CLIFTON CATHEDRAL, BRISTOL
In the summer of 1965 the Clifton Diocesan Trust approached. and then commissioned, the Percy Thomas Partnership to design a new Cathedral Church on a 4 acres site in Clifton. The subsequent decision to construct the boldly modern building using reinforced concrete was the result of considering a number of factors.
Firstly there was the need to consider the external extremes of hot and cold spells to minimise energy costs. It was appreciated that, if the building's fabric mass were warmed up sufficiently and fully insulate on the outside to retain heat, the fabric would then act as a thermal storage unit for the building. It was recognised therefore as being advantageous to create a fabric of dense mass.
Secondly, being sited by a busy road, the issue of sound control was fundamental. To prevent low frequency sound from entering the building it was again recognised that the fabric should be relatively dense in mass.
Thirdly, as the Structural Engineer was being asked to predict a 300 year life span for the building, it was agreed that the building should be a compressive structure and suggested concrete shells be used for the roof.
Reinforced concrete was chosen therefore for the building's structure because of its inherent thermal mass, acoustic insulation, durability and structural properties.
In recalling the construction phase of the project and the dedicated commitment of John Laing Construction Limited's craftsmen, Ron Weeks of the Percy Thomas Partnership wrote "no lecture was of more value than that given to the thirty or forty men on site one rainy afternoon in 1970. It explained with colour slides the early design drawings crystallising into the final design proposals and showing how the design detail had been developed by using a large balsa wood model. The balsa wood model was large enough for a person to put his head through the floor. It had been used to create the correct amount of lighting that was required within the building and it was how the Lord Bishop came to enter his Cathedral for the first time before any concrete was ever poured on site! This model was now on site and stayed on site for the duration of the contract. It had been the architect's design tool; it was now the contractor's reference point and aide memoire. It was possible in the talk to identify the steel fixer, the formwork designer, the carpenter and the concrete placer with their respective responsibilities in producing a perfectly cast concrete wall. Concrete was poured at three o'clock every afternoon and the formwork removed at eleven o'clock the following morning."
For the timber formwork, against which the insitu white concrete was cast, a Russian Redwood from the Kara Sea area was selected because of its consistent colour and figure variations. Particular attention was also given to the positioning of the expressed bolt ties that were typically located around the building's dimension module of one foot six inches. The fourteen Stations of the Cross set in the walls were designed and made by William Mitchell using “Faircrete". This material was a concrete and fibreglass mix and could be moulded by hand and with tools, but required the artist to work rapidly as it only retained its plasticity for approximately one hour after placing.
The insitu concrete structure externally is mostly clad in precast concrete panels with a coarse textured exposed aggregate finish. The Corrennie granite aggregate was selected to harmonize with the local Bath and Brandon stones of the surrounding buildings. The panels were cast face up, being a technique that allows the density of the aggregate to be maximised, the area of cement matrix apparent on the panel's exposed surfaces to be minimised thereby facilitating the consistency of their appearance and enhancing their weathering performance. The precast cladding panels were manufactured off site by The Marble Mosaic Company Limited and delivered on a just-in-time basis to suit the site's progress, thereby avoiding the need for any double handling or temporary storage of materials on site.
Recognising the achievements of Percy Thomas Partnership, Felix J Samuely and Partners as Consulting Engineers, John Laing Construction Limited as the main contractor and The Marble Mosaic Company Limited as the producer of the precast cladding panels, the Cathedral was judged to be the winner of the 1974 Concrete Society Awards for “outstanding merit in the use of concrete" and also received a Cembureau award for excellence.