The Holy Grail of the structural steel industry is a constructional form which eliminates the need for passive fire protection to the steel frames of typical multi-storey buildings. Many have tried, but, as tests at the Building Research Establishment's Cardington facility have shown, there is still a long way to go before the dream is realised.
Cardington research has confirmed, however, that there are ways of doing without fire protection to floor beams by using the protection afforded by the concrete floors themselves. Hiding the beams inside the floors brings other benefits, not least a valuable reduction in floor-to-floor heights and a significant saving of time on site.
British Steel's Slimflor system with the unique ASB asymmetric hot rolled I-section beam is perhaps
the best known example of the type, but it was not the first.
First to hit the market in the UK was the Swedish Thorbeam, a concrete- filled internally reinforced fabricated top hat section beam with floor units sitting on the lower flange. This was in the early 1990s. Now comes a new Scandinavian contender, the Deltadeck system from Finland, which is being marketed in the UK by BRC Square Grip.
Formally launched at a Concrete Society seminar in Cirencester last week, Deltatek has at its heart another fabricated concrete-filled beam, this time with more of a 'Quaker Hat' cross-section. While the Thorbeam top hat had no lid, in the Delta beam concrete is infilled via a row of holes in each inclined web, which can also be used as a route for services up to 150mm in diameter as well as continuity reinforcement.
These holes are actually punched through the webs rather than cut. Deltatek managing director Jorma Kyckling says the lips formed by the punching act as shear keys with the concrete inside. They also act as supports for the internal reinforcement, which is spot-welded into place in the factory.
One of the beam's most unusual features, says Kyckling, is its ability to be used with any type of column - insitu or precast concrete, or steel. And because it is a 'closed' box section it has much greater torsional capacity than either the Thorbeam or the ASB, he says.
According to Kyckling, inclining the side webs increases the protection they receive from the surrounding concrete and therefore enhances fire resistance. He also claims these webs are less prone to buckling than the vertical webs of the Thorbeam and similar designs and that the Delta design is much easier to pre-camber than the ASB.
Since launching the system in Scandinavia in 1990, Deltatek has supplied over 120,000m of beams to more than 700 projects. Ironically, the largest contract to date was in Thorbeam's home territory of Sweden, when 12,000m was supplied for a new hospital.
BRC Square Grip only came across Deltatek in October last year, 'almost by accident' on a visit to Finland, says regional director Doug Mirtle. Last Wednesday the company signed an agreement with Deltatek to market the system in the UK, with an option to produce the beams here if demand increases significantly. Negotiations to supply beams to the first UK contract are at an advanced state, Myrtle confirms.