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working details - A facade of self-supporting precast panels

Merrill Lynch HQ, City of London Swanke Hayden Connell Architects

The seven-storey building has a steel-frame structure enclosed with self-supporting facades of precast concrete column and spandrel units.

On the southern side, the units are faced with Portland stone; on the northern, stackbonded brickwork.

The facades comprise a series of storey-height precast concrete columns which support precast concrete spandrel panels.The columns are on a 12m grid, a width dictated by groundbearing problems.

At ground floor level the facade is set back so the columns create a colonnade, with precast arches spanning 12m between them. On the floors above, the spaces between columns and spandrels are filled with double-glazed windows.

The assembly was designed to maintain a constant 10mm-wide mortar joint and avoid intrusive silicone movement joints on the facades; joints between units are positioned at the backs of the columns.

Each precast arch was cast as two separate units - spandrel and soffit.To accommodate the relatively long span with the flat shape of the arch, the soffit is posttensioned by means of 50mm diameter cables housed in cast-in plastic tubes.

Each ground floor precast column is U-shaped, forming a Portland stone-faced front and sides; the rear Portland stone face is fixed by hand on to a metal sub-frame.To provide lateral restraint, the unit is bolted with brackets to the main structural column, a 356 x 406mm UC.

The 4 x 4m windows to the upper floors are supported by the precast structure; this minimises any movement to 3mm and allows the windows to be installed from the inside - site restrictions prevented the use of scaffolding.

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