The design, supply and installation of the new TFL underground entrances to the Main Entrance and Cowper Street Entrances to the Old Street underground station. The Main entrance consists of 98 tonnes of precast concrete, self-supported and cantilevered of a 20 tonne precast column. Each "arm" of the structure was unique in geometry with an angular twist from the trunk through to the corner. Bespoke cast in brackets were cast into the ends of each arm, with a second part cast into the perimeter beams which aids the support of the structure. Each corner bracket is cloaked by bespoke 4 part corner façade panels. The connection between each arm and the trunk was achieved by using a series of 10 couplers cast into the receiving face of the trunk, each coupler takes 6no 32mm and 4no 24mm dowel bars varying in length in each location which is received by a corresponding grout tube within the arm, and when set in position, each tube was filled with grout.
The chosen mix was a standard BLS 185, Portland stone coloured architectural mix with a combination of medium acid etch to the main structure.
The precast column design was undertaken alongside WSP and Morgan Sindall, with the Main Entrance being instructed based upon a concept, where the joint design team worked to prove the architects intent was buildable, and then how is can be turned into a workable engineered precast solution.
The installation was phased over a series of weekend possessions and with the aid of a bespoke support platform on each corner of the structure with a jack capable of minor adjustments to aid installation. Once the grouted connections had cured, the temporary structure was disassembled, allowing the corner façade panels to be installed.
