Seismic Performance of Hollow Core Floor Systems

The Guidelines are intended to be read in conjunction with NZS 3101:2006, the Concrete Structures Standard and apply to any building with hollow-core floors, whether new or existing. They apply whether or not the building is earthquake-prone or subject to alteration or change of use. The underlying theme of the guidelines is for the structural performance of the hollow-core floor system to comply with the Building Code requirements under full earthquake loading – whether or not this is achieved in a particular case. The Guidelines address the following issues:
• Structure type and implications: Earthquake performance of hollow-core floors is critically dependent on the displacement the structure experiences and the forces that are induced. For example, ductile frame structures will have higher inter-storey displacements than shear wall structures. In addition, supporting beams may be subject to beam elongation due to plastic hinge formation. However, even in shear wall structures, deformations of the structure can lead to excessive strains on the floor units and/or topping.
• Range of floor systems (floor units plus supporting structure): There is a variety of configurations used. Floor unit depth and span varies. Units may span past intermediate columns. They may be supported on walls or beams. Each configuration may have different implications for the structural performance overall.
• Seating details / performance: Seating details in buildings constructed over the last 30 years vary considerably – from those with generous overlap of unit with the support to those with “negative” overlap that rely on reinforcement in the cores of the units. The behaviour of the floor overall is critically dependent on the detailing of the seating details and associated reinforcement.
• Reinforcement details / performance: Reinforcement is needed for diaphragm action and for bending action in the units. The nature of hollow-core floor units is such that the placement and quantity of reinforcement has a critical influence on overall floor performance. More reinforcement does not necessarily give better performance due to the brittle nature of some failure mechanisms.

Liquid Concrete Pressures and Loads on Formwork