Fire Engineering in the ‘Cheesegrater’: London Landmark Case Study

Colloquially known as the ‘Cheesegrater’ the sloped glass and steel Leadenhall Building has fast become an iconic part of the London skyline.

Sat opposite Lloyd’s of London the building contains 610,000 sq ft of office space and incorporates 18,000 tonnes of steel with components made and assembled far and wide and developed by British Land and Oxford Properties.

In its concept, the structure was to have no central concrete core, with an external steel frame and central steel core providing lateral stability and 26 passenger lifts at its northern core. This presented numerous challenges through the construction phase, which from 2011 to 2014 preoccupied architects and engineers for Sherwin-Williams, the supplier of coatings for fire and corrosion protection.

The development’s tapering shape, which viewed from the west appears to ‘lean away’ from St Paul’s Cathedral, delivers varied floor plate sizes, all offering spectacular views over London.

The 52-storey skyscraper’s geometry makes it theoretically unstable, so exceptional engineering skills were necessary to develop a construction methodology that enabled the building to stay upright with tolerances of plus or minus 20mm required on all but five of its floors.

Other design features included a 100ft-high atrium including lawns, seating, trees, shops and an events space, all open to the public.

Solution

The development’s status as a major London landmark meant that Sherwin-Williams Protective & Marine Coatings’ fire engineering and technical services teams, in conjunction with structural steel specialist Severfield, had to maintain the highest standard of design and application from both a finish and aesthetic perspective as well as a protection point of view.

Many elements – including beams, columns and large transitional nodes within the mega-frame – were a complex bespoke design. They required specific and complex fire-protection analysis by the fire engineering team at Sherwin-Williams as well as coating application methodology in Severfield’s own facilities.

Further challenges were presented by floor-plate construction methods, from ‘slim floor’ construction in the basements to conventional down-stand beams in the upper floors.

Sherwin-Williams used building information modelling (BIM) on the floor plate fire design to seamlessly integrate coatings into the Severfield 3D model.

There was a need for different intumescent technologies given the large variance in the building’s environmental conditions. Needing to withstand wide temperature and humidity fluctuations the mega-frame exoskeleton required film epoxy fire proofing.

The steel remains wholly to view due to a glazed exterior. Thus the finish was architecturally high, another difficult achievement using a high thickness epoxy intumescent.

For fire protection, the whole structure was sprayed prior to site assembly using FIRETEX C69 blast primer, intumescent build coats of FIRETEX FX2002 and FIRETEX M95 epoxy intumescent with a top coat of Resistex C237 with a dry film thickness (DFT) of between 1-13mm, depending on the steel’s thickness. This provided 90 minutes of fire protection.

These coatings ensured that unusual design elements enjoyed optimum protection, but nevertheless fitted with the overall vision of an important visual landmark in the City of London.

Summary

Despite numerous challenges set by the designer and architects, the original vision has been achieved while ensuring the asset’s safety and protection with the optimum level of coating.

While the south side is dominated by the mega frame around which the building forms its leaning shape, the north side is a separate structure with a vivid ‘electric mustard’ yellow steelwork frame and alternating blocks of red and blue glass.

The 26 glass panoramic lifts are the fastest in Europe, taking just 30 seconds to reach the 45th floor. The 6,000 people working in the building can be seen going about their business daily behind the encasing glass.

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