If reinforced concrete has the required concrete cover, it performs very well in fire. Unfortunately, the steel reinforcement in many older and historically protected buildings does not have the required prescriptive concrete cover, this causes the steel reinforcement to heat up too quickly, lose its strength and leads to failure of the component. Fortunately, even with insufficient concrete cover, a hot calculation can often prove the component has much better performance than expected.
Fire design of concrete structures
All components of the reinforced concrete can be calculated: posts, beams, pedestals and one- and two-way slabs to name but a few. The effect of any coverings can be considered, whether MultiPur, Wood, Promat or any other covering, these can all delay the heat flow into the concrete and improve the fire resistance duration.
Fire safety design of ribbed slabs.
One particular area where we have lots of experience is in ribbed slabs (also known as Pi-Decken, waffle slab or Kaiser decken) especially for schools and industry buildings from the 60-70s. Typically the ribbed slab was originally installed with a hanging plaster ceiling which was fire tested and proven to give the required fire resistance. These hanging ceilings are then removed or compromised to allow for more services to be routed in the ceiling area, meaning the fire resistance is then lost.
As the concrete cover is often very small in ribbed slabs and the utilisation factor is relatively high the hot measurement calculation contains many challenges. However, we have experience in many projects where the requirement for additional passive fire protection coatings has been minimised or avoided altogether.
We have a large library of historical literature for ribbed slabs going back over 120 years. Therefore with some detective work we can also provide proofs for ribbed decks even when the original documents are missing.
One and two-way reinforced concrete slab.
Slabs are either reinforced in one or two axis and this strongly determines how well they perform in fire. Experimental Tests of real two-way reinforced slabs in fire have shown that they have a relatively high fire resistance duration. This is because when the steel relaxes, the slab acts like a hammock, which considerably reduces the stress on the steel and delays the time before failure. So even though the deformation may be very high, the slab will not collapse.
To a small extent one-way slabs can also behave like this, if supported on all edges, but this limited as usually there is less reinforcement in the minor axis.
