Gravity & seismic investigation and retrofit of loop bar double tee precast floors

Abstract

An investigation into the performance of double tee precast floor units that use the loop bar support detail was conducted as part of the ReCast Floors Research Program. Following the 2016 Kaikoura earthquake, concerns have been raised regarding the capacity of double tee units with the loop bar detail that are widespread in existing buildings across New Zealand. Guidelines have been provided for assessing the seismic performance of precast double tees, which required experimental data to confirm their accuracy. The objective of this investigation was to provide research and experimental data for the purpose of improving the assessment guidelines and validating retrofit concepts for double tees with loop bar support details. A database of existing double tees in New Zealand was created to determine the common design parameters for the double tee units and loop bar support details. Gravity and seismic loaded sub-assembly testing of single tee units with loop bar supports were conducted. The gravity tests focused on understanding the progression of damage at the support under increasing loads with the observed damage similar to theoretical failure modes outlined in the assessment guidelines. The gravity tests also explored the influence of the in-situ topping bond with the pre-cast unit, which was discovered to have only a minor impact on the support load capacity. The seismic tests aimed to understand the influence of seismic deformation demands (elongation and rotation) and the placement of the loop bars on the seismic performance of the unit. Units that were subjected to elongation demands, and those that were fabricated with undesirable loop bar placements, presented increased spalling and reduced drift capacity. The experimental seismic capacity was compared with the theoretical capacity calculated from the provisions in the assessment guidelines. The guidelines slightly underestimated the drift capacity of the tested tee units, but significantly underestimated the amount of unit and ledge spalling observed during the tests. Two retrofit concepts were designed and installed onto the seismic tested units, including a catch bracket and a hanger retrofit. Both retrofits were successful and able to support the unit during seismic loading when loss of support of the loop bar detail occurred.