It started life in the 1940s as accommodation for brave servicemen returning from the Second World War. But when Russell Square House in London’s Bloomsbury was remodelled in a £9 million speculative development to combine commercial and retail space, it revealed a hidden history for structural engineers Symmetrys to confront.
In September 2013, project design commenced on an ambitious plan to refurbish the nine-storey, 60,000 sq. ft. building on the northeast corner of the garden square. It would see a Category A refitting of the entire building and the creation of new lettable space at lower ground level, by partly enclosing an existing courtyard and adding a courtyard garden on the ground floor. In August 2014, work was ready to begin.
To give clear access into the newly created lower ground floor areas beneath the courtyard, the rear walls at lower ground level and some primary steel columns around the courtyard had to be removed. The reception was also to be enlarged by the removal of the walls on all sides. London-based structural engineering consultancy Symmetrys was charged by the client, EPIC, and architects Stiff and Trevillion, with the task. At the time, no one expected to unearth an even more challenging structural find.
As was common for its time, the building’s construction featured hollow clay pot floor slabs, supported on a concrete-cased steel frame. This comprised rolled sections with riveted flange plates and supported the floor slabs and masonry external walls. The hollow pot slab form of construction is notoriously fragile and difficult to repair and some of the upper floor slabs showed signs of distress. While the brickwork elevations did not support the floor slabs, they did provide sway stability to the rear of the building.
Symmetrys, also responsible for any repairs needed to the structural fabric of the upper floors, had to find a solution for removing the walls and primary columns at lower ground level while maintaining their vital support for the overlying nine, newly refurbished or occupied floors and minimising any significant distress to the fragile floor slabs above.
Having assessed options for the permanent works, Symmetrys recommended employing a series of steel portal frames, because the steelwork could be installed comparatively quickly and within the access restrictions of the site. The depth of the frames’ top booms would be kept to a minimum, to allow maximum headroom into the new extension. The intention was for these new frames to fit around the existing structure and foundations.
“Typically, the existing floor beams around the courtyard would have been retained to minimise any disturbance to the existing structure, with the portal frames installed directly beneath them,” Russell Thomas of Symmetrys recalls. “However, retaining the existing beam in the South West elevation was not possible where the existing column had to be removed. So the existing, overlying column and the floors it supported were supported solely on the new portal frame.”
This had an impact for the temporary works, which Symmetrys had also been trusted to undertake given its growing familiarity with the building. Structural movement of the overlying floors needed to be minimised, as this would cause cracking in the fragile, hollow pot slabs and new finishes – and jacking would be far from straightforward.
“Normally when the props are released, the weight of the supported existing structure causes a new beam to deflect,” Thomas adds. “But the existing structure had previously been supported by a wall that did not deflect, so new movement could cause cracking.”
Symmetrys considered a number of options to minimise the risk before recommending the introduction of a series of specialist jacks between the top of the new beam and the underside of the overlying existing wall. These were to be pumped to apply load to the beam and wall above, such that the beam deflected as it took the load, while the wall itself would stay in the same place. It was then the building’s past truly revealed itself.
As the existing structure became more exposed, it became apparent that Russell Square House had held back some secrets from the initial, exploratory works. Instead of the foreseen single column on its own footing, the main column requiring removal was part of a box frame, providing vital counter-balance to the party wall column’s foundation.
“The discovery of such a complex structural system in a 1940s building came as a surprise to us all and demanded an urgent, fundamental rethink,” Thomas says. “The counter-balance weight needed to be maintained to avoid settlement of the party wall column, even though the column normally providing that weight was being removed.”
The permanent works had to be redesigned to meet this new condition, yet within the restrictions imposed by the capacity of the foundations already installed and the space available around the existing foundations. For the temporary works, after extensive design reviews, Symmetrys advised the addition of a new grid of ground beams, against which the existing foundation could be jacked to reproduce the counter-balance load at all times during the removal of the column. At ground floor, the jacking would be upwards against the existing structure whilst, simultaneously, below ground the jacking would be downwards onto the foundations.
This introduced an additional frame of temporary works with a third set of jacks, each requiring a complex sequence of gradually increasing jacking loads to maintain the correct counter-balance force. Symmetrys had calculated the anticipated loads applied by the remaining, existing building and those required to be applied to the foundations via the jacks for each stage of the operation. The anticipated deflections of temporary and permanent works for each stage were also calculated – and with incredible accuracy.
“We monitored the movement of the existing and proposed structures and compared it to the expected value, so any unexpected behaviour of the building could be addressed before damage could occur,” Thomas explains. “It was realistic to expect some repair work would be needed, especially to the slabs, but the installation went exactly to plan.”
Symmetrys predicted the top beam would deflect 10mm at the key stage of releasing the propping; its actual deflection was 9.8mm – and the party wall column showed no signs of movement whatsoever. With everything behaving as expected, no cracking at all occurred in either the new finishes or the fragile, pot slabs of the overlying nine storeys.
What began as a complex remodelling of an intricate and fragile existing structure had been discovered onsite to be a great deal more complicated and sensitive, with twice the number of elements needing to be supported. Yet the ingenuity of the design and diligence of the installation had minimised the need for costly, additional temporary or permanent works or foundations, the reluctantly expected remedial works to nine floors of finishes and slabs, and the likely temporary relocation of tenants from occupied space.
“Ultimately, it meant we could help the main contractor avoid any significant programme delay – and help the client avert the financial consequence of postponing the newly refurbished premises from reaching the lettings market,” Thomas concludes.