CONNELL BROTHERS, having successfully completed the first ever demolition of a post-tensioned, pre-stressed beamed building at the former bomb-damaged Marks & Spencer complex in Manchester, are now leading the way with the demolition of Bernard House in Piccadilly, Manchester.

The project comprises of the demolition of Bernard House, a 10-storey, concrete structure with a further one and a half storeys to sub-basement level.

The building, a popular landmark in the centre of Manchester, has a unique roof which makes it a striking feature of the Manchester skyline. It is a timber structure of hyperbolic paraboloid form, comprising of a main rib element on each of the four axes of twin Glulam Beams. Each of these is formed in turn by two Glulam Beams, which are piggybacked and bolted together. The beams are then boarded and weatherproofed.

As seen by members of the public, the office structure rises above car deck level on a 9.75m square core and a post-tensioned, pre-stressed concrete slab cantilevers out to form the 24.4m square office floor plate. The cantilever slab, which is 1.5m thick at its root and 535mm at its edge, also supports the six levels of office floors and the roof above, through the reinforced concrete mullions located at 2.44m centres around the floor perimeter. During the construction of the slab, one hundred pre-stressing cable ducts were incorporated to receive the 7200, 0.276 inch (7mm) diameter pre-stressing wires, 72 wires in each duct, and temporary shuttering put in place as support. The cable ducts have been laid in a longtitudinal and latitudinal pattern producing a strong post-tensioned, pre-stressed concrete annulus around the core (which can be likened to a square doughnut). At three specific stages during the construction of the upper floors, the cables were tensioned and grouted up, allowing removal of the temporary supports and completion of construction.

The main objective of the project is to allow the safe demolition of the building by releasing the loads in the cables in a sequenced manner by employing the reverse of the construction phase. Connell Brothers will again be working in close liaison with Consulting Engineers, Allott & Lomax, in carrying out the exploratory works, monitoring of cables and their sequenced cutting allowing staged destressing. Preparatory works are currently being put in place by fully scaffolding Bernard House prior to removal of the hyperbolic parapoloid roof structure. Further preparatory works underway include the back propping of the upper floors, allowing the siting of plant at those levels.

Due to the location of the works, a planned sequence of weekend road closures has been implemented along York Street to allow for excavators to be craned up to roof level to deconstruct the building floor by floor. The Project Director appointed for the demolition is Mr Barry Connell, who is well experienced with city centre works, having overseen the demolition of the former Marks & Spencer complex in Manchester which earned Connell Brothers the prestigious ward of Demolition Contractor of the Year.

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