A concrete floor support unit fell in the 2016 Kaikōura earthquake. Photo: Statistics NZ
A new guide for assessing the earthquake strength of concrete floors in multistorey buildings has been released, aimed at helping with strengthening them better.
It makes more than a dozen changes to guidance rushed in after the 2016 Kaikōura earthquake damaged many buildings in Wellington, often cracking multiple floors.
New Zealand buildings have a lot of precast hollowcore concrete floors, and Kaikōura exposed hidden weaknesses in them.
As authorities grasped why floors fell in at Statistics House in Wellington, they realised they faced big gaps in engineering knowledge.
They hurried in stopgap changes in 2018, to improve the assessment and retrofit of buildings; in the meantime, seismic engineers set about a project, ReCast Floors.
"In contrast to most other deficiencies found in existing buildings, little international research is available regarding the adequacy of existing precast floors," said a ReCast project report in 2022.
Assessments carried on according to 2018's guidance, revealing in 2022 that about 150 buildings in the capital [https://www.rnz.co.nz/news/national/467993/scores-of-buildings-share-same-floor-weakness-as-education-ministry-s-office
shared a design flaw] in their precast hollowcore floors that had led to a government office block being evacuated.
The new guidance now supersedes what engineers called 'the Yellow Chapter' from 2018. It runs to 258 pages.
"Errors and internal inconsistencies identified in the November 2018 version have been corrected."
Engineers assessing many types of buildings are expected to use it.
The committee behind the guidance said it wanted to help make it easier to decide on retrofitting or not, and how to do it.
Since Kaikōura, some projects have worked but others have foisted heavy costs on building or apartment owners.
"It is increasingly evident that major - and sometimes controversial - issues can arise," said the new guidance.
This could even be as basic as "deciding whether the retrofit is needed ... and to what extent".
The committee aimed to help retrofits where necessary, "while limiting unnecessary disruption, demolitions and carbon impacts, [and] promoting continued use or re-use of buildings".
Retrofit studies had been limited, and the new research had brought to light much more about how to use or combine "selective" interventions - even "selective weakening" so that one overly-strong part of a building did not put pressure on the others - to make strengthening the floors work.
More new guidance is due later this year on geotechnical assessments and unreinforced and reinforced masonry.
The new guidance on concrete floors does not apply to buildings being assessed under legislation covering earthquake-prone buildings. The government last year extended by four years the various deadlines on building owners needing to strengthen these buildings.
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