The structural safety and acceptability of bell towers.

An exciting element of the Millennium celebrations in the United Kingdom is the reinstatement or renewal of peals of church bells in towers throughout the country to welcome in the year 2000. A sum of pound 3M is allocated towards this work. Many towers will need to be assessed structurally, to ensure their stability under the large dynamic forces arising from the full circle swing of heavy bells. To date the choice of critical stress or deflection parameter, and of a safe limiting value is unresolved. The majority of traditional bell towers are constructed from sandwich form masonry walls, with indeterminate stiffness and mass properties, resting on spread footing foundations of unknown size and depth, and on ill-defined soils. A survey of 19 active bell towers throughout the North of England provides a database of towers each of which is in satisfactory condition with respect to whole-circle bell ringing. Of particular interest is the tower of St Matthew's, Newcastle, in which the bells were initially hung, and rung, at the original designed height near to the top of the tower, however, the tower swayed so fiercely that it was designated unsafe in that condition, and the bells were rehung lower down the tower, where they are now rung without ill effects. The 19 towers were assessed structurally, using parameters of varying complexity. First, the towers were ranked based simply on tenor bell mass and tower proportions. secondly, the maximum ratio of observed sway displacement to overall tower height was calculated for each tower. Next the peak accelerations in the ringing chambers were compared. Finally, St Matthew's tower was analysed by finite element, with various stress parameters being calculated far the two cases of the tenor bell in its original position and in its present position lower down the tower. The conclusion from the study was that the preferred parameter for evaluation of acceptability of a bell tower is the ratio of maximum sway/tower height. A limiting acceptable level of 30 x 10(-6) is proposed for square topped towers. A tower topped with a spire may be competent to tolerate a slightly higher value of this ratio, because of its greater flexibility. The peak acceleration at ringing chamber should also be limited to 50mm/s(2).