Going up!

Going up!

The Leadenhall Building boasts state of the art construction and extraordinary craftsmanship, just as Arup designed

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Standing a statuesque 224 metres tall with 46 floors, this building provides the highest offices in the City of London. Developing ideas that can be traced back to Rogers’ and Arup’s early collaborations from the design of the Centre Pompidou in the 1970s, in Paris, The Leadenhall Building is an unashamedly ambitious exercise in architectural engineering. Here, three Arup directors dissect the various elements of the construct.

Room to breathe

By James Thonger, Director, Arup

As is usual in a building of this complexity, the design for The Leadenhall Building developed as the concept evolved. The ‘cheesegrater’ shape for this iconic form was established early in the design process, and was strongly influenced by the view from Fleet Street and the potential encroachment of the sight lines to the north of St Paul’s Cathedral. These constraints lead to the creation of the building’s distinctive triangular shape, with a vertical section on the north and an inclined façade to the south.

With this concept established, the design team had to tackle the challenges presented by this building form. The traditional location for a core in a building of these dimensions would be in the centre of the floorplate, however this would mean that at higher levels, the vertical core would protrude through the façade. Consequently the vertical distribution for both lifts and building services had to be concentrated on the north side of the building to achieve verticality.

Placing the vertical distribution on one side of the building left floor plates at upper levels clear, maximising space in the building’s interior. In lower sections of the building, the size of the floor plates meant that two fire fighting cores were necessary to remain compliant with building regulations. These cores were also able to house vital building services, including power, communications, water and drainage.

Conventional towers often have a centralised fresh air plant, either at the top of the building, or at a central point. By using the extended lift core to the north of the building, Arup was able to design a floor-by-floor arrangement for air supply. This means that each floor can operate its air plant independently, giving each tenant the ability to control the energy consumption by ensuring that the air supplies are only operational when the floor is occupied.

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Holding its own

By Nigel Annereau, Director, Arup

The design of Leadenhall Building incorporated a number of unique construction techniques and innovations, many of which were intended to promote off-site manufacture.  
There was no wet concrete used above Level Five; an extremely uncommon approach and one virtually unheard of in a structure this tall. Instead, Arup and Laing O’Rourke opted to use pre-cast concrete, usually associated with an in-situ topping that binds the individual planks together and creates a diaphragm. There was no such topping used on the Leadenhall project. The pre-cast planks were the finished article. The simplicity and benefits of the system surely means that this will gain support and be utilised on other future projects.

The advantages of this were numerous and included a reduced work force on site, which in turn increased safety. Noise levels were also diminished, as there were no concrete pumps, pokers or steel deck being cut. Having all component parts prepared, including pre-formed holes for cranes, also ensured optimum accuracy, whilst a lack of wet concrete meant the floor was ready to be trafficked the day after placement.  

The huge nodes on the exoskeleton structure, which was termed the ‘megaframe’ in the project vernacular, that connect the various structural members together were devised so that relatively quick connections could be made on site without the use of site welding. To achieve this, the connections had to be moved away from the centroid of the joining members to give room for the bolts. This gave rise to the meganodes, which were joined to their respective megaframe members with a small number of large diameter bolts which were tightened by jacking. This concept dramatically reduced the amount of time that operatives spent making the site connections at the edge of the site compared to welding the very thick sections.

The braced form of Leadenhall posed a further challenge. As the building weight increased during construction, it leant to the north. The challenge here was to make sure the building was as vertical as practically possible at the end of construction. The solution first offered was to construct the building deliberately to the south, in accordance with predicted movements. This would mean that it reverted to its desired position during construction.

In conjunction with the contractor, Laing O’Rourke, an alternative proposition was decided upon. A method was adopted whereby the building was built straight, with shim packs included in the diagonals of the main megaframe external structure. i.e. the diagonals were made deliberately short. At pre-determined stages of the construction the shim packs were removed and the diagonals jacked to reduce their length, the effect being that the building moved to the south. Through careful monitoring of movement during jacking, in tandem with computer analysis of predicted movement through the various construction stages, a very good degree of accuracy was obtained with respect to the building’s intended vertical alignment.

The north core steelwork (all of the yellow steel as seen on site) was a great example of offsite manufacture, not just for the steelwork, but also for pre-installed pre-cast concrete planks and services. With the benefit of large capacity cranes, the main sections of each floor of the north core were installed in three crane lifts, with handrails pre-installed and ready to walk on. There were huge benefits to this not least the contribution to site safety.

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Up, up and away!

By Julien Olley, Director, Arup

The passenger lifts serving the upper floors of the Leadenhall Building animate the distinctive north core. These are the fastest scenic lifts in the world, yet were not originally conceived as such.

When Leadenhall was originally designed and submitted for planning, the occupancy standard used for this, and for other buildings at this time, was one workstation per 14 m². The standard for lift performance was set at 6 m/s. As time passed however, developments started to use ever higher occupancy densities. This left Arup with the challenge of increasing the handling capacity of the lift system to accommodate one workstation per 10 m² in the low and mid-rise floors, and one per 12 m² in the high rise floors of the building.

Normally one would expect an increase such as this to require either bigger or more lifts. Unfortunately neither of these options were open to the engineering team, as changes would require new planning permissions. Whilst changes in lift formation will not always necessitate this, the very prominent location of the passenger lifts in the north core, meant that changing the size of these would certainly have required modifications to be made.

The only possible option left was to increase the speed of the lifts. This was not an issue for the low or mid-rise lifts as, even with increased speeds; they remained well within the accepted norm for high speed scenic lifts. The high-rise group however needed to be 8 m/s to achieve the performance required, which, at 18 mph is almost twice the average speed of road traffic in London.

The increase from 6 m/s to 8 m/s might not seem a great leap, yet the forces required to move objects increase to the square of these speeds. The increase was from 36 to 64 or almost doubled. The safety gear and buffers, which stop the lift in the event of an emergency have to bring a lift car and the people within it to a complete halt without causing injury. When the combined weight of these is almost five tonnes and its moving at almost 20 mph, this is no easy feat. Further challenges included the possibilities that wind noise of lifts passing would be intrusive, or that the mullions flashing past might cause susceptible people to have an epileptic fit.

The former was dealt with by lift manufacturer, KONE, which after experimenting with the application of wind fairings to the top and bottom of the lift cars, determined that these were not necessary to achieve the low noise levels required in the car.

To address concerns regarding flashing lights, Arup first researched existing scenic lifts such as those in the CN tower, Toronto, for any evidence of prior similar events. Although none were found, engineers wanted to be certain and also looked to reports of fits occurring to helicopter pilots. These are much better documented and indicate certain frequencies of the rotor blades passing through the visual field of the pilots can cause fits. By comparing these frequencies with those of the mullions passing the moving lift car, we were able to demonstrate that the problem was very unlikely.

All those involved with the design and manufacture of the Leadenhall lifts are immensely proud of the final installation and have learnt greatly from working on the project. Arup is passionate about the role we play in shaping the built environment. It is projects such as The Leadenhall Building that allow the opportunity to challenge conventional thinking and establish new technical benchmarks within our industry.

www.arup.com

 

Jade Tilley
Jade Tilley
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