12 May 2011
The Story
February has seen the completion of the scaffolding design work by 48.3 for a temporary roof over the Zone 2 Atrium at London’s King’s Cross Station. The scaffold design started in late September 2010 and was ongoing for just over four months until a final scheme was agreed and signed-off by Network Rail. The erection of the roof has been successfully completed by PHD Modular Access for Vinci Construction.
Peter Doody, Senior Engineering manager (CRE) for Vinci Construction described the task at hand, “A temporary roof was required to carry out timber repairs and strengthening works to the Grade I heritage atrium roof to Zone 2 of the Western Range Building, King's Cross Station. This was not a simple task as the structure was situated between two train sheds; the supporting walls were not in a very good condition, it was an irregular shape and had to be removable for cranage.”
“Working closely with 48.3 Scaffold Design Engineers, we were able to come up with an innovative design which was CAT III checked by JDL Consultants(covering the scaffold structure) and Waterman Consultants London Office (covering the scaffold and the existing structure) before being approved by Network Rail”.
The scaffold design posed two distinct design challenges. Firstly, the plan area of the existing roof was not rectangular - it is 18.5m wide at the northern end and 25.0m wide at the southern end. Secondly, the parapet walls upon which the whole structure is based are not capable of carrying lateral loading.
The temporary roof structure is Layher Lightweight Cassette system, which formed the solution to the first challenge. The increasing roof span north to south meant that the eave level had to be split into three separate heights. The supporting scaffold, which is built from tube & fittings, uses a horizontal spine beam with the supporting scaffold punched off to the necessary heights. The roof beams all have an individual span east to west, increasing north to south.
The condition of the parapet walls meant that the roof had to impose little, or no, horizontal reaction at its eaves. This meant that one roof support had to be a roller and the obvious choice for this was the eastern (straight) eave. The Layher rolling-roof castor and tri-beam formed the roller support needed with the transverse rolling lock remaining unfixed. A tie bar was then fixed below the roof to form a rigid frame and carry tensile loads under snow loading conditions and compression loads under wind loading conditions. The result was a near zero horizontal load reaction at the roof eaves – perfect!
The second aspect of the ‘lateral loading’ problem was trying to dissipate the horizontal wind loads on the supporting scaffold; the western face posed the greatest challenge. This face is 5.50m high at the northern end and 3.50m high at the southern end – but still 35m off the ground! A series of aluminium beams laid flat on the transoms, plan and ledger bracing formed a rigid diaphragm the full length of the scaffold. Lateral wind loads are transposed along the scaffold and into the north and south gables. Bracing in the gable transfers this load into anchors into the parapet walls: loading the walls along their length.
The safe erection of the scaffold was managed by PHD Contracts Manager Gary Greene: “The roof itself was fine to build once the supporting scaffold was set-out. Our only difficulty was finding the right harness for the scaffolders. The drawings were a great help, they were as clear as a bell, without them I don’t think we could have completed the job!”
For anyone travelling through King’s Cross or St. Pancras stations, the roof can be seen by standing at the eastern entrance of St. Pancras and looking northeast towards the King’s Cross train sheds.
