St Mary de Castro, Leicester, grade I - Spire Assessment and Dismantling
Situated in the heart of Leicester, adjacent to the Castle, this church of near Cathedral proportions dates back to twelfth century with various alterations over the centuries culminating in a major scheme of restoration in 1853 by Sir George Gilbert Scott.
The spire, over 35m tall sat atop the 30m high tower (note the past tense!). Likely to date back to the C18 alterations, an assessment of the tower showed it suffered from many defects, including fundamentally a weak sandstone, severe weathering, ovaling, lack of bonding, and extensive vertical fissures running to 6 to 8 meters in length.
My assessment concluded that the spire had a 'factor of safety' of one to withstand the typical wind loading imposed upon it. In other words it was dangerously close to collapse. Various repair options were considered in detail, including the introduction of an inner skin of brickwork, but working in partnership with the church architect and English Heritage, these were all discounted due to the severity of the accumulative effect of all the defects. The radical conclusion of my assessment work was: dismantle the spire.
The engineering challenges
The fundamental engineering challenge was how to you safely access and dismantle a structure which is in danger of collapse under wind load. At 65m tall any scaffold would not be self supporting, but rely on the spire for its own support. Compounding this, the presence of a scaffold increases the wind loading on the spire by over 50%. The final challenge was that circumstances dictated that the dismantling work had to occur in winter months.
Capstone prepared a carefully thought through 'performance specification' for the design of the scaffold, and the sequencing for its erection and dismantling. The heightened risk of collapse due to the presence of the scaffold was spelt out to all parties involved, including Building Control, the main contractor, the client, the scaffold designer, EH (the funder) and church architect. Capstone proposed risk mitigation measures, including vane anemometer wind speed readings, and daily tailored forecasts from the Met Office. Sharing the risk, all parties agreed to proceed. The scaffold was erected over a 7 week period and the spire was successfully dismantled in early 2014.
Client - The Church
Funder - English Heritage HLF
Contractor - Midland Stone Masonry