Quadrant Arcade, Regent Street, London
Quadrant Arcade, a previously underused retail arcade on Regent Street, has been opened-up into a vibrant arcade that links the famous central London retail street with the bustling regenerated Soho. Working alongside Contractor Forcia, Civic Engineers devised a scheme for the complex and challenging temporary works required to remove a significant central column carrying the Grade II listed front facade and eight storeys of live office floors. To achieve this, 200 tonnes of load was required to be transferred into a new 10-tonne plated box girder. The result being a new much larger welcoming entrance to a modern arcade.
The key to the success of the entire project, with its listed building status, complex structural interventions and 8 storeys of live office above, was to understand the exact loadings in the column that was to be removed. The existing building loads needed to be jacked into the new permanent structure to deflect and rotate the box girder transfer beam and load/settle the new piled foundations, prior to fully removing the existing column.
Prior to the Client and the design team submitting the proposals for planning permission and listed building consent, Civic Engineers were appointed alongside contractor Forcia to determine the feasibility of the proposals to remove the heavily loaded column. Working alongside permanent works Engineer Watermans, a thorough and detailed forensic analysis of the building was undertaken to establish the loads that would be transferred. This research, along with desktop-based studies of available structural information, enabled a detailed load takedown analysis to be carried out to establish the anticipated load needed to be transferred from removal of the column.
Weldability and chemical analysis testing were carried out on the existing steel frame to establish its suitability to be adapted for the connection of both temporary and new permanent elements. The outcome of this research was favourable, enabling us to design local strengthening and connections to the existing steel frame utilising site welding.
The building loads would ultimately be transferred into the permanent steel frame at high level using flat jacks. Strain gauges and movement monitoring of the enabling works frame showed when the loads were fully and safely transferred, at which point the column could be removed. The permanent steel frame was developed in line with the temporary works solution to create an efficient and effective structural solution that could be practically delivered on site. The load transfer process was key to the entire project.
Typically, jacking schemes are based on jacking a proportion of the theoretical calculated load in the building. Although detailed load take-down calculations were carried out to determine the theoretical load in the column to be removed, it was decided that a more sophisticated temporary works solution was needed to:
- precisely establish the actual loading in the column; and
- accurately monitor movement during the jacking operations.
A traditional solution to the problem would have involved propping the existing structure at 2nd floor, removing the existing column and providing the new supporting structure in the same location. This would have required significant damage to the façade from needles and would have required significant works within the tenanted office level. An interim stage jacking frame was designed, connected close to the base of the column at street level, to enable the column to be cut and the actual loads in the column measured.
The works were carried out directly adjacent to Regent Street and beneath eight storeys of live commercial office space. This two-staged process increased the overall safety during the execution of the load transfers and assisted with minimising the risk of damaging the listed building through ‘under’ or ‘over’ jacking.
The whole scheme and sequence were designed to minimise and simplify the temporary works. By retaining the existing column until the loads were transferred, no redundant transfer structures were required, bar the small steel temporary frame at the column base. Also, the permanent steel frame was fabricated in segments so it could be jacked sequentially into position. This approach also minimised health and safety risks.
The solution devised involved significantly strengthening of the connections between the existing columns and beams, with the new permanent support being put in position to either side of the existing column prior to any load transfer.
The shape of the beam and the relationship to its supports meant that the centre of gravity of the box girder was asymmetrical. We developed a balanced lifting plan which incorporated two additional columns to allow for incremental guided lift. Hydraulic jacks were utilised at basement level to lift augmented columns, which in turn raised the box girder from ground level to its permanent position beneath the existing arcade ceiling.