Ringing the changes
A new 27m-high bell tower in Basildon, Essex, has triple steel columns to resist the dynamic loads of pealing bells
PHOTOGRAPHS BY ROBERT GRESHOFF
Although Basildon is a New Town, built just after the Second World War, its parish church has a 'peal' of six historic bells, including the only Medieval bell to be cast by a woman bellfounder, Joanna Hille, in 1441. The church, at one side of Basildon's public square, facing the civic centre and theatre, was built in 1962 by architect Trina Cotton, but the bell tower had to wait for Millennium funding and is only just complete.
Designed by architect Douglas Galloway and engineer and lead consultant Buro Happold, with Fletcher Priest assisting in the realisation of the construction, the bell tower stands next to the west window of the church in a square at the west end of the main square. It is octagonal in plan, 27m high and 5.75m in diameter, with a copper-clad spire and frameless glass walls which allow the bell ringers and bells to be seen in action.
Two new bells, making a full peal of eight bells, have been cast by the Whitechapel Bell Foundry. The octagonal shape of the tower reflects this. The bells, mounted on steel cradles, face each other along the sides of the octagon on the top floor of the tower, the bell chamber. The floor below this is a sounding chamber. The ringing floor for the eight bellringers is on the first floor and is reached by a circular staircase from a lobby and exhibition space on the ground floor.
Designing a slender tower with around 2.6 tonnes of bells swinging 17 metres above its base posed a unique problem for Buro Happold. 'The worst case is when the tenor bell - the largest bell weighing a tonne - and the smaller bell opposite it are moving in the same direction at the same time,' explained Buro Happold's group director Stephen Tanno. To the dynamic forces generated by bell ringing, shown in the diagram opposite, had to be added the worst-case wind load; the deflection produced had to be controlled to 1-2mm at bell level to enable the ringers to control the moment of impact of the clapper on the bell mouth. The structure was analysed using etabs, a structural- analysis package for tall buildings which gives full dynamic modelling. It was clear that a conventional frame could not give the stiffness required. The solution was to use, instead of a single column at each of the eight corners of the tower, a triple column, consisting of three 139.7mm-diameter steel tubes tied together at floor levels by 20mm-thick Y-shaped steel brackets. One column sits within the frameless glass panels and is connected to a tubular ring beam. Tubular beams radiate from this to an inner ring beam to support floors. The steel brackets stiffen the column structure and hide the glass corner joints. Each tripartite column sits on a pair of 12m deep raked mini-piles.
The concept of placing bells within a lightweight construction, compared to a traditional massive stone tower, is unusual; it allows the building to be finely tuned, like a musical instrument, to control sound. The correct volume of sound from the bells is transmitted to the bellringers below by using the sounding chamber between as a sound attenuator. Access openings between each of the floors are normally closed off when ringing takes place. Sound transmission from the bell chamber is reduced by its construction; a 150.2mm thick concrete 'doughnut' slab, which supports the bell trestles, is separated by 25mm of cork insulation from the main 150mm concrete floor slab. The construction also absorbs some of the dynamic forces of the swinging bells.
Bell ringing is hard work, and the ringers generate their own warmth as they work. This, plus the fact that the tower is not used as a habitable space, meant that only trace heating was provided to temper the space in very cold weather and to limit condensation. Stack-effect natural ventilation can be used in the summer by opening vents at high and low levels.
At night the copper spire of the bell tower is washed with metal-halide lamps to create the effect of moonlight. The bells and the interior are designed to 'glow', as if lit by candlelight, by means of a fibre-optic lighting system. The optical fibres are taken up through the steel columns, where they are concealed and
LEAD CONSULTANT, STRUCTURAL & ENVIRONMENTAL ENGINEER AND QUANTITY SURVEYOR:
Buro Happold (Michael Dickson, Phil Tanner, Stephen Tanno)
Fletcher Priest (Shaun Earle)