Dutch researcher Joris Fellinger has developed a model that assesses and predicts the fire-resistant behaviour of concrete floor slabs. He discovered that the slabs crack after just 15 minutes but do not collapse. Fellinger, an adviser at the TNO Centre for Fire Research, tested how so-called hollow core slabs respond during a fire.
Fellinger has developed a temperature dependent model that predicts the development of cracks in the slab and the slab's collapse. At present legislation only covers the bending of hollow core slabs in the event of a fire. With the new model both the design of the slabs and the legislation can be improved.
During the fire resistance tests, Fellinger found that the slabs could rapidly bear less weight if they expanded due to the heat. After 14 to 16 minutes vertical cracks developed in the slab, even if Fellinger partially prevented the thermal expansion. During the first hour the load-bearing capacity of the slabs decreased rapidly and after this the decrease was slight. In addition to this he found that thick floors were more sensitive to thermal expansion than thin floors.
The new model is too complex to use in daily practice. Fellinger therefore advises that the maximum load tolerance of every slab that appears on the market should be determined beforehand.
He also determined the maximum load tolerance of the slabs. He discovered that this load tolerance plays an important role in determining when the slabs collapse. However, the load tolerance of the slabs scarcely has an effect on the formation of cracks and the slabs.
Hollow core slabs are made from concrete with strands of pre-tensioned steel. These slabs are popular in the construction of floors in both residential and office buildings. During a fire, floors made from these slabs must maintain their load-bearing and separating function for a given period of time, so that the building can be evacuated safely.