Historic Churches 2018

BCD SPECIAL REPORT ON HISTORIC CHURCHES 25 TH ANNUAL EDITION 21 CONSTRUCTION VIBRATION David Trevor-Jones I T MIGHT seem to be rather obvious that an ancient structure is vulnerable to vibration. But church towers have been moving in response to bell-ringing since bells were invented. So why do we need to worry about vibration now? The feature of the present era that has brought the question to a head, especially in the City of London, is adjacent redevelopment. The intensity of redevelopment has grown with the forest of cranes that has stood over the City since the early 2000s. Demolition of 1970s and 1980s buildings very often requires breaking massive concrete elements and thick reinforced concrete ground slabs. Huge amounts of energy are released into the ground, usually percussively, and sometimes insufficient thought is given to how far it might travel and what effect it might have. Vibration, like sound, can be deflected and reflected. Man-made and geological discontinuities in the ground influence the path of the three- dimensional sub-surface waves while a surface wave (called the Rayleigh wave) behaves more like the ripples on a pond. About 50 metres below the surface in the City the top of the London Clay beds represents a reflective surface to some ground wave modes. The result, spectacularly demonstrated during large scale demolition on the east side of Bishopsgate in the City at times over the past ten years, can be very long distance propagation. Vibration has been observed and measured at distances of the order of 100 metres from its source. It is not necessarily the development site next door that is generating the vibration detected in a building, and short-distance propagation might be weaker than long-distance. Modern office blocks tend to require very deep piling and very deep, multi-level basement excavation. While resultant ground movement is probably a greater long-term concern for neighbours, vibration can be a hazard in the short term for surrounding buildings. And whereas the response of relatively simple, stiff, engineered structures to ground vibration is reasonably predictable, that of ancient structures is not. ST HELEN’S, BISHOPSGATE It was the demolition of the massively over-engineered, reinforced concrete Crosby Square building from the 1970s to make way for The Pinnacle skyscraper (since re-cast as 22 Bishopsgate and presently under construction) that brought these issues into the author’s domain. The demolition started to generate unexpectedly high magnitudes of ground vibration and alerted the neighbourhood to the unusual local ground conditions that seemed to propagate it. The medieval church of St Helen’s Bishopsgate was affected and when redevelopment of the immediately adjacent southern block of St Helen’s Place received planning permission as well, a fundamental review of the threat to the ancient structure of the church (albeit as substantially restored by Quinlan Terry after the 1993 Bishopsgate bomb atrocity) became pressing. A vibration monitoring and management regime was included in the Neighbourly Matters Deed negotiated by the representatives of the church and the developer. The question was, how to specify meaningful, appropriate vibration level thresholds to trigger an escalating management response? The parties agreed to fund a limited literature search and consultation exercise to be conducted jointly by their respective expert advisors, the present author acting in that for the church. Standards addressing the thresholds of exposure to vibration that could indicate a risk of building damage have been developed over more than 80 years, starting with work in Germany before the Second World War. Part 3 of the current German standard, DIN4150, The church of St Ethelburga, Bishopsgate, City of London with 100 Bishopsgate demolition site immediately adjacent St Helen’s Bishopsgate (just right of centre) with St Helen’s Place immediately to its left (before its demolition) and the demolition site of 100 Bishopsgate further left (All photos: David Trevor-Jones)