Historic Churches 2018

BCD SPECIAL REPORT ON HISTORIC CHURCHES 25 TH ANNUAL EDITION 23 north wall. A vibration monitoring network was set up with a root monitor on the top of the masonry east wall that would provide measurements both of any motion available to transfer into the timber structure and at the same time, of any sway mode in the wall itself. Other monitors tracked vibration through the roof timbers to the rim of the dome. To date, the assumption that any imposed vibration measurable on top of the wall would disperse through the timber structure before reaching the dome appears to have been correct. WORKS OF ART Despite the dispersive effect of traditional timber framed structures, the risk remained that the frescoed dome at St Mary Abchurch could be affected by residual energy to some degree. How might works of art such as this respond to and be protected from vibration? Major art museums around the world have had to tackle the dilemma of how much of their collections can remain on display safely when major construction works, often to extend the museum’s own buildings, go on around them. This has been a matter of concern at the National Gallery, British Museum and Victoria & Albert Museums in London. The same issues have also been addressed at museums in the USA, and an interesting study has been conducted in Florence of the response of Michelangelo’s ‘David’ to vibration from pedestrian footfall in the gallery around it. The issues highlighted in research reports include attritional damage to painted gessoed timber (ancient Egyptian coffins seem to be singly the most vulnerable museum artefacts) and failure of old repairs. ‘Walking’, the vibration-assisted movement of artefacts off shelves is also highlighted as a real and potentially catastrophic risk. The relevance of the reports and papers on these museum collection studies to the British parish church is that our historic churches are themselves museums of works of art, from frescoes and murals to framed paintings on canvass or panels, commissioned and donated works of art and craft, statuary and memorials, and many others. Potentially these are all vulnerable to the effects of vibration. THE IMPORTANCE OF FORWARD PLANNING The most effective monitoring process is one that starts early. There is no British Standard historic church and there can be no standard handbook for designing a monitoring network for a particular building in its own particular circumstances. Neither are there standard alert thresholds or limits unless the structure is relatively simple, well-understood and in good condition. Every historic church will be different and must be evaluated and treated on merits. The input of a conservation architect advising on its likely structure and condition is invaluable. Every morsel of information from those familiar with the behaviour and quirks of the building, from the fabric warden to the quinquennial surveyor, is useful in determining its specific vulnerability and therefore the right regime of alert levels to select to protect it. No discussion of this kind is useful if the church concerned does not have the funds to pay for advice. That is where well-negotiated Neighbourly Matters Deeds are likely to be vital. The cost of setting up the kind of monitoring exercise outlined above will be in the low thousands, with an ongoing recurring weekly cost in the several hundreds of pounds. That cost would be an unsupportable burden on the average parish church, but it is likely to be minor in the context of a major construction project. On the other hand, the potential cost to a developer of causing irreparable damage to an historic building or work of art in consequent claims and reputational damage could be vast. The reality is that the alerts system is expressly intended not to impede work on site but to help with its management. A site manager with confidence in the system can adopt and respond to it in such a way as not to incur stoppages and delays. A well-conceived system and a well-run site operation will only trigger alerts exceptionally and the reporting regime is intended simply to provide an audit trail. In that sense it parallels a well-run health and safety policy. In the exceptional circumstance that an incident on site does result in damage to the church next door, the monitoring networks and audit trail would provide the evidence to support a claim. A thoughtfully designed monitoring and alert system coupled with an actions protocol should slip seamlessly into the construction management plan that is now universally applied on all major development projects. Open communications and openly accessible data foster confidence on all sides. As a result, neighbourly relations will be enhanced and the risk of a catastrophic outcome reduced to the minimum. DAVID TREVOR-JONES CPhys MInstP FIOA is an independent acoustician working both solely and in association with Sustainable Acoustics Ltd (see page 51). He has been involved in construction vibration and noise monitoring for more than 30 years and has worked to protect ten historic City churches to date, along with other ancient buildings. He acknowledges the assistance of Bob Wilson of Edwards Wilson, specialist ecclesiastical surveyors, in preparing this article. The frescoed dome at St Mary Abchurch, City of London (Photo by David Iliff. License: CC-BY-SA 3.0 https:// creativecommons.org/licenses/by-sa/3.0/)