030767C12.doc
SpencerHolmes
20th December 2011
consulting engineers - registered surveyors
PO Box 588
8 Willis Street
CBD Towers Limited
Wellington New Zealand
Ph :
04 472 2261
PO Box 40407
Fax :
04 471 2372
UPPER HUTT
email
[email address]
By Email:
[email address]
Attn: Dennis Cook
Dear Dennis
CBD Towers, 86-90 Main Street, Upper Hutt – Update to Seismic Assessment
We have undertaken a structural assessment of the building at 86-90 Main Street, Upper Hutt,
known as CBD Towers, in order to provide a more detailed assessment than the IEP undertaken
on behalf of the Upper Hutt City Council and to demonstrate the seismic strength of the building
relative to new building standard (NBS).
This assessment has been prepared for the sole use of Gillies Group, the reliance by other parties
on the information contained in this report shall, without the written approval of Spencer Holmes
Limited, shall be at such parties sole risk.
This assessment of the building has been limited to a review and update of the assessment
previously completed by Spencer Holmes Limited in October 2003, titled “Seismic Assessment
for Extensions and Alterations to Astral Towers, 86-90 Main Street, Upper Hutt for Gillies and
Mark Limited”.
The CBD Towers is an 8-storey building with a 2-storey podium constructed in 1975. The
podium is approximately 45.9 metres by 18.6 metres, whilst the tower levels are approximately
22.4 metres by 18.6 metres. The gravity support system consists of precast floor units with a
concrete topping slab. The building was originally designed for a seismic coefficient of 0.12g in
accordance with the requirements of NZS 1900:Chapter 8 1965. The seismic resisting system
consists of reinforced concrete frames in both of the orthogonal directions, where the north-south
direction has been described as longitudinal and the east-west direction as transverse.
The 2003 assessment stated that the building achieved an equivalent strength of that required by
NZS 4203:1992 in the order of 60-70% in the longitudinal direction, and 55-66% in the
transverse direction. The assessment was peer reviewed by ISP Consulting Engineers in the
attached letter dated 17th December 2003, which states:
“Based on our assessment of the documentation supplied by Spencer Holmes Ltd, on our own
check calculations and on the above we believe that the building satisfies a strength level of
approximately 70% of current code in the longitudinal direction and between 65% and 70% of
current code in the transverse direction.”
We have reviewed this assessment with a view to assessing the building strength relative to the
current code, NZS 1170.5:2004. The seismic coefficient for the building in accordance with NZS
4203 used in the 2003 report was calculated to be 0.18g, while under the current code, NZS
Spencer Holmes Ltd Directors Jon Devine, Ralph Jorgensen, Ian Leary, Philip McConchie, Hudson Moody, Peter Smith
2
1170.5, the seismic coefficient was calculated to be 0.15g. The seismic design load applied to the
building to under the current code requirements is therefore less than that required under NZS
4203. This reduction in the seismic coefficient is primarily due to the building having a “mid-
range” period of approximately 0.9 seconds and a capacity for limited ductility. It should be
noted that the current code NZS 1170.5 is considerably more onerous on low period structures of
nominal ductility, and near-fault factors applying only to tall structures with periods exceeding
1.5 seconds.
On this basis, we have calculated the building to achieve an equivalent strength of that
required by NZS 1170.5:2004 in the order of 75-80% in the longitudinal direction, and 70-
75% in the transverse direction.
As the assessment undertaken on the building is slightly above 70% NBS, the building would be
classified as grade B, or “low risk”. Under this classification, the NZSEE still consider
improvement to 100% NBS may be desirable, and recommends upgrading as is nearly as
reasonably practicable to that of a new building.
As the specific assessment undertaken on the building is higher than the 33% NBS threshold for
an earthquake prone building, and with the attached additional information providing the basis
for this evaluation, we are of the opinion that this is sufficient for the Upper Hutt City Council
to ensure this building does not appear on their EPB register.
We trust that this is all you require at this time, however, should you have any questions or wish
to discuss this please do not hesitate to contact the undersigned.
Yours faithfully
Spencer Holmes Limited
Vaughan England
Design Engineer
BE (Hons), GIPENZ
encl:
030767C12 Structural calculations C01-C05
030767R02 Seismic Assessment (2003)
030767B01 Structural Calculations (2003)
ISP Peer Review Letter
Spencer Holmes Ltd Directors Jon Devine, Ralph Jorgensen, Ian Leary, Philip McConchie, Hudson Moody, Peter Smith
