A particularly challenging aspect of the design for the suspended scaffold at St. Paul’s Cathedral, was the design of the mechanism that allowed the scaffolding structure to rotate. Amongst the many challenges to overcome, the three most significant were;
- Rotating the scaffold around an oval track
- The inability to establish an anchor pulling point for rotation
- Negotiating level changes in the step where the tracking system was placed
The support frames for the scaffold were designed to incorporate bespoke steerable bogies with solid steel wheels. The bogies travelled over tracks placed on the lower step of the whispering gallery. The tracking was designed as a series of 30mm thick steel plates. When abutted together this formed a faceted running surface to the full circumference of the whispering gallery step.
As it was not prohibited to make any intrusive attachments into the face of the cathedral, the tracking system was placed on fast setting, low shrink grout, placed on heavy-duty polythene. This enabled the removal of the steel plates and grout without causing damage to the fabric of the cathedral. The large changes in level around the stone step meant it was not possible to lay a single level track around the full circumference, as the thickness of grout required was too great. To overcome this, steps were incorporated into the tracking.
The ideal running line for the bogies was for the wheels to travel along in the centre of the steel plates. To achieve this a steering mechanism was incorporated in the front and back of each bogie. In all, there were seven bogie frames supporting the scaffold and each bogie frame had two steering assemblies. As the bogie frames moved inwards and outwards whilst travelling around the circumference, the supporting legs needed to be adjusted. This was achieved using a lockable slide mechanism, positioned at the top of the supporting legs connecting to the main body of scaffold.
The rotation of the scaffold was achieved by manually pulling the support frames and bogies around the tracking plates. The pulling system comprised of a ratchet chain pull. This was secured to the rear of the track with a steel wire strop and to the bogie chassis with a fibre strop. Motorised solutions were reviewed but found to be too costly, unreliable and unpredictable.
The tracking plates were used to provide a fixed anchor point for the movement of the scaffold, as it was not possible to use any part of the cathedral structure. Design development resulted in using one of the track plates positioned approximately 2.5m ahead of the travel bogie as the anchor. This provided a shallow enough angle to ensure an unacceptable uplift was avoided and the bogie was not pulled out of alignment. It also reduced the need to reset the pulling system.
A test arrangement was carried out where the access route onto the whispering gallery was located. Determining the actual pulling force to move the scaffold was difficult due to the many variables. However, using a rolling friction of 0.1 and a calculated weight of the portion of the scaffold supported on the bogies at 30 tonnes, the pulling force when rolling was 3 tonnes. Just using the rolling friction forces alone would not allow for the start-up forces. Using a typical start-up friction value of 0.25 resulted in a starting force of 7.5 tonnes.
There were 5 pulling locations beneath the main body of the scaffold (95% of the load), so a pull test of 2.0 tonnes was trialled on the tracking plates. In total, there were seven pulling stations, five beneath the scaffold quadrant and one each beneath the two legs of the scaffold cruciform. The anticipated maximum pulling force at each of the 5 pulling stations was 1.50 tonnes at start-up.
The rotation itself was carried out in 3 phases. The scaffold was first prepared by withdrawing any protruding scaffolding structures and platforms. It was then rotated through 90ᴼ and reinstated for the next phase of work.
The team of scaffolders carrying out the move became known as the ‘rowing team’. One operative was positioned at each of the seven pulling stations. Another monitored the rolling table at the junction between the cone scaffold and the dome scaffold. Another was on standby to assist at any location around the perimeter of the whispering gallery and me, the cox, gave the ‘pull’ instruction to synchronise the pull movement. By this means the whole scaffold assembly was manually moved through two full rotations of the dome. A total distance of 210 metres.
Steering the scaffold around the oval track was achieved by adjusting the track rod turnbuckles. These were positioned at the front and rear of each bogie assembly. Adjustments were made whilst the scaffold was in motion to guide the frames around the track. As the track radius changed and the wheels were no longer in the middle third of the track, the bogie assembly was realigned by adjusting the position of the support frame on the arm of the scaffold support structure.
On a personal note, the preparation of the scaffold for the initial pull and the first moments of the instruction to pull was always carried out with trepidation. The scaffold bedded down with inbuilt forces when it was locked in position between each rotation. However, once the scaffold started to move, everyone started to relax.