Originally marked by a square tower built in the 1890s, the crossing at St Anne's Cathedral has been crowned with a new spire. The architects envisaged a tapering silvery rod that rises high above the church, guiding the eye heavenwards.
The spire measures 50m from tip to pointed tip, rising 40m above the cathedral roof and suspended to 10m below. It is supported at its widest point, which is little over 1m in diameter. Ensuring sufficient rigidity was thus an important factor in our design.
We examined a variety of structural options before deciding on a tapering steel tube without internal support. Extensive modelling and very detailed dynamic analysis informed the design of the tube, which consists of butt-welded bands of varying thickness and height. Seams are polished to invisibility.
The spire is supported by four polished steel plate fins, 1m deep, extending from the internal corners of the tower's four walls. The tower roof is plate glass so that, from below, the spire appears suspended from the sky.