Structural Analysis of Historic Construction – D’Ayala & Fodde (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5

Structural interventions in English Cathedrals

G.A. CliftonDirector Historic Buildings – Gifford Ltd

ABSTRACT: The paper is an overview of the structural interventions made to English Cathedrals. It reviewssome of the most common problems and disasters and looks at how they have been overcome with reference toparticular events.

1 OVERVIEW

I aim to give an overview of structural interventionsthat have been made to our Cathedrals in England.Clearly this is a huge topic worthy of many volumes butI shall attempt to sketch a flavour of the some of the keyhistorical events and the common structural problemswith reference to some particular case histories.

Many of our Cathedrals in England were built, andin many cases rebuilt, during the period of great activ-ity in the 1100 and 1200’s. As architectural fashionschanged, new prelates were appointed or local saintsbeatified, there was a tendency to refashion the Cathe-drals in the latest style. The growing wealth of thechurch made all this possible, but excited the envy ofthe English monarchs. This led to Henry VIII abol-ishing the monasteries and acquiring their wealth in1536, at which point many of these fine buildingsbecame stone quarries for the locals. The depreda-tions continued during the civil war in the 1640’s, whenmost of the art work and fine interiors were strippedand the buildings were sometimes used as stablingfor Cromwell’s forces. During the Victorian period,in the latter part of the 19th century, there was a mas-sive programme of repair, renewal and restoration inour Cathedrals. This was funded by public donation,much from very wealthy individuals. Repair schemesincluded dramatic tasks such as the saving of the eastend of Winchester Cathedral, by the underpinning ofthe foundations by the diver Robert Walker. The workoften went well beyond restoration, with Gilbert Scottmaking many “Improvements” for example at ChesterCathedral.

Many of the restorations at the end of the 19thand early 20th centuries made use of the “magic newmaterial” reinforced concrete.A typical example beingthe tieing together of the towers and nave of LincolnCathedral with concrete beams and frames, togetherwith the tieing of walls with phosphor bronze rods andextensive cement grouting.

Figure 1. Chester Cathedral before restoration.

Figure 2. Chester Cathedral after restoration.

2 DISASTERS

The Cathedrals have survived earthquakes, fire, floodand man’s depredations over their 900 year history andsurvive today as superb examples of man’s endeavoursand his wish to glorify God.

The structural designs often pushed the limits ofstructural knowledge and collapses were not uncom-mon, a list just of some of the major spire and towerevents is instructive.

1107 Winchester Walkelin’s central tower fell and thepresent one built

89

1170 Gloucester South tower to the west frontcollapsed1175 Worcester Western tower fell into the river Severn1185 Lincoln The whole building collapsed during anearthquake1210 Chichester West tower collapses1238 Lincoln Central tower fell1248 Wells Central spire fell in an earthquake1272 Norwich Spire burnt down during rioting1322 Ely Central tower removed before it fell1362 Norwich Replacement timber spire blown downin a gale1407 York Central tower and Spire collapsed1463 Norwich Second replacement spire destroyed bylightningThe stone replacement from 1480 still stands!1548 Lincoln Timber spire (524 ft high) destroyed bylightening1561 Old St. Paul’sTimber spire destroyed by lightning1615 Ripon Central spire collapses1635 Chichester West Tower collapses again1640’s Durham West Tower demolished by Cromwell’sarmy1642 Lichfield Central spire demolished by Cromwell’sarmy1704 Canterbury Central spire removed due to stormdamage1786 Hereford Central spire removed after collapse ofwest front1807 Lincoln West front spires demolished as unsafe1832 St. Albans Spire removed as unsafe1861 Chichester Central tower and spire collapse1880’s Peterborough Tower replaced to prevent acollapse.

This is by no means an exhaustive list as it notdoes not include many of the major fires that havecaused untold loss to the Cathedrals, but does showthe determination to recover from disaster.

3 LONG TERM WORRIES

Over this time the buildings were subjected to muchrepair, renovation and alteration and we know, as didour forefathers, of the value of constant maintenanceand the dire consequences of its omission. We haverecords of many of these campaigns of repair andalteration and it is fascinating to find that many ofour current concerns are merely the continuation ofworries of the past generations.

I have personal experience of two very clear exam-ples of such ongoing concerns, one which, so far, hasrequired no intervention, but a second which resultedin significant works.

The retro choir at Wells Cathedral is a vaultedarea between the high altar and the Lady Chapel, thesupporting columns are very slender and support anunbalanced vault onto which are imposed large loads

Figure 3. Chichester spire collapse.

from flying buttresses. On visual inspection one isimmediately worried that the vault is in tension andthat the slender columns are undergoing excessivebending.

I have investigated these carefully and can showthat they have a factor of safety greater than 1.0 whichis just as well as they have been standing unchangedfor 900 years. On searching back through the archivesI find that every Cathedral Architect, Engineer, andbefore that the master masons, were all expressing con-cern about the retro choir and all setting up their ownmonitoring systems – just as I have done! Indeed theVictorians had a series of specially fabricated propswhich were kept close to the retro choir for instantinstallation if any cracks appeared. My own advice isthat if cracks appear everyone is to run, as there willprobably be localised and fast compression failures! Itgives one a profound sense of historical continuity tofind that we share the same worries as our forbears.

Another fascinating case is that of the Deans Eyewindow at Lincoln Cathedral.This is one of the top fiverose windows in Europe with fabulous 1220 stainedglass and a very ambitious form.

The companion window, the Bishop Eye collapsedwithin 50 years and the archives show a long historyof repairs to the Deans Eye, some successful and somea failure. A constant theme in Clerk of Works recordsover the centuries is how to keep the window stable.This was initially by adding cramps and lead fillingto the joints as they moved, then by bracing acrossthe central opening and latterly by adding major crossbracing behind the window. Until by the year 2000 no

90

Figure 4. Wells Cathedral retrochoir.

Figure 5. Deans Eye window before restoration – LincolnCathedral.

more patching was possible and replacement was theonly option. A complete reconstruction of the stonetracery was required and the opportunity was taken toinclude some hidden strengthening so that the internaland external cross bracing which had disfigured thewindow was no longer needed.

4 THE LAST 50 YEARS

In the latter half of the 20th Century there has oncemore been a significant round of repair and renewal,with priming funding from English Heritage, but withthe majority raised from appeals. As opposed to theVictorian repairs this time there was little “improve-ment”, the emphasis being upon conservation andrestoration.

There were major interventions to stabilise thestructure of these great buildings and four typicalexamples are:

1 Improving existing foundations by some means ofunderpinning, such as at York Minster.

2 Tieing elements together with concrete ring beamsat roof, clerestory and triforium levels and addingtie rods through towers and facades. Ely Cathedralbeing a good example.

Figure 6. York Minster structural restoration.

3 Stabilising towers and spires with concrete or steelframes, such as Salisbury Cathedral.

4 Reconstructions after fires, several examples unfor-tunately and York Minster suffered more than most.

5 YORK MINSTER

York Minster is built on an historic site and overlaysprevious constructions including a roman basilica.TheMinster was started in 1100’s but its present form wasfinished in 1472 – much of it having to be rebuilt afterthe central tower collapsed in 1407 whilst attemptingto remodel the tower piers.

5.1 Underpining

In 1956 a detailed survey showed a number of instabil-ities in the building. The central tower had differentialsettlement of 225 mm that was continuing and inves-tigations showed severely damaged foundations, theeast wall was 600 mm out plumb and still moving andthe building was suffering a number of other similarproblems. A major programme of works was under-taken of which the underpinning of the central towerpiers was probably the most significant. The intention

91

Figure 7. Fire at York Minster.

of which was to halve the bearing pressure below thefoundations to approx. 300 Kn/m2, given that the sup-porting soil is poor it is no wonder that it was stillsettling.

The principle employed was to join the footingsbelow each of the central piers to their adjacent naveand transept columns making one large footing twicethe size of that existing. The masonry footings weresurrounded by new concrete and the whole post-tensioned into a single composite foundation. In orderto ensure that the new parts of the foundation wouldcarry load, additional footings were cast below thenew areas, with flat jacks between them and the newfoundation. The jacks were then stressed in order toconsolidate the soil below these areas, when this hadhappened and all movement ceased then the gap andjacks were grouted.

This was a massive and complex task which wasundertaken successfully and has ensured the future ofthe minster.

5.2 Restoration after the fire

The instabilities were not the only problems at York.In 1984 a major fire resulting from a lightning strikedestroyed the South Transept. A significant recon-struction was then needed.

6 ELY CATHEDRAL

In the early 1970’s inspections showed that thestonework in the West Tower at Ely Cathedral wasloose and decayed with many cracks through the tower.The tower in common with many others is effectivelytwo almost separate shells, so that over time theymove independently, resulting in cracking and loss ofstiffness.

More detailed research showed that in the 1860’sGilbert Scott had carried out major repairs whichincluded the installation of massive wrought iron bars,

Figure 8. Ely Cathedral – West Front.

125 mm by 30 mm diagonally across the belfry andfour 60 mm diameter wrought iron ties across thetower at two levels, plus the inclusion of many otherwrought iron ties and straps throughout the tower.These were now exacerbating the problem as some ofthem corroded.

The work was carried out under the control ofJacques Heyman and was in several parts.

1 The external wrought iron ties and strapping thatwas corroding, and damaging the stonework, wasremoved, whilst all the internal ties that were still ingood condition were not disturbed.

2 The lower two thirds of the tower have been rein-forced by stitching with stainless steel bars andgrouting. It having been found that the core to thewalls was loose in areas with some voids.The stitch-ing effectively created complete ring beams at eachof four levels tieing together the two shells of thetower.

3 The geometrical stability of the tower was furtherassured by the provision of three sets of stainlesssteel ties passing from face to face of the tower andconnecting together the corner stairs. One of thesereplaced some of Scott’s work and the others wereadditions.

4 The octagonal belfry, which is of a lighter con-struction than the main portion of the tower was

92

Figure 9. Ely Cathedral – strengthening of the West Tower.

strengthened at its top by a new reinforced concretering beam, with stainless steel cables inserted andpre-tensioned at a lower level.

This is an excellent example of the progression ofrepairs to Towers in English Cathedrals. As we haveseen there were numerous collapses of towers rela-tively soon after their construction or alteration andthere was little that could be done with the technologyavailable at the time to save them. They were oftensignificantly under designed.

As the centuries passed by the continuing settle-ment of overloaded ground below the central pierscaused ongoing cracking of the major towers and thisresulted in loss of stiffness which caused further redis-tributions of load. This often increased the load on thepiers, with an ongoing racheting increase in crackingand movements. Sometimes this was added to otherinterventions such as the removal of the choir screenat Chichester which ultimately caused the failure ofthe spire. In other cases the inclusion of wrought ironties and strapping alongside the repair of crumblingstonework was sufficient to maintain stability.

Figure 10. Ely Cathedral – strengthening of the West Tower.

However, as is so often the case, whilst the inter-vention was successful it introduced other difficultiesthat in time also needed remediation. So that whilstin the original construction of these massive towerwalls timber ties were sometimes included to hold thewalls together, these rotted and the towers began tolose their integrity. So the wrought iron ties, in theirturn, decayed and needed replacing.

7 SALISBURY CATHEDRAL

A different approach was undertaken at SalisburyCathedral where there were serious concerns about thestability of the spire in the 1970’s. Salisbury spire hasa series of bands of open stonework at several levels inits height and the lowest level of stonework was decay-ing such that there were serious fears as to its abilityto continue to carry the wind loads and weight fromthe spire. It was not possible to individually replacethe stones as it was highly likely that the spire wouldcollapse with the removal of some of the stones. Vari-ous options were considered including the dismantlingand rebuilding of the spire – though needless to say thatwas thought of as a last resort.

The successful solution was to introduce a stainlesssteel frame inside the spire which effectively transmit-ted the load from the upper part of the spire into theframe and bypassed the open section. The frame wasthen jacked against the upper spire, to ensure that itwas taking some of the load from the open section.

8 REPAIR AND MAINTENANCE

Alongside these major interventions there is anongoing need for constant repair and renewals. All

93

Figure 11. Salisbury Cathedral.

Cathedrals have a long programme of stone replace-ment, working around the exterior over a 100 yearcycle replacing eroded stone. The work can varyfrom the straightforward on nave walls, to thecomplex on flying buttresses, to the difficult ontowers.

Wells Cathedral central tower requires replacementof areas of heavily eroded stones and other repairs onroughly a 120 year cycle; this being about the periodby which time pieces of stone are falling off to thedanger of the public below. In 1998 it was time to carryout such a task. The biggest difficulty was getting theaccess to all faces with a working scaffold as there is noaccess to the ground below any face. A flying scaffoldhad to be used supported from the transept, nave andchoir wall junctions, with an access bridge along theridge of the south transept from a lift/stair tower on thesouth transept gable.

The sequence in which the stone replacement iscarried out is important as with some areas requir-ing extensive repairs it is vital to ensure adequate loadtransfer at all times to avoid local overstressing.

As well as these major works, repairs and replace-ments there are always new constructions happeningat Cathedrals which impact upon the existing fabric.Many Cathedrals are improving their facilities suchas by the addition of choir rehearsal spaces, storage,education rooms, visitor centres and refectories. Theseall need to be close to the main building and oftenstructurally impact upon, it. Added to which are thedifficulties with the foundations, as invariably thesenew buildings will be located over areas previouslybuilt upon, so with important archaeological remainsbelow the surface. The foundations become a game offinding any spaces that are clear for support and thentrying to carry the building from these points.

Figure 12. Wells Cathedral – Tower repairs.

9 FINALLY WHAT OF THE FUTURE?

All over Europe we have repaired, strengthened andstiffened our major masonry buildings with concrete.Is this a good thing? Is it durable? What happens toour lovely flexible masonry buildings in the next 900years after they have been stiffened and tied together?That has to be the subject of a future paper.

REFERENCES

The Structural Engineer vol. 85 number 3 2007. The DeansEye Window – Lincoln Cathedral.

Institution of Civil Engineers proceedings 1971 paper 7415S. York Minster Structural restoration – Dowrick andBeckmann.

Proceedings of the Institution of Civil Engineers 60 1976.The strengthening of the West Tower of Ely Cathedral.

Thesis M. Kirby. Chichester Spire collapse. Medieval Cathe-drals their design, construction and structural problems

Int. journal of solids structures Oxford 1966. The StoneSkeleton.

J. Heyman ISBN 0-86078-597-1. Arches, Vaults andButtresses.

94