Between 1994 and 1996 a series of six fire tests were carried out on an eight storey composite building with metal deck floors at the Building Research Establishment test facility at Cardington in Bedfordshire. The test programme was divided into two parts; the first, comprising a single beam test and three large compartment tests was sponsored by the European Coal and Steel Community and Corus Plc. and was carried out by the Corus Technology Centre. A complementary programme, comprising two compartment tests, was sponsored by the Department of the Environment, Transport & the Regions and was carried out by the Building Research Establishment.
Cardington fire tests
The tests were carried out to determine if the fire performance of real buildings of this type is better than is suggested by tests on individual elements of construction. Evidence that this is the case had been provided by studies of actual fires in real buildings, tests carried out by BHP in Melbourne in Australia and also small scale fire tests and computer modelling of structural behaviour. In all these cases, composite floors had demonstrated robustness and resistance to fire far greater than was indicated by tests on single beams or slabs.
In order to determine a direct comparison, the first test was carried out on a single unprotected beam and surrounding area of slab. The results demonstrated that a failure deflection (normally considered to be Length/30) would have occurred at approximately 1000°C, far greater than the temperature of 700°C at which the beam would have failed if tested in isolation.
Further tests were carried out in compartments varying in size from 50m2 to 340m2 with fire loading provided by gas, wooden cribs or standard office furniture. Columns were protected but beams were not. However, despite atmosphere temperatures of almost 1200°C and steel temperatures on the unprotected beams in excess of 1100°C in the worst cases, no structural collapse took place.
Fire resistance of composite floors
Observations from the Cardington fire tests and other large building fires have shown that the behaviour of the composite floor slab plays a crucial role in providing enhanced fire resistance. Where significant numbers of beams are not protected, this has the effect of greatly increasing the distance which the floor slab spans in the fire condition. The Cardington tests demonstrated that, in these conditions, the slab acts as a membrane supported by cold perimeter beams and protected columns. As the unprotected steel beams lose their load carrying capacity, the composite slabs utilises its full bending capacity in spanning between the adjacent cooler members. With increasing displacement, the slab acts as a tensile member carrying the loads in the reinforcement which then become the critical element of the floor construction. In the case of simply supported edges, the supports will not anchor these tensile forces and a compressive ring will form around the edge of the slab. Failure will only occur at large displacements with fracture of the reinforcement.
Level 1 design guidance
The Building Research Establishment has developed a simple structural model which combines the residual strength of the steel composite beams with the slab strength calculated using a combined yield line and membrane action model designed to take into account the enhancement to slab strength from tensile membrane action. The Steel Construction Institute has developed this model into a series of design tables which were published in September 2000 in Fire Safe Design: A New Approach to Multi-storey Steel Framed Buildings. Use of these tables allow the designer to leave large numbers of secondary beams unprotected in buildings requiring 30 and 60 minutes fire resistance although some compensation features, such as increased mesh size and density, may be required. The publication also contains design examples and considerable information of the background to the tests. The recommendations of the guidance can be seen as extending the fire engineering approach and it is intended that designs carried out in accordance with these recommendations will achieve at least the levels of safety required by regulations. The process of creating design tables has resulted in some simplifications. Use of the BRE calculation method from first principles may lead to additional economies. The BRE calculation method may be used for fire resistance periods of up to 120 minutes.