PL EN. Widoczny [Schowaj] Abstrakt. Adres strony. Reliability of the thermal treated timber and wood-based materials in high temperatures. Pieniak, D.
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Structure and thermal stability of nanocrystalline materials. Laminate composite materials, Timber beams, Structural Download PDF. Baldwin RF. Plywood and veneer-based products, manufacturing practices An engineer's approach to the prediction of failure Reliability of the thermal treated timber and wood-based materials in high temperatures. Eksploatacja i Niezawodnosc — Maintenance and Reliability ; 15 1 : 18— The most significant problems were limitations regarding maximum dimensions of the components cross — section and capabilities of manufacturing of the large-scale components.
Durability and flammability of surfaces were the limiting factors as well. Nowadays, thermally treated wood and wood composites are more and more commonly used in the engineering constructions, such as: glued laminated timber GL , laminated veneer lumber LVL and thermally treated timber TT.
The timber undergoes a process of thermal degradation. In high temperatures timber structure is subject to simultaneous influence in the form of forces and thermal impacts. These factors influence stress distribution in the wood structure and limit its load capacity, reflecting structure decohesion.
The aim of the presented studies was to determine impact of increased temperatures on strength of the wood materials and wood-based composites. Additionally, based on the results of the strength studies, analysis of the probability of survival in high temperatures was performed.
Samples used in the static bending strength studies were made of the laminated veneer lumber — LVL, glued laminated pine timber — GL, and thermally treated — TT and non-treated spruce timber - NTT.
The samples were in a cuboidal shape with dimensions of 20x20x mm. Based on the obtained results a reliability analysis was performed. For the analysis a two-parameter Weibull distribution was applied.
In case of materials with laminated structure — LVL and GL, an increase in standard deviation of the results of bending strength in the successive temperature ranges has been observed. Increasing demands concerning the constructions result in necessity of improvement of mechanical — functional wood properties.
Undoubtedly, new technologies of production of wood and wood- based structural components, as well as new types of bolts, contribute to the development of the constructions made of these materials.
The wood is a flammable material undergoing thermal degradation. In the fire conditions a wooden structure is subject to the simultaneous influence in the form of forces and thermal impacts. Simultaneous interactions of these two factors influence stress distribution in the timber structure and cause a reduction of the load capacity of the construction.
High temperatures occurring during the fire cause decohesion of the structure. At a micro-structural level the wood is a non-homogeneous cell composite, composed of cellulose, hemicellulose, lignin and other minor constituents .
Cellulose is the largest part of volume of the timber. It consists of the long carbon chains, which are crucial for its strength. The hemicellulose consisting of branched amorphous polymers fills in the gap between the cellulose and lignin in the timber structure.
Construction timber has advantageous physical and technological properties, such as high strength and low deadweight. Till now, there have been some drawbacks due to limitations connected with maximum dimensions of the components cross-sections and manufacturing abilities of the large scale components.
Nowadays, a woodworking technology, development of the treatment methods, shaping of such type of materials as well as their aesthetic properties, favor production of this kind of constructions. Possibilities of application of the special agents protecting wooden structures against a negative impact of the environmental conditions are being continuously developed, resulting in larger applications range of timber materials . An application of water-resistant adhesives based on the synthetic resins together with a simple method of longitudinal joining by means of wedge joints, turned out to be especially important.
It enabled much faster building method with the use of glued structural components with dimensions larger than the natural input product . Joining of the wooden components by means of steel rod connectors gives more possibilities in the wood construction design and enables their local renovation with preservation of the overall structure . Due to necessity to obtain high aesthetics of the interiors, in the public utility buildings wood and wood-based structures of large span are applied.
In the buildings with compartments with high humidity swimming halls , the timber proves to be a very advantageous constructional material. New types of timber are getting more and more popular, due to both strength as well as fire parameters, which have not been used in Europe till now. The timber from tropical countries is an alternative for the domestic brands and certainly its further investigations are to be continued . In order to fulfill requirements of the modern architecture, where the most important is a freedom in the implementation of the architectural concepts, high fire resistance, necessity to obtain large structural spans, low maintenance, costs reduction and chemical resistance, the most commonly used materials are wood and wood-based composites, such as: glued timber GL , laminated veneer lumber LVL and thermally modified timber TT.
However, it should be noticed that the above materials perform different functions in the constructions. The glued laminated timber composites GL are used in single family houses and apartment houses, as well as largescale buildings such as production halls, market halls, sports halls, swimming pools or footbridges and bridges. The most commonly used type of timber for the production of glued components in Poland is pine or spruce, and rarely larch. A main adhesive used to produce this type of timber is a melamine adhesive, which is resistant to water and fire impact.
The alternative solution is resorcinol adhesive, which is especially resistant to humidity. Both types of adhesives do not release any harmful substances, even during the fire . The results of investigations have revealed that behaviour of GL in the conditions of higher temperatures strongly depends on the adhesive behaviour that bonds individual layers of the component .
The comparative studies of different types of timber, conducted with the use of conical calorimeter measuring heat release rate, have indicated that the charring rate of the glued timber samples decreases with the increase of timber density . On the basis of specific bending strength a glued timber was divided into 5 classes: from GL24 to GL Grading of the sawn wood was carried out according to the standard requirements, based on the visual evaluation for GL24 and GL28 and mechanically for the higher classes GL32 and GL Due to such method of grading the classes GL24 and GL28 are the most available.
The laminated glued timber after providing appropriate conditions such as chamfered edges and planed surface is resistant to fire. According to the guidelines of the Institute of Building, this type of components with a width below 12cm, are classified as low flame spread materials.
The components with the width above 12cm or below 12cm with additional fire-proofing agent are classified as fire retardants. Fire resistance of the glued timber in a range of R15 to R60 can be achieved at the design stage by the proper statistical analysis and proper selection of the cross-sections.
Nowadays, laminated veneer lumber is used in many ways, starting from the ceiling beams, and bridge engineering, ending with window and door components . Thanks to a laminar structure of the composite, constructional components based on LVL are very rigid, have a high resistance to fire and aesthetic appearance.
Such components due to their homogeneity have an excellent dimensional stability, and nowadays they are available in a wide range of dimensions . The adhesive resins bonding wood layers have a significant influence on the composite properties by decreasing adsorption of humidity, reducing impact of the acidic environment, and reducing curb weight .
Most often for the production of glued veneers phenol-formaldehyde adhesives are applied, while for bonding the external veneers melamine adhesives can be used as well .
In LVL composites the layers of the cut veneers of about 3—4 mm most often 3. The physical-mechanical parameters of LVL composite are mostly dependent on the type of the trees, from which the material originates, the type of adhesive, as well as a thickness of particular layers [1, 16]. Veneer layers in one structural component can be made of different species of wood.
It has been studied and found that the sequence of the particular veneer layers made of different wood species influences the bending strength and modulus of elasticity . A quality of timber used for the composite production and veneer species plays a significant role as well . Number of knots and their location in the material structure , as well as consistency of the fibers direction with the direction of the force vector, have been proved to have an impact on the strength .
Eksploatacja i N iezawodnosc — Maintenance and Reliability Vol. A thermally treated timber TT is widely used also in Poland. Most of the thermally treated timbers available on the market are the exotic types. Thermally treated domestic timber such as spruce timber can become an alternative for the exotic timber in the longer time horizon.
Modification of the timber structure improves some of its physical — mechanical properties, mostly hardness and wear resistance , it also improves dimensional stability of the timber components and its biological resistance, as well as reduces the amount of moisture absorbed by the wood [18, 27]. The improvement of this property occurs as a result of chemical composition, mainly due to degradation of hemicellulose . This process also improves resistance to the aggressive environment and wood decay, and what is important from the aesthetic point of view — enables obtaining dark decorative colour .
It is known that thermal treatment of timber in some cases, conducted at the specific temperatures and exposure times can lead to the reduction of the ultimate wood strength. On the basis of the obtained results from the conducted strength studies, a reliability analysis of the above mentioned wood-based composites and thermally treated timber in high temperatures was performed. In case of static structures a reliability evaluation is possible based on the probability of not exceeding of the limit state of the load capacity or failure of the construction .
Probability of failure or in other words probability of not survival can be determined based on the distribution of the random variable of material strength and distribution of this variable as a function of temperature in which the object is present. The above assumption brings reliability of the construction to the strength reliability and its components. The strength reliability specifies in both a synthetic and correct way, the essence of all studies and strength investigations as well as their final purpose .
Methods to Assess Load Capacity of the Old Wooden Building Components
The reinforcement effect was achieved by combining, with the assistance of the gluing technique, of a CFRP type carbon composite with wooden elements of the beam. The obtained results were presented in the form of fields of combined states of stresses and deformations in individual beam elements. Calculations were performed on numerical models employing the finite elements method FEM. The best known of these materials with which considerable hopes are associated include carbon or glass fibres [2, 3, 8]. These materials are characterised by very good specific tensile strength ratio of the tensile strength to density , high elasticity modulus, deformation linearity practically up to destruction as well as viscosity and resistance to chemical agents.
reliability of the thermal treated timber and wood-based materials in ...
Wooden constructions are very susceptible to damage caused by improper operating conditions. Depending on the work conditions of construction the consumption manifest itself through: growth of cracks, loose connections, exceeding the allowable diffracts, constant moisture, development of fungus and mold, the occurrence of pests. Determining the type of these defects and their impact on work conditions is the topic of this article. Determination of strength properties of the component consist on establishing the type and kind of wood used in the construction and its class making it is possible to assign standardized permissible stress values and the modulus of elasticity. Determination of the extent to which the original cross-section of the component has been damaged during operation shallow or deep crack, shrinkage crack, damage of internal structure of the wood by insects and fungus, moisture allows to determine how much the strength properties of the wood component has been deteriorated.
Abstract Wood maintains remarkable durability under optimum conditions. However, it erodes faster than other construction materials in the circumstances of average usage of civil structures. Therefore, built features made of wood, especially historical features and structures, require repairs and reinforcements more frequently than those made of other materials. Non-destructive testing is usually carried out in structures that are currently in use and is aimed at early detection of failures and taking appropriate remedial measures. It can be generally assumed that diagnostic testing of wood is a difficult process due to its structure. However, the ability to identify the current strength parameters of wooden structural components makes it possible to properly assess their suitability for further use and design appropriate reinforcement and reconstruction methods.
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