The performance of a glazed fire door is only as good as the method of glazing used. So the glass, glazing system and the beads all have to work in harmony if the protected space is not to be compromised.
Glass has been incorporated in fire walls and fire doors since such
fire barriers were required and, on the face of it, has the potential to severely compromise the fire resistance of the construction into which it has been installed. Up until the start of the 1980s only one form of fire resisting glass was available, known as Georgian Wired Plate (GWP) which was supplied in either an ‘as-cast’ state (with a ‘bobbly’ surface) or ground and polished on both faces (GWPP). The cast glass was translucent, rather than transparent and was not suited to many architectural applications.
Traditional wired glass has a soda/lime composition, a material with a very high coefficient of expansion and which is highly conductive. Unless the method of retaining glass in an aperture is kept to a minimum, the temperature difference between the glass exposed to the fire and the glass edge protected by the glazing system will cause differential expansion and, because of the lack of ductility in glass, inevitable breakage. The built-in wires prevent it from falling out.
When there are no wires, as is the case with many of the modern clear soda/lime composition toughened glasses, the temperature differential generated by the glazing system is critical to the performance and should form a major component in any risk assessment. With these monolithic unwired glasses the wrong glazing system (bead size, bead material or glazing medium) will reduce the fire resistance of the glass from both 30 and 60 minutes to less than five minutes. This represents a major compromise of any fire safety objectives, so the risk assessor should establish the critical factors from the glass supplier before approving it as being safe – especially if it is glazed into a timber construction.
Intumescent glazing medium
Glass also readily transmits heat to the unexposed face beads by radiation. As a consequence, any bead on the unexposed face which is installed directly in contact with the hot glass, or at best is only separated from it by an organic glazing tape, is exposed to rapid heating by this mixture of conduction and radiation. When the bead is of a combustible material, such as timber, this heating will fairly quickly induce smouldering on the protected side of the barrier before 30 minutes has elapsed. This smouldering produces quantities of smoke which reduces the tenability of the space, jeopardising the safety of people within it and providing a visual deterrent to persons entering it. This must be avoided if the space is a protected route or stair enclosure.
The eventual ignition of this bead will obviously create a more serious risk to people using the route. With timber glazed elements, a major function of a glazing seal is therefore to reduce the heat flow through the glass and onto the unexposed face retaining bead, in order to prolong the time before which either smoke production or flaming could occur. The primary function of the glazing medium is to insulate the bead on the unexposed face from the heat being conducted through the glass and also, if possible to ‘mask’ the bead profile from the radiant heat. Intumescent based sealants, strips or gaskets have been shown to have the capability of both insulating and ‘masking’ such beads.
Of more concern is the fact that should a non-intumescent sealant be used, this could be the cause of an even earlier ignition or excessive smoke production – especially if it is more combustible than the beads to which it has been applied. Oil based mastics or putties, for example, could make conditions significantly more onerous on the unexposed face if they are used in error. Re-glazing with an intumescent sealant is a preferred, if not vital requirement.
Insulated glass
Insulated or partially insulated glasses, which turn opaque and expand in thickness when heated, are not very edge sensitive and the beads require much less protection. However, the glazing pockets for these glasses must accommodate the expansion of the glass. If they don’t, the fracturing glass may crack coincidentally in a line just above the glazing beads, seriously weakening the glass which may cause it to fall out prematurely. It is preferable that the glazing medium does not apply pressure to the edge of the glass.
The comments above relate to single glazing for internal use. The need for reduced sound transmission has resulted in a growth of ‘twin-glazed’ partitions. Twin-glazing is very direction sensitive and if there is fire glass and non-fire glass adjacent to each other, it is critical that the non-fire glass is sacrificial and installed on the most likely side of fire exposure. If it is on the wrong side, then the performance of the fire glass can be seriously reduced due to overheating. If there is any doubt when twin glazing is installed, then specialist help should be sought.
Fire resisting glazing plays an important function in the design and construction of protected routes in buildings, corridors and stairways, but this critical function can be seriously compromised by the method of glazing and selection of the glazing medium. It is important, therefore, that the risk assessment of premises where fire resisting glass is installed reviews the presence or absence of, and the nature of, any glazing media used in such installations.
This article primarily addresses glazing systems, other than metal ones, which are very likely to have been glazed on site. It assumes that all metal glazing systems will have been factory made and installed by the manufacturer, or at least under their instructions. Metal glazed systems are, however, still very sensitive to the selection of the
glazing system.
This article is based on guidance from the Intumescent Fire Seals Association.
For the full IFSA guide to glazing seals. Go to www.fsa.org.uk
The performance of a glazed fire door is only as good as the method of glazing used. So the glass, glazing system and the beads all have to work in harmony if the protected space is not to be compromised.
Glass has been incorporated in fire walls and fire doors since such
fire barriers were required and, on the face of it, has the potential to severely compromise the fire resistance of the construction into which it has been installed. Up until the start of the 1980s only one form of fire resisting glass was available, known as Georgian Wired Plate (GWP) which was supplied in either an ‘as-cast’ state (with a ‘bobbly’ surface) or ground and polished on both faces (GWPP). The cast glass was translucent, rather than transparent and was not suited to many architectural applications.
Traditional wired glass has a soda/lime composition, a material with a very high coefficient of expansion and which is highly conductive. Unless the method of retaining glass in an aperture is kept to a minimum, the temperature difference between the glass exposed to the fire and the glass edge protected by the glazing system will cause differential expansion and, because of the lack of ductility in glass, inevitable breakage. The built-in wires prevent it from falling out.
When there are no wires, as is the case with many of the modern clear soda/lime composition toughened glasses, the temperature differential generated by the glazing system is critical to the performance and should form a major component in any risk assessment. With these monolithic unwired glasses the wrong glazing system (bead size, bead material or glazing medium) will reduce the fire resistance of the glass from both 30 and 60 minutes to less than five minutes. This represents a major compromise of any fire safety objectives, so the risk assessor should establish the critical factors from the glass supplier before approving it as being safe – especially if it is glazed into a timber construction.
Intumescent glazing medium
Glass also readily transmits heat to the unexposed face beads by radiation. As a consequence, any bead on the unexposed face which is installed directly in contact with the hot glass, or at best is only separated from it by an organic glazing tape, is exposed to rapid heating by this mixture of conduction and radiation. When the bead is of a combustible material, such as timber, this heating will fairly quickly induce smouldering on the protected side of the barrier before 30 minutes has elapsed. This smouldering produces quantities of smoke which reduces the tenability of the space, jeopardising the safety of people within it and providing a visual deterrent to persons entering it. This must be avoided if the space is a protected route or stair enclosure.
The eventual ignition of this bead will obviously create a more serious risk to people using the route. With timber glazed elements, a major function of a glazing seal is therefore to reduce the heat flow through the glass and onto the unexposed face retaining bead, in order to prolong the time before which either smoke production or flaming could occur. The primary function of the glazing medium is to insulate the bead on the unexposed face from the heat being conducted through the glass and also, if possible to ‘mask’ the bead profile from the radiant heat. Intumescent based sealants, strips or gaskets have been shown to have the capability of both insulating and ‘masking’ such beads.
Of more concern is the fact that should a non-intumescent sealant be used, this could be the cause of an even earlier ignition or excessive smoke production – especially if it is more combustible than the beads to which it has been applied. Oil based mastics or putties, for example, could make conditions significantly more onerous on the unexposed face if they are used in error. Re-glazing with an intumescent sealant is a preferred, if not vital requirement.
Insulated glass
Insulated or partially insulated glasses, which turn opaque and expand in thickness when heated, are not very edge sensitive and the beads require much less protection. However, the glazing pockets for these glasses must accommodate the expansion of the glass. If they don’t, the fracturing glass may crack coincidentally in a line just above the glazing beads, seriously weakening the glass which may cause it to fall out prematurely. It is preferable that the glazing medium does not apply pressure to the edge of the glass.
The comments above relate to single glazing for internal use. The need for reduced sound transmission has resulted in a growth of ‘twin-glazed’ partitions. Twin-glazing is very direction sensitive and if there is fire glass and non-fire glass adjacent to each other, it is critical that the non-fire glass is sacrificial and installed on the most likely side of fire exposure. If it is on the wrong side, then the performance of the fire glass can be seriously reduced due to overheating. If there is any doubt when twin glazing is installed, then specialist help should be sought.
Fire resisting glazing plays an important function in the design and construction of protected routes in buildings, corridors and stairways, but this critical function can be seriously compromised by the method of glazing and selection of the glazing medium. It is important, therefore, that the risk assessment of premises where fire resisting glass is installed reviews the presence or absence of, and the nature of, any glazing media used in such installations.
This article primarily addresses glazing systems, other than metal ones, which are very likely to have been glazed on site. It assumes that all metal glazing systems will have been factory made and installed by the manufacturer, or at least under their instructions. Metal glazed systems are, however, still very sensitive to the selection of the
glazing system.
This article is based on guidance from the Intumescent Fire Seals Association.
For the full IFSA guide to glazing seals. Go to www.fsa.org.uk
