30 Aug 2021
Alongside the Structural Engineering Society New Zealand (SESOC) and New Zealand Society for Earthquake Engineering (NZSEE), we've collectively released engineering advice about the use of hollow-core pre-cast concrete units in a New Zealand context. This advice is based on the latest research.
Hollow-core flooring has been in widespread use in New Zealand construction since the 1980s. We now know that hollow-core flooring is vulnerable during earthquakes, and has many failure mechanisms, including:
- loss of seating – where a unit comes away from its supports
- positive moment failure – where a unit cracks from underneath
- negative moment failure – where a unit crack from above
- web splitting – splitting webs along the length of the unit.
Methods for assessing these failure mechanisms are encompassed in the Earthquake Prone Building Methodology through reference to the ‘Purple Book’. Since the early 2000s, design provisions for new buildings have included measures that were believed to preclude the failure mechanisms from occurring.
Over the last 10 years, many buildings have been retrofitted to mitigate the known failure mechanisms during or after earthquakes. Such retrofits are not adequately considered in current design and assessment documents. Recent research has shown that while common retrofits provide for loss of seating, they cannot assure other known failure mechanisms, even where there is a comparatively small ‘storey drift’ (displacement between floors in a seismic event). A new ‘strongback’ retrofit shows considerable potential.
Additionally, this advice provides guidance for engineers on the adequacy of hollow-core floors with relation to new builds, assessments and retrofits. It also provides guidance for conversations with clients about hollow-core flooring and how to mitigate for known failure mechanisms.