Moorgate Exchange London, Case Study

Moorgate Exchange, London UK

Figure 1 General view of the Moorgate Exchange

Courtesy: HKR Architects

Summary

Moorgate Exchange is a 12 storey office building located in the Square Mile of the City of London, which was completed in 2014. It is a landmark structure with two storey-high V-shaped columns and an angled façade which forms a roof line with stepped landscaped terraces over six levels (Figure 1).

The distinct wedge-shaped form was the result of site constraints: the ‘rights to light’ of local residents, the height limitations of the St Paul’s viewing corridor and the partial overlap of the site’s footprint with the Crossrail tunnel.

The use of structural steel enabled the net lettable space to be maximised by the use of long clear spans, fire protection on the columns was minimised by infilling with concrete and a reduced overall floor depth allowed the incorporation of an additional storey. The total steel tonnage was 2,900 tonnes.

The building achieved a BREEAM “Excellent” and a LEED “Platinum” sustainability rating for its low impact on the environment through the use of high performance facades, high efficiency HVAC systems and other conservation measures. It is one of very few buildings in London to achieve these highest environmental standards.

The cost of the building was £56 Million.

Material Selection

Steel was chosen for the framing material because the construction time was shorter than if the frame was made of reinforced concrete. As a steel frame weighs less, a raft foundation could be used, which is faster and cheaper to construct than a piled foundation typically needed for a heavier concrete frame. A steel framed long span structure is also far more adaptable to future tenant changes than a concrete equivalent. Table 1 summarises the economic benefits of steel construction in office buildings.

The steel columns, supplied by Tata Steel, were hot finished circular hollow sections (CHS) made from grade S355J2H in accordance with European product standard EN 10210, which stipulates a minimum yield strength of 355 MPa and a minimum toughness of 27 J at -20ºC. This steel itself was microalloyed with 0.030% niobium by weight which enabled the strength and toughness requirements to be easily achieved. Furthermore, the use of niobium microalloying permitted a lower carbon content to be used which significantly improved the weldability and the finer-grains developed also enhanced its formability.

Factor	Improvement	Economic benefit
Speed of construction	20 to 30% reduction in construction time relative to site-intensive construction, depending on the scale of the project.	The economic benefit depends on the business operation. In terms of overall building cost, a saving of 1% in interest charges, and 2% in early rental or use of the space is predicted.
	Site management costs	Site management costs are reduced because of the shorter construction period, and the packaged nature of the construction process.	Site management costs can be reduced by 20 to 30% which can lead to a 3 to 4% saving in terms of overall building cost.
Service integration	The integration of services in the structural zone leads to reduction of 100 to 300 mm in floor to floor zone and hence to savings in cladding cost.	A 5% reduction in floor to floor height can lead to one additional floor in 20, and to a similar reduction in cladding cost, which is equivalent to about 1% in total building cost.
	Foundations	Steel construction is less than half the weight of an equivalent concrete structure, which is equivalent to a 30% reduction in overall foundation loads.	Foundation costs depend on the sub-structure and factors such as underground services  and represent up to 5% of the building cost.  A 30% reduction in foundation loads can  lead to a significant overall saving in terms  of construction cost.
Column free space	Long span steel construction provides more flexible use of space, which depends on the function of the building and its future uses.	A large column in the middle of the space leads to a loss of space of approximately 1m2, which represents about 1% of the floor area, and may lead to an equivalent loss of rental income.

Table 1         Summary of the economic benefits of steel construction in office buildings Courtesy: www.steelconstruction.info

Design

The 20,000 m2 steel frame building is arranged with two main cores to provide lateral stability and a central atrium which draws natural light into the adjacent office accommodation (Figure 2). Open space, long column-free areas were a priority, which led to a 15.5 m by 7.5 m structural grid. The design ensures flexibility, as all of the floors can be subdivided if necessary.

The steel frame was designed with ease of fabrication in mind, for example standard plate thicknesses were used wherever possible to fabricate a number of different beams.

In order to maximise the available space and achieve the required 90 minutes fire performance, the internal and perimeter columns are concrete-filled circular hollow steel sections, designed according to Eurocode 4  (EN 1994). The composite columns used around the perimeter of the building had a diameter of 457 mm (12.5 mm and 16 mm thickness). If a steelonly solution had been chosen, the diameter would have been 610 mm. The internal columns had a diameter of 508 mm and thickness of 16 mm and 20 mm.

The beams were cellular I-beams, generally of 550 mm depth, to allow integration of the services within the structural zone and hence increase floor-to-ceiling heights. This enabled an additional floor to be added to the top of the building, whilst abiding with the height restrictions. This floor system is used on all the floors, with shallower but heavier beams underneath the roof gardens, to allow for waterproofing and drainage.

To achieve an acceptable dynamic performance of the long span floor system, secondary or stiffener beams at midspan were inserted between the main beams, which increased the stiffness of the floor without adding much more mass and successfully reduced the response factor.

...