Understanding the cooling phase of fire in buildings
Improved understanding of a building’s behaviour during the cooling phase of a fire is vital to adapt building design and save the lives of firefighters. A joint project by the Universities of Manchester and Edinburgh in the UK (with industrial support from Ove Arup and Corus) involved modelling and testing the behaviour of composite steel-concrete floor plates as a fire cools down.
This structural element is used in a type of construction that is common, consisting of concrete floor slabs (sometimes with a small amount of reinforcement) attached to steel beams and columns.
Dr Martin Gillie of the University of Edinburgh, UK, says, ‘Our results back up things we thought we knew but didn’t have particularly strong experimental evidence for. For example, we established that large forces develop during cooling, particularly tensile forces as things shrink. We’ve now got some firm experimental evidence for that and can model that numerically with some degree of confidence’.
It was found that ‘the forces were sufficient to damage structural connections by overloading them under tension, which is not what they were designed to withstand’.
The timeframe between the tensile forces acting and collapse depends on the rate of the fire cooling, notes Gillie.
Professor Colin Bailey at the University of Manchester oversaw the testing. These structural elements were restrained at both ends. Sometimes such tests are carried out without these restraints, which means that the element is free to move, expand and rotate. Fixing the ends generates more realistic results by subjecting the part to the kind of forces it will be exposed to when part of a burning structure.
Gillie explains that there is now a trend within structural fire design to try and design according to real conditions, rather than the simplified conditions assumed in the past.
‘Traditional design for fire prevention ignored that structures cool at the end of a fire,’ he says. Yet, full-scale fire tests have shown that failures can occur after the temperature of a fire has peaked. ‘This [work] allows you to design for the cooling phase. Our experiments showed that computer models can [factor in] the tensile forces’ that occur during cooling.
Although he believes it is too early to say how the findings might change building design, the areas the research team thinks will need more attention on are the connections between structural elements and possibly the overall structural stability due to geometric changes on cooling.