The structural engineering work covered both major refurbishment and new build.
A steel composite floor was inserted into the existing building. New columns on hand-augered piled foundations were introduced to relieve the load on an existing 1.2m deep transfer beam. This allowed the beam to be reduced in size to achieve the required headroom.
The new steel and glass roof comprises bowstrung trusses spanning 21m supported by a 4m deep perimeter trussed girder. The girder also supports the plant deck and transfers loads to pre-cast concrete frame structures which form the new floors. Bearings are provided between the roof structure and the concrete columns to allow for thermal movements and prevent load attraction.
The vertical truss adjacent to the Brunel train shed supports a 'planar type' glass wall and had to be sufficiently stiff to prevent cracking of the glass. Castings were used to form the majority of the connections and were standardised wherever possible.
The adoption of a pre-cast concrete frame directly supporting the roof allowed the structure to be quickly erected on-site and ensured a high quality concrete finish was achieved. The frame was designed to resist lateral loads from the glazed wall along with any frictional forces developed by the movement bearings.
The pre-cast concrete frame is supported on bored piles that also support the ground floor slab. In areas affected by the proposed CrossRail development the ground slab is designed as a flat slab between large diameter piles. This is to enable the construction of the CrossRail ticket office at a later date without disruption of the recently developed Lawn area.
The lul ticket hall basement area was increased in size by using a grouted contiguous piled wall along the perimeter with an internal grained cavity wall to provide the required level of waterproofing.
Great Western Railway Hotel/BAA Check-in
The development of the gwr Hotel occurred over a number of years and occasionally in a piecemeal fashion. This resulted in a number of columns being located off grid and in an inappropriate position for the adoption of the required 27 baa check-in desks. In order to overcome this a series of transfer structures with new foundations were provided. To ensure only minimal movements of the hotel structure above, the transfer structures were preloaded by jacking.
In order to optimise the retail space link between Macmillan House and the new Lawn structures to the existing 1930s building, the existing floor structures at first floor mezzanine level were removed and replaced with new floors at appropriate levels. Again floors were of lightweight construction and of minimum depth, with holes for services penetrations built into the frame structure.
Large heavy goods lifts were introduced to fit in with the outline of existing foundations. A new steel frame deck was introduced at roof level to carry new plant for the Lawn and Macmillan House. An extensive part of the departures road facade was demolished and reconstructed to suit the access needs of the retail space inside.
David Dexter, WS Atkins Buildings Projects