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Delta floors are a new

technical

A new composite floor system to compete with British Steel's Slimflor and Slimdek (aj 3.7.97) has been launched in the uk by brc Square Grip.* The Delta floor system idea, like its rivals, started life in Scandinavia. It was launched by Deltatek of Finland in 1990. Assuming sales develop, brc will start manufacturing in the uk next year.

The main component is the Deltabeam, which lies within the depth of the floor plate. Providing an extended lower flange and inclined sides, it supports flooring of hollow or solid precast planks, in-situ concrete or a composite deck (there are just a few examples of the latter). Proprietary details exist for bolt or weld fixing of the beams to steel columns, and for concrete columns, cast-in shoes allow continuity of reinforcement floor-to-floor.

The beam has 150mm holes punched along its length, plus factory-fixed reinforcing bar inside as part of the structural and fire-engineering design. During on-site installation the beam is normally filled by pumping in concrete (there are also small holes at the top of the beam sides to allow air to escape). Continuity-reinforcement bars can be threaded across the beams into the cores of hollow floor planks or between planks on either side. This facility to include reinforcement also simplifies construction of in-situ areas such as cantilever balconies and borders to large floor openings.

The double support of the concrete floor (by the inclined beam faces and the extended web flanges), plus the in-situ concrete stitching, provide two-hour fire resistance with a flat floor soffit and exposed beam flange.

Punching the holes into the beam creates lips that act as shear keys with the concrete. Some of the holes can be used to provide transverse service routes.

Typically, beams have been used at around 10m span, though up to 20m can be viable. Recent buildings include the Myyrmanni Shopping Centre with 8.1m beams supporting 16.2m planks, and Terminal 2 at Helsinki Airport where 12m beams support 12m planks. Beam sizes available are 200, 300 or 400mm deep by 300. 400 or 500mm wide (plus web flanges). Dead-load deflections are accommodated by precambering the beams.

Opportunities

For a conventional steel frame where beams are not embedded in the concrete slab, recent fire tests at bre's Cardington test site have shown that when columns and primary beams are fire-cased, secondary beams can be left exposed. There is hope among the researchers that design guidelines can be developed which will allow primary beams to be uncased too, given suitable beam sizing and sprinkler protection. But this is not imminent.

The fire performance of systems like the Delta floor, combined with their flat soffits and shallow floor zones, offers useful additions to the construction palette beyond conventional flooring.

* brc Square Grip. For details tel 01623 440472

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