CHRIST CHURCH CATHEDRAL
STRUCTURAL STRENGTHENING / PARTIAL REBUILD option - sir miles warren scheme 2015
The Christ Church Cathedral was, and still is, one of the most important and well-known icons of the city of Christchurch. During the recent devastating Canterbury Earthquake sequence, the Christ Church Cathedral suffered significant damage. Since the earthquake sequence, there has been much debate as to what can be done to this historic building, and several schemes have been put forward.
Ruamoko Solutions is a specialist structural and seismic engineering firm, and has developed concept designs for the temporary stabilisation, permanent strengthening and partial rebuild of the Christ Church Cathedral as per the proposal put forward by the renowned architect Sir Miles Warren.
Managing Director Grant Wilkinson was the project director at Holmes Consulting Group for the design and installation of the seismic strengthening work on the Cathedral in 1999 which saved the building from complete collapse in the February 2011 earthquake. Grant has designed the strengthening for many heritage buildings in Christchurch, most of which survived the large earthquakes, albeit with various levels of expected damage given the magnitudes of the shaking.
Grant has developed a method of erecting primary stabilisation structures to the Cathedral with minimal risk to persons carrying out the installation. The proposed stabilisation structures will connect to the steel strengthening members that were installed in 1999, to secure the dangerous high level masonry of the nave and the transept walls. The stabilisation also includes the controlled deconstruction of the side aisle walls and the transept end walls down to window sill level. The proposed stabilisation work has been carefully considered by the contractors who strengthened the Cathedral in 1999, and a sequence and methodology has been developed that will allow all work to be carried out in a safe manner.
DECONSTRUCTION TO A SAFE HEIGHT
Careful deconstruction of the severely damaged and displaced masonry walls is a priority. The side aisle walls and buttresses and the north and south walls of the transept will be deconstructed to their window sill levels. The remnants of the nave west wall will be deconstructed to ground level, except for those wall sections that were strengthened with concrete shear walls in 1999. The slate roof and upper nave walls are generally lightly damaged and will be carefully deconstructed, as will the columns and arches that support those upper walls and the high level masonry walls and arches at the transept crossing. The only full height walls to remain will be the less damaged masonry walls to the apse and sanctuary.
BUILDING IMPORTANCE LEVEL
The Cathedral is defined by the New Zealand Standard for Structural Design Actions as “Importance Level 3” (IL3) because it will contain crowds of people and because the building and its contents are highly valued by the community. As a minimum, the strengthened and partially rebuilt Cathedral will have 30% more seismic strength than other “normal” use buildings.
SEISMIC DESIGN LOAD AND BUILDING STRENGTH
In 2011, the local seismic design loads for new and strengthened buildings increased as a direct result of the very large earthquakes. The increases were 36 percent and 80 percent for strength and stiffness respectively. Together with the increased importance level, this means that the strengthened and partially rebuilt Cathedral will be 177 percent stronger and 236 percent stiffer than equivalent “normal” use buildings constructed just prior to the earthquakes. This will result in a strong and safe Cathedral with low probability of structural damage in moderate to strong earthquakes with return periods of up to 1,000 years.
A major benefit of the Warren design is the removal of the massive and heavy construction at high level and its replacement with lightweight steel and timber-framed construction. This significantly reduces the structure and strengthening that would otherwise be required. Approximately 3,900 tonnes of masonry is deconstructed from heights of up to 23m. The replacement wall structure will be only approximately 250 tonnes. Similarly, the existing slate roofing will be replaced with new copper roofing which will reduce the roof mass by approximately 100 tonnes.
RECONSTRUCTION OF SELECTED MASONRY WALLS
The north and south side aisle masonry walls and buttresses will be reconstructed to their present height and fully supported by the concrete backing/bracing walls. The west wall will be rebuilt with a basalt stone and limestone veneer to replicate the original wall appearance and will be fully supported by a new 250mm thick concrete wall up to the rose window sill level, above which, the stone veneer will be supported by steel and timber frames. The rose window will be reconstructed in 11 segments by using white precast concrete that will be post-tensioned together into a single feature window fully supported by the heavy steel wall framing.
STRENGTHENING THE RESIDUAL MASONRY WALLS
Strengthening the masonry walls that are to be retained is reasonably straightforward. The walls will be drilled, grouted, pinned and braced before careful deconstruction of the inner limestone veneer and plaques. Supplementary foundations are required to be cast against and pinned to the existing foundations, and new concrete walls will be cast onto and pinned to the interior of the stone walls. The inner limestone veneers and plaques are then reattached to the new concrete wall to match the original layout. Elsewhere, the inner face of the concrete walls will be plastered with string courses and the like, to match the original surface features. The inner limestone veneers are then reattached to the new concrete wall. Similar strengthening work has been successfully completed on many heritage buildings including on four walls of this Cathedral in the 1999 strengthening, the strengthening of NZ Parliamentary Buildings, strengthening of the Canterbury Museum and many others.
The Cathedral was originally constructed on good foundation soils. Original plans show that the masonry walls were founded directly on top of the gravel layer that is approximately 1.7m below ground floor level. All existing masonry walls that are to be strengthened and reused will have new strengthened foundations that will also bear onto that gravel layer. A thick concrete raft slab under the entire Cathedral floor will be added for mitigation against any future differential settlements of foundations or floors and to ensure all foundations are well tied together.
BELL TOWER AND SPIRE
The bell tower will be founded on a thick concrete raft foundation that will be additionally supported by large concrete piles founded deep into the gravel layer below. The tower will have heavy concrete walls and buttresses that are covered with a basalt and limestone veneer to replicate the original tower shape and detail. The new reinforced concrete structure is taken to the viewing gallery floor level above the belfry. The spire frame will be constructed from steel and timber. Copper roofing will be used to further reduce the mass of the tower.