Slope Of Grain Research Articles

POSSIBLE DEFECTS IN WOOD, WOOD PARAMETERS VARIABILITY AND SOME OF ITS INFLUENCE ON QUALITY OF BUILDING STRUCTURE

Paper specifies the wood-framed with sheathing construction including phases of realization and exploitation. Methods of investigation based on practice and theory has been employed where practical identification of processes and their phases were presented with theoretical description of structure deformation within the exploitation period. Obtained results of investigations are presented in the form of technologic and mechanic of structure diagrams for buildings with adequate algorithms of analysis. Paper presents practical systematic of construction stages, technological problems and hazards in respect of loading and construction technology with method of computation of vertical deformations of building. Paper presents evaluation of contribution of wood defects in response to loading in the wood-framed residential building in exploitation process. Location of knots, allocation of pith in the elements cross section, defects of slope of grain and influence of moisture decreasing is examined in the paper. The wood-framed with sheathing in the form of large panel or modular three dimensional 3D elements are the most often used technology. This kind of buildings are actually constructed up to four story as a multifamily or varying universal buildings like school, kid-garden or offices. Low dead load from 0.30 to 1.00 kN/m2 and its favorable proportion to the live load increase energy saving factor in realization and within the exploitation time. Low own weight of structure enables complete prefabrication of wall, floor and roof panels, improving quality of construction and shortening time of construction. Process of prefabrication, wide assortment and variety of factory production does not require time consuming processes and complicated moulds or shuttering indispensable in precast RC structural elements.

Microwave scanning as an additional grading principle for sawn timber

Prediction of strength is an important aspect in machine strength grading of timber. This property is strongly influenced by grain direction: Deviations from the main direction can cause a dramatic loss of strength. Therefore, an essential demand of the timber industry is finding the direction of wood fibres in a fast and non-invasive way. For three decades now the use of polarized microwave radiation has been investigated, revealing the main direction of fibres in a non-contact and non-destructive way. This paper describes the use of an innovative prototype able to detect grain deviations on a local scale using a stereoscopic method. The results of this prototype are evaluated to identify specimens with severe local slope of grain. Additionally, the prediction of strength using the microwave unit is compared to existing grading principles concerning prediction accuracy and other aspects.

Numerical calculations of the influence of the slope of grain on the effect of knots

This research used the finite element method (FEM) to investigate the influence of the slope of grain (SoG) on the strength-reducing effect of face knots. The results showed that the SoG may reduce the modulus of rupture (MoR) of knotty beams by up to 50 %. In addition, this negative influence was found to be markedly dependent upon both the knot position and the side of the specimen where the SoG is perceptible. Centerline knots were more susceptible to the SoG influence than margin knots. For the face knots, this effect also became worse when accounting for the SoG at faces (SoGf) in comparison to the edges (SoGe). The worst case scenario occurred however, when both the SoGf and SoGe were assumed. Further investigations are suggested to improve timber efficiency as current lumber grading standards do not discern these features.

Improving strength grading of timber by grain angle measurement and mechanical modeling

Timber strength grading has become a major issue in the European Union during the last years, due to the introduction of the Eurocode 5 and all its related standards. Currently, the most performing strength grading machines are able to locally detect the boards’ knots sizes and positions and interpret this information through adapted grading models. The best lead to improve their accuracy seems to be the introduction of new information about the boards and adapt the mechanical model to take them in account. Small grain angle causes high reduction of clear wood's mechanical properties; local value of slope of grain appears to be of high interest. The aim of this study is to quantify the additional accuracy that grain angle information can bring to an optical scanner used as a strength grading machine. A specific grading model has been developed accordingly, and the results obtained for different machine/model/loading combinations are presented. These results show that slope of grain measurement can significantly improve the accuracy of the optical scanner, for both modulus of elasticity and modulus of rupture estimations.

Finite Element Modeling of Wood Bat Profiles for Durability

The bats used in Major League Baseball (MLB) are required to be turned from a single piece of wood. Northern white ash had been the wood of choice until the introduction of hard maple in the late 1990s. Since the introduction of maple to the game, there was a perceived increase in the rate of bats to exhibit multiple piece failures (MPF)—both ash and maple. These failures introduced a new aspect to the game that can be a significant factor during play, i.e. pieces of bats going into the field of play, thereby distracting fielders while reacting to the batted ball. Observations of bat breakage in the field and lab testing of bats in controlled conditions have shown the bat durability is a function of wood quality and bat profile. Wood quality is described by the density and the slope of grain of the wood used in the bat. The density and the slope of grain determine the effective strength of the wood. The bat profile is described by the variation in the diameter of the bat along its length. The wood densities and bat profiles which are preferred by players, are typically in direct contradiction with what makes for a durable bat. In this paper, the finite element method is used to develop calibrated models of the breaking of wood bats in controlled lab conditions. The modeling approach is then used to explore how bat profile influences bat durability and what potential changes can be made in bat profile to satisfy player desires while increasing bat durability.

Prediction of Strength and Elasticity in Oak Beams on the Basis of External Wood Characteristics

The purpose of the study was to search external features of timber useful for predicting modulus of rupture (MOR) and modulus of elasticity (MOE), both in axial direction bending, valid for a wide range of beam qualities. For the study, 23 European oaks (Quercus robur L.) were felled in northwestern Spain, where most of the oak stands in the country are located. The trees were sawn and a sample of quarter-sawn planks was selected. Planed and edged specimens (5×10×200 cm) were tested to obtain MOR, MOE, apparent density, wood moisture content, percentage of sapwood, ring width, slope of grain and size and position (faces, upper and lower edge, heads) of external defects. The MOR was correlated with the maximum face and edge knot diameter. However, the correlations were not high enough to justify construction of a predictive model of mechanical behavior on the basis of maximum knot size in the piece. The MOE was correlated with the size of knots and wane. Presence of sapwood did not exhibit significant influence on strength performance. Homogeneity in ring width was negatively related to MOR and mainly to MOE. Subject headings: Bending; Mechanical properties; Stiffness; Wood beams; Wood structures.

Strength Properties of Chestnut Structural Timber with Wane

Grading of timber for structural purposes allows material with similar physical and mechanical properties to be grouped together. The groups are defined by specific strength values and indicate the basic working stresses and stiffnesses to be used for structural design purposes. In particular, visual strength grading bases the assignment of each timber element to a grade on the characteristics that are visually measurable (i.e. knots, slope of grain, annual ring width). Very similar principles are implemented for the in situ assessment of timber structures, as recommended for example by the Italian standard UNI 11119, that specifies the procedures for the diagnosis of timber members. Currently, however, European standards and most of the national standards on strength grading refer to timber elements with rectangular cross section, while in existing buildings, members with irregular cross section are extremely common. Thus, 10 different provenances (5 from Italy and 5 from France) of chestnut timber were sampled. More than 600 structural elements with wane were visually examined in laboratory and all the characteristics were collected. Bending tests were then performed on each timber element until failure, and bending strength, as well as modulus of elasticity and density, were determined. Here, the limitations for the main strength-reducing characteristics and the characteristic values (fifth percentile for bending strength and density and the mean value for modulus of elasticity) of chestnut timber with wane are presented.

Experimental tests and numerical analysis of historic bent timber elements reinforced with CFRP strips

The paper presents the results of studies proposing to use carbon fibre reinforced polymer (CFRP) strips to reinforce beams with defects (biological corrosion, inclusions, slope of grain, cracks) in order to restore their load-bearing capacity. The proposed solution, consisting in bonding CFRP strips into the cross section, is suitable for restoration work in historic buildings. Moreover, owing to the fact that the reinforcing inserts are situated inside the cross section, the possibility that the adhesive strip–wood bond will delaminate is reduced and the fire resistance of the element is increased. The increase in the load-bearing capacity of the tested beams amounted from 21% to over 79%. The results of an analysis of the static behaviour of the reinforced cross sections, carried out using numerical modelling and the finite element method, are presented.

Caractérisation du Chêne sessile et pédonculé de France en vue de son utilisation en structure

Characterising the mechanical properties of oak intended for construction uses has become necessary to apply regulations concerning CE marking of construction materials as of 2012. To acquire the needed information, in 2002 FCBA engaged in a large-scale study of two species of French oak - Quercus petraea Liebl. and Quercus robur L.. 420 trees from the two species in the 25 cm to more than 70 cm range were harvested from 70 stands selected in seven French areas. Non-destructive and destructive tests were performed on a total of 3105 planks (50*150*3000 mm). Three main criteria (density, modulus of elasticity and modulus of rupture) establishing the strength classes according to EN 338 appeared of high value. The optimal grading gives 71% in D40 and 13% in D30 mechanical classes. However, correlations between special features (knottiness, growth ring width, slope of grain, etc.) and mechanical properties appeared low (R2=0.26). Therefore, visual grading according to the NF B 52-001 standard does not allow efficient grading of planks (26% in D30, 21% in D24 and 28% in D18). Using a crude statistical analysis of available data, a simple model has been developed to predict mechanical behavior on the basis of tree’s diameter at breast height and stem height from which plank is sawn.

The effect of fissures on the strength of structural timber

Detailed measurements of drying fissures were carried out on 28 pieces of gross cross-section structural Scots pine timber. Different parameters have been proposed to measure their magnitude: length, depth, thickness and slope of grain, as well as an index of area in the neutral axis plane (ratio of effective to total surface). The parameters of fissures have been related to bending strength, shear strength, modulus of elasticity and rupture energy obtained from mechanical tests, considering whether rupture is due to bending or shear. All of the fissures observed allows to classify the pieces as MEG according to the UNE 56544 standard. To quantify the magnitude of fissures the index of areas is proposed, while mechanical properties can be quantified by rupture energy. Analysis of the results does not show any relation between the fissures analyzed and the mode of failure, their magnitude and mechanical properties..

Method for predicting tension capacity of sawn timber considering slope of grain around knots

This study introduced a new parameter, the area reduction factor (ARF), to consider the effect of knots on the tension strength of timber. It is an improved version of the knot area ratio (KAR). ARF considers both the projected area of knots and the effect of the slope of grains around the knots. The tension capacity of a tested structural timber was predicted as a product of ARF, clear wood tension strength parallel to the grain, and the area of the cross section. ARF was determined as the minimum value obtained when a knot measurement window of 100 mm was slid along the plank. The prediction method was examined with 11 planks. The average ratio of the predicted capacity to the actual value was 1.11 with a coefficient of variation of 0.26. The average ratio obtained by using a KAR-based parameter, the clear wood area ratio (CWAR), was 2.15 with a coefficient of variation of 0.23. To study the reliability of ARF and CWAR as single parameters, the correlations of ARF and CWAR with the tension strength were determined for 57 planks. The coefficients of determination for ARF were slightly better than those for CWAR, although both of them seemed to be quite poor predictors of tension capacity when used alone. Therefore, a multiparameter model is preferred and should be a subject for further studies.

A method for under-bark detection of the wood grain angle radial dependence

Twist in wood, being closely related to spiral grain, may cause serious problems in building structures, furniture and joinery. It is therefore of great interest to sort out, at an early stage in the manufacturing process, trees, logs and boards that have an excess of spiral grain. The spiral grain pattern is described by a helical deviation of the fibre direction in relation to the longitudinal direction of a living tree or a log and seems to be an indicator for other defects such as compression wood. Remote microwave sensing of spiral grain has received a lot of interest during the past two decades. Its development has been impeded by the large variation with moisture content of the microwave properties of wood and by the complexity in modelling the electromagnetic field in a log with spiral grain. A review is presented of a direct method with no requirement for information on moisture content for boards. This procedure has recently been generalized to cylindrical logs and trees having a constant slope of the grain. A further generalization is presented here to allow for the normal spiral grain pattern with radially changing slope of grain in wood under bark. Based on this theory, a measurement procedure is proposed for the detection of wood grain angle with radial dependence, requiring no information on moisture content in the sapwood, which is also applicable to completely or partially frozen wood. A suitable application would be an instrument to use in the forest for measurements on living trees or logs.

Effect of a two-stage heat treatment process on the mechanical properties of full construction timber

This study investigated the impact of an industrially used two-stage heat-treatment method on the mechanical properties of full construction timber. Bending tests of full-size Norway spruce posts demonstrated changes due to heat treatment, such as a decrease in the density and bending strength, as well as an increase in the modulus of elasticity (MOE). The variability in bending strength and MOE appeared to be higher for treated posts and as a result the 5% values decreased, especially for the bending strength. The MOE was dependent on density for both treated and untreated posts, but this was not so for the bending strength. There appeared to be a relationship between the bending strength and the MOE, although it was rather weak, especially for treated posts. It was difficult to correlate the occurrence of natural defects in treated Norway spruce posts with an effect on the mechanical properties. Only a combination of several defects, such as large knots, enclosed pith and a deviating slope of grain, appeared to decrease the bending strength and MOE of treated posts, at least more than for untreated posts. The effect of a three year period of outdoor exposure on the strength properties of heat-treated terrace planking was limited. In particular, for the 5% value low strength range, where wood defects strongly determine the mechanical properties of wood, a three-year outdoor exposure did not change the bending strength or MOE of heat-treated wood.

Madera aserrada de gran escuadría para uso estructural y sus particularidades

The size effect on mechanical properties of timber is described including the recent evolution of testing procedures and stress grading. Further, a specifi c research work on large cross section timber is explained. A total of 395 pieces of sawn timber from three species (Pinus sylvestris L., Pinus nigra subs. salzmanii (Dunal) Franco and Pinus radiata D. Don) with cross section between 150x200 y 200x250 mm, have been visually graded according to UNE 56544 standard obtaining its output and mechanical properties. The percentage of rejected pieces is too high (38 to 45%) and therefore, a modifi cation of standard specifications is proposed. The output is highly improved in the proposal of standard (rejected percentage of 11 to 17%) whereas the mechanical properties are not modified in practice.A similar procedure was applied to 85 timber pieces from old timber structures of building demolition of Pinus sylvestris L. and Pinus pinaster Ait. The percentage of rejected pieces is 52% in a visual grading according to the application of all grading parameters of the standard UNE 56544:2003. This procedure is not practical for existing structures and therefore, it is proposed the application of grading considering only the parameters of knots size and slope of grain. In that way the output is improved reducing the rejected percentage to 10%.

Strength properties of thermally modified softwoods and its relation to polymeric structural wood constituents

Thermal modification at relatively high temperatures (ranging from 150 to 260 °C) is an effective method to improve the dimensional stability and resistance against fungal attack. This study was performed to investigate the impact of heat treatment on the mechanical properties of wood. An industrially-used two-stage heat treatment method under relative mild conditions (< 200 °C) was used to treat the boards. Heat treatment revealed a clear effect on the mechanical properties of softwood species. The tensile strength parallel to the grain showed a rather large decrease, whereas the compressive strength parallel to the fibre increased after heat treatment. The bending strength, which is a combination of the tensile stress, compressive stress and shear stress, was lower after heat treatment. This decrease was less than the decrease of only the tensile strength. The impact strength showed a rather large decrease after heat treatment. An increase of the modulus of elasticity during the bending test has been noticed after heat treatment. Changes and/or modifications of the main wood components appear to be involved in the effects of heat treatment on the mechanical properties. The possible effect of degradation and modification of hemicelluloses, degradation and/or crystallization of amorphous cellulose, and polycondensation reactions of lignin on the mechanical properties of heat treated wood have been discussed. The effect of natural defects, such as knots, resin pockets, abnormal slope of grain and reaction wood, on the strength properties of wood appeared to be affected by heat treatment. Nevertheless, heat treated timber shows potential for use in constructions, but it is important to carefully consider the stresses that occur in a construction and some practical consequences when heat treated timber is used.

The effect of global slope of grain on the bending strength of scaffold boards

A study is described, carried out on specially prepared specimens, that specifically examines the effect of slope of grain on the bending strength of scaffold boards. Each specimen was visually graded and both the modulus of rupture and modulus of elasticity in bending determined. From the effect of slope of grain on the reduction of bending strength revised visual grading rules have been suggested that include a slope of grain limit of 1 in 12. It has also been demonstrated that machine grading is as effective at determining the adverse effect of slope of grain on board strength as it is for other defects such as high rate of growth and knots.

Identification of Strength-Reducing Characteristics in Lumber Using Microwave Scanners

An application of the microwave technology to identify the strength-reducing characteristics in structural lumber is presented. The microwave scanners that have been developed at the Commonwealth Scientific & Industrial Research Organisation of Australia (CSIRO) in recent years can effectively detect two types of strength-reducing characteristics along the lumber: knot and slope of grain. The technique is shown to be simple, fast, safe, effective and reliable. Based on the data from the microwave scanners, an algorithm is then developed to identify juvenile lumber, which has very unfavourable physical and anatomical properties for appearances and structural uses. The use of the microwave scanners is found to be very useful in developing a strength prediction algorithm for stress grading of lumber. Some considerations for commercial uses and their expected benefits are presented.

Ultrasonic techniques for nondestructive testing of standing trees

The aim of this study is to assess wood quality of standing trees with ultrasonic velocity methods. The ultrasonic techniques currently used are: scattering based techniques that use travel time and other wave parameters for measuring the elastic constants of wood, and ultrasonic tomographic imaging techniques which seek to provide a high resolution picture of the defect. Scattering based techniques were firstly used as nondestructive techniques with increment cores of 5 mm diameter. On these specimens measurements with bulk waves––longitudinal and shear were used to detect the influence of sylvicultural treatment (pruning) on wood quality and also to detect defects (slope of grain, wavy figures). High resolution imaging techniques were developed for imaging abnormalities induced by biological attacks of fungi or insects in standing trees. In this case, ultrasonic tomography refers to cross-sectional imaging of trees from data collected by illuminating the tree from different directions. Ultrasonic images were reconstructed from the time of flight. The images were reconstructed from 120 measurements in situ and through the bark with direct transmission technique, using 1 MHz transducers. The resolution is 5 cm.

EFFECTS OF MACRO- AND MICRO-STRUCTURAL VARIABILITY ON THE SHEAR BEHAVIOR OF SOFTWOOD

Longitudinal shear strength and shear modulus of spruce and larch wood with a maximum of micro- and macro-structural variability was determined using a new testing method. Oven-dry density and slope of grain were measured after the shear tests. For the spruce wood samples, a data set of fiber and cell wall properties, i.e., lignin content, microfibril angle, fiber length, lumen diameter, cell wall thickness, latewood proportion, and ring width, was available. A multiple linear regression analysis of all fiber and cell wall properties showed a significant, but not very strong effect on the variability of shear strength (R2 = 0.21). It is thus demonstrated that micro-structural variability plays a minor role in the variability of shear properties. By contrast, a multiple linear regression involving shear modulus, density, and slope of grain as three independent variables revealed an excellent possibility to model the variability of shear strength (R2 = 0.72). This study demonstrates the potential for non-destructive evaluation of the shear strength of solid wood.

Mechano-sorptive bending creep of timber - influence of material parameters

In this paper mechano-sorptive creep test results from twelve Norway spruce trees are presented. The wood material was well defined with respect to growth conditions, position in the tree and material parameters, such as annual ring width, slope of grain, knots, compression wood, density and modulus of elasticity. The influence of these parameters on the creep behaviour was studied. The creep tests were conducted in a climate room with varying relative humidity of 30%-90% and the duration of the creep tests was at least 100 days. For seven of the twelve studied trees there was a radial variation in relative creep between the pith and the bark. The specimens cut near the pith showed larger relative creep than the specimens cut near the bark. When all the 59 specimens included in this study were examined together, the modulus of elasticity seemed to be the best parameter, measured in this study, to predict the magnitude of relative creep. The correlations between relative creep and material parameters varied between the specimens from the different trees. Also the appearance of the creep curves varied between different specimens. Some specimens showed large variations in relative creep within the moisture cycles, some specimens did not at all seem to be sensible to moisture changes. The reason for these differences were not explained successfully in this study.