Significant performance increase of slim-floor construction is achieved by an innovative and very economic activation of the in-situ concrete for the section properties.
In the past beam spans were economically limited to 7m. Nowadays, thanks to the Composite Slim-Floor Beam (CoSFB) development, spans up to 12m and even 14m are possible, and the beam remains rather light.
The steel consumption of CoSFB is generally between 20 kg/m2 to 25 kg/m2 of the floor area. Activation of the in-situ concrete for the load bearing resistance and the beam stiffness is simple. The connection of a hot rolled steel section and in-situ concrete is made by standard reinforcement bars placed on-site through existing holes in the web of the beam, before the in-situ concrete is poured. This ‘concrete dowel technology’ represents a well established and proven technology in bridge construction since decades and it is used for 25 per cent of railway bridges (filler beam bridges).
Enhanced performance without increasing the steel consumption
Thanks to ArcelorMittal, this technology is now also applicable to slim-floor construction. It should be emphasised, that the resulting CoSFB has an enhanced performance without increasing the steel consumption or the cost of fabrication and erection. Only the holes in the steel web should be drilled in the shop.
This construction type combines large spans with small floor thicknesses, low consumption of steel and overall a very efficient use of raw materials. The increase in span leads to a reduced quantity of internal columns and results therefore in large, column-free spaces, user-friendly with a maximum of flexibility at low costs. CoSFB is a perfect solution for open spaces, which aim on adaptability for the individual needs of the occupants and/or changeability of the building function. The efficiency of CoSFB is based on the optimal use of the applied materials steel and concrete: steel is mainly exposed to tensile stresses whereas the concrete is predominantly under compression and is applied for surface elements, as e.g. the slab. Due to the integration of the beam into the slab the beam is protected against heating in case of fire. Therefore, the requirements for fire resistance classes up to R60 are met without any additional fire protection, R90 can be achieved with low measures. Even higher fire resistance classes can be realised at low cost by protecting simply the slab-supporting plate.
The use of steel decks for the slab (e.g. Cofraplus 220®, German Technical Approval Z-26.1-55) leads to further optimisation of the overall weight of the floor combined with a high degree of prefabrication. This allows for an increased speed of erection, reducing the costs for labour and site installation. Thus, not only the consumption in resources and primary energy is condensed, but even the overall building cost. Furthermore the low material consumption also diminishes to a minimum the quantity of material delivered to the construction site and lowers significantly the emission impacts by the construction process. Low primary energy demand, the very low CO2-equivalent embedded in 1m2 of composite slim-floor construction, the high flexibility in use of the created open spaces together with high user comfort, as well as the large recycling potential of the structure and the integral assessment, leads to an extremely sustainable construction.
Slim-floor construction is characterised by the integration of the floor beam into the slab. This is realised by placing the slab on a supporting plate which is welded underneath the lower flange of a hot rolled section. It is possible to use a wide range of standard slab systems in slim-floor construction. The use of partially prefabricated slabs completed with in-situ concrete is a very common solution. The use of modern shallow steel decks (e.g. Cofraplus 220®) and steel panelled slabs (e.g. Cofradal 200® and Cofradal 260®) is significantly increasing. Also pre-stressed hollow core slabs with or without in-situ concrete are still used. The construction is very easy: the slab elements are directly placed onto the supporting plate which is welded under the beam. Therefore, fast and smooth erection is guaranteed. Further the building services can simply be installed below the slab, completely independent from the load bearing structure. In case of conversion they are easily accessible. Complex design of ducts is not required in slim-floor construction.
The planning phase and the construction phase are significantly simplified. In case of fire, slim-floor beams are well protected by the embedding concrete. In most cases, fire protection can be avoided (up to class R60 respectively R90).
In the conception phase special attention should be paid on choosing the slab span as large as possible. The ratio of the beam weight per square meter floor is generally between 20 kg/m2 and 25 kg/m2. The span of slim-floor beams without composite action constitutes to approximately L = h × 25 (h = beam height). For composite slim-floor beams with a shear connection between the steel section and in- situ concrete, spans can be increased to L = h × 35. Combination of CoSFB and Cofraplus 220® enables realisation of a structure with a column free floor area of 10m × 10m, a floor construction thickness of only 36 cm, and a weight of the CoSFB of only 22.5 kg/m2.
For more information telephone +352 5313 3020 or visit www.arcelormittal.com