Wood of Pilkington for all his help in writing this article.