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Consideration of Gustafsson's proposed Eurocode 5 failure criterion for notched timber beams
Abstract
Experimental results and theoretical predictions for failure of timber beams with end notches, or cuts on the tension face, are discussed. The validity of Gustafsson's formula and the underlying assumptions are examined in the context of structural design codes. Attention is drawn to the arbitrary dependence of theoretical predictions on the load and geometric arrangement of specimens, and the method by which fracture energy is estimated. It is not intended at this stage to give a complete solution for the problem, but it is concluded to be premature to incorporate the proposed Eurocode 5 failure criterion for notched beams into the Canadian Standard CAN/CSA 086.1. Key words: timber, fracture, notched beam, design criterion.
... This approach yields the same results as the approach for end-notched beams of the Canadian Standard CSA O.86, where the effect of the reduced stiffness at the transition between reduced and full cross-section as proposed by Gustafsson, [2], was neglected, [14]. Eq. (17) is similar to the approaches in the Australian standard AS 1720.1 and in the "Wood Handbook" of the Forest Products Laboratory, [10]. ...
... of Eq.(14) and (16) for a notch at an arbitrary position was proposed by Gustafsson as follows: ...
... A partir de ces essais, Larsen (1992) propose une corrélation entre l'énergie de rupture G c I et la masse volumique ρ du matériau, ce qui conduit à la mise en place d'un coefficient unique k n pour chaque type de matériau, Tableau 1.5, pour toutes les classes de résistance et toutes les essences de bois. Smith et Springer (1993) reprennent les travaux de Gustafsson et proposent une expression légèrement différente du taux de restitution G Ec5 , équation (1.11), où ils ne prennent pas en compte la réduction de résistance entre la section réduite et la section complète de la poutre. L'expression du taux de restitution G CSA écrite par Smith et Springer (1993) correspond à l'équation (1.14) : ...
... Smith et Springer (1993) reprennent les travaux de Gustafsson et proposent une expression légèrement différente du taux de restitution G Ec5 , équation (1.11), où ils ne prennent pas en compte la réduction de résistance entre la section réduite et la section complète de la poutre. L'expression du taux de restitution G CSA écrite par Smith et Springer (1993) correspond à l'équation (1.14) : ...
La problématique liée à la fissuration des éléments de structure en bois interpelle les professionnels depuis de nombreuses années. Les risques de fissurations sont intimement liés aux variations climatiques et notamment aux conséquences, en termes de teneur en eau du bois. De plus, en fonction de la géométrie de l’élément et des sollicitations appliquées, les fissures se propagent généralement en mode mixte et de manière intermittente.Cette thèse a pour but de caractériser les propriétés de rupture du bois et plus particulièrement du pin maritime, pour différentes teneurs en eau et pour différents modes de propagations de fissure : en Mode I (ouverture), en Mode II (cisaillement plan) et en mode mixte I+II. Cette caractérisation s’appuie sur la Mécanique Linéaire Elastique de la Rupture équivalente permettant l’estimation de courbes de résistance à la propagation de fissure (courbe-R). Par ailleurs, les propriétés de rupture étant influencées par la géométrie des spécimens d’essais nous avons fait le choix d’utiliser une unique géométrie (Mixed Mode Bending) pour caractériser les propriétés des différents modes de rupture. Ce choix entraine néanmoins un risque élevé d’instabilité de la fissuration dans bon nombre de configurations d’essais et nous avons donc dû recourir à un asservissement en déplacement par voie externe des essais afin de minimiser ces risques. Cette procédure conduit à un taux de réussite des essais (i.e., l’obtention d’une courbe de résistance « complète ») supérieur à 60 % pour les essais de Mode I (Double Cantilever Beam) et plus de 40 % pour les essais de Mode II (End Notched Flexure), toutes teneurs en eau confondues (5 % à 20 %).Parallèlement, nous proposons un modèle de courbe de résistance en mode mixte I+II basé sur les courbes-R des Modes purs I et II et leur dépendance vis à vis de la teneur en eau. Ce modèle, inspiré des modèles de zone cohésive, est composé de deux critères formulés en énergie : le premier est basé sur la résistance à la propagation de la fissure élastique équivalente et s’appuie sur la notion de taux de développement de la zone d’élaboration (FPZ) tandis que le second est fondé sur l’énergie nécessaire au développement de la zone d’élaboration. Le modèle proposé, décrit les courbes-R de mode mixte I+II expérimentales avec une précision satisfaisante et ce quels que soient la teneur en eau et le taux de mixité considérés.
... As is well known, a member that contains a sharp cornered notch will d e v e l o ~ stress concentrations that promote the development and growth of cracks. Lumber members are heterogenous and strongly anisotropic, and crack growth will occur mostly in the grain direction (Valentin et a1 1991; Smith and Springer 1993). The 1989 edition of CSA Standard 086.1 (CSA 1989 ) provides guidance in the assignment of factored shear capacities Can. ...
... It is assumed that S, values are normally distributed and M values are uniformly distributed. Pseudorandom normal and uniform deviates are used, respectively, to assign with the test observation by Smith and Springer (1993) that the ith selection of key physical properties. Based on inforthe critical energy release rate increases when there is a mation in the literature, mechanical properties are taken to square-cut notch having faces that are oriented other than be partially correlated to S, and M (Table 5 ). ...
The basis of new design provisions for avoidance of brittle failures in sawn lumber members with an end notch is explained. Linear elastic fracture mechanics and structural reliability concepts are combined with material property data to determine appropriate forms of equations for limiting the reaction force, when a notch is made on either the tension or compression face of a member. Following from this, the factored reaction force resistance of a member designed to the 1994 edition of the CSA Standard 086.1 "Engineering design in wood (limit states design)" depends upon the depth of the member and the geometry and size of an end notch if the notch is located on the tension face. A new property, the specified reaction force strength, which is a measure of the capability of the material to resist fracture, is taken to be independent of the timber species and stress grade of the lumber. Design variables such as duration of load and moisture service condition influence assignments of factored reaction force resistances for members end notched on the tension face. When a notch is located on the compression face of a member the resistance is simply the factored shear resistance of the residual cross section. As in previous editions of CSA Standard 086.1, notches are not permitted to have a depth greater than 0.25 times the depth of the section. Key words: lumber, fracture, structural reliability, notched beam, design criterion.
... In the formula for calculating the bearing capacity of notched beams [13], the increase Δδ in deflection is Δδ s + Δδ b + Δδ r to account for the increase Δδ r in deflection due to the reduced clamping stiffness at the transition between the full cross-section and reduced cross-section. However, Smith et al. [14] derived an equation for calculating the bearing capacity of a notched (5) ...
Shear experiments on mortise and tenon joints with top and bottom notches in the beam end were conducted with the length of the tenon as a variable. In addition, material experiments were performed to investigate the structural performance of the fracture modes of the joint. The experimental results show that when the lower notch at the beam end experiences splitting, the shear forces were identical for different tenon lengths and did not decrease. In addition, the deformation performance of the joint up to yielding was largely due to the compressive deformation perpendicular to the grain of the wood on the sides of the tenon. Based on the theory of the calculation method of the splitting strength of a notched beam, a formula for the splitting strength when the end of the beam is moment-resisting was proposed. It was confirmed that the proposed formula could estimate the shear force that caused the splitting fracture of the lower notch. In addition, the increase in shear force after the lower notch experienced splitting fracture was due to the increase in clamping force of the beam end due to rotational deformation.
... (3)) leading to the reduction factor k v for shear capacity in EC5 [6]. Smith and Springer [10] suggested to neglect an additional term used by Gustafsson and proposed an approach for notch capacity as given in CSA 086.1 [7] for solid timber (Eq. (4). ...
The load carrying capacity of glued laminated timber beams is reduced considerably by notches at the support on the tension side of the beam. Stress concentrations at the notch corner together with low shear and perpendicular to the grain tensile strength may lead to brittle failure. For the prediction of the load carrying capacity of notched beams, based on experiments and/or theory several design approaches have been made available each of them using different material properties as part of the model. However, specimen sizes in experimental tests, specifications of material properties used In theoretical approaches and
investigations on the influence of ambient conditions on notch capacity are often
not in line with the needs of the application in practice. A safe and practical design approach has to take into account all relevant parameters as e.g. geometrical and material properties, ambient conditions and load characteristics in a reliable way.
... 5) compared to other approaches not taking this reduction into account, like e.g. G Smith by Smith and Springer [26] (Eq. 6). ...
The present study proposes a new method to predict the ultimate load bearing capacity of structural notched beams subjected to splitting. Reinforced and unreinforced cases are investigated by using a FE-model for splitting failure. The contribution of mode I and mode II in the crack extension mechanisms is implemented in the numerical model by considering a mixed mode criterion established on the R-curves of wood. The influences of the notch and the reinforcements on the resulting quasi-brittle behaviour is analysed. The FE-prediction are compared to experimental data and the current design rules of the Eurocode 5.
The thesis describes the process of üre-design, the calculation and the description of
a design model for new types of timber-to-timber connections.
Based on the traditional mortise and timber joint the multiple tenon connection was
developed and experimentally investigated in this thesis. Two or more tenons enable to
share the total shear force into multiple contact forces. Thus, several possible crack layers
occur.
An optimised connection geometry was developed by applying FE-based optimisation
process. The result of an experimental test campaign was a three times higher load capacity
compared to the single tenon geometry. A detailed analysis of the strains and stresses in
the crack area showed the influence of transversal tension stresses on the fracture process
in mode I and shear parallel to the wood fiber in mode II.
Due to the comparability with end-notched beams, tenons could be analysed by the
energy-based approach and by the application of the finite element method (FEM) with
cohesive zones or the evaluation of the J-integral.
The J -integral is calculated out of the strains and stresses in a linear elastic model. The
fracture criteria is reached, when the J -integral exceeds the combined specific fracture
energy in mode I and II. After reliability and sensitivity of the model was checked, a para-
metric study was performed to carve out the main influences of the geometric dimensions
and relations of notched beams, tenon connections and multiple tenon connections. The
results of the parametric study were used to set up a simplified model to calculate the
load bearing resistance based on the decisive geometrical parameters.
The finally proposed model to design tenons and multiple tenons is based on the energy
conservation which was already used for the design of end-notched beams in Eurocode 5.
All principles of the energy-based method were applied on the mortise and tenon joint
and the multiple tenon connection together with mechanical consistent assumptions of
the crack propagating which were extracted from the experimental results and the FE
analysis.
All models were compared and showed very good results compared to experimental failure
loads of multiple tenon connections.
In the end a reliability analysis was performed to determine the main design parameters
for tenons and multiple tenon connections on the characteristic design level with reference
to the formulation of Eurocode 5. The design parameter k n was evaluated for both
connection types and considers the spreading of the material properties as well as the
model uncertainties. The given proposal enables the integration of the tenon connection
as well as the multiple tenon connection into the design formulation of end-notched beams
in Eurocode 5.
Currently, the design of CNC-manufactured dovetail connection is also referring to the
Eurocode 5. A collection of all experimental results under the application of a reliability
analysis could extend the given design proposal as well.
For the structural design, the J-integral model was presented. More applications for this
model are probable regarding the design and optimisation of new types of timber-to-timber
connections.
Auf Grundlage der traditionellen Zapfenverbindung wurde in dieser Arbeit die gereihte
Zapfenverbindung entwickelt. Durch die verteilte Kraftübertragung auf eine Vielzahl von
Zapfen kann die Tragfähigkeit entscheidend erhöht werden, mehrere potentielle Rissebenen
sind jedoch vorstellbar.
Für die optimierte Anschlussgeometrie der gereihten Zapfenverbindung mittels der Finiten-
Elemente-Methode (FEM) wurden unterschiedliche Zapfengeometrien und Einbauvarianten
im Entwurf berücksichtigt und experimentell untersucht. Obwohl es sich um ein sprödes
Versagen der Verbindung handelt, konnten, bei Bauteilen aus Brettschichtholz, durch die
Verteilung der Spannungen, sehr geringe Streuungen der Bruchlasten im Vergleich zu
ausgeklinkten Trägerauflagern oder der traditionellen Zapfenverbindung erzielt werden.
Die Auswertung der Verzerrungen an den Rissebenen konnte zeigen, dass Querzugspannun-
gen in Modus I sowie Schubspannungen in Modus II auftreten, welche das Versagen durch
die kritische Rissentwicklung unterhalb der Zapfen in Richtung der Holzfaser einleitet. Die
Rissentwicklung an Zapfen und ausgeklinkten Trägerauflagern sind vergleichbar, sodass
sich für die Berechnung von Bruchlasten das analytische Konzept der Energiebilanz, sowie
die Finite-Elemente-Methode mit Kohäsivzonen oder mit J-Integral eignen.
In dieser Arbeit wurden die drei verschiedenen Berechnungskonzepte weiterentwickelt und
auf die Zapfenverbindung und die gereihte Zapfenverbindung angewendet und verglichen.
Die Finite-Elemente-Methode mit Kohäsivzonen ermöglicht die Simulation der Rissentwick-
lung, wodurch eine realitätsnahe Last-Verformungskurve, sowie die Bruchlast, bestimmt
werden kann. Mit Hilfe der berechneten Bruchlast verschiedener Modellkonfigurationen
konnte die Anschlussgeometrie der Zapfen weiter optimiert und der Einfluss von Passunge-
nauigkeiten im Anschluss identifiziert werden.
Des Weiteren konnte das J -Integralmodell auf Grundlage der FEM weiterentwickelt werden,
um die Bruchlast von Riss behafteten Strukturen unter Beanspruchung des Modus I und II
zu ermitteln. Über die die kombinierte spezifische Bruchenergie konnte die Bruchlast von
ausgeklinkten Trägerauflagern, Zapfen und gereihten Zapfenverbindungen in umfangreichen
Parameterstudien zuverlässig bestimmt werden. Aus den Ergebnissen wurde ein vereinfach-
tes Handrechenmodell zur Bestimmung der Bruchlast von ausgeklinkten Trägerauflagern,
Zapfen- und gereihten Zapfenverbindungen entwickelt.
Ein weiters Bemessungskonzept für Zapfen- und gereihte Zapfenverbindungen basiert auf
dem Konzept der Energiebilanz, welches auf der Basis des ausgeklinkten Trägeraufla-
gers weiterentwickelt wurde. Der Vergleich experimenteller und berechneter Mittelwerte
von Bruchlasten zeigte hier die beste Übereinstimmung. Über die Anwendung einer Zu-
verlässigkeitsanalyse konnten die Proportionalitätskonstanten k n in Abhängigkeit der
Materialparameter für Brettschichtholz und Nadelvollholz bestimmt werden. Hierdurch
konnte ein Vorschlag für ein Bemessungsmodell für Zapfen und gereihte Zapfenverbindun-
gen als Integration in den Tragfähigkeitsnachweis für ausgeklinkte Trägerauflager nach
DIN EN 1995-1-1 gemacht werden.
Es ist denkbar, dass der erarbeitete Vorschlag auf weitere Verbindungstypen, wie den
Schwalbenschwanzzapfen, und auf andere Werkstoffe angewendet werden kann.
Zudem bietet das entwickelte J -Integralmodell vielfältige Möglichkeiten zur Modellie-
rung von Holzverbindungen und zur Bestimmung von Bruchlasten sowie zur weiteren
Formoptimierung von Holz-Holz-Verbindungen.
Formschlüssige Holz‐Holz‐Verbindungen haben aufgrund der einfachen Herstellung und Montage eine lange Tradition und gleichzeitig ein großes Potential im modernen Holzbau. In den vergangenen Jahren wurde u.a. die Zapfenverbindung sukzessive an die CNC‐Produktionstechniken angepasst und weiterentwickelt. Mit schwalbenschwanzförmigen Zapfen wurden so die Montageabläufe und bei der gereihten Zapfenverbindung die Tragfähigkeit entscheidend verbessert. Experimentelle Untersuchungen zeigten, dass bei allen auf der Zapfenverbindung basierenden Verbindungstypen das Versagen von der Rissentwicklung am Zapfen eingeleitet wird. Hohe Querzug‐ und Schubspannungen in der Rissebene führen dann, wie bei ausgeklinkten Trägerauflagern, zum Versagen des Bauteils.
In dem folgenden Beitrag wird eine Bemessung von Zapfenverbindungen mit Bezug zu dem Konzept vorgeschlagen, welches der Eurocode 5 für die Ausklinkung bereithält. Dies basiert auf der bruchmechanischen Auswertung des Rissprozesses und wird dann zu einem Nachweis mit Bezug auf die Schubfestigkeit umformuliert. Das entwickelte Modell zeigt sehr gute Übereinstimmung mit experimentellen Ergebnissen. Durch die Analyse der Ergebnisse aus experimentellen Untersuchungen an Zapfenverbindungen konnte eine Zuverlässigkeitsanalyse der aufgestellten Bemessungsgleichung durchgeführt werden. Das vorgeschlagene Bemessungskonzept für den Grenzzustand der Tragfähigkeit ist als Erweiterung der Bemessung für ausgeklinkte Trägerauflager konzipiert und ist auf unterschiedliche Zapfenverbindungen anwendbar.
Abstract:
Tenon connections in timber constructions ‐ fracture mechanic analysis and proposal of a new design model:
Timber‐to‐timber connections benefits in terms of the production and assembly process in structural timber engineering. In the past few years, the traditional mortise and tenon connection was adapted step‐by‐step on the CNC production process. New connection types as the dovetail connection enables a better installability and multiple tenon connections show significant higher loading capacity. Experimental investigations showed that all these connection types fail due to the crack propagation in grain direction. Transversal tension stresses combined with shear stresses as observed at the notch results in failure.
In this contribution the failure load of tenon connections will be developed with the energy‐based fracture mechanic method used in Eurocode 5. The new model shows good accordance to experimental investigations on tenon connections. A normalization of the experimental test results enabled a reliability analysis of the developed design equations. The proposed design equation is a modification of the Eurocode 5 formulation and allows to calculate the ultimate limit state of tenon joints.
Le renforcement des assemblages et des éléments structuraux en bois
vise à dépasser les limites de résistances du matériau, en assurant le transfert d’efforts
plus importants dans des zones de faiblesses pouvant être la source de fissurations
prématurées dans les ouvrages. Les renforts utilisés peuvent être en acier, en matériaux
composites, voire même en bois. Leur accroche peut être mécanique (organes
vissés) ou par adhérence (collage structural : goujons collés par exemple). Dans les
deux cas, le transfert des sollicitations reste mal connu, et les effets d’amorce ou
de déviation de fissure ne sont pas bien appréhendés. Dans les techniques de l’ingénieur,
la résistance offerte par le bois dans la zone renforcée reste négligée, ce qui
va dans le sens du principe de précaution. Actuellement, les investigations scientifiques
s’intéressent à la résistance de ce type de techniques sans pour autant se
pencher sur les interactions entre le comportement quasi-fragile du bois et celui des
renforts qui régissent le gain en performance mécanique. Or, ces solutions peuvent
aboutir à une rupture ultime provoquée par le fendage progressif du bois et la perte
d’ancrage du renfort. Il semble donc dans un premier temps utile de proposer des
prédictions de la résistance à court terme au fendage des poutres renforcées et sans
renfort, qui peuvent servir à l’exploration ultérieure des mécanismes de ruine pour
les comportements à long terme. C’est pourquoi, dans cette étude, un modèle global
de prédiction de la résistance ultime des composants de structure soumis au fendage,
renforcés et non-renforcés, a été développé. Il considère l’aspect quasi-fragile du bois
et la propagation de fissure en mode mixte, à l’aide d’une loi de mélange établie
sur les courbes-R. La pertinence de cette modélisation est ensuite comparée aux
méthodes de dimensionnement actuellement proposées dans les Eurocodes 5, pour
les poutres entaillées, via des campagnes expérimentales à différentes échelles.
Traditionally unreinforced tension-side notches at supports of glued-laminated-timber (glulam) bending members have been designed in Canada assuming shear capacity is reduced in proportion to the square of the residual depth ratio. The origin of that practice lies in World War II era studies in the USA on solid wood members. More recent research in Canada, US, and Europe has focussed on application of linear elastic fracture mechanics (LEFM) methods to such problems, reflecting that such approaches can account for effects of variables other than notch depth ratio. Canadian wood design code CSA Standard 086 “Engineering Design in Wood” first capitalized on availability of LEFM approaches in 1994 by adopting such a method for design of sawn lumber members with tension-side notches at supports. The same was not done then for glulam members because of paucity of supporting test data. That gap has now been filled and the 2014 edition of CSA Standard 086 employs consistent LEFM based design approaches for both sawn lumber and glulam members with tension-side notches. New provisions for glulam recognize influences that notch length and shape, and how laminations in members are fabricated have on member capacities. Discussion here addresses the logic that underpins the new glulam provisions and how application of those provisions impacts design solutions. Overall impact of the new design provisions is to discourage use of relatively large tension-side notches at supports of larger glulam bending members, while not discouraging use of notches in smaller glulam members. ©, 2015, National Research Council of Canada. All Rights Reserved.
Tests were carried out on softwood structural glued-laminated-timber (glulam) members to calibrate a linear elastic fracture mechanics design method for bending members having tension-side end notches. Fifty-eight specimens with depths up to 646 mm represented 11 combinations of member size, notch depth, member geometry, loading arrangement, and glulam type. Specimens were loaded until they sustained significant damage at or near notch locations, but none of them failed as bending members. Notch damage started with creation of short stable cracks parallel to the laminations and ended with episodically crack growth. Typically, crack initiation is at about half the load level associated with episodic crack growth. Contrary to current design practice loads positioned close to notched end supports were found to influence strengths of members significantly, and their effects should be taken into account. The findings have been implemented in the 2014 edition of CAN/CSA Standard 086 “Engineering Design in Wood”. ©, 2015, National Research Council of Canada. All Rights Reserved.
COST Action FP1004, Enhance Mechanical Properties of Timber, Engineered
Wood Products and Timber structures, has a broad mandate. Clearly timber
suffers in comparison with homogeneous, isotropic materials such as steel. There
are many ways of addressing the variability timber, which is a result of it being a
natural, harvested product. There are techniques for moderating the effects of
timber’s different strength and stiffness at angles to the grain. The objective of the
COST Action is to bring together an understanding of the state of the art in
research in these areas and to identify particular gaps in knowledge.
At the first Management Committee Meeting of the COST Action, in November
2012, there were two important decisions. Firstly, the delegates agreed to focus,
initially, on a selection of the key issues outlined in the Memorandum of
Understanding, which underpin the scope of the project. Secondly, in response to
COST Action Strategic intent, delegates agreed to promote a conference that
would focus on Early Stage Researchers, giving them the maximum opportunity to
become involved in the COST Action and helping them to promote their research
and network with one another and to meet experts in their field of work.
The result of these two decisions was the Early Stage Researchers Conference in
Zagreb, held on 19 and 20 April 2012. The template for the call for papers was an
extended abstract and the papers selected for presentation are published in these
proceedings. They form an excellent basis for understanding the State of the Art
in these important areas of research. The COST Action will proceed to explore
these and wider areas of research in the field of enhancement of performance of
timber.
Attendees at the Conference, many of whom were presenting at an international
conference for the first time, were able to enter into dialogue with their peers and
with international experts. Scientific collaboration will develop from these
exchanges and the many delegates will look back on Zagreb as an important
milestone in their careers.
Notches are details that significantly reduce the load-carrying capacity of beams made of timber. The amount of reduction was the subject of investigation in numerous studies from an experimental or theoretical point of view. The singular stress distribution around the notch corner can be described by equations based on linear elastic fracture mechanics, fracture energy, or complex solutions. From experimental tests, simplified equations can be proposed. Standards, codes, and handbooks for the design of timber structures are based on such equations. When comparing the dimensions of notched beams tested in experiments with those that are relevant in practical applications, specimens of small dimensions were primarily tested. The application of design approaches verified by tests on specimens of small dimensions and the use of material properties determined from small clear specimens do not account for size effects that are important when working with beams of large dimensions. The influence of duration of load, moisture variation, and cyclic loading on the fracture of the notch has to be studied. An extension of tests on notched beams and relevant material properties that are now gained from small clear specimens to those of large dimensions is necessary.
Read More: http://ascelibrary.org/doi/abs/10.1061/(ASCE)ST.1943-541X.0000838
The long-term loading strength of end-notched beams made of glulam and LVL was tested. The beams were of various sizes, with
and without a moisture sealing at the notch. Tests were conducted in open shelter climates, and at constant and cyclic relative
humidity. The short-term strength was tested after conditioning in the various climates. Both the short-term and long-term
strength of beams without moisture sealing was significantly affected by transient moisture conditions, e.g. the moisture
variations due to change of the time of the year. The strength was only slightly affected by the magnitude of the humidity,
if this was kept constant. Duration of load strength reduction factors were evaluated for 6 months of loading. Average reductions
in ultimate failure strength ranged from 0.68 for small LVL beams without moisture sealing tested during spring and summer
to 0.81 for large glulam beams with a moisture sealing at the notch.
Die Zeitstandfestigkeit keilverzinkter Träger aus Glulam und LVL wurde geprüft. Die Balken waren unterschiedlich bemessen
und mit oder ohne Endversiegelung versehen Die Prüfung erfolgte sowohl in natürlichem, geschützten Klima als auch bei konstanter
Feuchte und im Wechselklima. Die Kurzzeitfestigkeit wurde jeweils nach dem Konditionieren geprüft. Alle Ergebnisse an nicht
versiegelten Proben waren deutlich beeinflußt durch Feuchteübergänge, z.B. die Feuchtewechsel innerhalb eines Jahres. Die
Festigkeit hing nur in geringem Maße vom Wassergehalt ab, wenn dieser konstant gehalten war. Reduktionsfaktoren (k
mod) wurden für ömonatige Belastung ermittelt. Die durchschnittlichen Werte für die Bruchlast lagen im Bereich von 0,68 für kleine
LVL-Träger ohne Feuchteversiegelung (geprüft im Frühjahr und Sommer) bis zu 0,81 für lange Brettschichtholz-Träger mit Feuchteversiegelung
der Keilzinken.
Since Porter (1964), some authors have applied fracture mechanics concepts to wood by assuming that the material is orthotropic and elastic. In order to verify these assumptions, the objective of this paper is to compare experimental results and finite elements computation.
This paper presents a method of simultaneous evaluation of bending and shear moduli of wooden beams. The method, which is based upon the measurement of two natural frequencies of a beam, is simple to perform. Existing techniques for determining shear moduli of wood based materials are either too cumbersome to carry out or prone to measurement errors. Using this method tests were conducted on matched groups of clear and knotty specimens. It was found that for clear specimens a value of 20 can be assumed for the E to G ratio. For specimens which contain knots the variation was so large that no corresponding figure can be suggested with confidence. But the ratio for knotty materials is though to be higher than 20.
Algor processor reference manual Method for determination of the softening behaviour of wood and the applicability of a nonlinear fracture mechanics model
- Algor Interactive Systems
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ALGOR Interactive Systems, Inc. 1989. Algor processor reference manual. Pittsburgh, Pa., August. Bostrom, L. 1992. Method for determination of the softening behaviour of wood and the applicability of a nonlinear fracture mechanics model. Lund Institute of Technology, Lund, Sweden, Report TVBM-1012 (Ph.D. thesis).
Common unified rules for timber structures
- P Crubile
- J Ehlbeck
- H Briininghoff
- H J Larsen
- J Sunley
Crubile, P., Ehlbeck, J., Briininghoff, H., Larsen, H.J., and Sunley, J. 1987. Common unified rules for timber structures. Report prepared for the Commission of the European Commu-nities, Brussels, Belgium. Draft Eurocode 5. CSA. 1989. Engineering design in wood (limit states design).
A study of strength of notched beams. Meeting of the International Council for Building Research Studies and Documentation Working Commission W18 -Timber Structures (CIB-W18)
- P J Gustafsson
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