Summary

Over the last 30 years, considerable use of precast concrete has been made in the construction of buildings in New Zealand for floors, beams and columns. The use of precast concrete has been particularly common in flooring systems, replacing the traditional and more labour-intensive cast in situ concrete. Units incorporating hollow cores to reduce weight without significant loss of strength or stiffness are structurally efficient and competitive in cost. They became popular for a wide range of buildings in New Zealand and overseas. Reliance for overall continuity and integrity of the floor system is on reinforcement placed in the cast in situ topping slab 50 to 65 mm thick. Over the years, the thickness of hollowcore units increased from 150 mm to 300 mm and more recently even 400 mm, with corresponding increases in spans.

Questions were raised about the integrity of hollowcore floors, particularly after some failures in the Northridge earthquake in California in 1994. In October 2001, load tests on a full-scale model of a hollowcore floor assembly at the University of Canterbury [1] indicated potentially serious gaps between assumed and actual behaviour of hollowcore floor systems in strong earthquake shaking. The hollowcore units collapsed on to the test floor at lower levels of load than expected, and exhibited brittle failure mechanisms in some elements. The results caused considerable concern among structural designers, territorial authority officials and manufacturers. However, importantly, the intermediate column did not have the required reinforcement tying it back to the floor and to other columns. Had the test been done on a floor with the same support details, but with the requisite intermediate column tie-back in place, it is highly likely that the performance of the hollowcore floor would have been significantly improved. Subsequent tests incorporating the requisite tie-back, and revisions to support and perimeter detailing, showed a marked improvement. A further test incorporating reinforced ties in the voids at the ends of the units showed even better performance.

In April 2002, the Cement and Concrete Association of New Zealand and the New Zealand Society for Earthquake Engineering set up a Technical Advisory Group representing industry, research, consulting engineering and local authority interests. The Group’s role was to interpret the outcome of the University of Canterbury tests, disseminate information and indicate necessary directions of future research and design practice. In August 2002 and October 2003, this Group reported on the test and recommended changes in design approaches. They recommended changes to hollowcore seating and connection details. Changes to the Concrete Design Standard, NZS 3101 [2], were made effective in March 2004, and cited as a means of compliance with Clause B1 Structure of the Building Code in March 2005.
A report by Sinclair Knight Merz to the Building Industry Authority, submitted in November 2003 [3] and prepared in response to the Scarry Open Letter [4], had as one of its recommendations that a survey be conducted ‘to determine the extent of the hollowcore deficiency that may lead to building failure in a major earthquake event’.

In July 2003, the Building Industry Authority started a review of the use of hollowcore floor systems in New Zealand. The objective was to determine the extent and nature of the use of these systems nationally, to relate that use to particular concerns raised by the University of Canterbury tests and to advise the industry of any concerns. This work has been continued by the Department of Building and Housing.

In May 2004, a report entitled Review of Hollowcore Flooring Performance in Recent Earthquakes was produced for the Building Industry Authority [5]. This reviewed the design details and performance of hollowcore floors in overseas earthquakes and related the information to New Zealand conditions. Relevant information was limited, but the report commented that hollowcore floor systems in low-rise structures similar to New Zealand buildings performed well in at least one severe earthquake.
A report on a pilot survey in Christchurch was produced in October 2004 [6]. This survey reviewed building consent documents and indicated that 16 percent of hollowcore flooring in Christchurch was in a category for which further investigation was considered prudent. Two buildings were identified as being of potential concern, and these were referred to Christchurch City Council with a request that the owners be advised to make more detailed checks.

In February 2005, a similar review was reported on for Wellington [7] where, by reviewing building consent documents, 9 percent of hollowcore floors were assessed to be in a category for which further investigation was considered prudent. Thirteen buildings were brought to the attention of Wellington City Council with a recommendation that the owners be advised to check them in more detail.
A report investigating the potential performance of hollowcore floors in Wellington, Christchurch and Auckland was produced for the Department in February 2005 [8]. The report examined the likely displacements of buildings designed to the limits of the current Standards and compared them to those considered acceptable in the light of the Canterbury tests. The conclusion was that, for a design earthquake in Wellington, displacements of flexible (frame) buildings designed to the limits permitted by the loading Standard could exceed the lateral displacement limits for buildings with hollowcore floors now in the concrete design Standard as amended in March 2004. In Christchurch, it was found that a significant number of such buildings could exceed the maximum displacement level now considered advisable. For Auckland, the report concluded that the displacements of such buildings would generally be less than the limits now considered advisable, and that a detailed survey of Auckland hollowcore use was not warranted.

Since the initial test at the University of Canterbury, two further full-scale tests were performed [10, 11]. Each used details recommended in the revisions to NZS 3101 [2]. Both tests showed markedly improved performance from the original detail.

In June 2005, the Department issued Practice Advisory 5 highlighting issues of concern and providing recommendations for designers, builders and territorial authority officials.

As a result of its investigations, the Department will:

• release the results of its investigations
• recommend that owners with concerns seek professional advice
• recommend that territorial authorities require a report from a qualified structural engineer when buildings with hollowcore floors are subject to significant alteration
• support the development of guidance material for designers and territorial authorities
• support Standards New Zealand in making any necessary amendments to NZS 3101 in the light of research and investigations completed since March 2004
• continue to support and encourage territorial authorities in their interaction with owners and designers on buildings they identify as requiring further investigation.