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The engineering design was driven by the need to achieve the spaces envisaged by the architect, while taking account of the significant constraints imposed by the site, and the adjacent Grade I-listed buildings.

The close proximity of the National Portrait Gallery and the National Gallery, and their sensitivity to movement, led to a lightweight composite steel structure founded on tripod bored piles, independent of the existing buildings, with the major column loads located away from the existing facades.

The distinctive form of the new building's upper levels is achieved by supporting the majority of the weight of the floors from a storey height truss spanning the length of the yard, between two columns. This truss forms the side wall of the second floor Tudor gallery. The first floor balcony gallery is hung beneath the second floor beams. At third floor, the beams cantilever over the top chord of the truss and support the framing for the rooftop restaurant and kitchen. The clerestory facade, bringing light down into the entrance hall, is also hung from the tip of these beams, in order to avoid transferring load to the National Gallery parapet.

Careful structural surgery was required to the existing building (originally constructed in 1895) in order to provide circulation routes to link the new building with the existing galleries, to improve disabled and fire access, and to create services routes to the basement plantroom. Reinforced concrete-framed openings were created in the stone and masonry facade and internal walls, and the main stair core was totally demolished and re-built to accommodate new lifts and stairs. The structural solutions had to be developed in conjunction with outline construction sequences, in order to ensure that clear loadpaths were maintained and movement minimised during the various stages of construction, as well as in the final condition.

The air conditioning plant that served the original building had to be completely rebuilt within the existing basements in order to provide space for the new galleries. This was achieved while maintaining the system operational at all times. A separate steel gantry structure near the entrance to Orange Street was designed to provide additional plant area for the extension. Services are routed from this into a service wall, which extends the full width and height of the new building, supplying air to the galleries. The lecture theatre is serviced independently from a plant room beneath the lecture theatre seating.

Every millimetre counted during the design stage. It was essential to maximise the available space while ensuring that it would fit together on site. The end result is an achievement in terms of integrated design and a result of a successful and enjoyable collaboration with the architect.

Jo da Silva, Associate Director, Ove Arup & Partners

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