Filigran slabs


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Filigran slabs are prestressed (prefabricated) concrete slabs that serve first as a casting base for the monolithic part of the structure and as part of receiving the tensile forces of the composite structure after the reinforced casting is made. The working principle of a filigran slab is shown in the figure. E-Betoonelement started manufacturing filigran slabs in 1999.

The first construction project incorporating filigran slabs was the Stockmann car park and additional storeys of the shopping centre. Filigran slab combines the advantages of precast reinforced concrete elements and cast-in-situ reinforced concrete.

The use of filigran slabs is recommended in cases where using hollow core slabs is complicated.

For example:
- in the case of large openings and cut-outs;
- in the case of big point loads;
- if there is a need to install communication accessories under the floor since they do not fit in the cast-in-situ topping of hollow core slabs;
- if the thickness of ceilings needs to be reduced;
- if there is a need to make the ceilings more soundproof without increasing their thickness;
- in the case of the system of cast-in-situ beams and suspended ceiling panels.

Technical data
of filigran slabs:
width 1,200 mm
thickness 70, 90, 100, 120, 150 mm,
length can be up to 10 m but the recommended length is 7.2 m

The slabs are longitudinally prestressed with cables of 12.5 mm in diameter. The cables are selected according to the eventual load on the composite structure. In addition, filigran slabs are reinforced with connecting loops which ensure the co-operation between the topping and the slab. The connecting loops are used for lifting the filigran slabs.
The minimum thickness of the topping is 70 mm and the strength class of concrete is C25/30 - C33/37.
The topping is reinforced according to the calculations.

Labelling of filigran slabs
The label of a slab consists of an alpha-numerical combination. Letters represent the filigran slab and numbers represent the thickness of the slab in mm, the thickness of the composite structure, reinforcement, and the slab’s sequence number in the project.

For example:
– KP – filigran slab
– 90 – thickness mm
– 160 – thickness of the composite structure in mm
– 13 – number of strands (determined by the manufacturer)
– 101 – project sequence number

The underside of slabs is a moulded steel surface which complies with the requirements of Class 2 of BY 40 (classification guide for concrete surfaces). The modular width of the slab is 1,200 mm and it can be cut and adjusted when necessary. The smallest recommended width is 400 mm. When necessary, slabs with expanded polystyrene insulation can be provided.

Until the cast-in-situ concrete topping hardens, the filigran slabs are supported from below in one or, depending on their length and cut-outs, two or more points. The designer of the composite structure will determine the location of proppings and indicate them on the detailed layout drawings of filigran slabs. In the case of certain spans, filigran slabs do not require supports during the construction phase, but in such case no high requirements can be set with regard to the flatness of the ceiling.

The design of openings and penetrations is more flexible than for hollow core slabs. For large openings, a thicker slab should be chosen. Openings are surrounded with additional reinforcement which has been verified by calculations. Filigran slabs can be used as cantilevers both longitudinally and transversely. The reinforcement for cantilevers is determined by calculations.

The minimum order quantity of filigran slabs is approximately 1,000 m3. For designing and choosing the structure, please address the specialists at E-Betoonelement. In co-operation with the designer of the building or the client, they will make the calculations and design the structures related to the filigran slabs.

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