Burning issues

Few can doubt that we need to take radical action if we are to prevent damaging and irreversible changes to our environment. Building designers and engineers have a significant role to play in driving forward this agenda, at least as far as the built environment is concerned.

A fire in a building will discharge smoke and hot gases into the environment. The bigger the fire, the greater the potential for widespread pollution. The products of combustion discharged into the atmosphere are dependent on the materials involved but all building fires will produce large quantities of hydrogen chloride, hydrogen cyanide and carbon monoxide. From this perspective, it may be considered that a truly sustainable building is one which doesn’t suffer a fire during its lifetime but where, if there is a fire, the discharge of effluent into the environment is limited and relatively benign.

The drive towards environmentally sustainable design (ESD) has been gathering pace for some time. But it is important to ensure that this rush to sustainability doesn’t come with any unforeseen consequences for the building’s users. ESD and fire engineering have developed independently as disciplines, and little work has been done to explore any synergies or conflicts. It is entirely possible that a building designed to be environmentally sustainable may compromise the fire safety strategy or, conversely, that steps taken to provide an acceptable level of safety may contradict the environmental aspirations of the design.

One aspect that may appear to present a potential conflict is the use of double- and triple-glazed units on a building facade. This type of glazing is intended to reduce heat loss and/or reduce solar gain, and it is not clear how it will react in a fire. With such uncertainties, the use of fire engineering techniques to calculate the requirements for structural fire resistance may be questioned, as these generally assume that a proportion of the glazing will fail, thereby providing a means of venting the energy from the fire to atmosphere.

Although there has been little if any structured research in this area, provisional results from tests by a leading glass manufacturer seem promising. In a number of tests, double-glazed units have failed quicker and in a more predictable manner than an equivalent area of single glazing. Although the exact mechanism has not been fully explored, it would seem that the increased insulation results in a greater heat gradient across the inner pane glass, causing it to fail within six minutes or so, compared to between five and 20 minutes for a single-glazed equivalent. The second sheet of glazing is then instantaneously subjected to the hot gas, again causing it to fail very rapidly.
In these tests, the results of which are still to be published, double-glazed units not only failed in less time, but the way the glass failed and fell out of the frame was much more predictable than with single-glazed specimens. Although not yet tested, it is reasonable to assume triple-glazed units will behave in a similar manner.

These results suggest that the provision of double-glazed units may actually complement the use of fire engineering. However, there is some research to the contrary and, until we have definitive results, the use of models that rely on the failure of glazing must be approached with caution in buildings with insulated facades. It is quite likely that this difference in results is due to the use of low emissivity glass in one series of tests, and further research is required.

Increasing the thermal performance of the building envelope is generally seen as a positive enhancement but in the event of a fire will result in more of the heat being retained in the building – increasing the potential for flashover. If flashover does occur, the damage will be greater than would otherwise have been the case. In a similar manner, sealing the building envelope may prevent a fire from getting access to sufficient oxygen and, consequently, the accumulation of the unburned products of combustion and vitiated air may result in a smoke explosion, or backdraught, the consequences of which often prove fatal for firefighters.

“Green roofs” add a certain aesthetic element to buildings as well as having other environmental advantages. Normally the structural elements supporting a roof would not be provided with any fire protection and the failure of a roof during a fire is often seen as a positive event, as it allows the hot smoke and gases to vent to atmosphere, reducing the potential for lateral fire spread and improving conditions within the building for fire crews.

Where the roof supports an increased load, such as sedum planting, should the supporting structure now be protected from thermally induced failure, although not strictly required by the Building Regulations? What are the consequences for fire crews should the green roof collapse into the building? If the roof does not vent during a fire, will we see a change in firefighting tactics similar to those in the US and parts of Europe, where firefighters need to actively ventilate through a roof using chainsaws, disc-cutters and, in some areas, detonating cord?

Powerful alpha emitters

There is one simple adjustment that building engineers can make that will assist in improving our environment in the long term, and that is in recommending against the use of smoke detectors that contain an ionisation sensing chamber. These have been used quite extensively, including for domestic smoke alarms. However, the presence of a source of ionising radiation in these detectors, although small, provides significant problems when it comes to their disposal. Special measures and contractors must be employed and, at the end of the day, do we want these small but powerful alpha emitters buried beneath our feet? There are detectors with the same performance characteristics but without the inherent hazard and it would seem prudent to specify these, provided overall effectiveness is not adversely affected.

The Fire Service may use many thousands of litres of water when extinguishing a fire and any fire water run-off will contain toxic and polluting chemicals which may be carried away into rivers and water courses. Protocols exist for the management of water from firefighting, and the Environment Agency takes a dim view of organisations that contribute, however unwittingly, to the pollution of rivers.

It is incumbent on building operators to take reasonable steps to ensure that a fire involving their building will not cause widespread environmental damage but, unfortunately, the test of reasonableness will usually only be resolved in a court of law after the fact. There is also a strict liability in tort to safeguard your neighbour from any losses resulting from pollution escaping from your land, and the financial implications can be very severe, covering any losses as well as the cost of remediation.

The simplest and most effective solution is not to have a fire in the first place, but some things are outside our control. The second most effective solution may be to install a fixed fire suppression system as, generally, these will control a fire using only a fraction of the water used by the Fire Service. With reduced water usage, the potential for polluted run-off is minimised, but not removed. Perhaps the most effective system would be a water mist system, with conventional sprinklers trailing in the rear. Both usually rely on pumps and generators, adding to the building’s carbon footprint, but the reduction in expected fire size and severity more than compensates.

Gaseous fire-extinguishing

The installation of an extinguishing system may allow the reduction of other passive fire safety measures, such as fire doors, fire-resisting walls and partitions. This in itself will have an effect on the building’s energy usage.

Gaseous fire-extinguishing systems have received a lot of attention in the past, largely as a result of the phasing out of halogenated hydrocarbons. The new generation of chemical gases can be shown to present less of a risk to the environment, although some residual risk still remains, with regard to their global warming potentials and atmospheric lifetimes. A better solution may be to use one of the inerting gases – as these are predominantly nitrogen, which in itself is relatively benign or to consider the use of hypoxic generators which can prevent fires from starting or can be used to extinguish a fire once it has started.

A true comparison should involve consideration of the efficiency of each type of gas, as the longer it takes to extinguish the fire, the more damage to the environment from the fire effluent. There may be a compromise involving the use of a slightly more polluting gas on the basis that it will extinguish the fire quicker than less polluting alternatives ironically, one of the most effective gaseous fire suppressants was Bromotrifluoromethane (BTM 1301), which has been phased out under the Montreal Protocol.

The answers to many of the questions raised are not readily apparent. It is entirely possible that most if not all ESD solutions will have no significant impact on the safety of building users in the event of a fire. However, care should be exercised to ensure that any innovative design solutions will not pose a threat to the safety of building users under both normal conditions and during a fire, and this may require closer collaboration between the parties involved.