In the latter half of the 20th Century there has once more been a significant round of repair and renewal, with priming funding from English Heritage, but with the majority raised from appeals. As opposed to the Victorian repairs this time there was little "improvement", the emphasis being upon conservation and restoration.
There were major interventions to stabilise the structure of these great buildings and four typical examples are:
1 Improving existing foundations by some means of underpinning, such as at York Minster.
2 Tieing elements together with concrete ring beams at roof, clerestory and triforium levels and adding tie rods through towers and facades. Ely Cathedral being a good example.
3 Stabilising towers and spires with concrete or steel frames, such as Salisbury Cathedral.
4 Reconstructions after fires, several examples unfortunately and York Minster suffered more than most.
York Minster is built on an historic site and overlays previous constructions including a roman basilica. The Minster was started in 1100's but its present form was finished in 1472 - much of it having to be rebuilt after the central tower collapsed in 1407 whilst attempting to remodel the tower piers.
In 1956 a detailed survey showed a number of instabilities in the building. The central tower had differential settlement of 225 mm that was continuing and investigations showed severely damaged foundations, the east wall was 600 mm out plumb and still moving and the building was suffering a number of other similar problems. A major programme of works was undertaken of which the underpinning of the central tower piers was probably the most significant. The intention
of which was to halve the bearing pressure below the foundations to approx. 300 Kn/m2, given that the supporting soil is poor it is no wonder that it was still settling.
The principle employed was to join the footings below each of the central piers to their adjacent nave and transept columns making one large footing twice the size of that existing. The masonry footings were surrounded by new concrete and the whole post-tensioned into a single composite foundation. In order to ensure that the new parts of the foundation would carry load, additional footings were cast below the new areas, with flat jacks between them and the new foundation. The jacks were then stressed in order to consolidate the soil below these areas, when this had happened and all movement ceased then the gap and jacks were grouted.
This was a massive and complex task which was undertaken successfully and has ensured the future of the minster.
The instabilities were not the only problems at York. In 1984 a major fire resulting from a lightning strike destroyed the South Transept. A significant reconstruction was then needed.
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