Mott MacDonald carried out detailed option studies to determine the most cost-effective structural solution for T5’s basement structures. We determined that a composite steel system was to be most economical in combination with the 9m by 18m grid of piled foundations and also the most flexible, allowing for possible changes to the building, including floor penetrations.

Floor beams consist of welded plate girders, all 1,200mm deep, giving a consistent structural soffit level to simplify co-ordination and installation of building services, which are supported beneath. Different loading requirements are incorporated by changing the girder plate thickness.

Construction of the floor forming the roof over the rail station box comprises precast concrete inverted T beams with an in situ cast reinforced concrete deck, spanning between steel girders. The exposed soffit of the precast beams provide a high quality finish over the station.

For T5B and T5C we adopted a regular foundation and column grid of 9m x 9m. Floors are of flat slab construction. Mott MacDonald incorporated services openings early in the design. And for speed of construction we used precast concrete T section beams to form the roof over the tracked transit system tunnel and station in T5C.

Standardisation

The extensive areas of wall and slab construction enabled significant standardisation of reinforcement and formwork. Most of the reinforcement was prefabricated in the nearby Colnbrook Logistics Centre, to which all construction materials were delivered. Complete cages and mats of reinforcement for slab construction were transported to site on a just-in-time basis. This speeded up construction, enabled higher quality to be achieved as well as improved safety.