St John’s Church
1 October 2012
Assessment of Earthquake Resilience Proposed Strengthening Measures forSt John’s in the City Church Building
These notes are further to a presentation made to members of the St John’s congregation after the worship service on 30 September 2012 and summarise the presentation and discussion.
An Initial Evaluation of the earthquake resistant strength and resilience of the church building undertaken on behalf of WCC several years ago rated the building Earthquake Prone at 21%NBS.
A Detailed Seismic Assessment of the building has since been carried out and the results of that have just become available. The following notice will be displayed at the front entrance to the church. It describes the outcome of the detailed assessment.
Note that “%NBS” refers to the proportion of the New Building Standard (the strength and resilience requirement for a new building today on the same site) that a given building possesses.
Risk to the Building
The building will suffer significant structural damage in a 25%NBS earthquake, with a high risk of irreparable damage.
Risk to Life
But it is unlikely to collapse until an earthquake of 50%NBS earthquake occurs, which has an average annual probability of 1/100 or a 40% chance of being exceeded over 50 years.
Our structural engineer, Dunning Thornton, states:
“….the risks in the building are significantly lower than many other buildings inWellington”
This is because:
- The building is used for a very limited amount of time each week, very much decreasing the risk exposure
- The building possesses a high level of resilience beyond the point of first structural damage
The timber building above has been damaged structurally (in the Christchurch earthquake of Feb 2011) to the point where it would certainly be considered uninhabitable. However, it has not collapsed and is sufficiently intact that life safety was preserved.
Many pre-1965 timber houses in Wellington would not be rated significantly higher than our church building in terms of earthquake strength and resilience.
Summary of proposed strengthening measures
The 3D representation of the church building structural framework shows the buttresses along the sides of the main hall and the buttressed bell tower and spire.
- The attachment of the spire to the bell tower requires strengthening. This can be simply achieved internally.
- The timber ground floor of the church requires bracing in many locations. This is domestic type work and can be simply achieved.
- Improvement of the longitudinal connection along the eaves on each side of the roof space will be investigated.
Currently, the timber buttresses along the walls and at the corners of the towers sit on concrete pads without significant attachment. The buttresses require “holding down” by drilling ground anchors into the good ground at 6-8m depth so that if the church is shaken in a transverse (across the building) direction (from Willis St towards the courtyard and vice versa) the buttresses along the walls and on the towers will be able to brace the building. This will prevent the walls toppling or leaning excessively, so that the roof structure will not be disturbed.
In addition, some further investigation of the attachment of the buttresses to the top of the walls will need to be undertaken, with possible improvement in these connections required.
The buttress locations requiring anchoring are highlighted on the plan above. This work can be undertaken from outside and is expected to have a very minor effect on the building aesthetics.
Expected cost of strengthening
The proposed strengthening works are expected to cost in the range $300k - $500k.
They will most likely achieve between 70% and 90%NBS.
Possible programme
Because structural engineers are difficult to commission at present, largely due to demands arising out of the Christchurch Earthquakes, it is likely that there will be some delay before Dunning Thornton is able to complete the detailed design of the strengthening measures they have proposed.
The requirements of the church administrative structure will also play a part.
We estimate that the design might be completed by September 2013 with the physical works tendered and commissioned by the beginning of 2014. Completion of the strengthening should be possible by the end of 2014.
Questions and Answers from Church EQ resilience and strengthening presentation, 30 Sept 2012
- Explain %NBS as a Richter magnitude
Richter magnitude is a measure of the energy released at the source of an earthquake. This could be 10km away and at 10km depth or hundreds of km away and at 100km depth. The Richter magnitude alone is therefore not an adequate representation of an earthquake at a particular site.
The seismic loading code specifies a 1/500 year return period earthquake as the 100%NBS earthquake which will be generated by a 7.6 Richter Magnitude earthquake on the Wellington Fault which runs along the foot of Tinakori Hill and along the Hutt Valley motorway. It is not a simple matter to interpolate for lesser magnitude earthquakes and the level of shaking experienced at a particular site is dependent on many factors including depth to source and location of the source along the fault line.
- Is the church founded on good soils?
This requires a little research on my part for addition to an information sheet. However, the general idea is that there is sufficiently good ground at 6-8m depth to allow effective anchoring of the buttresses.
- Are the internal steel/iron posts well enough connected to the gallery?
The posts are sufficiently well connected at both head and base to allow them to prop the gallery while “going for the ride” under earthquake shaking. The gallery floor and soffit timber sarking ensure that they are braced to the walls and buttresses.
- How about the vertical shaking that occurred in Christchurch? Will the strengthening cope with that?
The level of vertical shaking in Christchurch was unexpected and unusually high, partly due to amplification from the deep soil “basin” under the Christchurch CBD. However, ground anchoring of the buttresses along the sides of the church and at the towers will provide resistance to vertical shaking effects.
- Is the organ well restrained, and will that be considered?
The organ pipes are tied together to provide some restraint. However it will be necessary to consider this more closely and possible work on an organ re-build might afford an opportunity to brace it fully.
- Are the spotlights and other fittings in the church ceiling likely to fall in a large earthquake?
The general lights and the ventilation grilles are well-fixed and should not fall out. It will be necessary to consider augmenting the spotlight fixings with safety chains.
- What is the best thing to do if an earthquake strikes while we are in the church?
In New Zealand we have relatively good standards of construction overall. It is not advisable to attempt to exit a building during earthquake shaking. You are much more likely to be hit by falling pieces of cladding etc than to be crushed by a building collapse. You should get down between the pews as first preference. They are very solid and will offer the best available protection from falling items.
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