Nelson Residents Association

YOUR CIVIC GUARDIAN

TC - Let D Braithwaite

17 Champion Tce

Moana,

Nelson

 

25 March 2014

 

Dear Kerry

You asked for my opinion of the conclusions in the Tonkin and Taylor (T&T) report to Arthouse Architecture dated 12 March 2014, in response to Arthouse request of 11th March.  The T&T report has been published by NCC for public information in “Live Nelson” issue 372 dated 24 March 2014.

I have read the T&T report.  I have to admit some surprise to read that two new weaknesses, not previously reported, are now reported by T&T just one day after being requested to do so.  If the piles under the main hall have insufficient capacity to resist the loads they will be subjected to in an earthquake, and are weakened by corrosion, as now inferred by T&T, it would be hugely expensive to replace them, and the building would therefore have been condemned long ago for this reason alone, without worrying about the risk of liquefaction damage and various other structural weaknesses, which have been investigated and reported many months ago.  I find it very surprising that T&T have raised the new issues of lack of capacity of the piles and their weakness through corrosion so quickly at this late stage in proceedings.

Pile Capacity

I take “pile capacity” to mean the strength of the piles to resist without failure all the stresses which the applied loading will create.  T&T specifically exclude, in this regard, the bearing capacity of the piles on the ground on which they are founded.  What they are talking about is the structural strength of these elements of the building.  I had thought that all aspects of the strength of the structure has been examined and reported by Holmes Consulting Ltd (HC) in their report dated February 2014.  HC clearly considered the structural strength of the piles under the building was within their brief.  They state that the piles under the northern extension have a capacity equivalent to 100% NBS (HC cl. 4.1.5 (no worries!).  As to the southern extension, they say that the piles supporting transverse loading have a capacity of 30% NBS (HC cl.4.3.4 (no good!). As to the piles supporting the main hall, HC make no comment (Why?)  This absence of comment could indicate either (1) that they saw no weakness that needed them to comment, or (2) they have inadvertently omitted to report on this part of their strength analysis.  HC could be asked to resolve this matter.

Corrosion of Reinforcement in Piles

T&T have presumably conducted some tests on which they base their opinion that some piles may be weakened by corrosion of the reinforcing steel.  Such corrosion is promoted by the presence of both oxygen (in the air) as well as water, especially salt water, and is not uncommon in concrete bridge and wharf piles where they are exposed in the tidal range and are subjected regularly and alternately to wetting during submergence followed by air drying.  The piles under Trafalgar are embedded in ground where free groundwater and air flows are unlikely, in my opinion, to promote equivalent deleterious conditions, and there T&T’s opinion is overly and unnecessarily cautious.  I will stand corrected if T&T testing that I presume they have done shows that ground chemistry and permeability of the concrete and the depth of cover of the reinforcement are each or all significantly unfavourable to the durability of the piles.

 

David Brathwaite.

 

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