Thursday, 24 November 2016

Wellington post-quake visit

 

IMG_0285Yesterday I was out and about looking at damaged buildings around central Wellington to help understand why they were damaged. (This is what designers do.)

As you can see, there are three main clusters of damage: on fill at the waterfront and Te Aro flat, and above Waterloo Quay in Thorndon:

 

 

Several things struck me (observations rather falling masonry, fortunately):

  • Deloittes-23.11.16Perhaps the greatest thing to notice is not so much that so many buildings were damaged, but that so few weren’t! Around 60 buildings are on the official list of damaged buildings, and there is talk that maybe four will be demolished. Only four in a city of thousands! Focus on that and you realise what a job those engineers did.
  • It’s true however that not all damage has yet been found (not all buildings have their structure as openly exposed as the Courtenay Place/Reading Cinema carparks), nor that all of it is visible from outside. Yesterday, for example, when I walked past the Asteron Centre in Featherston St, it was still considered sound. Overnight however cracks were found in the stairwell, and the building is now officially declared unsafe. The only damage seen from the outside however is a plywood sheet over a windowpane – and much the same can be said of the officially-damaged Deloitte’s building on Brandon St, below, which aside from a few fallen ceiling tiles looks undamaged from the exterior. And as for the pipes and cables under Featherston St, well, that’s another story altogether …
  • That said, there was talk about town that the building in Featherston St had suffered in the 2013 Seddon earthquake, and surprise that it was not on the ‘damaged list’ this time.
    And this was a theme with several of the damaged buildings I saw: new cracking evident from the outside (in the precast panels of the Revera building in Mulgrave St for example, seen below) had clearly joined hands with cracks from earlier times that had been cleaned up and painted.
Revera-23.11.16

Cracking evident at spandrel/mullion joints on precast panels at the closed Revera Centre in Mulgrave St

  • Let’s also celebrate the crews out cleaning up all the debri. This is work they hadn’t programmed, and yet just ten days after the quake the damage to the city is clearly well contained. Glass and tiles fell from tall buildings along The Terrace, for example, but the fast cleanup leaves the streetscape looking as clean as it always has – and much the same can be said of the rest of the city I saw.
    Intergen-20.11.16
  • For the most part, the effect of the quake (if any) on smaller buildings was barely evident in most parts of town I saw. Those suffering damage were primarily taller buildings. That said however one of the worst damaged buildings, the comparatively new Statistics House (below, behind a severely damaged low-rise building on the disturbed ground around both), is low-rise. The effect of poor ground conditions here at the severely damaged Centreport compound– sitting on fill that has clearly suffered – is undoubtedly a large factor. So the role of all Welllington’s reclaimed land is clearly a factor to bear in mind.

StatsHouse-23.11.16The much-discussed Statistics House, sitting on failed reclaimed land next to a severely damaged low-rise
building on the right. Note the drop in ground level by the lower building’s entrance step.

  • The building code requires that “significant” buildings which includes council buildings, defence buildings, archives etc. be given a higher level of seismic performance. Yet both the 2007 Defence and main Archive Building (first picture below), and both Wellington Council and Greater Wellington Regional Council Buildings, were all damaged – just Christchurch’s council building was damaged in the Canterbury quake. There are good reasons for the damage to all of these, but perhaps the damage to these last three especially might suggest that the expertise assumed to reside in these buildings about these very issues may be very much less than many might suppose.

DefenceHouse-23.11.16Damaged Defence Building, above left; damaged Archives Building, centre; undamaged private apartment building, right.WRC-23.11.16

Greater Wellington Council and Wellington City Council buildings (above and below) both closed for business.
Not altogether a ringing endorsement of the expertise alleged to lie within.

Council-23.11.16
  • Remember, there is no such thing as earthquake-proof. The first responsibility of the structural designer is primarily to maintain the building’s integrity in  quake sufficient to get the people out, resulting in what might be called “controlled damage.” Although the quake happened at midnight rather than midday, and at one-third of the intensity the structures were designed to withstand, this was achieved all across town.
    Damage to secondary elements however, caused by failure of partitions, windows etc. to move with a building’s more ductile structure has sometimes caused significant failure that could, if the damage had occurred at midday, been catastrophic. Falling wall tiles, falling fittings and falling panes of glass from tall buildings for example: several buildings suffered in this way, and all of these would have been deadly to anybody in the street. (While not itself very tall, the new BNZ building on damaged ground at Waterloo Quay, below, exhibits this kind of failure, with internal infill walls cracked and window rubbers and external fittings and finishes popped out or unfastened.) This is not so much an engineering failure as one of the designers of those secondary elements – a warning to all designers to understand a building’s earthquake response, and also a reminder of the risk of potentially dangerous finishes and styles of buildings in a city located on a significant fault line that has itself not yet ruptured.

BNZ-23.11.16
BNZ Harbour Quays building on damaged reclaimed land beside Waterloo Quay

StatsHouse-23.11.16-2
Statistic House exterior: stiffer secondary structures (wall, glass curtain walling system)
crushed and/or buckled by movement of more ductile structural frame

  • This, in other words, is a warning that should be heeded.

[All pics by PC, unless otherwise noted]

to be continued …

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3 comments:

  1. Quite right. Despite 30 seconds of shaking, there is 20 seconds of shaking after the big one, only a handful of buildings failed. Oddly, none are earthquake prone so the engineers will have to explain that.

    As for this nonsense about the council. Its responsibility is for the public spaces after which individual landlords must assess their own buildings after hiring legions of engineers.

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  2. The location of damage seems to support the observations from Christchurch that ground conditions and depth to groundwater level are crucial, and in many respects more important than the size or intensity of the earthquake and the building design. Christchurch suffered acceleration many times in excess of the design criteria, several times, but the western half of the city sitting on good gravels and >1m to groundwater were largely unscathed, but the eastern half sitting on soft unconsolidated sediments and/or shallow groundwater were the worst affected.

    Interestingly the models used to predict damage from liquefaction used to be insensitive to groundwater depth. They were also insensitive to the depth of the liquefiable layer (i.e. whether the soft sediment is 1m down or 10m down didn't matter too much). The Christchurch experienced showed however both these were crucial, and the new classification system developed by Tonkin & Taylor based on observations in Chch now take both these variables into account.

    To emphasise how much of an inexact (and usually conservative) science this is, one particular site in Cantebury was considered red zone (can't be built on) under some of the older models, but then ended up being TC1/TC2 (almost as good as you can get) under the new Tonkin models. I also learnt that tweaking a few variables in your model can produce wildly varying results, which I suspect also applies to climate models.

    The take home lesson is that ground conditions are crucial, but even if you have poor ground you get massive improvements in performance by adding about 1m of granular fill over the top of it to increase the depth to groundwater, and provide a 'raft' over any liquefiable layers.

    This is largely in the context of low rise, but results in Chch's CBD suggest it probably applies to high-rise too. It's hard to make too many generalizations when dealing with complex geology, but Colombo St in the centre of the Chch CBD represents the approximate point that groundwater depth starts to increase as you head west, and the high-rises east of that line suffered a lot more than those west of that line.

    ReplyDelete
  3. I would venture that much of the Wellington reclaimed land is absolute rubbish as it stems from the 1800's. I watched the Queens Wharf Event Centre being built and the long piles were darned easy to punch in - there was a layer where a tap of the hammer saw the piles go in about 1 meter or more. It seems to me that Wellington has a few (newer) medium height buildings built on very long legs so it follows that they are like big boobed women on long legs with very high heels - they stagger about and trip over a lot.

    3:16

    ReplyDelete

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