08.10 Fire resistant glazing

Categories: Glass & Glazing

Introduction
The use of a fire rated glass in an appropriate fire rated framing system can prevent the spread of fire and even heat for periods up to 2 hours.  For comparison, ordinary  annealed glass in a non fire rated frame will break in a matter of minutes.
 

Glazing safety in a fire situation
Reaction of building elements to fire is now classified with two ratings - integrity and insulation.  Integrity is the ability of a wall or roof to retain structural strength during fire and prevent the passage of smoke or hot gases.  Insulation is the ability of a wall or roof to resist the transfer of heat from the fire to the remote, cool side, of the element.

For all transparent glazings sufficient infra-red radiation may be transmitted through the glazing to cause ignition of materials on the other side.  To prevent spread of fire in this way or to provide protection to a fire escape route it is necessary to use glazing materials that provide insulation to maintain acceptable temperatures on the cool side.
 

Glass behaviour
The failure of glass during a fire occurs principally because of thermal stresses created in the glass.  The centre of the pane warms and expands whilst the edge of the pane is protected in the glazing rebate and remains cooler, image.  The resulting differential expansion leads to large tensile stresses around the edge of the glass, image,  which can cause the glass to break.  The performance of the different glasses is as follows:

Annealed glass
Annealed glass reacts to elevated temperatures by rapid expansion, which results in failure due to excessive stresses being generated at the edges of the glazing.  Annealed glass should be considered unsafe in all fire situations.

Wired glass
Wired glass is designed to give fire resistance.  The performance obtained depends upon the area of the glass, the type of framing and the glazing details.  Fire-resisting wired glasses should be specified carefully.

Heat strengthened, toughened and heat soaked toughened glass
These glasses behave similarly to plain annealed glass, although higher stresses may be tolerated.  Note that body-tinted glasses will absorb more radiant heat.  Toughening the glass only increases the stresses that the glass can withstand prior to fracture.

Borosilicate glass
Ordinary glasses are manufactured from soda lime silicates.  Borosilicate glasses offer the advantage that they have a lower coefficient of thermal expansion and a higher viscosity at elevated temperatures.  This means that they can withstand the high temperatures of a fire for longer periods.  These glasses are generally supplied in a heat strengthened or toughened state.  These glasses remain clear during a fire situation, and do not protect passers-by from the effects of infra-red radiation.

Laminated glass
In a fire situation laminated glass may break and hang loosely from its frame.  Softening of the plastic interlayer may allow such broken laminated glass to continue to fall from a slope glazing system for a significant period of time after a fire has been brought under control, and as such presents a serious hazard to fire-fighters.  Proper selection of the glass can help to control this problem, for example by incorporating borosilicate glass.  The local building control officer shall be consulted before selecting laminated glass for use in overhead glazing situations.  The use of some kinds of laminated glass may be prohibited in some areas, or may require additional precautions such as automatic sprinkler systems.

Intumescent glazing
Intumescent glazing is a special form of laminated glass in which the interlayer is a material which becomes opaque when subjected to excessive heat.  This opacity is accompanied by fracture of the glazing, but the glazing remains in place.  Intumescent glazing thereby protects passers-by from the effects of intense infra-red radiation.  Intumescent glass is available with several intumescent interlayers, depending upon the level of fire protection required. The stages of intumescence can be seen here, stage 1, stage 2, stage 3.

Intumescent glass can be susceptible to clouding under UV exposure or the presence of moisture in the rebate.  Intumescent glasses may be supplied with internal coatings to block UV radiation, and so must be installed the correct way round, and with tapes to protect the edges of the intumescent layers from water - these tapes must not be removed.

Plastics glazing materials
In a fire situation some plastics may burn or melt and release particulate contaminants or noxious gases.  The use of the building should be considered.  Areas used for food or pharmaceutical preparation may exclude the use of plastics glazing materials on the grounds of the high cost of replacing contaminated stock should a small localised fire occur.
 

Framing systems
Glass performance is only as good as the framing system allows.  Frames for fire resistant glazing are usually of steel or hardwood although softwood may be used.  In all cases the frame, beads and glazing system have to be specially designed as a complete fire rated system.  The assembly has to be either tested or assessed as a whole to determine its ability to give the required performance in the event of fire.

Steel frames are prone to distortion in the event of fire, image, and unless correctly designed may cause the glass to break.  Glazing details may be required to clamp the glass into place to prevent it sagging away from the top glazing rebate, image, and consideration has to be given to the use of fire rated glazing tapes and seals and possibly the use of intumescent seals.  The fixing of window frames into a wall plays a role in preventing distortion of the frame, image.  The most difficult combination is a door with little restraint around the edge of the door leaf, image.

Timber frames will char and the glazing beads may fail at an early stage.  Glass can be retained in place by the use of metal profiles behind the beads or by the screws used to fasten the beads.
 

Testing
Assemblies, doors and windows, are tested in a furnace wall, typically up to 3000mm x 3000mm, image.  A predetermined time/temperature regime is achieved and the time to loss of integrity is recorded.  Also the time to loss of insulation if required.