Structural safety in wooden beams under thermal and mechanical loading conditions

Abstract

The main objective of this paper is to identify different analytical methods which permit the calcula- tion of the stress level in wooden simply supported beams, due to mechanical and thermal loading conditions. Two different wood species, with different cross-sections, will be presented. The fi re resistance, the charring depth layer and the charring rate will be determined using the fi nite element method with Ansys® program. To characterize the stress state in wooden beams, all elements are subjected to mechanical load considering the reduction of the cross-section, infl uenced by thermal action. Another purpose of this work is to identify the ultimate safe load-bearing capacity in wooden beams, subjected to uniform load simultaneously with the thermal effect. All numerical results per- mit the specifi cation of simple design calculation methods, simplifying the verifi cation of the fi re safety of wooden beams. Wood is a renewable resource, recently attracting public attention, as an environmentally friendly material. This product is a building material with attractive attributes such as archi- tectural and structural characteristics. Wood is classifi ed in two different botanical terms. The botanical terms, softwoods and hardwoods, indicate the basic structure and cell type of mois- ture within the tree. Softwoods generally come from the coniferous species (pines, fi rs and spruces, for example) and are generally fi ne textured. Hardwoods (eucalypts and oaks, for example) have broad leaves and the texture ranges from fi ne to coarse. The types of wood include softwoods, hardwoods and glued laminated woods, in the forms of solid wood, ply- wood and wood-based panels. Due to large variation, type and wood quality, a system of strength classes was established. Each grade of classifi cation is a function of the physical and wood properties. The wood when exposed to accidental actions, such as ficonditions, pre- sents a surrounding charring layer. However, this layer can delay the heating process from the exposed side to the wood core section, acting as an insulating layer. The wood core section may remain at low temperatures, depending on the fi re exposure and element size therefore. It is important to calculate the value of charring rate and determine the thickness of char layer formation through the section. These parameters are important in fisafety design because they determine the residual load-bearing cross-section, due to critical external conditions. Safety rules and guidelines should be useful for different wooden structures. The high vulner- ability of wood, with respect to firequires a rigorous thermal and mechanical analysis. The study of fi re resistance of wood structures is therefore a topic of great interest. Several researchers have presented experimental and numerical models for the study of wood in the presence of high temperatures (1-3). The charring rate of softwood or hardwood material, exposed to fi conditions, has been studied in different countries, (4-11). Some empirical models for charring rate calculation have been developed by other researchers (4-6). The wood species considered in this work are the Fir subalpine and Redwood, from Northern Europe. These conifer species are widely used in construction, textile, paper- making, resin