What is a ribbed cassette floor?
A ribbed cassette floor is an insulated precast floor unit consisting of a thin prestressed concrete slab on top, thicker load-bearing ribs along the long edges and an insulating underside of expanded polystyrene (EPS). The Dutch name is ribcassettevloer; it is also simply called a ribbed floor.
The principle is efficient: concrete sits where compression is carried, and in the ribs where the bending moment is greatest. EPS fills the rest and provides the thermal insulation. Self-weight stays low, around 210 kg/m², while the span can be substantial.
Key figures
- Maximum span — about 6 to 7 metres
- Widths — available in 900, 1200 and 1800 mm
- Structural topping — usually a reinforced screed of around 50 mm cast on site
- Bearing lengths — indicative values are about 100 mm on masonry, 90 mm on concrete and 80 mm on steel; always follow the manufacturer’s figures
- Line support only — the units must bear on walls or beams, not on isolated points
Application
The ribbed cassette floor is used above all as an insulated ground floor over a crawl space, in housing and light commercial buildings. The insulation is already in the unit, so no separate layer underneath is needed.
Advantages
- Insulation and structure in one unit, so one operation fewer on site
- Low self-weight, which is kind to the foundations
- Openings for services are formed in the factory
- The floor is craned in in a single operation, which is fast
Limits and points to watch
- Spans over roughly 7 metres are not possible
- Notify openings in advance; cutting a prestressed unit afterwards is not permitted without the manufacturer’s approval, because you may cut the prestressing strands
- An accessible and ventilated crawl space must remain below
- Make sure bearing surfaces are flat and to level; adjusting afterwards is awkward
Related terms
- Hollow core slab floor
- Coffered ceiling
- Crawl space
- Formwork
- Reinforcement
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