Historic Churches 2018

22 BCD SPECIAL REPORT ON HISTORIC CHURCHES 25 TH ANNUAL EDITION is an important authority. So is the corresponding British Standard, BS7385-2, but neither addresses potentially fragile, ancient and non-standard structures other than rather unhelpfully broadly. The outcome of a wide-ranging consultation with UK-based building conservation and special interest groups was that surprisingly little information was available. It seems that development including demolition has taken place close to ancient buildings without much attention to their vulnerability to ground vibration, not least in London. The sites yielding the greatest case study information were in Southampton, where the West Quay shopping centre development had impinged on the medieval city wall, and at Christ Church Greyfriars where the construction of an office building had impinged on both the church (maintained as a controlled ruin since destruction in the blitz) and part of London’s Roman wall. Eventually the synthesis of standards, documented evidence (scant though it was) and anecdotal experience led to the relevant parties agreeing amber and red alert threshold levels along with an absolute limit value for vibration monitored at points on the medieval rubble north wall of St Helen’s. The term ‘limit’ is widely used in noise and vibration monitoring work, but it is only meaningful if breaching it has a consequence. In the case of managing vibration to avoid building damage, the limit value must represent the onset of a real risk and therefore has a real significance. At St Helens, the amber and red threshold levels were established to help manage the processes generating the vibration, so that the absolute limit would never be exceeded. Amber alerts would be made available only to the construction site team to advise them that significant but sub-actionable vibration was occurring, escalating to red alerts, usually shared among all interested parties, to warn that vibration was approaching a significant risk level. A protocol was established to set out a hierarchy of actions to be taken in response, from noting and recording to suspending work to review methods. The alert and limit values are specified as velocity values. Vibration may be measured as a displacement, velocity, acceleration or even jerk (acceleration of acceleration) and the proprietary analytical software packages now commonly available with monitoring instruments allow transformations between them to be made rapidly and easily. The advantage in referring to velocity is that the values are meaningful in both building damage risk and human perception applications. Geophones, which are velocity transducers, are often used for this purpose. The alerts are sent out from vibration monitoring devices via mobile phone or wi-fi networks. The alert levels are programmed in and a message is automatically sent to a list of registered recipients if they are exceeded. As a rough order of magnitude indication, amber alert thresholds for churches are usually in the 2–5mm/s range and red in the 3–8mm/s range, with the absolute limit in the 5–15mm/s range. The choice of actual threshold depends upon the specific vulnerability of the subject building. The positioning of monitors is guided by an International Standard (BS ISO 4866) but in the cases of historic buildings generally and of churches in particular, locations must be chosen to reflect the specific sensitivities of the subject. Monitoring on memorials fixed to the church walls can be helpful both because they can be seen in some degree as the canary in the coalmine in respect of the building’s response to vibration, and also because historic memorials are valuable works of art and sensitive to vibration in their own right. Although unlikely, the nightmare outcome could be a precious work of art ‘walking’ off its wrought iron fixings under vibration excitation. Conventional masonry and especially framed buildings tend to sway in response to ground vibration, both because energy in the ground naturally rocks them about a rotational axis in or below their footings or foundations and because of the relief of the inertial mass imposed by higher storeys with height up the structure. The monitoring at St Helen’s was designed around that proposition with monitors placed high up and low down, and distributed along the length of the medieval north wall, including on one of the important memorials fixed to it. The results, which included documented incidents of serious impact and loading, showed that the random stone facing with a well-consolidated rubble core, all bonded with a lime mortar of some kind, reacted dispersively – that is to say, the vibration energy dispersed through the slightly elastic lime mortar, absorbed along the way by the frictional and dynamic losses in the wall much as water wave energy is dissipated by a boulder beach defence. The structure itself was initially investigated and a watch was then maintained by a specialist team of conservation professionals during demolition work. The wall did not sway as a modern one would and reacted to at least one major accidental impact just locally, the impact energy absorbed by the complex three- dimensional web of mortar in the irregular joints between facing stones and bonding of the random rubble core. TIMBER STRUCTURES The St Helen’s experience can be extended to timber structures. At St Mary Abchurch, adjacent to and partly over London Underground’s works to extend Bank Station, the principal aim of vibration monitoring was to protect Wren’s frescoed saucer dome ceiling and the timber roof structure supporting it. It was speculated that the sheer complexity of the timber structure would be key. Timber moves naturally under varying humidity, temperature and wind loading so to some degree the roof and the dome suspended from it have probably rocked and swayed significantly over the centuries (it even survived the Blitz, albeit with considerable damage to the ceiling). The key differences between natural perturbation and the forces that can be imposed by demolition and construction are that the latter can include sudden repeated shocks and relatively long-term repeated excitations. A veritable forest of timbers in the roof of St Mary’s with very many complex joints provides a dispersive system similar in concept and probably in behaviour to the three-dimensional net of lime mortar bonding all of the elements in St Helens’ Triaxial geophone mounted on a bracket high up on the north wall, St Helen’s Bishopsgate, City of London Triaxial accelerometer mounted on top of the Throckmorton memorial, St Katharine Cree, City of London