Robert Kliger

Quality of structural timber-product specification system required by end-users

Interviews conducted with the end-users of timber products revealed that their knowledge of timber is very poor. However, building contractors are not satisfied with the quality of timber, primarily as a result of its excessive warp. Their requirements are not passed on to the producers in the forest and sawmill industry. The quality of a timber product should be described in a specification of requirements for each individual product. In this paper, a systematic analysis of the end users expectations is presented and is illustrated using a product specification for wall studs. The proposed acceptance levels are based on interviews with contractors and derived from the requirements set for the wall. The current grading rules are more generous than our proposal. Field tests by carpenters showed that our proposal limits appear to be reasonable. A study of commercially-available studs showed that it should be possible both technically and economically to produce studs which fulfil our requirements. If timber is to hold its own as a building material, the forest and sawmill industry will have to produce products which match the end users requirements.ZusammenfassungUmfrageergebnisse unter Enduntzern von Bauschnittholz zeigten, daß deren Kenntnisstand über diese Produkte sehr gering ist. Sie sind jedoch nicht zufrieden mit der Qualität des Bauholzes, vor allem aufgrund starker Verformungen. Die Anforderungen an das Bauholz werden nicht an die Hersteller im Forst und in Sägewerken weitergegeben. Die Qualität dieser Produkte sollte durch spezifische Anforderungsprofile beschrieben werden. Im vorliegenden Bericht wird eine systematische Analyse der Erwartungen der Endverbraucher vorgelegt und am Beispiel einer Produkt-Spezifikation für Wandpfosten erläutert. Aufgrund der Befragungen und der Anforderungen für den Holzbau werden bestimmte Akzeptanz-Grade vorgeschlagen. Die derzeitigen Sortierungs-Regeln sind großzügiger als die neu vorgeschlagenen. Felduntersuchungen bei Zimmerleuten ließen die neuen Grenzwerte als vernünftig erscheinen. Eine Untersuchung handelsüblicher Pfosten ergab, daß es sowohl in technischer als auch in ökonomischer Hinsicht möglich sein sollte, Bauschnittholz entsprechend den neuen Anforderungen herzustellen. Wenn Schnittholz seine Eigenständigkeit als Baumaterial beibehalten soll, müssen Forst- und Sägebetriebe Produkte liefern, die sich an den Erfordernissen der Endnutzer orientieren.

Distortion of Norway spruce timber Part 1. Variation of relevant wood properties

Picea abies) trees from one fast-grown and one slow-grown stand in southern Sweden. From the trees 240 studs (45 × 70 × 2500 mm) were taken for measurement of distortion. Wood properties were measured on small specimens (13 × 13 × 200 mm) cut from the studs. Spiral grain angle was found to vary from approximately +3° (left-handed) close to pith to zero 150 mm from pith with a strong individual variation. The material from the fast-grown stand had a larger spiral grain angle compared with the slow-grown material. Spiral grain was poorly correlated to other parameters. Presence of knots had a substantial influence on longitudinal shrinkage (αl) measurements. Specimens with large knots (KAR > 33%) had almost 100% higher longitudinal shrinkage than specimens without knots. It should be pointed out, however, that measuring shrinkage in small specimens containing even small knots can create a problem with regards to the obtained results, especially results of αl. It was found that presence of compression wood in several growth rings more than doubled the longitudinal shrinkage. For the radial and tangential direction the presence of compression wood decreased shrinkage with about 30%. The ratio between tangential and longitudinal shrinkage was 49 for normal wood whereas for compression wood the ratio was 13. These results confirm the theory that the microfibril angle governs shrinkage. Longitudinal shrinkage decreased slightly with increased distance from pith whereas radial and tangential shrinkage did not display any substantial radial variation. The fast-grown material had generally a higher longitudinal shrinkage and lower transverse shrinkage than the material from the slow-grown stand. About 50% of the variation in longitudinal shrinkage was explained by radial position, density and ring width. Density and ring width did explain 60% of the variation in radial shrinkage but only 30% of the variation in tangential shrinkage.Picea abies) von einem schnell- und einem langsamwachsenden Standort wurden verwendet. Daraus wurden 240 Kanthölzer der Abmessung 45 × 70 × 2500 mm geschnitten und daran die Verwerfung bestimmt. Holzeigenschaften wurden an kleinen Proben (13 × 13 × 200 mm) gemessen, die aus diesen Kanthölzern hergestellt wurden. Der Faserwinkel variierte zwischen +3° in der Nähe der Markröhre bis 0° bei 150 mm Abstand vom Mark mit großer individueller Streubreite. Die Proben vom schnellwachsenden Standort hatten einen größeren Faserwinkel als die vom langsamwachsenden Standort. Die Korrelation des Faserwinkels zu anderen Holzeigenschaften war nur sehr schwach. Anwesenheit von Ästen hatte einen bedeutsamen Einfluß auf das longitudinale Schwinden (αl); es lag bei hohen Astanteilen (KAR > 33%) fast 100% höher als bei astreinen Proben. Es allerdings muß betont werden, daß bei der Messung an kleinen Proben schon kleine Äste problematisch sind, speziell für αl. Weiter zeigte sich, daß Anteile von Druckholz in einigen Jahrringen das longitudinale Schwinden mindestens verdoppeln. Radiales und tangentiales Schwinden wird durch Druckholzanteile um etwa 30% verringert. Das Verhältnis zwischen tangentialem und longitudinalem Schwinden betrug 49 für normales Holz, bei Druckholz lag dieses Verhältnis bei 13. Die Ergebnisse stützen die These, daß der Mikrofibrillwinkel das Schwinden regelt. Longitudinales Schwinden nahm mit zunehmender Entfernung vom Mark langsam ab, während radiales und tangentiales Schwinden keine wesentlichen Änderungen aufwiesen. Schnellwachsendes Holz zeigte allgemein höhere Schwindwerte als langsamwachsendes Material. Rund 50% des longitudinalen Schwindens werden erklärt durch die Parameter radiale Position, Dichte und Jahrringbreite. Dichte und Jahrringbreite erklären 60% des radialen Schwindens, aber nur 30% des tangentialen Schwindens.

Distortion of Norway spruce timber Part 2. Modelling twist

Picea abies). Several material parameters were also measured, such as spiral grain angle, shrinkage in all three directions, annual ring width and density. Twist in the studs was measured at four different times at different moisture contents. The amount of twist correlated well with the moisture content and was reversible throughout several moisture changes. When the moisture content decreased, the twist increased and vice versa. About 50% of the variation in twist could be explained by a single parameter, i.e. the average growth ring curvature. All studs with severe twist were cut with its centroid within a radius of 75 mm from the pith. A statistical analysis of the data shows that growth ring curvature and spiral grain angle together explained about 70% of the variation in twist. Other parameters, such as shrinkage strains, density and ring width, did not increase predictability. When using a model developed by Stevens and Johnston (1960), about 66% of the variation in twist could be explained. The model also explained twist quantitatively well. The model included curvature of the growth ring, spiral grain angle and the tangential shrinkage strain.Picea abies). Mehrere Materialeigenschaften wurden ebenfalls gemessen, und zwar: Faserwinkel, Schwinden in drei Richtungen, Jahrringbreite und Dichte. Die Verdrehung der Kanthölzer wurde zu vier verschiedenen Zeitpunkten bei unterschiedlichen Feuchtegraden gemessen. Das Ausmaß der Verdrehung war gut korreliert mit der Feuchte. Mit abnehmender Feuchte stieg die Verdrehung und umgekehrt. Rund 50% der Verdrehungswerte sind durch einen einzigen Parameter erklärt, nämlich die Jahrringkrümmung. Bei allen Kanthölzern mit starker Verdrehung lag die Mittelachse innerhalb eines Abstandes von 75 mm von der Markröhre. Die statistische Analyse ergab, daß Jahrringkrümmung und Faserwinkel zusammen ca. 70% der Variation der Verdrehung erklären. Andere Parameter wie Schwindspannungen, Dichte und Jahrringbreite erhöhten die Vorhersagbarkeit nicht. Mit Hilfe des Modells von Stevens und Johnson (1960) konnten rund 66% der Verdrehung erklärt werden. Dieses Modell lieferte auch zufriedenstellende quantitative Ergebnisse. Berücksichtigt werden dabei Jahrringkrümmung, Faserwinkel und tangentiale Schwindspannung.

FE analysis of stringer-to-floor-beam connections in riveted railway bridges

The results from ten fatigue tests performed on six full-scale riveted stringers are presented and discussed. The stringers were taken from a 100 year old bridge, and the tests were performed at two different stress ranges: 60 and [similar to] 100 MPa. The tested stringers showed an excellent fatigue performance and their fatigue lives were in correspondence with or above the fatigue design curve given by the codes. Furthermore, the efficiency of using stop-holes to retard fatigue crack propagation in riveted girders was examined. The method was found to be an effective measure when aimed at temporarily arrest the crack propagation in riveted girders.

Material properties and their interrelation in chemically modified clear wood of Scots pine

Abstract The mechanical and physical properties of modified wood were assessed experimentally. Timber studs with a cross-section of 45 mm×70 mm were modified on a semi-industrial scale by four different methods: 1) acetylation (AC) with acetic anhydride; 2) modification with methylated melamine formaldehyde resin (MMF); 3) heat treatment in vegetable oil (HT); and 4) furfurylation (FA). Sapwood of Scots pine (Pinus sylvestris) with test specimen dimensions of 10×10×200 mm3 was investigated. A total of 2449 specimens were included in the study. The following properties and their correlations were studied: density, modulus of elasticity (MOEdyn) measured dynamically, equilibrium moisture content (EMC), and swelling behaviour. The properties were measured at humidity levels of 30%, 65%, and 90% RH, while the temperature was kept constant at 23°C. Significant changes in material properties took place. AC increased the density and reduced MOEdyn, EMC and swelling strain. FA increased the density and the swelling coefficient and reduced EMC and swelling strain. HT reduced the EMC, while MMF modification increased it.

Experimental and Numerical Investigation of the Behaviour and Strength of Composite Steel-CFRP Members

The use of advanced composite materials to strengthen and repair existing structures is increasing rapidly. One specific area in which the technique has recently been introduced is the strengthening of metallic structures with bonded carbon-fibre laminates. In this paper, the behaviour of composite steel-CFRP members is studied experimentally and using FE analysis. A new type of test specimen has been developed to examine the effect of various material parameters on the behaviour and strength of bonded steel-CFRP elements. Finite element analysis of the tested elements was also employed to examine the type and magnitude of interfacial stresses in the bond line of these elements. Moreover, different types of fracture mode could be examined by testing composite elements with different combinations of CFRP-laminates and adhesives. The tested composite elements displayed different behaviour and a large difference in strength and ductility could be observed.

Stress Distribution in Adhesive Joints with Tapered Laminates — Effect of Tapering Length and Material Properties

One major problem when using bonded fiber-reinforced plastic laminates to strengthen and upgrade existing structures is the high stresses in the adhesive layer, in the area close to the end of the laminate, which might govern the failure of the joint. One method that has been put forward as a means of reducing the stress concentration in this area, is to taper the end of the laminate. Although this method has been suggested by some design guidelines, no specific information is usually provided about the tapering type, required tapering length, and limitations associated with this method. A parametric study has been carried out to investigate the effect of tapering length and the material properties of joint constituents, i.e., stiffness of the laminate and adhesive, on stress distribution in adhesive joints using the finite element method. Two different configurations, including normal and reverse tapering, were considered. The results indicated that the effect of tapering on stress distribution is highly dependent on the stiffness of the laminate and the adhesive used in the joint. It was concluded that tapering is more effective in joints with softer laminates and stiffer adhesives. Reverse tapering was found to have more favorable effects on stress reduction in comparison to normal tapering.

Bending Creep of High-Temperature Dried Spruce Timber

Summary In this paper, bending creep test results from high-temperature (HT) dried Norway Spruce timber are presented. The results are compared with creep tests of conventional low-temperature (LT) dried timber. The HT drying was performed at 115°C and the LT drying was performed at 70°C. The creep tests were performed in a varying climate with 20°C and 30–90% relative humidity. A total number of 24 specimens, 45 × 70 × 1100 mm, were loaded in bending during approximately 240 days. Both the HT-dried and the LT-dried material were cut from the same four trees to have as equal raw material as possible for the two drying methods. The results show smaller bending creep deformations of HT-dried timber (approximately 30%) as compared to the LT-dried timber. The variation in magnitude of creep for HT-dried timber was equal to or smaller than for LT-dried timber. Additionally, the variation in deformation between the moisture cycles was smaller for HT-dried timber. These observations indicate a less pronounced mechano-sorptive effect for HT-dried timber. The free shrinkage in the grain direction was significantly smaller for the HT-dried material than for the LT-dried material.

Assessment of Density in Timber Using X-Ray Equipment

Wood density has a strong relationship with several mechanical properties, such as strength and stiffness. An x-ray image calibration procedure, which enables the determination of density properties for the in situ assessment of timber structures, has been developed. This nondestructive method is useful for evaluating the internal condition for global assessments of the structure. For this study, 14 wood specimens with significant density differences were exposed to x-rays. The specimens were set up in a number of image configurations. The images were analyzed and evaluated using an image toolbox. A similar procedure was applied to six built-up blocks with different thicknesses in order to calibrate the thickness of the specimens. Finally, a calibration wedge was set up to verify the procedure. The density calibration procedure from x-ray images was verified on a timber beam specimen, resulting in good agreement and an average accuracy of 97%. The values obtained from the image calibration presented a very good linear correlation between the measured density and the grayscale from x-ray images with coefficients of determination (R2) ranging from 0.90–0.98. The main advantages compared with conventional techniques are the detection and quantification of internal damage, defects, disturbances, and deterioration that may reduce the mechanical properties of the structure. This study shows good potential when it comes to the development of a viable tool for in situ assessments of timber structures. This technique could be used indirectly in analyses of structural behavior.

Comparison between HT-dried and LT-dried spruce timber in terms of shape and dimensional stability

Abstract The performance of timber studs from Norway spruce (Picea abies) in terms of shape and dimensional stability was evaluated. The shape stability studied included three modes of distortion, i.e. twist, bow and spring. The dimensional stability was assessed by measuring longitudinal shrinkage and swelling properties. The study comprised 96 studs measuring 45 mm×70 mm×2500 mm from 15 butt logs. Half the studs from each log were dried using a high-temperature method (HT) at 115°C, while the other half were dried using a conventional low-temperature method (LT) at 70°C. Distortion in the studs was measured at moisture contents of 14% and 9%. The HT-dried timber had significantly lower distortion values than the LT-dried timber. As a result, 80% of the high-temperature-dried studs were able to pass the limits for distortion, while only 60% of the low-temperature-dried studs passed the same limit. Studs cut close to the pith displayed the largest twist, independent of the drying method. A simple model for predicting twist that was used in previous studies was also valid for HT-dried timber. The most important parameters in this model for both HT- and LT-dried timber were grain angle, annual ring curvature and tangential shrinkage.