Every once in a while the opportunity to work on an iconic building comes along. Such an opportunity was the design of an internal suspended access scaffold, used for the cleaning and restoration of the dome and cone inside of St Paul’s cathedral.
Most projects present a variety of challenges but the design and construction of this access system presented a mass of challenges not normally encountered in one single project. The initial list of included:
- To avoid all intrusive attachments to the fabric of the building as it is Grade 1 listed
- To avoid any scaffold supports that would inhibit public access
- To only cover one quadrant of the dome at any one time to minimise impact on the paying public when viewing the dome
- To enable the scaffold to be relocated from one quadrant to another without dismantling or re-erection
- To enable the scaffold to provide 300mm unobstructed clearance to the whole inner face by enabling the scaffold to be free standing over a height of 35m
- To enable erection to take place during night time
- To avoid any materials storage within the cathedral
The final design of the suspended scaffold was a 98 degree segment which allowed overlap through the rotations. The outer face matched the curvature of the dome and cone structure, which provided an inherently stable frame that would provide the 300mm clearance zone. It was agreed that the suspended scaffold would be incrementally rotated through 90 degrees resulting in the full inner surface of the dome being accessed in 4 rotations. Central support was provided from the lantern at the highest point of the cathedral and peripheral support was provided from the whispering gallery steps.
Further practicality and constructability factors were accommodated into the design due to abnormalities of the cathedral and with working in a high profile environment. This included material logistics, providing all structural elements piece small, the limited capacity for mechanical handling, ensuring safety during erection and dismantling, the ovality of the dome and the uneven whispering gallery steps.
The working design solution
Working with the limitations for support it was decided to suspend the scaffold like a pendulum using a central wire suspended from the lantern. To provide lateral stability for the pendulum and support to the outer edge of the scaffold structure, a series of posts were placed around the perimeter of the whispering gallery steps.
Discussions with the client highlighted some concerns with the failure of the central suspension wire. As assurance the diameter of the wire was increased, however this created issues with installation as it lacked any flexibility. The design resulted in a more flexible 3 wire system which enabled the system to be easily brought into the cathedral and raised into position.
The suspension support system was raised as small items through an 800mm square opening and bolted together to form a cruciform. The arms were supported on hardwood packing positioned on the stone sills in the side of the lantern structure. In the centre of the cruciform a large diameter bearing was installed from which the suspension wires were supported. This bearing allowed free rotation of the suspended scaffold without generating a twist in the suspension wires.
The bottom of the suspended scaffold was erected at cathedral floor level with the extremities of the frame temporarily removed to enable the scaffold structure to clear any protuberances. A 5 tonne SWL drum winch was placed centrally beneath the dome and was used to raise the base level of the suspended scaffold up to the whispering gallery level. The smaller scaffolding sections and frames used to construct the dome and cone access scaffolds were winched into position using a 100kg WL Turfor Winch.
Access to allow the scaffold materials into the cathedral was provided through the north transept doors and a cordoned off temporary storage and erection area was created in the centre of the cathedral, directly beneath the dome.
Suspended scaffold access levels
A central structural column was used to provide lateral stability at levels 7 and 14 and was constructed in aluminium modular lattice beams. A steel cube frame was positioned at the base of the column in the centre of the cruciform.
Access to the inner face of the cone, between levels 15 to 20, was constructed to the full 360 degrees. As the dome scaffold rotated through the series of 90 degree rotations, the cone scaffold remained static. This was achieved by creating a 2.5m diameter turntable on top of the central column at level 16, the second level in the cone. The bottom level of the cone access was suspended downwards with the upper access levels constructed in reducing diameters.
The quadrant, where access to level 1 to 14 was provided, was infilled with spokes that radiated outwards to the perimeter. The basic structure of the cruciform and working platforms were also constructed in aluminium modular lattice sections and covered with corrugated steel decking overlain with a plywood wearing surface.
The scaffold access levels were constructed using aluminium tube and standard fittings with fire treated scaffold boards. Access to the surface of the dome was achieved using cantilever transom units that were withdrawn as required, either for working past a level in the scaffold or for the rotations.
The outer edge of the cruciform was fitted with a thrust roller to track along the face of the dome to both guide the scaffold and provide lateral support during the scaffold rotation. The vertical wheel loads were carried on a plain steel plate track placed on a levelling grout which was separated from the steps within heavy duty plastic sheeting as it was not acceptable to fix to the fabric of the cathedral. The whole of the support frames was encased in plywood to prevent access onto the scaffold by members of the public.
During the life of the suspended scaffold it was rotated through two full rotations, 720 degrees, and a further section of suspended scaffold was erected onto the base of the quadrant to enable one particular mosaic to be fully refurbished. Upon completion of the restoration work the scaffold system was dismantled by the reverse process of the construction. An added challenge at this stage was to avoid any accidental contact with any part of the refurbished surfaces.