Too often our designs remain hidden behind the fabric of a building. These finishes obscure our designs and limit an engineer’s ability to express our ingenuity and creativity.
So how would it feel to be able to design and detail a building where all the structure was exposed and what would be the benefits?
The Quadrant:MK at Milton Keynes provided this opportunity on a grand scale, with most of the structure remaining on display.
Clad or Expose?
Buildings traditionally have an internal veneer of finishes for a number of reasons. Finishes can accommodate complex architectural detailing and use a broad range of materials. The finishes hide the building services and allow for greater flexibility of servicing. The structure can be designed to be utilitarian, focusing on function only. Finishes can be easily replaced if damaged.
These benefits provide a strong impetus to design a traditional building but there are some significant advantages to be gained by omitting the finishes. These include a reduction of fit out costs and programme, less working at height, improved durability and a merging of form and function.
The main incentive for choosing exposed structures is sustainability. Exposed structures tend to require less material, less construction processes, fewer trades and fewer deliveries and therefore usually have a lower embodied energy. In addition, concrete floor slabs have thermal mass, which can be mobilised to modulate the internal temperatures of a building and reduce or avoid the need for supplementary air conditioning.
Using the Thermal Mass
Thermal Mass is the term used to describe the process of mobilising the mass of the building to provide inertia against temperature fluctuations. This is usually achieved using exposed concrete frames. There is some debate as to whether profiled or flat soffit slabs provide the optimum performance, but the current thinking is that a flat soffit provides the optimum surface to harness the cooler night-time air by avoiding trapping pockets of warm air.
The form of passively cooled buildings needs to be carefully tailored to be able to harness and store the cool night-time air. The Quadrant:MK buildings are designed with internal atriums that project above the roof level. At night perimeter vents are opened up on the external façade and the window is open at the top of the internal atria. This venting arrangement creates a stack, which passively draws in cool night-time air at each floor level and vents the warmer internal air through the atrium. This cools the floor slabs sufficiently to allow them to absorb the warmth generated during the day.
The resulting buildings are passively cooled by the thermal mass of the exposed concrete, requiring no supplementary cooling except in meeting and server rooms. Using the exposed frame in this way dramatically reduces the energy use in the building. This low energy consumption has significantly contributed to the attainment of a BREEAM ‘Excellent’ designation for nearly 400,000ft2 of office space.
Form and Function
The best exposed structures seamlessly combine form and function. Bridges often provide the purest and most legible expression of the merging of form and function. The two pedestrian bridges at The Quadrant:MK are exposed steel trusses and hence the elegance of the structural form would be important.
The key functions of the bridges are to provide a clear span across the atrium, to enhance connectivity between the buildings and to create a strong physical and visual link within the central atrium.
Typical bridge forms include arch, single or paired beams or trusses and cable stay options, all of which have their own distinctive appearance.
The 18m bridge span was achieved with an inverted bow string truss projecting diagonally from one corner of the bridge deck to the other. The bow string is a single solid steel bar which is suitably slender to maximise views through the truss. The diagonal orientation of the bow-string creates a dynamic view of the bridge soffit which changes depending on where it is viewed. The final solution represents a link between form and function.
Speed and Safety
The central atrium roof was installed at a 20m height between the office buildings and so it became important for safety to deliver a design which would reduce the number of trades working at height. The steel truss solution involved craning in prefabricated truss units into position between the buildings and enabled a modular integrated panel system to be lowered on top of the trusses. This significantly minimised the numbers of trades working at height as separate trades were not required to install insulation or apply finishes such as plaster.
The roof steelwork will be a combination of exposed and clad steelwork. The trusses have been detailed to support a Barrisol tensile fabric surround. These are clipped to the trusses to create the dramatic volumetric shapes defined by the form of the trusses.
There is a natural elegance to well-designed structures which often mirrors the beauty and strength found in nature. We set out to design a shading canopy for Southern Elevation of The Quadrant:MK which achieved this natural beauty.
The entrance shading canopy is designed as a series of half portal frames. The depth of each portal varies to suit the spans of the rafters. The portal frames are formed from tapering box girders, with the angle of taper varying slightly on each column to suit the larger forces generated by the larger spans.
The canopy frame design is a simple geometric expression of its structural needs. The simplicity of form is maintained by the absence of any cross bracing. The canopy has a diagonal intersection with the office buildings. We were able to mobilise this geometry to stabilise the canopy without the need for any cross bracing. The images below show how a combination of the slender tapering frames and the absence of braces or trusses help to create a dramatic entrance for the building.