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Balfour’s balancing act on listed urban lighthouse

A super-sensitive rebuild over a tube line in central London required extremely careful additions and subtractions to its loading.

Project: Lighthouse Building, King’s Cross
Client: UK Real Estate
Region: London
Main contractor: Balfour Beatty
Structural engineer: Ramboll
Architect: Latitude Architects
Quantity surveyor: Potter Raper
Start date: January 2014
Completion date: May 2016

“It was full of pigeons and rats and was literally falling down,” says Latitude Architects director Andrew Gilbert of the Grade II-listed 19th century Lighthouse Building, which resides a stone’s throw from the splendour of the rejuvenated King’s Cross station.

The flat-iron building, which hosts the landmark feature that gives it its name, had found itself on Historic England’s At Risk register. Its basement sits on two train tunnels, meaning little weight can be added to the structure, ruling out major additions of sellable space and thus making the significant cost of refurbishment hard to recover.

Enter developer UK Real Estate, which is overseeing an intricate, lightweight scheme that will allow a storey-and-a-half to be added – with the half to be used as a plant room – as well as a mixture of historic restoration and modern finishes.

The former residential block between Pentonville Road and Gray’s Inn Road will become home to a ground-floor restaurant and four levels of office space, with impressive views and an outstanding range of transport options leaving no excuses for tardy employees.

Technically challenging site

“It’s a prime location for a development but a really difficult site, with roads all around and tunnels underneath it,” says Ramboll associate Jackie Heath. “The concept was keeping the historically significant parts – the lighthouse itself and the external masonry walls – and combining that with the contemporary roof.

“The design was based on not exceeding weight restrictions. It was a real balance between adding space and limiting weight”

Andrew Gilbert, Latitude Architects

“The strategy was to take out the heavy internal masonry cross-walls then put in a lightweight structure. The key thing was that for it to be commercially viable we had to add additional storeys.”

Mr Gilbert adds: “The design was based on not exceeding weight restrictions. It was a real balance between adding space and limiting weight.”

Once the designs were approved and the project got on to site, main contractor Balfour Beatty faced major challenges.

Balanced loadings

The biggest issue was that as well as not adding weight to the existing structure, the site team had to be ultra-careful not to remove too much weight from it. This could have led to the problem of heave, as the existing building was pressing down on the masonry cut-and-cover Circle line and Thameslink tunnels beneath.

The contractor devised a carefully planned construction sequence, splitting the building into seven ‘bays’ separated by the existing cross-walls. All the demolition and rebuilding work required in one bay had to complete before any could begin on the next.

This ensured the proportionate change in load was minimal – but led to an unusual construction programme where sections of upper-level floors were completed before adjacent sections of lower-level floors had even started.

Confined site

The team selected bay five – a central section of the building – to be cleared first, so it could house the project’s tower crane. The crane had to sit on a structure that transferred load to the basement walls, rather than straight down on the tunnels beneath.

“Given the confines, there was no location for the tower crane other than within the footprint of the building,” says Balfour Beatty senior quantity surveyor Darren Simpson. “Once we established ourselves on site, removed asbestos and made the structure safe for works to commence, we started the soft strip then floor removal in bay five.”

Installed flying shores and wall props between the cross-walls allowed them to be cut with a full-height slot so that new steel columns could be fed in. Steel beams were then put in to connect the columns.

“All this had to be co-ordinated to make sure the temporary works did not prevent installation of the new structure,” Mr Simpson says. “We had worked with a lot of the subcontractors before and held daily meetings to make sure everyone knew what was happening the next day.”

Metal trough decking installed for the floors featured steel reinforcement to reduce the weight of concrete needed. With the structural work of one bay complete, focus moved to the next. The process quickened both through repetition and as the building narrowed towards the lighthouse.

Central London logistics

As well as sitting on top of a tube and a train line, the Lighthouse Building is wedged between two major London highways. This made organising a construction site somewhat challenging.

“It’s a very confined site that poses a number of logistical challenges,” Mr Simpson explains.

With space in short supply, the developer was handily able to offer room for a site office in an adjacent property it owned on King’s Cross Bridge.

Additionally, TfL granted permission for an existing loading bay on Pentonville Road. “It’s a one-way street and the bay is wide enough for one vehicle, we can’t use articulated vehicles and we book deliveries 48 hours in advance,” Mr Simpson says. “We distribute materials from there using a tower crane located within the building.”

Nonetheless, the project still required lane closures to get the crane and its housing structure delivered to the site. “There have been complex logistics that have required us to keep residents and other stakeholders informed.”

Level monitoring at pre-determined points in the basement has recorded a maximum 5 mm, meaning that a planned emergency process of adding load to the building was proved redundant. “We had triggers: if two adjacent points moved beyond these levels for two weeks then we had to explain why to Network Rail or Transport for London,” Ms Heath says.

“Generally it moved within what we expected, which was fine. We had a year’s-worth of monitoring, and where it wasn’t, we understood [there would be] seasonal movements. It was always explainable and we never had to stop.”

Isolated frame

Balfour Beatty retained the façade by tying it back to the new concrete floor slabs using angle brackets fitted with bespoke elastomeric pads made by CDM UK. This kept the internal frame isolated from the vibrations of the trains on the existing frame.

It also used pads between the column base plates and the existing concrete foundations. Extensive tests at this stage ensured future occupants will benefit from the view and accessibility of King’s Cross, but not suffer from the relentless noise.

The pads – used in various parts of the structure – help isolate the existing frame from the new internal steel frame. This prevents loud vibration from the trains below reaching the office space, which on the day of CN’s visit was notably quiet compared with the site office, which is situated outside the footprint of the building. Other measures such as secondary glazed windows also keep out street noise.

With the façade tied to the new frame, site workers could remove the existing cross-walls by hand, before the floor slabs were completed. Temporary works could then be removed.

The final stage of the frame consisted of intricate steelwork to create a stepped roof, the design of which reflects the nearby train stations and the heritage of the building, while also hiding plant and keeping weight down.

“The roof was curved on two planes, which made it tricky,” Mr Simpson explains. “But it all went smoothly.”

Lighthouse restoration

Timber in the lighthouse itself had to be carefully restored by hand. The story goes that this was once used to advertise an oyster bar in Victorian times, but in the future it is more likely to be a stunning background to office events held on the fourth-floor balcony.

With Historic England involved in the design of works to the lighthouse, the team issued drawings to a Camden Council conservation officer, who kept an eye on the construction phase. This work saw zinc cladding added and the weathervane restored.

“It has been a one-off project and a learning curve. But the end result is one we can be really proud of”

Andrew Gilbert, Latitude Architects

Site workers demolished and rebuilt sections of the façade at 370 Gray’s Inn Road and 283 Pentonville Road, with the brick retained to reconstruct other areas of the building. Other areas of façade benefited from repairs and cleaning in the meantime.

Work also included repairs to existing Stucco window surrounds on the first, second and third floors, along with the addition of several new precast concrete Dormer window surrounds.

Operatives also installed zinc diamond-shaped tiles on the curved section of the roof and a zinc standing seam on the flat section. Secondary glazing as well as double-glazed sash windows on the first three floors created a cavity to give maximum soundproofing within the weight allowances of the project.

By early February contractors were painting staircases and adding final touches around the building. Completion is expected within the next few weeks.

“It has been a one-off project and a learning curve” Mr Gilbert says. “But the end result is one we can be really proud of.”

Critical load planning

Extreme care was taken to ensure the load of the Lighthouse Building didn’t cause problems for London’s critical transport infrastructure running beneath it either at construction stage or in final use.

“We initially had hand-drawn coloured-in drawings,” Ms Heath says. “Laser scan surveys were carried out of the outside of the building and of the tunnels. These were put together so we could position the building very accurately on top of the tunnels.

“We could then superimpose the design of the new structure over the existing façade.

“We modelled the load as it was; the intermittent load with partial demolition; then the final building load. This meant we could predict the movement and stress on the tunnel to present to Network Rail and TfL.

“This all meant we had gone the extra mile to ensure we knew what the loads were and what the expected movement was.”

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