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An extension has been created by adding two floors to the library of Imperial College, a three-storey post-war precast-concrete building. The existing lifts were re-orientated within the core areas to give a central lobby at each level. The new floors have double-volume spaces naturally lit by three large 3.2m-square rooflight lanterns.

The new floors are constructed from a steel frame which at roof level supports a Kalbau aluminium trapezoidal sheet deck, two layers of insulation and a Kalzip roof covering of stucco-embossed aluminium sheet with standing seams.

Each lantern consists of a cruciform frame of 125 x 175mm steel T-sections which sits on steel flats welded to the top flange of a 200 x 75mm rsc ring beam. The ring beam is bolted to the main structural steelwork. The lantern walls are glazed on all four sides with natural aluminium curtain- wall transom sections at head and foot. The cruciform frame is braced at the eaves by a 100 x 50mm rsc ring beam. The curtain-wall system is supported by 100 x 100mm rsa cladding rails which are welded to the upper and lower ring beams. Fascias are clad with natural anodised aluminium.

The curtain wall's double-glazed units comprise an outer layer of 6mm toughened glass, coated to reduce solar gain, and an inner layer of 8.8mm laminated glass. The junctions at the corners of the lantern are sealed with black silicone.

The lantern roof is solid - it has an outer covering of natural anodised aluminium sheet with standing seams, 90mm rigid insulation and an inner Kalbau liner of profiled aluminium which helps to brace the lantern structure. The internal soffit is drylined.

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