Lincoln Cathedral’s magnificent Dean’s Eye Window is being pieced back to restored and conserved condition, watched closely by Geoff Clifton, engineer for Lincoln and Wells cathedrals. Jon Masters reports.
It would be a brave soul willing to stand on a floor consisting of the old stone tracery of Lincoln Cathedral’s Dean’s Eye rose window. By modern reckoning though, when restoration is complete, the window will have to be as strong as a floor to withstand the wind 30m up on the cathedral’s North west Transept.
This has presented an intriguing problem for Lincoln Cathedral engineer and Gifford chairman Geoff Clifton. Until it was taken down earlier this year, the old tracery was held up by iron bracing and compression from a sagging masonry arch above the window. The new stonework has got to go up to the same pattern, without any bracing and very nearly to the same dimensions.
Clifton has been able to slightly thicken the tracery by 20mm and with application of some modern engineering, solve the problem. The results should be impressive. Glaziers of the cathedral’s works department have finished conservation of the window’s 77 panels of 13th century stained glass and this year, its masons have started rebuilding the 7.5m diameter tracery, in situ and with around 100 newly worked and carved stone pieces.
“The Dean’s Eye is unusual. It has relatively thin spokes in comparison to most other rose windows and it has a quatrefoil instead of a central circular panel, which has complicated load paths through the structure,” says Clifton.
“The window was built in 1220 and braced shortly after and at around the same time as the Bishop’s Eye Window of the South west Transept collapsed. These may have been similar, but are now a different design.”
The top of the Dean’s Eye Window is immediately below a masonry arch supporting the gable wall of the North West Transept. The arch has spread, allowing it to sag and ovalise the window.
“The iron bracing and compression from the sagging arch was all that was holding the Dean’s Eye together,” says Clifton. “The old tracery was in a poor state with a lot of cracking and clean breaks in the stonework, particularly in horizontal members which have experienced less compression so suffered more bending stress due to wind loads.”
The masonry arch was propped and the old tracery entirely removed earlier this year. Cathedral conservators and the consultant archaeologist have laid out the individual pieces to their original pattern on the floor of the North West Transept. Lead dowelling shows where the stone work has been repaired since 1220 and in places erosion of the tracery has reduced its original 210mm thickness down to 170mm.
The old tracery would not inspire much confidence as a floor, but Clifton and his team at Gifford have designed the new window structure to withstand dynamic loading equivalent to a floor load. This comes from the window’s 500 year design life and calculation of a corresponding wind speed. The peak predicted design load, at 30m above ground and around Lincoln Cathedral’s specific shape and location on an escarpment surrounded by flat and low lying land, is a suction of 2.5kN/m2.
Design deflection is virtually nil and the stonework will not work in tension. So the window could not be assumed to act as a beam across the opening and had to be analysed as a 230mm deep flat arch with large resulting outward thrust forces. Complicating matters further, the structure surrounding the window is stable but with little capacity to resist outward thrust beyond that already applied by the gable arch.
Clifton has solved the problem by considering the outer roundels of the tracery as a ring around a central disc of stonework. The outer roundels are being fixed to the surrounding masonry with Cintec anchors, so can act in bending and allow the window to be analysed in section as a flat arch spanning between two cantilevers.
When the ‘outer ring’ is complete, sections of electro-polished stainless steel will be placed to form a continuous and corrosion resisting steel ring hidden between the tracery’s outer and inner sections, to contain arch thrusts as hoop stresses.
“The steel ring’s efficiency in tension will ensure minimal deflection, and tensions generated within the ring by wind induced arch thrusts will act to clamp together the stonework of the inner disc,” says Clifton. “This stiffness and the action of the metal ring will also distribute bending forces around the full perimeter of the window, instead of concentrating them at the ends of the four main diagonals crossing the centre circle.”
The new Dean’s Eye central quatrefoil will be a single stone-clad piece of the same electro-polished steel, which will also help distribute forces better, says Clifton. And steel ties through the spokes from each outer roundel will give additional bending capacity and stiffness at the join with the inner section.
“These are not theoretically needed, according to the analysis, but they are there as my insurance policy and help me sleep at night,” he says.
The emphasis of construction, says Clifton, is on detailed initial planning followed by getting it right one stage at a time with advice from the cathedral’s masons. Individual pieces of tracery are being conventionally winched up to the scaffold, before being positioned with a specially built stone manipulator and lasers used to check alignment.
“A detailed photogrammetric survey was carried out before the window was dismantled. From this survey exact templates were prepared for each stone and the masons used these to produce the new work. However, as a precaution and to build in some tolerance, the last stones to be fixed have been left oversized in readiness for trimming to fit,” Clifton says.
After the outer ring of stonework is complete with all 16 roundels anchored, the steel ring and central disc of the tracery will be built from the bottom upwards. The heaviest piece is the 750kg stone clad quatrefoil, followed by the outer roundels, which weigh 270kg each.
All of the tracery pieces are now ready for fitting after a year of masonry and detailed carving work by the cathedral’s own team of eight masons. Carvings include a winking figurehead of the current Dean of Lincoln and have been worked in the same style rather than to match the original detail, most of which has been lost to centuries of erosion. French Anstrude Roche Claire Oolitic Limestone is being used, after an extensive search for a stone compatible with the Lincoln stone.
“Lincoln Limestone has excellent qualities but the cathedral is sat on the only beds of this material more than 900mm thick. Mining beneath the cathedral was not an option so Anstrude was chosen for its equivalent strength and occurrence in thicker beds.”
Clifton is expecting the masonry for the Dean’s Eye tracery to be complete by the end of the year. The conserved glass will then be put back in place with Isothermal glazing on the outside of the tracery to protect the 13 Century glass.
“The fully restored window should look fantastic,” Clifton adds.
Box – Cathedral Engineer Geoff Clifton
Geoff Clifton, 57, is in pretty good shape. As engineer for Lincoln and Wells cathedrals, he climbs a lot of spiral stone staircases and arrays of ladders to inspect the inner and outer areas of mediaeval structures.
The post of cathedral engineer is held individually. For legal purposes, consultancy is provided by Gifford. But as cathedral engineer, Clifton is responsible for advising the Deans and Chapters of Lincoln and Wells on maintenance of cathedral fabric.
Roaming sections of both cathedrals with a keen eye for any structural changes is a regular duty. So is pouring through cathedral architects’ quinquennial reports on both buildings to identify areas that need engineering attention.
Work carried out since Clifton took up his post at Lincoln in early 2000 has included monitoring cracks in the main walls of the Nave and Central Crossing Tower. Results, says Clifton, showed the tower to be rotating and the walls moving through repeating cycles.
“People sometimes forget that these buildings have stood up for 900 years. Both Lincoln and Wells cathedrals are structurally sound and a long term view has to be taken towards monitoring because the behaviour of such massive buildings tends to be seasonal.”
Box – Lincoln Cathedral
Lincoln Cathedral dates back to the 11th Century, but only parts of the West Front remain from the original structure consecrated in 1092. Fire and earthquake destroyed much of the first building during the 12th Century and it was then rebuilt and extended in stages, reaching its current form in 1400.
The cathedral, built from local Oolitic Limestone, is a double cross in plan and 147m long from its West Front to the rear of the Angel Choir. The North and South Little Transepts are towards its eastern end and the Great Transepts meet between the Nave and the Chancel and either side of the 83m high Central Crossing Tower.
Lincoln cathedral also includes a Wren designed library over the Cloister, which is overlooked by the North West Transept and the Dean’s Eye Window. The Cloister abuts the decahedral Chapter House where in 1301 Edward I proclaimed his fourth son, the future Edward II, to be the first English Prince of Wales.
A glass act
Gifford
Published in New Civil Engineer, 16 September 2004