Introduction and background
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. Early forms of precast floors consisted of reinforced or prestressed solid slabs about 100 mm thick with 50 to 70 mm of cast in situ concrete topping. Many variations were developed to meet market demand.
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 70 mm thick. Over the years, the thickness of hollowcore units has increased from 150 mm to 300 mm and more recently even 400 mm and more, with corresponding increases in spans.
Typically, the units have no reinforcement protruding from the ends to provide positive connections with their supporting beams. The units are produced by an extrusion process and have no transverse or shear steel.
Questions were raised about the integrity of hollowcore floors, particularly after some failures in the Northridge earthquake in California in 1994. There was a concern that strong earthquake shaking could cause loss of support to the ends of the precast units in ductile frame structures due to elongation of beams parallel to the floor units. This concern led to a test programme being undertaken at the University of Canterbury in Christchurch.
In October 2001, load tests on a full-scale model of a hollowcore floor assembly at the University of Canterbury indicated potentially serious gaps between assumed and actual behaviour of hollowcore floor systems (precast units and in situ topping, together with the surrounding and supporting beams) in strong earthquake shaking [1]. The hollowcore units collapsed on to the test floor at lower levels of load than expected, and exhibited brittle failure mechanisms in some elements. In view of the amount of hollowcore floor in existing buildings, and its ongoing common use, the test results caused considerable concern among structural designers, territorial authority officials and manufacturers. Importantly in this test, the intermediate column did not have the required reinforcement tying it back to the floor and to other columns. The absence of this steel resulted in very significant sideways displacements that were detrimental to the structural performance of the floor. Had the test been done on a floor with the same support details, but with the requisite column tie-back in place, it is highly likely that the performance of the hollowcore would have been significantly improved.
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 tests, disseminate information and indicate necessary directions of future research and design practice. In August 2002 and October 2003, this Group reported on the University of Canterbury test and recommended changes in design approaches. They recommended changes to hollowcore seating/connection details for structures where the inter-storey displacement was expected to be greater than 1.2 percent of the storey height. Changes to the concrete design standard, NZS 3101, were initiated to reflect these recommendations and amendments were made effective in March 2004 [2]. They were cited as a means of compliance with the Building Code in March 2005.
Concerns regarding the University of Canterbury tests coincided with the issue, in December 2002, of an Open Letter by structural engineer John Scarry [3] expressing concerns on the state of the structural engineering industry in New Zealand. A report by Sinclair Knight Merz to the Building Industry Authority, submitted in November 2003 [4], was prepared in response to the Scarry Open Letter. One of its recommendations was that a survey be conducted ‘to determine the extent of the hollowcore deficiency that may lead to building failure in a major earthquake event’.