The Background

The magnitude 7.8 (Mw) Kaikoura earthquake in 2016 was the second largest NZ earthquake recorded since European settlement with in excess of $1.8 billion in insurance claims received. The earthquake resulted in two deaths in North Canterbury.

In Wellington many buildings were damaged and several beyond economic repair. Significant damage to pre-cast floors in multi-storey buildings was a common occurrence. An advisory committee was formed to tackle the issue of assessment and retrofit of pre-cast floor units, with specific initial focus on hollow-core.

The committee is comprised of members from Wellington City Council, Engineering consultants, Contractors, Concrete NZ, Quake Centre and QuakeCoRE. A project known as “ReCast Floors” has been initiated.

The First Year

ReCast Floors[1] is a three year joint project being undertaken at the Universities of Auckland and Canterbury, and BRANZ that is funded by BRANZ[2], EQC through UC Quake Centre, and the Concrete New Zealand Learned Society. Details of the aims of the project have been published previously [1,2].

The collaborative nature of the project may be a first for New Zealand structural engineering, extending to University of Canterbury students undertaking testing at the University of Auckland and vice-versa.

The first year of the project has seen substantial progress being made, driven by the four post-graduate students who comprise the project’s ‘backbone’.

The focus to date has been on the behaviour and retrofit of hollowcore floors, though it is intended that the project also eventually consider double tee floors.

[1] Retrofit and performance of existing preCAST concrete FLOORS

[2] from the Building Research Levy

Physical Testing

The initial experimental focus of the project has been on testing at the University of Auckland of sub-assemblies comprising a single hollowcore unit and a beam stub to represent the support conditions of the floor.

Preliminary results of testing undertaken by Mike Parr[1] suggest that existing retrofit supports ‘hard against’ hollowcore units may be less concerning than previously thought, provided the support is ‘flexible’.

Mike’s testing has also demonstrated that post-installed extensions to starter bars can satisfactorily suppress negative moment failure.

More details of these results along with testing currently being undertaken by Frank Bueker[2] (focussed on the impact of ‘hairpin’ reinforcement on negative moment failure) will be presented during October at the Concrete New Zealand Conference in Dunedin [3].

Mike has also been leading design of a ‘super-assembly’ specimen currently being constructed in the University of Canterbury structures laboratory (Figure 1). Conceptually similar to the specimen tested by Matthews [4], Lindsay [5], and Macpherson [6] during the 2000s, this specimen will allow the efficacy of retrofit techniques to be demonstrated with realistic accounting for the influence of three-dimensional effects that can affect precast floors.

Testing is anticipated to commence during the third quarter of 2019, with the intention that the specimen will be refurbished for at least a second test to be completed by Frank during 2020.

[1] PhD Candidate, University of Canterbury

[2] PhD Candidate, University of Auckland

Figure 1: Partially constructed super-assembly specimen in the University of Canterbury structures laboratory (courtesy of Mike Parr)

Other Workteams

Aside from physical testing, ReCast Floors also includes workstreams focussed on finite element modelling of precast floors, and forensic investigation of precast floors damaged by earthquakes.

Ana Sarkiz Fernandez3 is leading efforts to develop ‘solid’ finite element models representing hollowcore floors. Ana recently spent time at the Eucentre Foundation, Pavia, collaborating with Professors Nascimbene and Brunesi to develop an understanding of finite element modelling of New Zealand hollowcore units.

Preliminary results show a promising match to experimentally observed shear strength, with work now focussed on extending the analysis to account for support conditions of the hollowcore [7].

If successful, this work will dramatically improve our understanding of the behaviour of hollowcore floors, permitting investigation of the impact of a wider range of parameters than could practically be consider in laboratory testing and with more precise control of variables such as material properties.

Forensic investigations of damaged floors are currently focussed on the BNZ Harbour Quays building that is currently being demolished due to damage incurred during the 14 November 2016 ‘Kaikoura’ earthquake. Mohamed (‘Mo’) Mostafa4 is coordinating investigations aimed at intimately documenting the condition of the building as revealed during ‘strip-out’ prior to demolition.

Insights into the real-world behaviour of precast floors will subsequently be gained by correlating the observed condition of the building with accelerometer data recording the motion of the building during the earthquake [8].

The ReCast Floors team would be delighted to hear of other opportunities to investigate the properties of precast floors in other buildings, whether earthquake-damaged or not.

Close

A key aim of the ReCast Floors project is to promulgate results to industry. Preliminary results have been presented to Wellington Structural Group (June 2019) and the SESOC conference [9].

It is intended such update presentations will occur regularly. Members of the project would be pleased to present to other regional groups if approached and are also able to act as a ‘frequently asked questions’ sounding board for (high level) queries pertaining to assessment and retrofit of precast floors.

References

[1] Henry, R. S., Parr, M., Brooke, N. J., Elwood, K. J., Liu, A., and Bull, D. K. (2018) Progress Towards Experimental Validation of Precast Floor Retrofit Solutions. in Proc. Concrete New Zealand Conference, Hamilton, New Zealand.

[2] Brooke, N. J., Elwood, K. J., Bull, D. K., Liu, A., Henry, R. S., Sullivan, T., Hogan, L. S., and del Rey Castillo, E. (2019) ReCast Floors - Seismic Assessment and Improvement of Precast Concrete Floors. SESOC J.32(1), pp.50–59.

[3] Bueker, F., Parr, M., Elwood, K. J., Brooke, N. J., Bull, D. K., Hogan, L. S., and Henry, R. S. (2019) Development and Testing of Retrofit Solutions for Hollow-core Floors in Existing Buildings. in Proc. Concrete New Zealand Conference, Dunedin, New Zealand.

[4] Matthews, J. (2004) Hollow-Core Floor Slab Performance Following a Severe Earthquake (PhD Thesis). PhD Thesis, The University of Canterbury, Christchurch, New Zealand. 521p.

[5] Lindsay, R. (2004) Experiments on the Seismic Performance of Hollow-Core Floor Systems in Precast Concrete Buildings (Master’s Thesis). Master’s Thesis, The University of Canterbury, Christchurch, New Zealand. 190p.

[6] MacPherson, C. (2005) Seismic Performance and Forensic Analysis of a Precast Concrete Hollow-Core Floor Super Assemblage (Master’s Thesis). Master’s Thesis, The University of Canterbury, Christchurch, New Zealand. 246p.

[7] Sarkis, A. I., Brunesi, E., Sullivan, T., and Nascimbene, R. (2019) Numerical Seismic Performance Assessment of Precast Pre-stressed Hollow-core Concrete Floors. in Proc. Concrete New Zealand Conference, Dunedin, New Zealand.

[8] Chandramohan, R., Ma, Q., Wotherspoon, L. M., Bradley, B. A., Nayyerloo, M., Uma, S. R., and Stephens, M. T. (2017) Response of Instrumented Buildings Under the 2016 Kaikoura Earthquake (including Appendix A published online only). Bull. N. Z. Soc. Earthq. Eng.50(2), pp.237–252 and A1–A10.

[9] Brooke, N. J., Elwood, K. J., and Bull, D. K. (2019) ReCast Floors - Progress Towards Retrofit of Precast Floors. in Proc. SESOC Conference, Auckland, New Zealand.