Most commercial multi-storey buildings are built to a standard design. They typically consist of heavy concrete floor slabs supported on vertical columns (mullions) and horizontal beams (transoms).
The mullions and transoms interlock and are usually made from extruded aluminium. They tend to weaken if the temperature rises above 200°C, and melt between 550°C and 650°C.
The facades, though, are the weakest link in the chain. They are almost always lightweight skins – usually a metal carrier framework holding prefabricated glass panels.
Typical forms of curtain walling
The Loss Prevention Council (LPC) carried out surveys of UK multi-storey buildings that use this kind of curtain wall system. The Council identified many inherent weaknesses – relating both to materials used and the method of construction.
It found that the cavity between the front of the floor slab or beam and the curtain walling can act as a flue. This is usually sealed with fire-stopping, but shrinkage of the fire-stopping material, or it coming loose as the facade moves, can create gaps that allow hot gases to move between floors in a fire.
Problems were also found with glazing materials. Curtain walling systems are normally double-glazed but seldom made of fire-resisting glass.
When ordinary glass is exposed to fire, the temperature differential across the pane causes stresses that can only be relieved by cracking. This leads to glazing failure and creates new ventilation passages for fire gases.
Opaque spandrel panels made from toughened glass are often used to hide the junction between the ceiling and the floor. They are especially vulnerable to failure because their proximity to the ceiling exposes them to higher temperatures.
As a consequence, buildings with toughened glass bridging the gap between floors are more likely to suffer from fire climbing up from floor to floor. A fire can pass in this way and ignite the floor above within minutes.
The test programme
The LPC set up a three-storey test rig, built to mimic the behaviour of a typical office building (figure 1). A series of different panel combinations was subjected to a standardised fire load of 420 MJ/m2 – representing an "average" office fire.
Exposed wire thermocouples were placed around the test rig at 0{5 m vertical intervals on two "measurement trees", transoms, mullions, brackets and glazing – to monitor temperature changes. The LPC also used infra-red photographs to assess temperature gradients across the glazed surfaces.
The research found that windows can smash in as little as five minutes (see table 1), suggesting that fires in heavily glazed buildings can easily pass from floor to floor before fire services arrive.
This means that insurers' traditional assumption of only one or two floors being lost in a fire needs to be revised. A revision which will doubtless push up premiums for such buildings, and may help to persuade clients to invest more in fire protection.
Overall, according to the LPC, there are "inherent structural weaknesses" in tall curtain wall buildings with glazed facades. There is also "a tendency to allow spread of fire from storey to storey".
These issues have more of a bearing on protecting buildings than preventing loss of life – the emphasis of fire protection measures in the Building Regulations. Indeed, the LPC argues that the Building Regulations should be broadened to cover building protection as well as life safety.
Scope for action
In summary then, how can building designers cut the risk of fire jumping from floor to floor in tall office buildings?
The most effective method of preventing fire from smashing through glass on one floor and entering the one above is to use sprinklers. Sprinklers' effect on the source of the fire is irrelevant: simply cooling the gases stops them from breaking through fire stopping or glazing.
Sprinklers may even be aimed directly at glazing to keep the glass cool and prevent it from breaking. Such "window drenchers" may be fitted internally or externally, but they do not prevent the build-up of hot gases near the ceiling which could pass through gaps in fire-stopping.
The fire-stopping between the floor and building skin is essential and must be correctly fitted with no gaps. Without fire-stopping, hot gases transfer fire upwards through the building. Even when fire-stopping is properly installed, metal components of the curtain wall may deform and create gaps that allow hot gases through.
It is dangerous to assume that fire-resistant curtain walling is the answer to all problems. Even with this type of cladding, fire can climb up between floors very fast. In the most common form of construction for tall office buildings – double-glazed float glass systems – fire can break through glass in just 5-13 minutes.
Remedial measures
The LPC tests led to a series of recommendations for preventing the spread of fire in tall office buildings. In short, the LPC concluded that:
- in non-sprinklered buildings, the assumption that only one or two floors will be lost to fire must be revised;
- aluminium brackets used to support framework should be included in the same quality assurance procedures used to assess cladding systems;
- glazing by normal float glass was the weakest component in the system because of its early failure time and its tendency to promote flashover and backdrafts;
- glazed spandrels are a weakness in cladding systems, and fire resisting glazing should be considered;
- window drenchers on the inside or outside of the curtain wall prevent the spread of fire.
However, they do not prevent a hot gas layer from forming at ceiling level – opening up the possibility of fire spreading through gaps in fire-stopping.
Source
Building Sustainable Design
