Built in 1920, the jetty at Portsmouth naval dockyard was the designated site for two Queen Elizabeth aircraft carriers currently under construction. The jetty had to be upgraded and strengthened to support the two 66,000 tonne vessels; this required the provision of suspended access to 5,000m2 of the underside of the concrete structure.
The requirement set by the client was to provide fully suspended access scaffold that could only be supported from the underside of the 600mm thick concrete deck. The scaffold was to be built as a light duty scaffold with a loading of 0.75kN/m2 with the ability to upgrade the scaffold as required and with minimal effort to a loading of 2.50kN/m2.
The working platform would be set approximately 2 metres below the underside of the jetty, putting the scaffold platform within the tidal zone twice per day.
The final restriction was that due to the amount of work required to the underside of the concrete deck, no horizontal tubes could be installed within 2 metres of the underside of the concrete deck.
Denholm Industrial Services (DIS) faced a series of unique challenges, which required serious consideration for a scaffold that would be in place for over a year.
Before addressing the method of suspension of the scaffold, consideration was given to the type of decking to be employed. Normal scaffold boards or steel staging would be required to be fixed in place. They would also be subject to additional loading from tide and wave action, as well as deterioration from marine algae, which would also cause unsuitable conditions underfoot after only a short period to those using the scaffold.
Denholm Industrial Services' solution was to install GRP open grate decking. This was lighter than conventional decking and was constructed with 22mm square openings to allow unrestricted flow of water during tidal conditions. The GRP also provided a non-slip surface throughout the period of occupation with no deterioration of the product. The GRP was secured with sacrificial ties so that in the event of a foreign object becoming trapped below the scaffold, only the decking would be displaced and the structural integrity of the scaffold frame would remain intact.
As the deck was exposed by the outgoing tide the GRP was simply repositioned to allow works to continue.
With the decking solution in place Denholm Industrial Services worked closely with one of our suppliers to enhance an existing scaffold fixing by providing a single point support system. The fixing bracket used for the first time on this project was tested to 40kN and, when bolted directly into the underside of the concrete jetty, provided a loading of 15kN per location, installed on a 2m2 grid throughout the 5000m2.
During the development of the fixing it was also demonstrated that this solution eliminated the requirement to lace tubes to the underside of the concrete deck from which the scaffold would be suspended.
Having satisfied the client that Denholm Industrial Services had achieved a viable set of solutions, the operations team - working with daily tides - employed installation techniques developed over years of marine experience to erect the scaffold.
This project brought together the skills, knowledge and experience within the Denholm Industrial Services organisation, working alongside our client's team of international engineers to provide a unique and safe solution.