Precast concrete floor unit support and continuity report on the performance of current details: effects of frame dilation and translation on extruded hollow core flooring

Abstract

Three tests were undertaken to investigate the adequacy of typical precast Hollow Core flooring support details used by structural designers throughout New Zealand. The emphasis of these tests was to determine the ability of standard details to withstand displacement type loadings associated with the differential movements of support members during a severe earthquake. The testing regimes were so arranged to examine two separate types of frame-to-frame or frame-to-wall support displacement formats, as follows:

The first two tests involved direct loss of support, as caused by the axial dilation of plastic hinges in seismic load resisting frame members, or any similar movements caused by volumetric changes which may result in a loss of support integrity. These tests examined the common support detail with its full reliance on composite topping bond, as well as a special reinforcing detail recommended by Firth Stresscrete for short seatings. These tests followed a format earlier employed at the University of Canterbury to verify the performance of recommended reinforcing details for maintaining structural integrity at supports. Provision was made for an applied vertical load to test the shear performance of these details.

The third experiment involved a new test assembly which specifically examined the ability of Hollow Core units to sustain excessive compression stresses in the exterior web and soffit regions under translational displacements. This assembly also demanded that a degree of bond performance must occur between the precast unit and composite topping to sustain the lateral load, and thus examined the ability of the topping slab to transfer diaphragm shear as assumed in design. A vertical load was applied at mid span prior to the main test to cause cracking along the support lines, thus simulating the formation of cracks caused by service loads and volume changes in a real structure. Measurements of starter and continuity bar stresses were taken at all these stages of loading to assess the expected levels of "top steel" stress in the floors of real structures under service loads. The applied axial compression load used in this test was based on the geometry of a Los Angeles building that exhibited failure of Hollow Core units in the January 1994 earthquake, and was calculated from the seismic loading provisions of the New Zealand loadings code.