Dynamic loads exerted by pealing bells have given engineers working on a new bell tower in Basildon, Essex, an unusual set of challenges. The free-standing 30m high glass-clad tower will support a peal of eight bells - six medieval, two new.
'The bells are arranged radially, and the worst case is when the largest - the 1t tenor bell - and the smaller bell opposite are moving in the same direction at the same time,' says Buro Happold's facade engineering group director Stephen Tanno.
'If these forces cause too much sideways deflection of the tower at the wrong moment, there is a serious risk of the bells actually swinging right over the top and staying there - which would rather put the ringers off their stroke.'
Thus the maximum permitted sway under the worst combination of wind and bell loads is 2mm. To achieve this degree of stiffness with more than 2.6t of swinging bells mounted nearly 17m above ground level on a structure only 6m in diameter at its base required something out of the ordinary in terms of structural form.
'The octagonal layout was chosen by architect Douglas Galloway long ago to reflect the number of bells,' Tanno says. 'But at scheme design stage it was obvious that a conventional frame would never be stiff enough. So instead of single columns we went for a triple tube design that is between two and three times stiffer than a single, larger tube.'
The main vertical elements are now made up of three 140mm diameter steel tubes arranged in a triangular plan form. Each 'column' sits on a pair of 15m deep raking minipiles. Above the bell level the two outer tubes curve sideways to form elegant Gothic dormer arches, while the inner tube sweeps upwards and inwards to support a copper-clad spire topped by a stainless steel cross.
Essential stiffening is supplied first by radial and perimeter bracing at each floor level. At first and second floor levels these diaphragms are connected only to the inner vertical tubes, with floor loads transferred to the outer tubes by batten plates at 3m intervals. Forces from the belfry floor are taken directly by all three tubes.
Cross-bracing on all faces of the octagon by steel rods inside the glazing adds more stiffness. Substantial steel fins on the inner tubes also help, although their primary purpose is to locate and carry the weatherproofing for the 15mm toughened glass panes chosen for the glazing.
Even this will deflect 25mm under maximum wind load, Tanno says. He adds: 'The other problem is that bell ringing is a very physical activity, the tower is open to the south so there'll be significant solar gain, but it isn't used often enough to justify air conditioning.
'So in summer we'll rely on the stack effect to draw air in through low level vents and exhaust it through vents in the dormers, and in winter there'll only be trace heating, to keep down condensation.'
Controlling sound within the tower is almost as important as damping down temperature variations. Just the right volume has to be transmitted down the tower to the bell ringers below. 'Our acoustic consultant David Fleming worked closely with the Whitechapel Bell Foundry, which is producing the two new bells,' Tanno says.
'They calculated the area of the openings in the floors needed to achieve the right sound levels on the bell ringing floor. The sounding floor between the bells and the ringers also acts as a sound attenuator.'
Central access openings in the floors are normally closed off when ringing takes place. The lower two floors are of relatively straightforward timber construction, but the complex design of the belfry floor is intended to achieve more than just sound absorption.
A 150mm thick concrete 'doughnut' slab carries the eight legged steel bell support structure. This doughnut sits on 25mm of cork, which in turn is supported by another 150mm thick concrete slab. There is no through connection between the two concrete elements, a feature which obviously helps to reduce sound transmission.
'It also absorbs some of the dynamic forces from the bells,' Tanno adds. 'These can be vertical as well as horizontal, so the cork filling in the concrete sandwich will have an important cushioning effect.'
Architectural detailing was by Fletcher Priest, acting as subconsultant to Buro Happold. Main contractor Mowlem poured the ground floor slab in early October.
Erection of the Tubeworkers-fabricated frame was well advanced before Christmas. Temporary setting-out steelwork is needed to locate the frame components precisely during erection - 'glazing bosses have to be positioned to within 2mm', Tanno points out. A jig was used to pre-assemble the spire framework before it was lifted into position on top of the vertical inner tubes the week before Christmas.
Alpha Plana is due to start installing the 3m by 1.2m panes of glass next week. Mechanical and electrical services, including installation of a complex fibre optic lighting system, are the responsibility of EIC.
The tower sits outside the west window of St Martin's, Basildon's parish church. When the first bells ring out, some time in March, they will signal the end of an odyssey that began back in the late 1970s.
Canon Lionel Webber of St Martin's has been campaigning and fund raising for the bell tower since then, but it took a Millennium Commission grant of £329,000 towards the £670,000 cost to bring the dream to fruition.
Buro Happold has also made a major contribution to the project, donating much of the design time involved.