Advanced Materials Research Vol. 778

Abstract: 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.

Abstract: The aim of this preliminary study was to investigate differences in mechanical and fungicidal properties of three different wood species (English oak (Quercus sp.), common beach (Fagus sylvatica) and Norway spruce (Picea abies)) specimens that were in indoor use for several decades, compared to control specimens of freshly cut timber. The collected material was cut to smaller specimen and conditioned in standard climate prior to further analysis. Mechanical, fungicidal, and sorption properties were determined according to the standard procedures. Our results show that mechanical properties of oak wood do not deteriorate in the investigated time frame. On the other hand, resistance of the wood against fungi decreases over time. The reason for this is yet to be confirmed, it could be due to the degradation of secondary metabolites. Similar results were observed for spruce wood as well. There were no statistically significant differences in mechanical properties of old and new spruce wood. In contrary to oak wood, there were also no significant differences in fungicidal properties, considering that spruce wood has lower durability than oak wood. Same as with oak wood, G. trabeum was found to be the most aggressive fungus. Ageing of beech wood resulted in a big drop in all of the tested mechanical properties, but showed no significant differences in fungicidal properties. Here is where the low durability of beech wood really shows. The old beech wood specimens were moderately deteriorated by insects and fungi, which was the reason for loss of bending and compressive strength.

Abstract: In the context of a more wide research study for in situ mechanical identification of ancient timber structures by non-destructive techniques, the paper illustrates the experimental activity developed on structural elements made of old chestnut timber, 4x4x76 cm size: 24 squared specimens were extracted by six elements in actual dimensions, already used in a previous tests campaign. Non-destructive (ND: hygrometric, sclerometric and resistographic) and destructive (D: bending) tests were performed, the latter according to UNI EN 408 (2004) standard, in order to assess the mechanical properties and the collapse mechanisms of chestnut timber. Data gathered are presented in detail and, NDT-DT correlations, obtained through a linear regression approach, are provided to predict wood density, strength and modulus of elasticity of the tested members.

Abstract: Besides the difficulty of assessing an existing timber structure on site, the efficiency and accuracy of visual inspection is often compromised by its subjective nature inherited by the level of expertise of the inspector. This often leads to conservative predictions of the mechanical properties, even with the use of specific visual grading norms. The main objectives of this work are to assess the effectiveness of visual inspection as a method to define different classes of strength and stiffness and to provide a statistical analysis on its subjectivity. For that aim, visual inspection using Italian standard UNI 11119:2004 and bending tests of 20 old chestnut beams (Castanea sativa Mill.) at different scale element, were carried out. Comparisons and effectiveness of visual inspection is analyzed within and between different scales of the timber members, and also regarding different level of expertise of inspectors. The results evidence similar percentages of segments classified with higher and lower visual inspections classes and proved to be a good qualitative indicator of bending strength between sawn beams. An overall 42% accuracy of the most experienced inspectors was found with better differentiation between visual classes, whereas lower level inspectors scored approximately less 5%. Lower level inspectors also evidenced higher concentration of values around a higher mean for each class denoting a more conservative approach. Regardless of the inspector level, knot size was considered the main limiting visual parameter with higher influence in small scales of the timber elements. When studying the characterization of a single knot, coefficients of variation of 15.7% and 21.8% were found for measuring the minimum and maximum diameter. Bayesian probability networks were considered as to individually assess the accuracy in stiffness prediction of different level of inspectors, and by combination of their information, evidencing that parallel combination for prior information may allow the increase in visual inspection accuracy.

Abstract: This research highlights the constructive features of Juniperus thurifera timber visible in the Western Mediterranean (from the island of Sardinia to Morocco). The use of this material is above all related to vernacular and traditional architecture, in the coastal areas as well as in the interior of the analyzed countries. An initial taxonomy of its use presents us with the issue of understanding the main structural performance of this type of timber, focusing in particular on its structural use and behavior. The results, visible throughout the research, are related to the resistance of the timber and its performance. On the other hand, this raw material is valued in vernacular architecture because of its strength when exposed to sea water or harsh weather conditions. Juniper beams and pillars are the most frequent solutions, but the use of branches and roots is also common, as the research demonstrates. In this framework, the text underlines the results of the research based on a geographical survey of juniper as a construction material, and on the first attempts to study its mechanical performance.

Abstract: In Italy timber and wood products are becoming increasingly used for building constructions due to their excellent physics and mechanical properties. International Codes require the use of wood previously graded according to the current regulation. This paper reports the preliminary results of an experimental campaign aimed at verifying the reliability of the use of Sardinian timber as structural material. For this purpose Maritime Pine boards from two different Sardinian areas have been analyzed and visual strength graded. Physical and morphological properties (density, knots, clusters knot, resin pockets, deviation of the grain, annual ring width position of board respect to the pith, humidity, etc.) along with mechanical and non destructive properties (elastic modulus, tensile strength, ultrasonic pulse velocity) have been checked. Timber properties have been statistically evaluated in order to identify the performance of the base material. Regression analyses have been carried out by studying the correlation between non destructive parameters and mechanical properties in order to define a criterion for predicting the strength class of the base material.

Abstract: The Wood Penetration Test described in the present paper was set up almost 20 years ago to evaluate the timber vaulted roof decay of Palazzo della Loggia in Brescia (Italy). The test can be regarded as an extension to timber structures of the dynamic soil penetration test, as it is based on the insertion into timber structural members of a steel graduated rod, which advances by means of repeated constant energy blows transmitted by a rebound hammer. The test makes possible to distinguish between different degrees of decay as a function of the number of blows necessary for each centimetre layer penetration. This technique proved to be effective and reliable for investigating the extent and depth of wood decay and thanks to the assessment campaign the possibility of restoring the ancient vault gained strength. The test was then frequently proposed and adopted by the Structural Engineering Laboratory of Brescia University and several ancient building timber structures were checked by means of this methodology. Some significative examples will be briefly described. A recent investigation has been done on the rim beams at the basis of the Palazzo della Loggia timber vault by using the penetration test and the Resistograph and the comparison of the results is also illustrated. The test is easy to perform and the output data can be interpreted very clearly by structural engineers: it is possible to determine the depth of the decayed layers and consequently the reliable dimensions of the resisting timber sections.

Abstract: The paper presents a novel concept and a prototype of a diagnostic tool for in-situ assessment of timber in existing structures and buildings. The device enables direct determination of conventional compressive strength and modulus of deformation in arbitrary depth along timber profile. The measurement of the stress-deformation relationship is performed in a prepared hole of a very small diameter by means of a special small size symmetrical loading jack. Testing and verification of the new device behaviour during loading of wood along the fibres has been carried out on fir which is the most frequent wood species in European buildings. The comparison of stress-strain diagrams acquired by means of the new device with those measured in compliance with the European standard for testing of wood shows a very good correlation including the determination of the compressive strength values. This semi-destructive method causes a very gentle damage and can be also used for the assessment of historic timber structures.

Abstract: Withdrawal resistances of wood have been applied for in situ assessment of wood in existing timber structures. The author had proposed method to estimate shear strengths of wood from measured withdrawal resistances of probes which are screwed into wood. In order to verify the accuracy of these estimated shear strengths by proposed methods, withdrawal resistance measurements and shear loading tests were conducted for wood. Single withdrawal resistance measurement was applied for wood specimens, and estimated shear strengths from withdrawal measurements were compared to the measured shear strengths by shear loading tests of wood. Correlation between the estimated shear strengths and measured shear strengths of specimens was reasonably good (R2=0.73). Multiple coaxial withdrawal resistance measurement which can provide distribution of shear strengths in cross-section of wood was also proposed. The average of estimated shear strengths by single withdrawal resistances was 7 percent less than that of measured shear strengths. The average of estimated shear strength by multiple coaxial withdrawal resistances was 3 percent greater than that of measured shear strengths. The single withdrawal measurements and multiple coaxial withdrawal resistances are available to estimate shear strengths of wood and shear strength distribution in the cross-section of wood. Estimated shear strengths obtained from these methods will be valuable for strength based in situ assessment of wood.

Abstract: The paper presents a survey of state-of-the-art nondestructive and semi-destructive diagnostic techniques of testing timber structures and examples of their application. Nondestructive testing is a field of study which covers: identifying the mechanical and physical properties of materials and structural components, detecting material defects and discontinuities, and measuring the geometric dimensions of objects without affecting their performance. The nondestructive and semi-destructive methods are described in a systematic manner and the relevant equipment, technology and applications are discussed. The paper focuses on acoustic methods, resistance drilling techniques and radiography, which are suitable for detecting internal defects, decay and cracks, determining the location and dimensions of degraded areas and assessing the mechanical properties of structural timber members. Much attention is devoted to the techniques that are used in practice and have shown significant promise for future development. The results of several drilling resistance tests carried out on historic timber structures are presented.