4 NEW ZEALAND TIMBER DESIGN JOURNAL VOL 19· ISSUE FIRE PERFORMACE OF TIMBER-CONCRETE COMPOSITE FLOORS* James O'Neill 1 , David Carradine 1 , Peter J. Moss 1 , Massimo Fragiacomo 2 , Andrew H. Buchanan 1 1 Dept. of Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand. 2 Dept. of Architecture, Design and Urban Planning, University of Sassari, Piazza Duomo 6, 07041 Alghero, Italy. Email: jwn17@uclive.ac.nz ABSTRACT The use of structural timber systems in large multi-storey timber buildings is restricted due to current fire regulations and a perceived level of risk. This research investigated the fire performance and failure behaviour of timber-concrete composite floors currently under development in New Zealand. Furnace tests were performed on two full-size floor specimens at the Building Research Association of New Zealand, both achieving fire resistance ratings of greater than 60 minutes. The test data and visual observations recorded aided in the development of an analytical model for evaluating the fire resistance of the floors. This was implemented in a spreadsheet that is able to predict the expected fire resistance of these floors for different load combinations, spans, floor geometries and materials. KEYWORDS: Composite, Concrete, Connection, Fire Resistance, Laminated Veneer Lumber, Timber. INTRODUCTION 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. The benefits in aesthetics, sustainability and economical savings due to fast erection time will undoubtedly be a significant factor to their widespread use in the future. Timber-concrete composite structures are not a new technology, and arose in Europe in the early twentieth century as a means of strengthening existing timber floors by the addition of a concrete slab. Due to the many advantages they possess over traditional timber floors, they are now being used in new construction [1-3]. This is currently under investigation in many parts of the world such as Sweden [1-3], the United States [4], Germany [5, 6], Switzerland [7] and New Zealand [8-11]. There are many different types of composite flooring design, the main two categories being either solid timber slab type designs or beam type designs. Each main type has its advantages and disadvantages with respect to constructability, cost, strength and aesthetics. Beam type designs consist of timber beams (either sawn timber, glulam or LVL) being used as floor joists, upon which a solid membrane (usually a plywood sheathing or steel deck) is fixed and a concrete slab is cast above. The forms of connections between the timber and concrete are extremely varied, some of which are glued, non- glued, and notched connections [8]. Glued connections consist of a form of steel reinforcement (rebars, punched metal plates, steel lattices) which is glued into the timber members and continues out into the concrete slab. Non-glued connections can consist of screws partially screwed into pre-drilled holes in the timber, a punched steel profile screwed into the timber, inclined steel bars driven into tight holes, or shear studs screwed into the timber member. Notched connections consist of a notch cut out of the timber member which the concrete is cast into, and a stud can be incorporated in the notch for better performance. A number of these different types of connections have been investigated by Ceccotti [12], Lukaszewska et al. [1, 2] and Yeoh et al. [10]. Solid timber slab type designs are generally composed of a solid timber decking from nailed timber planks with a concrete slab cast directly on top. Slab type floors generally utilise a grooved connection, the concrete is cast into grooves or trenches in the top of the timber decking which allows a large shear area of concrete to timber to be utilised, resulting in a very stiff and complete composite connection. Kuhlmann and Michelfelder [5] have conducted extensive research on the strength and stiffness of grooved timber slabs. The type of composite floor under study was a semi- prefabricated beam type system comprising of "M" panels that were built with laminated veneer lumber beams sheathed in plywood as permanent formwork for the concrete. The plywood had holes cut to accommodate the shear connection between the beams and the concrete slab. Both notched connections and * Presented at World Conference on Timber Engineering (WCTE) 2010, Italy. Reprinted here with the authors permission.