striking £65m commercial redevelopment in central London, the brief for Moorgate Exchange focused on two core aims. The first was to provide a dynamic and high quality commercial building that made a modern architectural statement, while maximising lettable area. The second was to produce a structure that achieved the highest BREEAM and LEED sustainability ratings possible.
Situated just south of Moorgate station, Moorgate Exchange is bounded by roads on the east, west and south sides. Underneath the site lie Crossrail tunnels and other significant structures, while an invisible but inviolable boundary exists above the site in the form of a sloping rights of light envelope. Due to the proximity of the Barbican overlooking Moorgate Exchange, the roof had to be designed as a ‘fifth elevation’ with as much care as the other four principal elevations. All of these factors placed limitations on how the building could be founded, yet the response of the structural design to these constraints helped shape the architecture of the building, both internally and externally.
The two storey sequence of ‘vee’ columns running along the north and south elevations of the building form a dramatic architectural presence at street level, but began as a structural response to constraints within the ground. At the south west corner of the site an underground chamber encroaches onto the site and precluded founding columns under that corner of the building. Various options were considered, including cantilever beams, inclined hangers, large trusses, and raking columns. The raking column solution looked promising and was developed further.
Firstly, to avoid placing significant horizontal forces on the second floor plate, the single raking column originally proposed was coupled with an adjacent triangular arrangement of columns to efficiently resist the overturning forces; this idea was then repeated at the eastern end of the elevation to open up the corner entrance to the building, resulting in a run of vee columns along the entire length of the south elevation. Once this was settled as a concept for the south elevation, it was mirrored on the north elevation to add symmetry to the building.
Structural design
Having been fixed as a concept, the form of the vee columns was developed to address fabrication and construction requirements. The first design developed for the vee columns comprised H section steel columns with concrete cast around them to form tapered cylinders. Whilst very efficient as a pure structural design, the weight of these composite columns would have been too great to lift into position as part of the erection of the steel frame, and an alternative solution was required to ensure buildability. The final design comprises slender tapered steel box section columns with GRC surrounds, which match the doubly curved GRC cladding around the vee column nodes.
Internally, the building has two wings running along the north and south sides of approximately 16m in width with the two cores and an atrium located in the central third of the plan. The ideal floor plan for a speculative office would have no internal columns, and the structural design developed for Moorgate almost meets this ideal. A long span steel floor spans between the central cores and a line of columns just inside the north and south facades; this effectively creates column free office space. In conflict with the demand for column free spaces, which would naturally result in a deeper floor structure, was the desire to minimise floor to floor heights to achieve greater net floor areas.
Minimal impact
These two constraints resulted in a long span shallow floor structure which would be susceptible to floor vibrations, such as people walking across the floor, and countering this could be costly. An alternative solution was developed that aimed to mobilise a larger amount of mass within the structure to counter the dynamic forces applied. This was achieved by placing transverse beams at key points between the main floor beams, which once optimised turned out to be very effective; the resulting floor structure has a steel depth of only 550mm but actually achieves a peak Response Factor below 4, which is less than half the threshold of 8 recommended by the British Council for Offices.
The upper floors of the building have a larger footprint than the ground floor of the building, requiring a transfer structure at either first or second floor level to step the columns out. The brief also called for a dealer floor at first floor level requiring a deeper ceiling zone for services, so we integrated the transfer structures with the increased services zone so that the deeper structure could be accommodated without impacting the spaces below. Having established the second floor as the transfer level, elements of the first floor structure were also hung from the second floor to minimise the number of columns at ground level. From this, a very clear diagram for the building structure emerged became the organising principle for developing the structural design.
To minimise the size of the columns adjacent to the facades, and to avoid the use of unattractive intumescent paint, composite columns consisting of circular steel sections filled with grout in-situ were used. The resulting sections have far greater fire resistance than plain steel sections and allow the section size to be reduced and fire protection to be omitted.
The roof, or ‘fifth elevation’, was defined by the sloping rights of light envelope, and this led to a series of terraces as successive floors step back to stay within the envelope. The first consideration with the terraces was to maximise internal floor area, and to achieve this, a series of 1.5m wide lightweight frames with very shallow roof beams were added to the main structure at each terrace level to increase floor area. Having maximised the floor area, the terraces were then developed to achieve several objectives namely biodiversity, solar control, support to the Building Maintenance Unit, amenity space for the offices and a second means of escape.
Sustainability strategy
This brings us to the sustainable aspects of the build. In addition to the roof terraces invaluable habitat for local ecology, nearby tube access at Moorgate, Liverpool Street and Bank stations is supplemented by 136 bicycle parking spaces intended to encourage greener travel for office users. Rainwater harvesting coupled with onsite attenuation tank and greywater recycling plant reduces storm-water runoff and mains water consumption. Energy-efficient LED light fittings, with daylight sensor control, will ensure that lights are switched off when adequate daylighting is available and the building fabric u-values and infiltration exceed Part L 2006 requirements. Passive design measures, high-performance facade types, high efficiency HVAC systems and other energy conservation measures have together indicated a potential reduction of 30% below the Corporation of London 2001 Energy Code of Practice benchmark for commercial offices over 2000m2. Such factors led to the buildings award of BREEAM Excellent and LEED Platinum sustainability ratings.
Moorgate Exchange successfully resolves the constraints of a complicated site into a very efficient speculative office building that makes the most of its setting. The structural engineering solutions developed in response to the site constraints and brief made an important contribution to the development of the architectural design and are legible in the final building form.
