The issues concerning glass and fire hazards are to be taken very seriously, even if we understand the normal functions of glass. The consequences of being complacent can be severe, including loss of life and costs to organisations that use the buildings.
The transparency of glass is a major potential drawback in developed fire conditions. Even if glass stays in place, dangerously high levels of heat can be transmitted by conduction and direct radiation, with additional contributions from re-radiated and convection from a hot glass surface on the non-fire side. That in turn risks secondary fire generation and the production of smoke from smouldering floor coverings, producing a very hot un-breathable, choking atmosphere. In an enclosed environment – such as internal escape ways and lobby areas – transmitted heat can therefore cause unpleasant, untenable conditions, which create unacceptable risks for those trying to get out of the building. Since standard glass products are vulnerable to the smallest of fires, and cannot survive intact for any practically safe length of time, it is essential for designers to have an understanding of glass behaviour. This is so that fire-resistant glass can be used efficiently and effectively within a dominant transparent design concept.
Where fire safety in glass construction is concerned the designer is faced by essentially four levels of choice, and accordingly four very different levels of risk. A designer has four levels of choice with respect to fire safety in glass construction. Accordingly, they carry four very different levels of risk:
• Minimise the use of fire-resistant glass barriers, placing reliance implicitly in a faith that fire will not break out and that the fire load can be controlled to minimise the extent of likely fire damage. However, good practice modern fire safety relies on integrated fire safety, dependent on interlocking different fire safety systems in a holistic fire safety concept. This choice requires a high degree of confidence that the design and its implicit assumptions are faithfully reproduced as intended through the (usually complex) supply and build chain where cost considerations too often dominate over technical performance considerations. It also requires that the occupation of the building in its lifetime keeps in line with the original design concept – without significant change to use, layout and fire load in a way that conflicts with the basis of the design blueprint.
• Standard glass products such as tempered – that is, toughened – glass may be used perhaps in combination with sprinklers as substitute fire barriers. This is a high risk option since tempered glass and water do not combine well under fire conditions. Tempered glass may fail catastrophically and without warning in the face of fire due to thermal stress and shock. Commercially produced tempered glass is not produced consistently enough, with sufficient attention to the critical detail that affects sensitivity to failure. Designers may consider that the use of standard pvb (polyvinylbutyral) safety laminated glass presents a lower risk option – only to replace one critical mode of failure in fire with another. Such laminated interlayers burn vigorously and catastrophically when exposed to heat from a fire, within ten minutes of flame exposure. In the event of fire, chance may substantially determine the outcome. If the flow of water is not complete and continuous over the whole glazed area then there is likely to be a heightened risk of glazing panel and assembly failure.
• Integrity fire-resistant glass can be used to hold back the fire for a relatively short period (up to 30 minutes effectively, in developed fire conditions), with a risk that levels of heat on the protected side become too high for people and fixtures, fabrics and fittings. Consideration of transmitted heat levels may cause both the pane size of integrity glass, the extent of glazed area and application to be limited. The level of protection may only be sufficient for the purpose of rapid evacuation of the building in short time (for example, 20 minutes), on the assumption that:
a) The alarm is given and acted upon relatively promptly.
b) The occupants are familiar with the building, can move readily and calmly, without support and supervision, in conditions that are not too crowded and congested.
c) The fire and rescue service receives the emergency call in good time for rapid response, and can get promptly to the fire within a matter of only some minutes.
• The highest level of protection – the lowest risk profile – and the highest degree of confidence in effective fire protection is obtained by the use of a fire-resistant glass that provides reliable insulation performance (together with integrity). Large areas and extensive use of such a fire-resistant glass can be incorporated in the design, in a variety of applications, for the purposes of escape, access and prolonged protection against high fire intensity and hours of exposure. Insulation performance is therefore particularly of value, and used extensively, because of the need for a high level of assurance given the high risk of damage should fire break out and the element of unpredictability that essentially accompanies fire events. Insulation is therefore the most reliable choice and the one that offers maximum peace of mind, especially if the objective is to have a substantial measure of confidence against fire during the working life of the building. It is a level of performance that is essential to guard against the risk of prolonged or intense fires, and the only choice that provides a sufficient safeguard against extensive damage to the building and surrounding buildings from extensive fire development.
Fire Safety Design with Glass
The role for fire-resistant glass is an essential one in maintaining the transparent design concept. Structural resilience in resisting fire and limiting fire spread is a basic prerequisite, and in those respects fire-resistant glass is very effective as part of a fire safety compartmentation and fire separation strategy using the construction and layout of a building to contain and isolate fire.
Fire-resistant glass is also used in partitions or doors to provide protected corridors and transit points for quick and safe evacuation or access by fire and rescue personnel. That includes lobbies around escape stairs, the stairs themselves and refuges as places of relative safety en route to a place of safety outside the building. The sense of openness created by glass constructions also has an advantage: it helps to reduce panic and encourage orderly evacuation by allowing occupants to be aware of what is going on to move away from the seat of the fire.
For the glass designer, there are three particular areas that merit particular consideration: the combination of fire resistance with sound insulation; the use of integral loaded fire-resistant glass floors to maintain the transparent design core theme; and the use of fire-resistant glass in facades to prevent break out and spread via the facade.
Combining Fire and Sound Insulation
Plastic organic interlayers are commonly used in glass laminates to help achieve acoustic targets. Nevertheless these have no significant practical resistance against fire and constitute a danger through ignition and vigorous flaming of the interlayer when exposed to fire. Thermal deterioration of organic layers becomes increasingly rapid from about 150oC onwards, with the generation of heavy smoke and fume before flaming.
Acoustic insulation can readily combine with certain types of fire-resistant glass, e.g. those that produces fire resistance by the use of an inorganic glassy interlayer (such as Pilkington Pyrodur integrity, with additional benefit of 15 minutes insulation rating, and Pilkington Pyrostop full range of insulation or inorganic gel layers (such as the Contraflam range). It is quite possible to effectively design with fire-resistant glass types at various levels to achieve very good sound insulation together with reliable fire resistance.
Loadbearing Fire-Resistant Glass Floors
Without the option of fire-resistant glass floor constructions, the transparent glass design concept cannot be said to be complete. Fully tested integrally loadbearing solutions are now available using products from major glass manufacturers, and particularly good developments have been made with a 60 minutes rated insulated and integrity structure, using either steel or timber supports.
Such options also combine robust fire protection with effective acoustic insulation. The sound insulation Rw index for the Pilkington Pyrostop glazed floor structure, for example, is 48 (-1;-5), reference EN ISO 717. This is a remarkable result – providing a tremendous combination of fire resistance, acoustic performance, loadbearing capacity, and design capability. The loadbearing capability of the floor must be demonstrated by fire testing under load. The applicable test standard in Europe is EN 1365:2000, part 2, Fire resistance tests for loadbearing elements: floors and roofs, subject to loads determined in accordance with EN 1363-1. An applicable successful test report is essential for all fire-resistant glass systems.
There has to be insulating protection within the floor system against heat transferring into the floor structure. The same is said for the glass used. Un-insulated integrity fire-resistant glass, on the other hand, risks structural instability under load and fire exposure due to the possibility of heat transfer into the structure.
A major risk concerning fire spread is the threat of fire break out from inside the building through the glazed façade either to transfer fire to facing buildings or to allow ready fire transfer up the face of the same building jumping from floor to floor, which is a particular concern for skyscrapers and tower blocks.
Such a threat can easily be countered by incorporating sections of fire-resistant glazed systems in the façade. In most situations a high performance integrity glass in a double glazed unit would suit, whilst in others the risks and value of the building and surrounding buildings may also justify the use of a higher performance level of fire protection, using insulation performance (for example for sections of atria adjoining lobby, escape and refuge areas).
However, the whole façade does not need to be constructed from fire resistant glazing for this design strategy to work. Bands of fire-resistant construction at regular intervals running up the building would be appropriate, to act as a fire break according to a well tried and tested principle. Such installations should also be considered at particular hazardous locations or critical safety points. Such locations, for example, would be external glazing at re-entrant corners and angles in the façade to prevent fire jumping across the gap on the same floor level, bypassing internal compartmentation.
Looking to the Future
The trend in building design is increasingly towards the more complex, crowded, taller and individualistic, at the lowest possible overall cost and optimum efficiency, whilst seeking to create sustainable building value and optimum function. There is increasing pressure on space utilisation as owners look for ways to get the maximum use out of their buildings. These trends increase the threat of fire, and glass has a major role to play in modern design. This means that the role of fire-resistant glass design will always be a vital one.
Mike Wood is a technical consultant on glass and fire. He is chair of the UK’s Passive Fire Protection Federation (PFPF) and a member of the Fire Sector Federation Executive