The 2016 Kaikōura Earthquake highlighted issues with older precast concrete flooring. Photo / 123rf, File
The 2016 Kaikōura Earthquake highlighted issues with older precast concrete flooring. Photo / 123rf, File
Opinion by Nic Brooke
OPINION
Over the past few weeks, Kiwi engineers have flocked to seminars aimed at helping building owners make offices and apartments safer in earthquakes.
The seminars represent the culmination of the four-year ReCast research project that’s been undertaken since the 2016 Kaikōura Earthquake highlighted issues with older precast concrete flooring.
Precast floors are constructed by taking a concrete plank that is cast off-site, lifting it into position to be supported by walls or beams, and then placing more concrete as a topping that ties the floor to the building.
The practice was enthusiastically adopted in New Zealand in the 1980s to enable buildings to go up quickly and more cheaply.
Some forms of this building method, however, have been proven unsuitable for seismic conditions in places such as New Zealand.
Essentially, it means buildings are built like a set of shelves placed onto brackets within a frame. When you shake the frame, the shelves are at risk of detaching.
While various types of precast floors are still used today and, if designed appropriately, are expected to be robust, building standards for precast floors have become progressively more stringent since the early 2000s.
As such, industry advice now recommends against the use of previously prevalent “hollow-core” planks.
Rather than pull down all buildings with precast flooring, structural engineers across industry and academia have spent the past four years developing world-leading ways to retrofit offices and apartments.
The ReCast Floors Project cost $2 million, with major contributors including Branz with funding from the Building Research Levy, the Earthquake Commission, QuakeCoRE (New Zealand’s centre of research excellence for earthquake resilience), and Concrete NZ, which represents more than 500 industry members.
It was an unprecedented work programme that brought together two of New Zealand’s major universities – Auckland and Canterbury – and organisations from across the construction industry.
Damage to the Wellington waterfront after the Kaikōura earthquake. Photo / University of Canterbury
We had little existing guidance to go on.
Our focus was on developing robust retrofit solutions, taking them through testing in a spectacular, purpose-built test frame at the University of Canterbury.
The facility is basically a representation of a substantial part of a single floor in a building – six columns with beams joining them together and whole floor units spanning across.
It is one of the larger test specimens that’s ever been assembled in New Zealand, with complex control systems to ensure the right parts go in the right directions at the right times.
Nic Brooke is the coordinator of the ReCast Project and an associate professor at The University of Auckland. He is recognised as an international expert on the performance of concrete structures. Photo / Clive Copeman
We’ve used it to test and verify the retrofit solutions, which included steel bracing and bolting systems that are fitted underneath the existing flooring.
Design guidance was then developed for the different solutions that will enable practising structural engineers to implement them.
Until now, building owners in Wellington, Auckland, and throughout New Zealand have been waiting for verified, tested retrofit methods before going ahead to upgrade their buildings instead of demolishing and sending them to landfill.
This research will give them the confidence that retrofitting can work in ways that comply with regulations, and in turn helps offer their tenants peace of mind.
In short, those of us who have been involved in the project are confident that the research has produced robust ways to greatly improve building performance in the event of a future major earthquake.
As the ReCast Floors Project has demonstrated, the future resilience of New Zealand’s built environment can in many instances be achieved through proven and elegant modifications to existing buildings.
Nic Brooke is the coordinator of the ReCast Project and an associate professor at the University of Auckland. He is recognised as an international expert on the performance of concrete structures.
