VOL 27· ISSUE 1 » NEW ZEALAND TIMBER DESIGN JOURNAL 6 CREEP IN TIMBER: RESEARCH OVERVIEW AND COMPARISON BETWEEN CODE PROVISIONS G. Granello 1 & A. Palermo 2 1 PhD, Lecturer, Department of Civil Engineering and Natural Resources, University of Canterbury. E-mail: gabriele.granello@canterbury.ac.nz. 2 PhD, Professor, Department of Civil Engineering and Natural Resources, University of Canterbury KEYWORDS Long-term behaviour of timber, Creep, Visco-elasticity ABSTRACT The design of timber beams and joists is often governed by serviceability requirements, i.e., meeting deflection limits over the life of the structure. Since timber is a visco-elastic material, it is subjected to creep when permanent loads are applied. The amount of creep depends on several variables, e.g., load-to-grain angle, moisture content of timber and stress level, which therefore strongly affect the design. This paper presents an overview of the creep properties of timber, including conceptual models of the material, constitutive laws, and analytical approaches proposed by current building codes. The purpose is to provide a link between the advances in terms of experimental results and theoretical formulations to common design calculations. Redirections to the most relevant studies are reported for the reader interested in a more comprehensive knowledge of each specific topic. Finally, the long-term deflection of a Radiata Pine LVL beam calculated by four different procedures specifically according to New Zealand Standard 3603, Eurocode 5 (European code), National Design Specifications for Wood Construction (North-American code) and Toratti’s model is reported. The aforementioned models seem providing consistent results, however the uncertainty in the deflection estimation appears growing in case of more extreme environmental conditions. These discrepancies are believed dependent on how each procedure takes into account the mechano-sorption effect. GENERAL PROPERTIES OF CREEP Timber, due to its particular molecular architecture, presents strain–stress behaviour dependent on time: if subjected to a constant stress, such as permanent loads, it reacts as a viscoelastic material, increasing its deformation with time. This effect is known as creep (Morlier 2004). The long-term behaviour of timber can be divided into three main phases (Findley and Davis 2013), graphically presented in Figure 1: 1. Primary creep: the deformation rapidly increases before reaching a more stable rate; 2. Secondary creep: the rate of deformation is fairly constant or decreasing; 3. Tertiary creep: the deformation rapidly increases leading to the failure of the material. Whether or not the tertiary phase occurs depends Figure 1: Three phases of creep: 1) primary, 2) secondary and 3) tertiary. on the stress level σ. To avoid the occurrence of the tertiary phase, and therefore the material failure, Standards, for example Eurocode 5 (2004) (European building code), New Zealand Timber Standard 3603