Spotlight on precast concrete floors

By Rob Gaimster, Cement and Concrete Association of New Zealand and NZ Ready Mixed Concrete Association.

THE 7.8 MAGNITUDE (MW) Kaikoura earthquake of November 14, 2016 – a complex multi-fault rupture with prolonged ground shaking – was a wake-up call for Wellington.

As a result of 25 to 30 seconds of strong motion in the CBD, with localised amplification on deep deposits near the waterfront, structural damage tended to be concentrated on moment-resisting concrete frame buildings between six and 15 storeys.

The outcome has been a number of official recommendations intended to ensure more rigorous regulations and address public safety concerns. Issues with the performance of precast concrete floor systems are reflected in these recommendations.

While the spotlight is warranted to a degree, it is important to consider some qualifying factors.

For instance, the codes in force at the time a number of these buildings were designed (eg, NZS 3101:1995 Concrete Structures Standard) did not account for the unique nature of the earthquake. These codes have been, or are in the process of being, updated with total concrete industry support.

Moment-resisting, concrete frame, multi-storey buildings comprise a significant proportion of the Wellington CBD, and would therefore inevitably be amongst any damaged stock by sheer volume.

We must also not have our attention drawn away from the more pressing challenges posed by unstrengthened unreinforced masonry buildings.

Another outcome of the Kaikoura earthquake from which valuable lessons can be learnt is the debate around safety versus amenity. It seems that although, with the odd exception, our buildings perform as designed, public expectations are not being met.

The disruption caused by building repair or demolition has seen a growing call for damage resistant design, an approach which allows for ‘life safety’ as well as ‘building survivability’. Through PREcast Seismic Structural System (PRESSS) and base isolation concrete is ideally positioned to help meet this demand.

Post-quake discussion has focused on two areas. Firstly, an investigation into Statistics House – a six storey reinforced concrete office building built in Centreport’s Harbour Quays business park in 2005 – which suffered the partial collapse of two floors.

Secondly, Wellington City Council’s (WCC) Targeted Assessment Programme, which sought to address public safety issues by confirming the structural integrity of multi-storey buildings that had experienced significant shaking in the Kaikoura earthquake.

Statistics House

The Ministry of Building, Innovation and Employment (MBIE) ordered an independent investigation into Statistics House to understand the implications for the building regulatory system.

The report concluded that a combination of four factors contributed to the partial failure of lower floor segments. Two of the factors – the flexible frames and style of floor construction – combined with significant shaking for up to 120 seconds, and localised amplification of the shaking, to compromise the support of the lower precast concrete floor units on their framing.

WCC targeted assessment programme

WCC recently undertook the inspection of buildings that were of a like profile to Statistics House, and others of similar form that were known to have sustained significant non-structural damage. In total 64 buildings (from about 600 across the Wellington CBD) were subject to targeted damage evaluations.

A range of damage levels was identified within the buildings, from isolated and local damage to damage that was more distributed throughout some buildings, and with varying degrees of severity.

For precast floors, most of the damage was observed in ductile frame buildings with hollowcore flooring that were constructed in the 1980s.

Recommendations

The MBIE investigation made four key recommendations – investigate similar buildings to Statistics House in the Wellington region, notify industry about issues with existing buildings with precast floor systems and frames that may be affected by beam elongation, access expertise to consider the implications for this type of design for new buildings, and undertake research into Standards to ensure they reflect current best practice.

The MBIE report noted that the combination of the causal factors in the partial floor collapses of Statistics House was not foreseen by Standards when the building was designed. As such, NZS 1170.5 – Earthquake Actions Standard and NZS 3101 were identified as requiring review. In fact, Amendment 3 to NZS 3101 will be published soon, addressing design issues in this area.

The WCC report offers nine recommendations in such areas as building owner responsibility, post-earthquake assessment  and instrumentation to gauge shaking impacts. A number of the recommendations also relate to precast floors, including the call for a testing regime, standardised details and guidance for practitioners.

However, the report mentions that while the focus of the assessment programme was on buildings most affected in this earthquake, it is important that structures vulnerable in earthquakes, such as unstrengthened unreinforced masonry buildings, remain on the radar.

Moving forward

These reports set a path forward for the wider Wellington region, one which will require collective buy-in from all stakeholders if the recommendations are to ensure a more robust building regulatory system and address public safety issues.

The concrete industry is fully committed to this process; participating in technical discussions, driving Standards revision and exploring new low damage approaches to seismic design.

References

Kestrel Group. (May 2017). Wellington City Council Targeted Assessment Programme following the Kaikoura Earthquake of 14 November 2016 – Summary Report. http://wellington.govt.nz/~/media/about-wellington/emergency-management/files/targetedassessment-programme-summary.pdf

Ministry of Building, Innovation and Employment. (March 2017) Investigation into the performance of Statistics House in the 14 November 2016 Kaikoura Earthquake. http://www.mbie.govt.nz/publications-research/publications/building-and-construction/investigation-into-the-performance-of-statistics-house.pdf

• From 1 July 2017, there is a nationally consistent approach to the assessment and management of earthquake-prone buildings, along with a standardised notice and national public register of earthquake-prone buildings.