St Wulfram’s Spire - full report
Church architect Graham Cook has written a report on the history and future of restoration work at St Wulfram’s Church.
Here is Mr Cook’s report in full:
The spire of St Wulfram’s church, in common with most stone spires, is constructed of a relatively thin (about 6 inches or 150mm) octagonal, tapering hollow skin,capped at the summit with a solid section of stone, in this case 5.4 metres (about 17 ft 6 inches) deep. This solid top section is clamped together, both here and normally,by a sturdy metal rod (the holding-down rod) tightened above and below the solid top stonework and carried on down to a pair of strong metal cross bars known as cross trees.
These are built into the masonry lower down in the spire, in this case a further 3.1 metres (10 ft) below the underside of the solid top section of masonry.
This metalwork has the effect, when tightened up, of tying together the top 8.5 metres of stonework enabling it to act as one solid mass and better resist the effects of strong winds.
The summit of the spire of St Wulfram was found to be in a parlous condition during the war and in 1945, while the Church Architect Lawrence Bond was away on war duty, an inspection was carried out by Mr Robert Godfrey, the Surveyor at the time at Lincoln Cathedral. He had taken on the care of St Wulframs while Mr Bond was away. In essence he advised that the top 40 ft or so of the spire probably needed rebuilding at that point.
Much of this damage was probably due to the fact that the earlier holding-down rod, and other metalwork described above, would have been made of iron which is vulnerable to rusting in this exposed situation. The resultant rust expansion often causes extensive splitting and opening up of such stonework.
By the time consideration was being given to starting this work, in 1946, Mr Bond had returned from the war and it was amicably agreed that both Mr Bond and Mr Godfrey would collaborate in seeing the project through. When the scaffold was erected they jointly inspected and decided that it was in fact only the top 16 ft or so which needed rebuilding. That work was done very well with much new stone and a new bronze holding down rod and cross trees, all of which still appear in good condition.
It is clear from notes made at the time that the 24 ft or so of the spire below the rebuilt section (effectively down to the cill level of the upper spire lights) was closely inspected and although a few rusting iron cramps were cut out and replaced in non-ferrous metal, nothing more extensive than that was felt to be necessary. This decision was jointly made by Mr Bond and Mr Godfrey and is recorded in notes.
To be fair to those who made the decisions in 1946 there would probably have been no open joints visible at that time (over 65 years ago), and they did not have the advantage of “cover meters” to test for the presence of buried metalwork.
The exterior of the spire was previously last inspected by steeplejacks in 1987 and no evidence was found at that stage of any significant problems. As a further five yearly general inspection of the church was under way it in Autumn 2011, it was decided that it would be wise to carry out another external inspection of the spire and Cedar Steeplejacks of Sewstern were commissioned to do this.
When Cedar Steeplejacks erected their ladders to inspect, the initial reaction was that there was not much wrong. However, although most joints look perfectly sound on the outside elevations Cedars did raise concern about a few external fractures at high level, for which there was no obvious cause, and it was then decided that we needed to look inside to see what was causing these.
After some delays due to high winds, close inspection of the internal stonework from the level of the top spirelights to the underside of the solid masonry at the summit was carried out on 14th October 2011 from a scaffold platform erected inside and accessed through the upper spire light openings.
I went up on the external ladders, and thus into the interior at the summit, to look at the condition myself with the steeplejacks. We also used a “cover meter”, a metal detector which gives a reading for the depth of any concealed metal behind the surface.
In addition we brought in a specialist Structural Engineer from London, Ed Morton, who has wide experience in dealing with problems with church spires. He climbed the spire with us, subsequently carried out various sophisticated computer aided calculations based on his findings and produced a report with recommendations for action in late November 2011.
Visually it can be seen, much more clearly on the inside of the spire than the outside, that there are a number of places where embedded iron cramps in the 24 ft deep zone below the 1946 rebuild have now rusted to the extent that the stone has lifted a little and spalled the inner face of the stone.
There are also some fine vertical cracks between these lifted joints and in some places the lift has been sufficient for a thin sliver of daylight to be visible from inside. Testing with the “cover meter” has shown that virtually every course of the stonework in this particular section of the spire is tied together with embedded iron cramps, a lot of which will be rusting to some degree or other.
There have been two previous recorded rebuilding campaigns at the top of this spire. The first followed a partial collapse of the spire in 1797. The second was in 1884/8 which was overseen by the Architect at the time, Oldrid Scott. The strong suspicion is that the embedded ironwork currently causing so many problems dates from the 1797 rebuild but historical research into this question is ongoing. Cast iron was a relatively new material at that stage and the long term effects of its subsequent corrosion were probably not fully appreciated.
The discovery of this rusting ironwork means that, although the top 16 feet of stonework is secure and tightly cramped together with non ferrous metalwork, unfortunately that mass of stonework is now sat on a deteriorating band of masonry 24 ft deep below that. In some courses where the cracks are most pronounced a lot of the weight of the stones above is carried on the rusting iron cramps themselves rather than, as originally, the mortar bed around them.
This means that effectively a lot of the bonding effect of the mortar in the joints between stones has been lost and a tendency has been introduced for courses of stonework to want to “rock” slightly on the expanded ironwork. This obviously significantly weakens the structure under wind loading. Also the 24 foot deep band containing the rusting ironwork also contains the upper spire light openings which, in themselves, create one of the weak points in a spire structure.
The calculations carried out by the Structural Engineer confirmed our fears that there was a risk of instability in the upper parts of the spire, in its present weakened condition, if wind speeds approached 100 miles per hour. In the period running up to Christmas 2011 a threat arose that winds of that sort of speed, already being experienced in Scotland, might be heading southwards. Serious concerns were obviously raised about public safety in that period before Christmas when the church was in heavy use most days and also an awareness of the need to prevent any potentially catastrophic collapse or disturbance of the high level stonework with threats to the roofs and other fabric of the building below.
A rapid decision was therefore made to carry out the Structural Engineer’s emergency recommendations for temporary strengthening to the summit of the spire and these were very efficiently put into effect by Cedar Steeplejacks in the early part of December 2011, working in difficult weather conditions.
This involved the erection of a grid of bracing scaffold poles inside the top part of the spire combined with external boarding and strapping which, together, clamped the structure together more firmly and thus made it better able to resist the buffetting effects of strong winds.
This has bought us time but a major rebuilding exercise, to the whole top 40 ft of the spire (down to the level of the present, temporary high level scaffold platform), is going to have to be faced as soon as sufficient funding, through donations and grants, can be raised. That fundraising process is now well under way and has been given significant impetus by the offer, in December 2012, of a substantial grant towards the costs from English Heritage / Heritage Lottery Fund, applied for in June 2012.
However that leaves the best part of £250,000 of funds still to be raised by the end of 2013, by which time we have to have completed a series of further investigations, agreed specifications for the rebuilding work with English Heritage and the Diocesan Authorities, agreed a tender list with English Heritage and obtained and reported competitive tenders for the necessary scaffolding and repair works and negotiated all necessary permissions for the repair work to proceed in 2014. The dismantling and rebuilding work will take all of the time between early spring and late Autumn 2014, the work having to be completed by the time the winter weather arrives in late Autumn as we are using lime mortar which needs time to cure properly before it is exposed to hard frost.
In the meantime consideration has had to be given to the now regular, annual return to the walkway at the top of the Tower of the nesting Peregrine Falcons. Once their nesting box has been put in place, in early spring, it becomes a legal offence under wildlife protection legislation to disturb or go near the birds until the young have flown in July. We are carrying out all of the high level investigations and measurements, needed for preparation of the specification, this month so that we will be out of the way during the critical nesting months for the birds.
Steeplejacks will, by the end of February, ensure that all boards at high level, which might be disturbed by winds, have been removed so there should be no need to ascend the Tower or Spire until the birds have flown in the Summer. The SOS banner flown from the top of the Tower will also need to be taken down for the duration of the birds nesting period but will be put back again once the young Peregrine Falcons have flown the nest this summer.
The investigations being carried out at present include a detailed survey of the whole of the stonework to produce accurate measured drawings and record in detail the full extent of the iron cramps and resultant damage, the strength of the various mortars used, the condition and type of the stone used (earlier work was almost certainly in Ancaster stone but at least some of the later work used Clipsham stone) and decide on the most appropriate, safe and cost effective method to carry out the necessary scaffolding and repairs.
Graham Cook, Church Architect ~ February 2013
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Thursday 23 May 2013
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