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Buildings & Earthquakes Context Our past and why now Risk 2 - - PowerPoint PPT Presentation
Buildings & Earthquakes Context Our past and why now Risk 2 - - PowerPoint PPT Presentation
Buildings & Earthquakes Context Our past and why now Risk 2 examples from Christchurch July 1843 7.6 Whanganui October 1848 7.4 Marlborough January 1855 8.2 Wairarapa October 1888 7.1 North Canterbury November 1911 7.1 Cheviot
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- 1935 after the Napier earthquake – first entry of seismic
requirements into the building code.
- 1950’s tectonic plate movement becomes orthodox
geology –the cause of earthquakes is understood.
- 1960’s onwards NZ engineers visit all major earthquake
sites immediately after they happen to measure and record building performance.
- 1976 – first major change to the loading codes. Design
values increase. Loading codes changes occur periodically until 2008.
- Post 1976 to 2006 – materials codes are revised to
incorporate new techniques to improve the behaviour of buildings in earthquakes.
- Christchurch was the first test of the new methods. It has
reinvigorated seismic engineering thinking.
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Understanding the Risks
Activity
Indicative risks/Fatality risk per year
Smokers:20 cigarettes a day
1 in250
Drinkers: 1 bottle of wine a day
1 in300
Drivers: motor cycling sports
1 in 700
Car drivers: 20 to 40 year olds
1 in 5,000
Pedestrians, Household workers
1 in 10,000
10,000km/year car travellers
1 in 10,000
Hikers in the mountains
1 in 20,000 similar risk to 34%NBS
Air travel: plane crashes per flight
1 in 100,000
Living in buildings: death by fire
1 in 100,000
Death by lightening strike
1 in 1,000,000
Death by earthquake historically in NZ
1 in 1,000,000
Winning Lotto (odds of winning not death)
1 in 4,000,000
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Compliance versus Performance
%NBS Risk Grade
LOW RISK
>100 <1x A+ 80 - 100 1 - 2x A 67 - 80 2 - 5x B ERB 34 - 66 5 - 10x C EPB 20 - 33 1 - 25x D <20 >25x E
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- This building was built in 1984.
- Yellow stickered during the emergency period.
- Limited access for engineering assessment.
- The interior was a mess. Upstairs the glass partitions
between offices were shattered and on the floor.
- All the neighbouring buildings were empty.
- Some had already been demolished.
- There was a heritage building next door which was
being made secure.
- The early assessment of the visible damage suggested
the building could be repaired.
- An IEP rated the building at 33%NBS.
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- The general area became classified TC3 – the ground
is highly susceptible to liquefaction and settlement.
- Measurements showed the floor slab to have settled
differentially up to 100mm.
- The insurer required to know what the cost would be
to repair to both 34%NBS and 67%NBS.
- This necessitated a detailed seismic assessment (DSA)
- The DSA reduced the %NBS to 30%.
- Using the plans and modelling the structure some
additional critical weaknesses had been found.
- Given the known ground conditions no strengthening
was feasible without attending to the foundations.
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- A geotechnical report was commissioned.
- Site testing around the building was done using a
cone penetrometer.
- It confirmed a high water table at 1.2m and
liquefiable soils to a depth of 13m.
- It predicted earthquake settlements of 70mm to
141mm.
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- The geotechnical report recommended piling as the
- nly viable solution for stabilising the building against
future earthquakes.
- A preliminary design using 50 14m long micro piles
was done and cost estimated.
- The estimate of cost for repair to 34%NBS was
$630,000 and $750,000 to 67%NBS.
- However, even if the building was strengthened it
would always be out of square and out of level.
- The building was demolished.
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- The building was given a yellow sticker.
- In reality there are three structures in two buildings.
- The larger building was of unknown age and there
were no drawings of it available.
- It consists of a two storied reinforced concrete frame
with a flat roof in front of a single storey concrete frame with a pitched roof. Both frames are extensively in-filled with brick panels.
- The other building was built in concrete masonry in
- 1984. There was a set of structural drawings available.
- Attempts were made to ascertain what the concrete
reinforcement was by destructive and non destructive means.
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- IEP’s rated the two buildings at 6%NBS and 75%NBS
respectively.
- The older building was earthquake prone.
- In view of the relatively light damage suffered by the
structure , particularly in view of its proximity to the February 2011 epicentre, a DSA was done.
- A DSA of the old building re-rated it as 40%NBS.
- The tenant moved back in.
- Structurally this building posed a number of issues,