A Double Tee has a top flange and two webs. The units are prestressed, are relatively lightweight, with a high load capacity and can be used for long spans. They are suitable for long-span structures such as multi-storey car parks. Units are generally 2400mm wide with ribs at approximately 1200mm centres.
Effective load sharing between units is achieved by welding together cast-in plates and brushing grout into the shaped longitudinal joints. The units may be used without a topping or, for enhanced performance, they may act compositely with an in situ topping to create a robust floor. The use of a structural topping overcomes possible difficulties with differential cambers between adjacent units.
Typically spans of up to about 17m can be achieved (for an imposed loading of 5kN/mm2) with an overall depth of about 900mm.
Requirements for the basic performance criteria of double tee units, and the evaluation of conformity, are given in BS EN 13224: 2004, Precast concrete products – Ribbed floor elements.