St James Twin Chapel Spire

St. James Twin Chapel Spire

A case study report in Structural Conversation submitted by Mariana Isa towards the degree of Master of Science in the Conservation of Historic Buildings at the University of Bath.

Department of Architecture and Civil Engineering.

Session 2005-2006

St. James Twin Chapel Spire

Acknowledgement The help, advice and support from the following are gratefully acknowledged: Dr. Michael Forsyth Nok Ali Amran David Stapleton Teq Hud Hud Juliane Eichner Bath North East Somerset Archive Haycombe Cemetery Office Classmates of Msc. Conservation of Historic Buildings 2005-2006

St. James Twin Chapel Spire

Synopsis

This report is a structural case study on the St. James Twin Chapel Spire, which was dismantled due to its condition that was at severe risk of collapse if there had been a strong gust of wind. This report covers the structural condition of the spire before it was dismantled in 1978 based on documents found in the Bath City Archive.

This report consists of two sections. The first section includes a structural analysis and diagnosis of the spire’s structural defects and the second section is a discussion on the proposed intervention and the philosophy of conservation behind it.

ii

St. James Twin Chapel Spire

Contents Page No. Acknowledgement

List of Illustrations

i

Synopsis

ii

1.0

Introduction

1

2.0

Published Records

3

3.0

In situ Survey

4

4.0

Structural Analysis

5

4.1

Structural Description of the Spire

5

4.2

Structural Condition of the Spire

6

4.3

Wind Action onto the Spire

6

4.4

Defect 1: Crack at Base of the Original Anchor Block (F/G)

7

4.5

Defect 2: Crack at the Base of Tapered Octagon

9

5.0

Proposed Structural Intervention

11

5.1

Stone Treatment

11

5.2

Strengthening the Spire – Post Stressed System

11

6.0

Philosophy of the Structural Repair Techniques Applied

13

7.0

Conclusion

14

Notes

15

Bibliography

16

Appendices Appendix A – Drawings Appendix B – Working Procedure Report

St. James Twin Chapel Spire

List of Illustrations

Figure No.

Title of Illustration

Illustration Credit

1

The present St. James Twin Chapel.

Author

2

Location map of St. James Twin Chapel in Bath.

Author

3

Twin Chapel Spire before and after demolition in 1978.

The Creasy Partnership Guildford & Epsom

4

Stonework of the spire stored in the East Chapel. Markings on the stones are still visible.

Author

5

A conceptual north-south section of the Twin Chapel Spire.

Author

6

Cracks at the mid height of the Twin Chapel Spire

Bath City Archive

8

Wind action effect to level F/G and T/U.

Author

9

Defect at base of the original anchor block (F/G).

Bath City Archive

10

Cause of structural defect at F/G.

Author

11

Defect 2: Crack at the base of tapered octagon (T/U).

Bath City Archive

12

Cause of structural defect at T/U.

Author

13

Proposed Intervention to restore the Twin Chapel Spire.

Author

i

St. James Cemetery Twin Chapel Spire

St. James Cemetery Twin Chapel Spire 1.0

Introduction

Figure 1: The present St. James Twin Chapel.

1.0.1 St. James Twin Chapel in the middle of St. James Cemetery was opened in 1861 but has now closed. It is located ¾ mile west of the Bath City Centre, on the south of the Lower Bristol Road.

1.0.2 Built in 1860, the St. James Twin Chapel was a mortuary building designed by Bath’s City Architect at that time, Charles Edward Davis1. The chapel is of medieval architecture, consisting of two twin chapel complexes built using Coombe Down Stone. The two chapels are placed on the West-East axis, separated by a twin-carriage porchway.

1

St. James Cemetery Twin Chapel Spire

Figure 2: Location map of St. James Twin Chapel in Bath.

1.0.3 A slender stone spire was placed above the twin carriage porch-way of the chapel. Unfortunately, the spire’s structural condition was deemed to be dangerous and was carefully dismantled in 1978 by the Bath City Council2.

1.0.4 During the spire’s demolition, all the stonework were measured, recorded and subsequently numbered. The stonework are now stored in the two cemetery chapels for future reuse3. The stump of the original spire is temporarily capped-off at the top of its pedestal level, just above the ridge of the Twin Chapel’s roofs.

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St. James Cemetery Twin Chapel Spire

Original Spire

Pedestal Level

Figure 3: Twin Chapel Spire before and after demolition in 1978.

1.0.5 A grant aid of £37 500 had previously been offered to the City Council by the Department of the Environment under Section 4 of the Historic Buildings and Ancient Monuments Act 1953, as the Department of the Environment regarded the Twin Chapel as an outstanding historic building4. However, the restoration project never came to being due to funding difficulties5.

2.0

Published Records

2.0.1 There are little published records on the St. James Twin Chapel. However, as the building was designed by the City Architect in 1860, Charles Edward Davis, original scaled contract drawings and specifications still exist in the Bath City Council Archives6. These drawings however are not available to the general public.

2.0.2 A detailed measured drawing of the spire and photographs of the Twin Chapel before the spire was dismantled in 1978 were obtained from the Bath Council Archive, showing the form, details and structural defects of the spire. The drawing and photographs are used as primary reference to the spire’s original design for this report. 3

St. James Cemetery Twin Chapel Spire

2.0.3 The most comprehensive document found on the spire’s structural history is the Feasibility Study on the Restoration of The Twin Chapel Spire Report by Creasy Partnership Guildford and Epsom, published by the Departmental of Environmental Services of Bath City Council. This report contains detailed description of the spire’s characteristic features, structural analysis, structural calculations and technical details of the proposed spire restoration in 1985. Detailed recordings of the original spire together with its drawings are also included.

2.0.4 It has been noted that a structural report on the Twin Chapel Spire was submitted by David Mclaughlin, a Msc. Conservation of Historic Buildings student at the University of Bath in 2005. However, this report was not used as reference.

3.0

In situ Survey

3.0.1 The first site visit was on 12 November 2005 to investigate whether the Twin Chapel was suitable for the assignment. The building’s cemetery compound was found to be well looked after and the chapel is locked at all times to keep safe the spire’s original stonework.

3.0.2 The second visit on the 20 December 2005 was accompanied by a representative from Haycombe Cemetery Office. Investigation into the chapel found the whole building’s structure to be in a sound condition. There were hardly wall cracks or timber rafter decay.

3.0.3

The long effects of storage onto the spire’s stored stonework have appeared to be mild with little sign of severe chemical erosion from the local atmosphere. The markings on the stones are still visible.

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St. James Cemetery Twin Chapel Spire

Figure 4: Stonework of the spire stored in the East Chapel. Markings on the stones are still visible.

3.0.4 As a whole, the Twin Chapel building is thought to be well maintained despite the fact that it has been closed for a few decades.

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St. James Cemetery Twin Chapel Spire

4.0

Structural Analysis

4.1

Structural Description of the Spire 4.1.1 The spire of St. James Twin Chapel was a tapered octagon structure, hollow at its stem and solid at its apex. The spire rose to a height of 86ft (27 metres) above the ground. The vertical axis of the spire was placed at the middle-point of the longitudinal axis of the twin chapel complex7.

4.1.2 The base of the spire’s pedestal is formed in cross section as a solid cruciform. Part of the pedestal’s base is carried in suspension, below the pitch of the porch-way roof and above the carriage way, by a system of Wrought Iron Beams. The pedestal emerges through the roof into an exposed

extension

of

a

similar

cruciform cross section, carrying a 4column

unit

of

an

open

bell

8

chamber .

4.1.3 The bell chamber supports the lower stem of the tapered octagon spire. The lower stem is the first unit above the base of the spire which is formed as a hollow octagon. The stem itself is parallel sided but with four diagonal open latticed panels. These four Figure 6: A conceptual north-south section of the Twin Chapel Spire.

latticed panels are what support the upper portion of the tapered octagon9.

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St. James Cemetery Twin Chapel Spire

4.2

Structural Condition of the Spire

4.2.1 A structural inspection of the spire, carried out in 1978 revealed two major structural cracks at the mid Crack 1 (F/G)

height of the spire (Figure 7). Scaffolding was erected around the spire to its full height to help investigation.

Crack 2 (T/U)

4.2.2 In general, the original spire was found to be in good condition, but the structure was inclined to sway in high winds and there were significant external damage to several areas of the stonework. Both defects were found to be inherent in the construction of the spire10. 4.2.3 The decision was to take down the spire to its pedestal level, capping off the exposed stone with a

Figure 7: Cracks at the mid height of the Twin Chapel Spire.

temporary covering of lead, and to make a detailed record of the dismantled stonework11. 4.3

Wind Action onto the Spire

4.3.1 The city of Bath is located within a moderate zone of wind intensity. The maximum basic wind speed which will blow continuously for a gust duration of 3 seconds is 42 m/sec over a period of 50 years12. 4.3.2 National Code Recommendations suggested that the spire would safely resist an equivalent static pressure of 25 psf (0.5N/m2)13. 4.3.3 As a whole, the spire was quite stable under average intensities of wind pressure, but liable to suffer damage from high pressure gusts which are repeated over a long term. High gusting had induced corresponding fluctuations in uplift and eventually rocked the spire and generated hammer blows on the Figure 8: Wind action effect to level F/G and T/U.

windward side of the stonework14. 7

St. James Cemetery Twin Chapel Spire

4.4

Defect 1: Crack at Base of the Original Anchor Block (F/G)

Crack at F/G

Figure 9: Defect at base of the original anchor block (F/G).

4.4.1 External cracking appeared at level F/G at the base of the original anchor block. It was assumed that this defect (Figure 9) was due to the considerable oscillation caused by wind action against the height of the spire, and worsen by the ineffective stone anchor which was suspended internally from the apex by an iron finial crossed by two linked iron bars15. It is likely that the architect used this ‘pendulum’ device in an attempt to counteract the effect of wind sway by holding down the head of the spire. 4.4.2 This ‘dead-weight’ anchor block, resting on the internal base at level F (Figure 10) was not heavy enough to hold down the spire. Its influence was particularly insignificant at the positions where it most needed. The size of the ‘kentledge’ provided was only 2% of that required to stabilize the most sensitive components of the structure16.

4.4.3 In a high gust wind, the suspended anchor block is liable to lift and swing against the internal faces of the spire with corresponding damage to the stone’s at both the support level at the apex of the spire.

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St. James Cemetery Twin Chapel Spire

Figure 10: Cause of structural defect at F/G.

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St. James Cemetery Twin Chapel Spire

4.5

Defect 2: Crack at the Base of Tapered Octagon (T/U)

Crack at T/U

Figure 11: Defect 2: Crack at the base of tapered octagon (T/U).

4.5.1 There was a severe horizontal crack through the stonework of both the solid and lattice face where the base of tapered octagon is. The crack was most significant on the north east of the structure and probably accords with the direction of the prevailing wind.

4.5.2 A flat band of iron to encircle the structure at the base of the octagon was introduced to restrain the outward thrust on the stonework of the open belfry from the weight of the spire above. This precautionary measure was not adequate to prevent a fracture of the stones at T/U.

4.5.3 Level T/U at the base of the tapered octagon is a position of exceptional sensitivity and is particularly vulnerable to above average wind gusts. The four latticed panels of the lower stem, supporting the tapered octagons are relatively weak in diagonal tension and provide only a fraction of the desirable efficiency. A stabilising component was inserted horizontally at the mid height of the lower stem to stiffen it17.

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St. James Cemetery Twin Chapel Spire

4.5.4 The bell chamber is arranged as 4 wide panels on the square sides of the octagonal cross-sections without any form of restraint on the diagonals. In its present form the efficiency of the bell chamber unit is very low and largely indeterminate. Improvements to ensure that the 4 columns act together as a fully braced frame is possible by inserting a horizontal cross frame at mid height.

Figure 12: Cause of structural defect at T/U.

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St. James Cemetery Twin Chapel Spire

5.0

Proposed Structural Interventions

5.0.1 In 1985, two methods were proposed to restore the spire back to its original position and improve its structural stability. The proposed structural intervention included treatment of stonework and strengthening the spire by introducing a full height system of central anchor rods which are post stressed from the existing base18.

5.1

Stone Treatment

5.1.1 The spire’s restoration report states that at least 80% of the stone from the original spire can be re-used after relatively minor treatments.

5.1.2 The proposed guidelines for the spire’s stone treatment are as follows: i.

Natural erosion of the stone face and corners are to be accepted. Stones which may have been used in positions of face bedding are to be avoided in the process of restoration.

ii.

The removal of structural defects (rusting from previous fitments, cracks, blemishes and spalling) will only involve those threatening the stability of the restored spire. This will involve squaring up of the existing stone block and importing additional material to match.

iii.

The stonework of the spire, including the base and pedestal are to be face cleaned. All traces of old mortar are to be removed for the stability of the structure, especially in the position of the anchorage seating.

5.1.3 The existing metal cross which surmounts the spire can be reused. However, the existing apex seating was found to be very narrow and appears to have been damaged by erosion and relaxation in the anchorage. The top anchorage was redesigned to avoid these troubles. A limited amount of new stonework will probably be required in this sensitive position. 5.2

Strengthening the Spire – Post Stressed System

5.2.1 The dead weight of the stonework towards the spire’s apex was insufficient to hold down the structure against the above average wind gusting. The loose anchor block which was originally inserted at mid height to stabilise the structure, was also inadequate for this purpose, both in weight and the insecurity of the anchorage within the main stonework19.

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St. James Cemetery Twin Chapel Spire

5.2.2 To stabilize the reconstructed spire, the ‘Kentledge’ concept was proposed but with significant modifications.

5.2.3 It was proposed to anchor all intermediate positions to the base of the structure by a system of central anchor rods. From this base, the full weight of the stonework could then be effectively utilised to offset the gust effect on the more sensitive levels of the structure. The anchorage rods are post stressed individually at each stage (refer Figure 13).

Stage 3: Level F –A2 - At this level there is a sharp change in cross section. A reinforced concrete sealing slab is introduced. The original anchor finished at the apex. But the stone base is very narrow and severely damaged.

Stage 2 Level 6- F - To stabilise the spire at these levels, it is necessary to apply an appropriate Anchor Force at a Level F. - Original structure has already been formed with a convenient internal corbel. A coupling is provided to form the base of Stage 3.

Stage 1 – Alternative type 1 – Central Anchorage Rod (C.A.R) - Pass a central anchor rod through the solid hub of the cruciform base and pedestal and anchor it into a new steel grillage (positioned at just below the existing grillage). - If access hole does not exist, a vertical clearance hole will be drilled along the axis of the Spire stump and cut horizontal access holes. - An alternative and simpler solution may be to insert a corner anchor rod in the 4 reentrant angles of the cruciform cross section.

Figure 13: Proposed Intervention to restore the Twin Chapel Spire. 13

St. James Cemetery Twin Chapel Spire

6.0

Philosophy of Structural Intervention Techniques

6.0.1 The restoration of the spire is measured against English Heritage’s principles of conservation; minimal intervention, like for like, do nothing, preserve as found, sympathetic and reversibility.

6.0.2 The spire has a significant historical and structural value to the community, hence, its reconstruction is deemed substantial.

6.0.3 It is fortunate that the spire’s stonework was carefully marked during the demolition and is still securely kept. This, together with the availability of the spire’s measured and original contract drawings, allows the spire to be restored to its original form righteously - conservation wise.

6.0.4 The existence of 80% of the spire’s original stonework enables the retention of its historic authenticity. Replacement of blemished stones with the same material is a like for like intervention. 6.0.5 The spire’s major problem was the structural design itself. If reconstructed to the exact original, the spire is expected to encounter the same complications. The proposal to strengthen the structure by inserting iron anchor rods, is not a minimal or reversible structural intervention. The new anchor rods will change the spire’s load path onto the masonry beneath it. The complications that this proposal would create for the masonry structure must be taken account of. The tendency for the iron rods to rust must also be noted. 6.0.6 It is difficult to comprehend the reasoning behind the spire’s demolition before the structure had broken down in 1978. The question whether the proposed anchoring to conserve the spire could have taken place before demolition arises. Measures should have been taken to restrain it from breaking down, not to bring it down before the structure fails. If the structure has managed to stand against time, it gives an indication that it is capable of standing even longer. Calculations are sometimes unreliable in cases such as this.

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St. James Cemetery Twin Chapel Spire

6.0.7 The decision to dismantle the spire does not comply with any of the conservation principles. It seems that this decision was heavily influenced by the lack of existing funds.

6.0.8 The decision to record the stones and store the spire’s stonework however, opens the possibility for future restoration when technology has advanced and funds have expanded.

7.0

Conclusion

7.0.1 The assumption that the spire was a dangerous structure and was demolished before it actually failed is unaccountable. If the spire has found a way of standing for several decades, an explanation on why it has stood must be looked into. If analysis suggests that the spire does not work, the analysis is at fault, not the structure.

“A structure only falls down when it has exhausted all possible ways of standing up.” Walley’s Theorem

7.0.2 However, it is understood that the stonework was dismantled as a precaution measure to avoid damage, in-case the structure does fall down. Limited funding implies that there is no guarantee that the structure will be able to undergo restoration in near future.

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St. James Twin Chapel Spire

Notes 1.

Bath City Archive Records

2.

The Creasy Partnership Guildford & Epsom (1985), p 4

3.

Honeycombe Cemetery Office Records

4.

The Creasy Partnership Guildford & Epsom (1985), p 4

5.

Honeycombe Cemetery Office Records

6.

Bath City Archive Records

7.

The Creasy Partnership Guildford & Epsom (1985), p 5

8.

The Creasy Partnership Guildford & Epsom (1985), p 5

9.

The Creasy Partnership Guildford & Epsom (1985), p 5

10.

Honeycombe Cemetery Office Records

11.

The Creasy Partnership Guildford & Epsom (1985), p 4

12.

The Creasy Partnership Guildford & Epsom (1985), p 6

13.

The Creasy Partnership Guildford & Epsom (1985), p 6

14.

The Creasy Partnership Guildford & Epsom (1985), p 7

15.

The Creasy Partnership Guildford & Epsom (1985), p 8

16.

The Creasy Partnership Guildford & Epsom (1985), p 9

17.

The Creasy Partnership Guildford & Epsom (1985), p 11

18.

The Creasy Partnership Guildford & Epsom (1985), p 11

19.

The Creasy Partnership Guildford & Epsom (1985), p 12

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St. James Twin Chapel Spire

Bibliography 1.

Beckmann, Poul. Structural Aspects of Building Conservation. Mcgraw-Hill Book Company, London, 1995.

2.

Brebbia, C.A. Structural Studies of Historical Buildings IV: Dynamics, Repairs & Restoration Volume II, Computational Mechanics Publication, Southampton, 1991.

3.

Brebbia, C.A. Structural Studies of Historical Buildings IV: Dynamics, Repairs & Restoration Volume IV, Computational Mechanics Publication, Southampton, 1995.

4.

Bussel, Michael. Lecture Notes for Msc. Conservation of Historic Buildings at University of Bath (19.10.05), session 2005-2006.

5.

Courtenay, Lynn. T. The Engineering of Medieval Cathedrals Volume 1. Ashgate Publications, Aldershot, 1997.

6.

D’Ayala, Dina. Damage, Appraisal and Repair of Historic Structures. Lecture Notes for Msc. Conservation of Historic Buildings at University of Bath, session 2005-2006.

7.

Fidler, John. Stone: Stone Building Materials, Construction and Associated Component System: Their Decay and Treatment Volume 2. English Heritage, 2002.

8.

The Creasy Partnership Guildford & Epsom. St. James Cemetery Bath Twin Chapel Spire Feasibility Study Restoration of the Spire. Department of Environmental Services, 1985.

Website Resources 1.

http://www.bathnes.gov.uk/BathNES/lifeandleisure/cemeteriesandcrematorium/ cemlist.htm. List of Cemeteries in Bath (Accessed (29.11.2005)

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