A comprehensive research programme is underway to scientifically validate and, where possible, enhance the retrofitting techniques used to improve the performance of precast concrete floors in multi-storey buildings, particularly those installed in prior to 2006.

Concerns over the seismic vulnerability of such floors – which feature extensively in the country’s multi-storey building stock – surfaced after the 1994 Northridge earthquake in Los Angeles and have subsequently been borne out in local seismic events.

With industry having progressively developed retrofit techniques for affected buildings based largely on “sound principles”, Compusoft Engineering principal and ReCast Floors project manager Dr Nicholas Brooke says formal validation was long overdue.

“The retrofit techniques being used in theory work very well, but there might be unresolved issues such as exactly how wide a piece of support steel should be or whether it needs to be hard against an existing floor unit or set down slightly,” he says.

“So, some of it is validation of techniques that everyone expects to work, some of it is refinement of the methods of designing and installing these techniques.”

With a $2 million budget – funding provided by BRANZ from the Building Research Levy, the Earthquake Commission and a number of other organisations – the ReCast Floors project formally commenced in October 2018.

Due to be completed by October 2021, the collaborative work is being directed by an eight-person research team.

Due to be completed by October 2021, the collaborative work is being directed by an eight-person research team, representing the key New Zealand groups involved in precast floor research.

Driven by four full-time PhD students at the universities of Auckland and Canterbury, with another to join soon, the project is also being guided by an industry advisory group consisting of other key stakeholder representatives.

Encompassing several different investigations being undertaken at a number of facilities, the project’s planned research is split into two streams of work.

One of those streams, laboratory and analytical research, entails:

• sub-assemblage testing of single units,
• super-assemblage testing of multiple units supported by moment resisting frames; and
• computer simulation by developing finite element analysis tools.

The other stream revolves around the interpretation of real-world performance, and comprises:

• processing of damage assessment data,
• forensic analysis of case study buildings,
• documentation of damage in buildings; and
• in-situ testing of precast concrete floors (dependent on opportunities).

Dr Brooke describes the super assemblage test being built specifically for the project at the University of Canterbury as “spectacular”.

As well as seeking to validate current retrofitting techniques, the project will test new and refine existing methods.

“It is basically a representation of a substantial part of a single floor in a building – something like six columns with beams joining those together and whole floor units spanning across."

“That’s one of the larger test specimens that’s ever been put together in New Zealand. We’ll have some really quite complex control systems to ensure the right parts go in the right directions at the right times.”

At the time of being interviewed halfway through the project, Dr Brooke says “many good findings” are consistently being revealed.

“For example, there were concerns that some of the retrofits already done could have actually made certain aspects of seismic behaviour worse. However, that, in most instances, now seems unlikely to be an issue.”

As well as seeking to validate current retrofitting techniques, Dr Brooke emphasises the project will test new and refine existing methods, potentially leading to the development of less-expensive solutions.

“You hear ideas or suggestions that people are contemplating but may not actually implemented because they are uncertain about how that approach might perform. A number of these ideas are being fed into the research, developed further and be proven to be effective (or not) by the ReCast Floors project."

“In particular that could result in less-invasive solutions. A typical retrofit solution involves some relatively low-cost steel work to basically provide the load paths that aren’t reliably there. The cost of the structural parts would only be something like 20 percent of the total, whereas the disruption to the existing fit out would be the other 80 percent."

“So, there is a drive to look for retrofits that can either be done from underneath or above a floor – but not both – as obviously, that gives you much less disruption.”

"One of the overarching goals of the project is for it to serve as the focal point for the collation and investigation of issues related to precast concrete floors and the dissemination of findings to industry," says Dr Brooke.

Overview of research projects, oversight arrangements and expected outputs

“At the time that the project was conceived there was a concern that different organisations seemed to be picking up different parts of precast floor research – a lot of the same people were involved. However, it was less an issue of conflicting advice, more a risk of being inefficient."

“The project has succeeded in pulling everyone together under one roof and becoming the focal point it was aimed to be.”

Dr Brooke says another potentially “ground- breaking” aspect of the project is the work being undertaken by one of the PhD students at the University of Canterbury to develop complex analytical models of the hollowcore floors.

“If that is successful, then it will basically allow consideration of a far wider range of floors than is possible in a lab.”

Emphasising the “excellent job” being done by all students involved, Dr Brooke says the collaborative nature of the relationships forged between the universities, industry and academics is a particular highlight of the project.

“As far as I know it is probably the first time in structural engineering and, maybe more generally, where a large programme of research is being run at two universities in parallel, with day-to-day collaboration.”

IMPORTANT MESSAGE

An important message that Concrete NZ’s Technical Director Dave McGuigan hopes the ReCast Floors project will deliver is that industry is prioritising the issue of life safety of buildings with potentially vulnerable floor systems in a seismic event.

“The key output of this project is that it restores confidence amongst building owners and practitioners that we are making sure the building stock achieves life safety objectives,” he says.

“The Canterbury and Kaikoura earthquake events demonstrated the vulnerability of these buildings – in particular Statistics House in Wellington where three floor units lost their support."

“While the focus is on addressing something that could have major life safety consequences people still need to be mindful that these buildings will have performance characteristics that might render them uneconomic to repair after an earthquake.”

Taken from Concrete Magazine.