Located in the heart of King's Cross and dating back to 1875-1895, the Lighthouse building is a local landmark. It was left derelict for many years until developers; UK Real Estate took the bold decision to redevelop the Grade II listed building that was on Historic England’s Buildings at Risk Register. The building now features an additional storey-and-a-half, conserved and repaired masonry façade and the restored timber lighthouse, ensuring its great character was retained and its long term viability secured.
A building, whose entire footprint sits directly over two underground tunnels
The Lighthouse building’s footprint sits entirely over two tunnels, one TfL District Circle and Metropolitan, one Network Rail Thames Link “the basement slab steps up to pass over the crown of the TfL tunnel, it is that close” comments Jackie Heath, Conservation and Structural Engineer at Ramboll. Coupled with roads surrounding the site on all sides, this made for a hugely complex and constrained project.
With the addition of the storey-and-a-half and new roof structure, it was crucial to avoid changes to the load passing down onto the two cut-and-cover masonry tunnels below. The strategy adopted was to remove heavy internal masonry, replacing it with a lighter weight structure, enabling the additional storeys to be added for the equivalent weight.
Preparation
The concept was checked with a preliminary model examining the effect on the tunnels to ensure the strategy would work. Further advanced engineering analysis was used to provide confidence to TfL, Network rail and the client that the strategy could be achieved.
Ramboll also carried out a sensitivity analysis of movement predictions using a range of soil stiffness to give confidence that the movements would be modest, generally less than a few millimetres. Further measures were taken including; movement monitoring, an emergency preparedness plan and sensible action trigger levels for early indication in case movement was greater than the agreed trigger levels.
The movement of the tunnels was monitored for nearly a year before the works began and then throughout the programme. Overall movement was within TfL and Network Rail allowances and within a fraction of that predicted.
Construction
In order to maintain the existing magnitude of load applied on the tunnels and the existing load paths, the demolition and construction sequence was designed carefully and then developed further by contractors; Balfour Beatty into a detailed sequence, working bay by bay to remove floors and then insert steelwork, construct floors and remove crosswalls.
Conservation, façade retention and restoration
The recognisable landmark of the timber lighthouse at the apex of the building has been completely restored. A timber survey of the complex timber structure, identified decayed timber, which was locally repaired with pieced-in new sections. The diagonal sheathing boarding was repaired or replaced and clad with pre-weathered zinc. The dome was similarly repaired with pre-weathered zinc, capped in lead and the crown topped with the original restored weather vane. The existing timber structure to the nose of the building continues through the top floor and has been treated as a standalone, period restoration, with lime plaster on laths.
Latitude Architects’ vision for the new curved roof references the large vaulted roofs of the nearby train stations. It is stepped to bring daylight into the plan, lowering at the front to maintain the setting of the original lighthouse and keeping it distinct and separate from the new roof.
The façade comprises Gaunt bricks with painted stucco window surrounds and sills, replaced by precast concrete surrounds in the new areas of façade and new double glazed timber windows with secondary glazing.
The original bressummer beams over the shop front, which support the façade walls were retained and strengthened where required.
The façade was monitored weekly throughout the works with prism targets and total station. The monitoring was an effective way of providing reassurance to the design team, contractor and rail authorities without overly conservative and restrictive precautionary temporary works.
Extensive anti-vibration measures
As well as the issues over load consideration, creating a good environment, suitable for its end use and future occupants was very important. Extensive anti-vibration measures were designed and implemented to overcome the issues of train vibration.
Inside the Lighthouse building, close proximity of the very shallow tunnels was continually evident by very regular noise and vibration of the underground trains. Considering future tenants and end user comfort, the new structure has been isolated from the tunnels. The principle adopted is that all retained building fabric continues to vibrate just as it did before, but the new steel frame, windows, floors and finishes are all isolated from the vibration.
This was achieved by installing elastomeric bearing pads under the column baseplates, between the façades and the lateral restraint fixings, and at all other points of contact. Great care was taken to isolate all fixings and finishes to achieve the level of isolation required. This included gutters, substation, dormer surrounds and pilasters.
The result has reduced the natural frequency to 10 Hz and is now within the BRE office guidance levels of 46-48 dBA.
A stunning result
The increase to five storeys, plus mezzanine roof space for plant and new roof has increased the lettable floor area. The redevelopment has resulted in the creation of interesting and varied spaces, each with their own individual character, making attractive commercial and retail spaces.
The flatiron building with its curved nose is a landmark that is instantly associated with King’s Cross, people know it and love it. The entire project team is incredibly proud to have resurrected a derelict listed building on Historic England's Buildings at Risk register, by conserving the most significant parts and enhancing the building with modern, sympathetic contemporary design. The Lighthouse building also achieved a BREEAM “Very Good” rating.
“One of my bigger passions is to restore old listed buildings, bringing them back to life to suit the present times, whilst at the same time maintaining the building's historic integrity. The Lighthouse has been a very complicated project, one that has taken nine years to reach fruition. I have had the help of an inspiring professional team as well as a great contractor. It is a project that I will forever be proud of” Nick Capstick-Dale, UK Real Estate.
