Finsbury Square
Less is beautiful
London
50 Finsbury Square is an excellent example of a building with an exposed primary structure and partially concealed curtain walling. The primary structure, clad with natural limestone, is connected to the internal beams and slabs by means of projecting concrete and stone clad steel beams. Horizontal stone transoms, spanning from column to column have then been placed in front of the curtain walling. The setting out of these horizontal stone transoms allows a perfect view from inside of the building to the outside. The external stone screen created by the transoms guarantees a high internal privacy and at the same time assures a good solar control when used with the high performance glass, without using any additional internal shading device.Credits
Architect: Foster and Partners
Client: Standard Life Insurance
Photographer: N. Young
The tecnology
3000 x 4500 unit structural curtain walling Glazed atrium Aluminium c.w.
The Entrance The 4.2 m high bi-parting door is the highest in Europe. Adjacent to the bi-parting door a double storey height of curtain walling 7.2 m high is a main feature of the ground floors transparency. This external façade which is characterised by 19 mm thick glass 1.5 m wide and 7.2 m high exceeded the standard dimension of float glass production, is supported by means of alternate aluminium and stone mullions without any intermediate transoms. The extremely large sizes and weights of these glass panes has necessitated the modification of specialist lifting equipment during all the processes from initial production, to edgeworking, transport and up to final installation. The Main Curtain Walling One of the main architects requirements that has characterised the curtain walling design and development, was the possibility to replace the elevation glass from the inside of the building. This is mainly because external access from outside is difficult due to the external stone screen. The designed solution is a factory-assembled frame of two glazing panels 1500 mm wide and 3200/4200 high. The possibility to replace the glass from inside has been achieved by designing a frame which has a removable intermediate mullion. In the unlikely event of glass breakage, this allows sufficient space to change the glass from inside the building although the glass must be passed through the opening to the outside first. The units are glazed with full height double glazed units, which are bonded with structural silicone to an aluminium carrier frame, this frame is then retained into the main mullion and transom grid. The projecting beams which penetrate the curtain walling required a ‘non conventional’ approach in the detailing, particularly due to the very demanding architectural requirements. The Atrium The internal atrium is the real core of the building because of its technical/architectural solutions and for its functions. In particular it provides zenith of natural lighting and connects the two north and south wings of the floor plan. From a geometrical point of view the atrium, which faces east and is not visible from outside the building, has an elliptical shape in section and trapezoidal shape on plan. Externally the atrium is flanked partially by the factory assembled transparent glazed curtain walling, and also by aluminium cladding. Internally each floor has vertical curtain walling glazed with frameless monolithic 15 mm toughened glass, protecting the occupants from the enormous space below. The atrium is supported by means of a steel structure composed by horizontal pre-cambered circular steel trusses connected to the slabs and by vertical rectangular curved shaped transoms connected to the circular trusses by feature stainless steel pins. Above the curved steel structure an aluminium curtain walling is connected by means of satin stainless steel brackets. The curtain walling is a panellised system with triple glazed insulated units with high performance glass. From bottom to top the glass panes have a graduated frit from 80% at the bottom reducing to 10% three quarters of the way up. The frit consists of horizontal stripes of white colour spaced at different centres, in order to reduce the effects of the view of the adjacent building. To the underside of the steel structure the cleaning cradle brackets are fixed. They support the cleaning cradle rail, which allows the motors to lift the cleaning cradles from top to bottom for cleaning and maintenance purposes. When the cradles are not in use they are stored at the top of the atrium behind electrically operated openable panels. The Bridges The atrium is crossed by pedestrians at various levels by 6 vertically offset Bridges which connect the two wings of the building. The support structure consists of rectangular hollow section steel beams spanning between the floor slabs. The floors and the balustrades of the Bridges are fully glazed to create an almost transparent structure. The main features of the Bridges are:
- The handrail is integrated within the 15 mm toughened glass balustrade. This was achieved by using a pre-tensioned steel flat sandwiched between the aluminium channel handrail and the notched balustrade glass. The channel is structurally bonded to the top of the glass balustrade.
- The glazed pedestrian floor is made up of a solid thickness of 8mm glass and 2 No 19mm glass panes laminated together. When the lighting to the edge of the Bridges is switched on the glass floor becomes illuminated throughout the thickness of the glass. This effect is obtained by means of a particular fritting to the top and bottom surface of the laminated glass which ‘divert’ the horizontal light transmitted by the glass to a vertical path illuminating the surface of the glass. The final agreement of this solution required both numerical evaluations and a full size mock-up developed in our factory making use of various glass finishes to achieve the architectural requirement, and with close collaboration of Zumtobel Staff Lighting who were responsible to the Architect for the buildings lighting design.
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