The Fire Performance of Timber-Concrete Composite Floors

Abstract

Timber-concrete composite floors are a combination of timber joists and concrete topping, creating a flooring system to best utilise the advantages each material has to offer. Timber is used as the main tensile load bearing material due to its high strength-to-weight ratio, while concrete is used in floor slabs for its advantages in stiffness and acoustic separation. The strength of the system is dependent on the connection between timber and concrete, thus the connection must be strong, stiff, and economical to manufacture, to ensure that the flooring system is economically viable.

This research investigated the fire performance and failure behaviour of timber-concrete composite floor systems currently under development in New Zealand, resulting in a calculation method for evaluating the fire resistance of these floors. Furnace tests were performed on two full-size floor specimens at the Building Research Association of New Zealand (BRANZ). Both floor specimens were 4 m long and 3 m wide, consisting of 65 mm concrete topping on plywood formwork, connected to double LVL floor joists. They were tested over a 4 m span, subjected to a nominal design live load of 2.5 kPa. Both floors were subjected to the ISO 834 test fire for over 60 minutes. Two separate connection types were tested; concrete notches cut into the timber beams with an incorporated shear key, and metal toothed plates pressed between the double beams.

It was found that the reduction in section size of the timber beams due to the fire governed the failure mode of the floors. Due to the composite action achieved by the connections, the floor units were able to withstand prolonged exposure to the test fire, well exceeding one hour. The test data and visual observations aided in the development of a numerical model for evaluating the fire resistance of the floors. This was developed in a spreadsheet that is able to predict the expected fire resistance of these floors, taking into account some major time dependent variable properties that can have an effect on the overall performance. Load-span tables have been produced to give the estimated fire resistance of floors with differing floor dimensions, span lengths and applied loads.