Asticus building jacking – Westminster, London

The scope

Freyssinet was appointed to design and deliver the temporary works and to jack the structure, allowing a new floor to be inserted between the existing eighth and ninth levels. The extension was carefully planned to preserve the building’s original architectural character while creating additional office space through the introduction of an extra floor.

The planned extension works were to maintain the integrity of the original architectural features while providing an extra floor and additional office space.

The solution

The solution was proposed by Freyssinet as an alternative to the demolition of the roof and the building of new floors from the bottom up which has significantly reduced the environmental impact of the project.

According to the initial plan, the solution was also intended to limit the disconnection of the building’s air-conditioning system placed on the roof and the building itself (air-conditioning system could therefore only be disconnected for the duration of the jacking and reconnection of the pipes, 5-6 days).

Freyssinet developed the temporary works design and jacking sequence to enable the installation of the additional floor, collaborating with contractor Iconic on the innovative structural extension.

As preliminary operations, the last two floors (n.9-10) were laterally restrained by six radial restraints anchored on floor n.8 and then disconnected from the lift shafts. Two radial restraints anchored to the lift shafts were then added along the disconnected edge.

Building jacking

Jacking was carried out with 400t capacity primary jacks installed in the axis of all building columns. Steel clamps working in friction and prestressed against the column by PT bars were installed below and above the column portion to be cut and removed for to allow the installation of the primary jack. 150t capacity secondary jacks were installed between the two levels of clamps for unloading the column before cutting.

The lifting was carried out by a sequence of jack extensions and contractions intercalated with steel support insertions and bracing installation. Due to the nature of the building (reinforced concrete frame with solid slabs), relative vertical movements between the columns had to be maintained below the limit of 2mm and this was achieved by a combined use of a synchro-pump and a laser-based monitoring system following the progress of the lifting.

A handling system, composed of an electric hoist on a rail anchored on the level 9 soffit and by bespoke trolleys and rails allowed the team to move sections of tower legs from the floor to final position inside the jacking system without manual handling.

During the vertical movement, a system of radial jacks sliding against the lateral restraints ensured that all restraints remained engaged (even in storm conditions) to maximise the horizontal stiffness of the restraint system.

At the end of the jacking process, the whole vertical load was taken by the tower externa legs and secondary jacks only and the tower central leg and primary jacks were removed to provide room for building the new reinforced concrete column portion. The load was then gradually transferred from the tower to the new column by deflating the secondary jacks in stages.