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Ref: 7265 L01
10 November 2012
Bloxam Burnett Olliver
Hamilton
Attn: Graeme Jamieson & Chris Dawson
Dear Graeme & Chris
Temple View – David O McKay Building
Structural Engineering Review
Many thanks to you both and to the Church for hosting me on the 24 August and
for showing me around the David O McKay building.
You have asked me to provide you with an opinion on the likely performance of the
structure during a major seismic event and in addition, the ramifications of the
building undergoing a change-of-use.
You have advised me that the building (and the surround campus) is no longer
being used as a school and that the Church wishes to remodel and redevelop the
campus as an area of housing with a rest-home, a new chapel and a historic park.
The opinions I have given in this report are of a strategic rather than detailed
nature. I have not undertaken any structural analysis however I have inspected
the building reasonably thoroughly and have viewed the original drawings.
The original drawings are dated 1955 and appear to have been competently
prepared. The construction too appears to have been generally competently
executed. I understand that they were largely built with missionary labour but in
any event it is reasonably obvious that the work was generally supervised by
competent tradesmen.
In 1955, both in New Zealand and in USA (where the building was designed),
there were requirements for building designers to make due allowance for the
effects of earthquakes. In New Zealand this had been formalized in 1935 with the
issue of the New Zealand Standard Model Building By-law, NZSS No.95. This
applied restrictions on the use of some materials and required the designer to
design the structure for a lateral (horizontal) load of 0.1g (10 percent of gravity).
This was not mandatory unless the local authority had adopted the by-law.
Consulting Structural Engineers
94 Dixon Street, PO Box 27-153, Wellington 6141
Telephone (644) 385-0019, E-Mail: [email protected]
Temple View – David O McKay Building, Page 2
By 1955 understanding of structural behaviour of reinforced concrete, during
seismic shaking, had not progressed very far, particularly in relation to post-elastic
performance. Certainly there was little or no knowledge of the importance of
ductility or of the necessity for satisfactory post-elastic behaviour. Some useful
improvements were made in the 1965 code but it was not until 1976 that ‘modern’
seismic design philosophy, including requirements for ductility and for a desirable
hierarchy of strength (Capacity Design) was codified.
So when considering the David O McKay building structure, understanding the
code requirements and the state of seismic engineering knowledge at the time of
design and construction is important when determining its likely performance.
Unfortunately the facts that the drawings were competently done and that the
construction appears generally workmanlike and robust give no surety of
satisfactory performance in a major seismic event.
When reviewing the existing structure against both current code principles and
good practice it is readily apparent that it has a number of significant
shortcomings. These include:
It is highly irregular, vertically.
It is highly irregular, horizontally.
It has many heavy wall elements with limited out-of-plane restraint.
In a number of locations a satisfactory load-path to transmit lateral forces to
ground, is not readily apparent.
The reinforced concrete members (columns beams and walls) are generally
not detailed for satisfactory post-elastic performance (not detailed for
ductility).
The likely performance of the ground supporting the structure also needs to be
considered. The recent geotechnical investigation carried out by Tonkin & Taylor
concludes that in the area of the David O McKay building the supporting soils have
an estimated liquefaction potential index of 4-14, at the Ultimate Limit State. This
translates as a low-high risk of liquefaction potential and estimated settlements of
between 67-167mm. This level of settlement may not, on its own, induce structure
failure or instability however even for a superstructure that fully meets current
seismic codes, high damage ratios may be expected in an event that is less than
the ULS “design event”.
The effect of the structural shortcomings and the underlying liquefaction potential
is that the structure has numerous vulnerabilities, typically referred to as critical
structural weaknesses. Not withstanding that the seismic hazard for the area is
relatively low, compared to say Wellington, I believe it is likely that the building is
earthquake prone and that its performance during severe shaking is likely to be
very unsatisfactory with a reasonably high probability of localized out-of-plane
failure and some areas of more global collapse.
If the building is to be recommissioned for a new purpose then I would expect the
Building Consent Authority [BCA] would regard the building as undergoing a
change-of-use. This would necessitate significant seismic strengthening to
achieve dependable performance as near as reasonably practicable to 100% of
New Building Standard [100%NBS]. This would involve considerable intervention
to the existing form and fabric and most likely would include:
Consulting Structural Engineers
94 Dixon Street, PO Box 27-153, Wellington 6141
Telephone (644) 385-0019, E-Mail: [email protected]
Temple View – David O McKay Building, Page 3
New primary bracing elements (shearwalls and braced frames)
Improvement of roof and floor diaphragms
Strengthening of walls for out-of-plane actions
Improvement of existing structural elements to achieve dependable ductility
Enhanced connections between members and diaphragms etc.
Considerable improvement of foundations and sub-foundation materials.
This may include retrofitting of deep piled foundations.
The cost of this work would not be insignificant. I am loath to give any sort
estimate without undertaking specific analysis of the existing structure and
conceptual strengthening design. However, based on other recent projects you
might expect the cost to be in the range of $1000-$2000/m2. Foundation work,
necessitated by the liquefaction potential would push it towards the upper bound
and possibly beyond.
The nature of the work that I anticipate would be required is complex, messy and
time-consuming. It will also have some effect on the existing planning and
inevitable some effect on the appearance of the building, both externally and
internally.
I would note that strengthening to 100%NBS does not imply that the building
structure is equivalent to a new building, in all respects. The building will still
contain old, brittle materials. So while the risk of collapse of a well retrofitted
building should be no greater than for a new building, the onset of damage
resulting in costly repairs or demolition may be sooner than for a more modern
building. I should also note that current code intent is focused on life-safety rather
than property protection so even new buildings may be fit only for demolition
following a large “design event” earthquake, unless specific damage avoidance
design (in excess of the requirements of the code) has been undertaken.
You have specifically asked me to comment of the safety of the auditorium ceiling.
From my brief inspection it appears that the ceiling consists of a cement-based
plaster on a steel lath which is suspended from the roof structure above. I don’t
consider that it is a major seismic risk relative to the remainder of the structure.
However, I understand that small portions have delaminated and bits of plaster
have fallen. My advice is that the whole ceiling should be inspected/tested for
drummy areas and delamination.
You have also asked of the practicality of retaining some portions of the
structure/building for incorporation into the new development. It is of course
possible but the individual existing elements will still require local strengthening. In
addition the effect of incorporation may well compromise the global seismic
performance of the new structure and its structural efficiency and cost. I suspect
too that the very bits that may be most desirable to retain may be the high risk
elements of the existing building.
Finally you have also asked me whether the building should continue to be used
on an ad hoc basis: The risk of a severe seismic event within any short, time
period is small and so occasional, continued use, as an interim measure, can
probably be morally/ethically justified while redevelopment or re-strengthening is
planned. The timeframe for such an interim arrangement should be as short as
Consulting Structural Engineers
94 Dixon Street, PO Box 27-153, Wellington 6141
Telephone (644) 385-0019, E-Mail: [email protected]
Temple View – David O McKay Building, Page 4
possible, say 12 months maximum. If the occasional use were to carry on,
effectively indefinitely, then I would suggest that this not justifiable.
I hope that this provides the information that you require. I would be happy to
provide elaboration should the need arise.
Yours faithfully,
Adam Thornton
DIRECTOR
121110
Consulting Structural Engineers
94 Dixon Street, PO Box 27-153, Wellington 6141
Telephone (644) 385-0019, E-Mail: [email protected]