6 Jun 2019
Ribbed slabs have started to be more popular again after many years when they were somewhat out of fashion. About fifteen years ago, I was working on a PFI hospital project where we, as the structural engineers, had proposed a ribbed slab.
Our clients, the PFI contractor, requested that we provide an alternative solution of a flat slab. Our argument that this would give a floor system that was more sensitive to vibration was nullified by the half a million pound saving that the change gave due to time savings for the construction of the slab and the sealing of partition heads. That changed our preferred solution for several projects following that one.
So why would you specify a ribbed slab? There are many long-term benefits of having a ribbed slab, these include material efficiency, visual interest, better thermal mass and vibration control.
Although ribbed slabs are deeper than flat slabs, the amount of concrete and reinforcement are considerably less, with a saving of about 20% in the volume of concrete and 10% in the weight of steel. This means that the loads on the columns and foundations are smaller resulting in slimmer columns and less extensive foundations. Like flat slabs, ribbed slabs can be post-tensioned. This will result in further material efficiency with the volume of concrete reducing by a further 10%, a total saving of 30% on a typical flat slab solution.
Ribbed slabs are being used by architects to give visual interest when exposing the slab soffit. The coffers formed by the ribs can provide visual interest and can be used as the reflective surface for lighting and chilled beams.
Concrete’s high thermal mass, which helps to stabilise the ambient temperature within a building, is a well-known benefit of using exposed concrete. Ribbed slabs increase that benefit by increasing the surface area of the soffit so that the heat can pass into the thermal mass more efficiently.
Concrete has very good vibration characteristics for areas sensitive to vibration such as laboratory buildings and hospitals. This is due to its high mass and relatively high stiffness. Ribbed slabs provide a stiffer floor than the equivalent flat slab and are therefore better at reducing foot-fall vibration or vibration from equipment.
The Concrete Centre held a ribbed slabs seminar in May, underneath the stunning ribbed slabs of the recently-completed Royal College of Pathologists HQ in London. This fantastic venue was a perfect place to effectively convey the many benefits of ribbed slabs, and exposed concrete in general, providing a modern look and comfortable conditions for delegates at the seminar.
A full article, which presents more on design considerations and details for specifying ribbed slabs is in our latest Concrete Quarterly magazine, as the summer issue’s regular Structures piece. Read more.
Photos Cordek & Hufton + Crow (c)
Written by - Jenny Burridge, head of structural engineering at The Concrete Centre