It would be a brave soul willing to trust their weight solely to the old stone tracery of Lincoln Cathedral's Dean's Eye rose window, 30m above ground level. Until it was taken down earlier this year for much needed restoration, the old tracery was held up by iron bracing and compression from a sagging masonry arch above the window.
Yet when restoration is complete, the window will be strong enough to withstand the winds that slam into the cathedral's north west transept, and more than able to support the weight of a man.
Restoration of the window has presented an intriguing challenge for Lincoln Cathedral engineer and Gifford chairman Geoff Clifton.
'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, at around the same time as the Bishop's Eye Window of the south west transept collapsed.'
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.
'Iron bracing and compression from the sagging arch was all that was holding the Dean's Eye together, ' recalls 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.'
With failure of the window expected at any moment, the masonry arch was propped and the old tracery entirely removed earlier this year. Cathedral conservators and the consultant archaeologist laid out the individual pieces to their original pattern on the transept floor, and got to work assessing the toll taken by nearly eight centuries of wear and tear. Lead dowelling showed where the stonework had been repaired since 1220. In places erosion had reduced the section of the tracery from 210mm to as little as 170mm.
The new window structure has a 500 year design life and will be able to withstand dynamic loading from the increasingly violent storms expected over the next half millennium. Peak load, generated as winds rush at and around the cathedral and the escarpment on which it sits, is a suction of 2.5kN/m 2.Deflection of the window is virtually nil as the stonework tracery cannot be subjected to tensile forces. To analyse the effects of suction forces, the window had to be treated as a 230mm deep flat arch with large outward thrust forces. To complicate matters, 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 thickening the tracery ever so slightly - by 20mm - and by considering the outer roundels of the tracery as a ring around a central disc of stonework. This outer ring is being fixed to the surrounding masonry with Cintec anchors.
When the outer ring is complete, an electro-polished stainless steel hoop will be placed between the tracery's outer ring and inner sections, containing arch thrusts from the upper part of the roundel.
'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.'
Stainless steel rods will also form an armature for the new Dean's Eye central quatrefoil to help distribute forces, while stainless steel ties will provide extra bending capacity between the spokes and outer roundels.
'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.
Construction is being guided by the cathedral's masons.
Individual pieces of tracery are being winched up to the scaffold, before being positioned by a specially built stone manipulator. Alignment is checked with the aid of lasers.
'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.
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 elements are now ready for fitting after a year of detailed masonry. Carvings include a winking figurehead of the current Dean of Lincoln and have been worked in a modern 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 13th century glass.
'The fully restored window should look fantastic, ' Clifton adds.
Clifton has been able to slightly thicken the tracery by 20mm. It is being placed by a purpose-built stone manipulator.