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Innovative, hybrid, reinforced-concrete superstructure frames were tailored specifically for this project and developed, from conception, by close cooperation between the structural engineer, Pell Frischmann, and Bouygues' technical/construction teams. The hybrid frames comprise a high proportion of precast concrete, stitched together with insitu concrete columns, walls and slab topping.

This form allowed rapid construction to a high quality and with a high level of accuracy. The precast units were delivered using 'just in time' techniques and only on rare occasions were the tower cranes unable to continue installation.

This hybrid form of construction allowed floorplates of 4,200m 2 to be constructed in an 11-working-day cycle, which is extremely fast.

This speed of construction enabled Bouygues to complete the concrete works within programme and within budget.

Pell Frischmann's substructure design re-uses the existing basement rafts and the retaining walls that remained following the demolition of the three old tower blocks. The design solution was environmentally beneficial and allowed significant economies to be made in terms of both construction time and cost. The mixture of demolition combined with construction of new foundations on top of the existing foundations has, however, posed a challenge for the geotechnical engineering team, which has carried out detailed analyses of the foundation movements to predict likely differential ground movements and ensure that the new structure can accommodate these movements.

New works began on site in December 2002 and the structural frames were completed by the end of March 2004, a total period of 16 months. Construction was phased around the demolition of massive existing wartime concrete structures on the site, which took longer than expected. Demolition was not completed until September 2003, which indicates the speed at which construction of the frame was progressed and completed.

Innovative use of prefabricated steel shuttering by Bouygues on site allowed precast beam units to be supported on shuttering while allowing in-situ cast concrete to be poured to form columns and beam stitches.

This helped to speed up cycle times.

The use of steel shuttering to all in-situ and precast concrete works has provided an excellent quality of concrete finish, which only requires a skim coat of plaster followed by a final coat of paint to complete the finishing works.

The concrete material provides adequate fire resistance and therefore there is no requirement to install any dry walling or partitioning, leading to significant savings in both cost and time.

The project illustrates European cooperation, with precast-concrete procurement open to tender with European suppliers. Precastconcrete floor planks were supplied from Belgium and precast-concrete beams from France. The suppliers were able to provide a high level of quality control in concrete production and the finish was to a very high standard.

Roger Hewitt, technical director, Pell Frischmann

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