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In-plane mechanical properties of birch plywood and its performance in adhesively bonded connections
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.ORCID iD: 0000-0002-6698-3770
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Birch (Betula spp.) is a hardwood species with a wide natural distribution on the Eurasian continent, especially in northern Europe. Compared with conventional plywood made from softwood, birch plywood has favorable mechanical properties that could be used in new types of efficient connections for timber structures, and thus enable a substitution of current systems using slotted-in steel plates. Such new connections could result in significant advantages in terms of environmental impact and economy as well as ease of prefabrication and assembly. However, birch plywood has rarely been utilized in connections, and therefore, there is a lack of knowledge necessary to design safe timber structures applying such connection systems. In particular, there is a need for increased knowledge of the mechanical properties of birch plywood and its structural performance under various loading conditions. Current connections in timber structures usually also involve mechanical fasteners, e.g., steel screws and dowels, but there is limited use of adhesively bonded (glued) connections. 

The aim of this thesis is to gain new knowledge required for the development of adhesively bonded connections using birch plywood as gusset plates in structural applications. Examples of such structural applications are timber trusses and portal frames. In this context it is necessary, first, to characterize the in-plane mechanical properties of birch plywood, and second, to investigate its performance in adhesively bonded connections.

The results of the mechanical testing show that birch plywood possesses the highest and lowest tensile, compressive, and bending strength and elastic modulus at 0° (parallel) and 45°, respectively, to the face grain (the fiber direction of the face veneers). The opposite findings were noticed for the shear strength and the shear modulus. All these strength values are similar to or higher than the corresponding strength values of common softwood structural timber in its longitudinal direction. Moreover, a size effect on the in-plane bending strength property was observed at 0° and 90° to the face grain but not at other angles, which is attributed to different failure mechanisms. Based on the experimental work, both analytical and numerical models to predict the in-plane mechanical properties of birch plywood are proposed.

Three different adhesives systems were used in the studies: melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF), and a two-component polyurethane (2C PUR). All adhesives used show adequate bonding strength between birch plywood and spruce glulam. However, the use of the adhesive systems should be further investigated in the future. The different manual pressing methods investigated show no significant influence on the bonding strength. Moreover, the bonding strength changes within a relatively small range when the loading direction is varied from 0° to 90°, which is beneficial for the design of birch plywood in adhesively bonded connections. A clear correlation exists between the bonding strength and the shear strength of the weakest wood adherend.

In addition, the moment capacity and bending stiffness of adhesively bonded connections using birch plywood were determined experimentally as well as by analytical and numerical models with a satisfactory agreement.

In timber connections, especially those that are prevalently loaded in tension and/or compression (e.g., in timber trusses), the contribution of the plywood width on the load-bearing capacity needs to be quantified. The results show that the tensile strength of birch plywood within the bonded area shows very low angle-dependence. This is possibly due to the restricted crack propagation at 22.5° and 45° when the gap between the bonded regions is small. The tensile capacity of birch plywood loaded at 0°, 22.5°, and 45° reaches a plateau at certain widths of the gusset plate, which can be well predicted and explained by the spreading angle theories proposed in this study.

In the future, more studies are required for the further development of the adhesively bonded connections with birch plywood. Some preliminary studies serving this purpose have been presented in the thesis as on-going work.

Abstract [sv]

Björk (Betula spp.) är ett lövträdslag med bred naturlig utbredning på den Eurasiska kontinenten, särskilt i norra Europa. Jämfört med konventionell plywood tillverkad av barrträ har björkplywood gynnsamma mekaniska egenskaper som skulle kunna användas i nya typer av effektiva förband för träkonstruktioner och därmed möjliggöra en substituering av nuvarande system med inslitsade stålplåtar. Sådana nya förband kan resultera i betydande fördelar vad gäller miljöpåverkan och ekonomi samt enkel prefabricering och monterbarhet. Dock har björkplywood sällan använts i förband, och därför saknas kunskap som krävs för att utforma säkra träkonstruktioner med sådana förbandsystem. I synnerhet finns det ett behov av ökad kunskap om de mekaniska egenskaperna hos björkplywood och dess strukturella beteende under olika belastningsförhållanden. Nuvarande förband i träkonstruktioner involverar också vanligtvis mekaniska fästelement, t.ex. stålskruvar och -dymlingar, medan användningen av limmade förband är begränsad. 

Syftet med denna avhandling är att ta fram ny kunskap nödvändig för en utveckling av limmade förband med skivor av björkplywood i bärande träkonstruktioner. Exempel på sådana träkonstruktioner är fackverk och portalramar. I detta sammanhang är det nödvändigt att dels karaktärisera de mekaniska egenskaperna i planet hos björkplywood, dels att undersöka dess prestanda i limmade förband. 

Resultaten av de mekaniska testerna visar att björkplywood har den högsta och lägsta drag-, tryck- och böjhållfastheten och elasticitetsmodulen vid 0° (parallellt med) respektive 45° relativt ytfanérens fiberriktning. De motsatta resultaten noterades för skjuvhållfastheten och skjuvmodulen. Alla dessa hållfasthetsvärden är liknande eller högre än de motsvarande hållfasthetsvärdena för vanligt konstruktionsvirke av barrträ i dess längdriktning. Dessutom observerades en storlekseffekt för böjhållfastheten i planet vid 0° och 90° relativt ytfibrerna men inte vid andra vinklar, vilket tillskrivs olika brottbeteenden. Utifrån det experimentella arbetet föreslås både analytiska och numeriska modeller för att förutsäga de mekaniska egenskaperna i planet hos björkplywood. 

Tre olika limtyper användes i studierna: melamin-urea-formaldehyd (MUF), fenol-resorcinol-formaldehyd (PRF) och tvåkomponents polyuretan (2C PUR). Alla använda limtyper uppvisar tillräcklig styrka mellan björkplywood och limträ av gran. Dock bör användningen av limtyperna utredas vidare i framtida studier. De olika manuella limpressmetoderna som undersöks visar ingen signifikant inverkan på limfogens styrka. Dessutom ändras limfogens styrka marginellt när belastningvinkeln i förhållande till plywoodens yta varieras från 0° till 90°, vilket är fördelaktigt vid utformandet av björkplywood i limmade förband. Det finns en tydlig korrelation mellan limfogens styrka och skjuvhållfastheten hos det svagaste träsubstratet.

Vidare bestämdes momentkapacitet och böjstyvhet hos limförband med björkplywood experimentellt samt genom analytiska och numeriska modeller med tillfredsställande överensstämmelse.

I träförband, särskilt de som belastas i drag och/eller tryck (exempelvis i träfackverk), måste plywoodens bredd i förhållande till bärförmågan beräknas. Resultaten visar att draghållfastheten hos björkplywood inom det limmade området inte påverkas av ytfanerens fiberriktning i förhållande dragriktningen. Detta beror möjligen på den begränsade sprickutbredningen vid 22,5° och 45° då mellanrummet mellan de limmade områdena är litet. Draghållfastheten hos björkplywood belastad vid 0°, 22,5° och 45° når en platå vid vissa bredder hos plywoodskivan, vilket förutsägs väl och förklaras genom de teorier för spridningsvinkeln som föreslås i denna studie.

I framtiden krävs fler studier för vidareutveckling av limmade förband med björkplywood. Vissa preliminära studier som tjänar detta syfte har också presenterats som pågående försök i avhandlingen.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2024. , p. 87
Series
TRITA-ABE-DLT ; 244
Keywords [en]
Birch plywood, in-plane mechanical properties, adhesively bonded connections, gusset plates, load-to-face grain angle, spreading angle
Keywords [sv]
Björkplywood, mekaniska egenskaper i planet, limmade förband, belastningsvinkel (relativt ytfanérens fiberriktning), spridningsvinkel
National Category
Civil Engineering
Research subject
Civil and Architectural Engineering, Building Materials
Identifiers
URN: urn:nbn:se:kth:diva-344306ISBN: 978-91-8040-872-1 (print)OAI: oai:DiVA.org:kth-344306DiVA, id: diva2:1844065
Public defence
2024-04-12, Kollegiesalen, Brinellvägen 8, KTH Campus, public videoconference link https://kth-se.zoom.us/j/64622085558, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20240314

Available from: 2024-03-14 Created: 2024-03-12 Last updated: 2024-03-25Bibliographically approved
List of papers
1. In-plane mechanical properties of birch plywood
Open this publication in new window or tab >>In-plane mechanical properties of birch plywood
2022 (English)In: Construction and Building Materials, ISSN 0950-0618, E-ISSN 1879-0526, Vol. 340, article id 127852Article in journal (Refereed) Published
Abstract [en]

There is an increasing demand for engineered wood products in modern structures. Birch plywood is promising in structural applications, due to the combined advantages of its superior mechanical properties and the cross lamination configuration. However, the off-axis mechanical properties of birch plywood have not been thoroughly investigated. The aim of this paper is to establish a comprehensive experimental dataset that could serve as the input in the analytical or numerical models to design birch plywood under various load conditions. Specifically, tensile, compressive and shear tests were conducted under five different angles to the face grain, i.e., from 0 degrees (parallel) to 90 degrees (perpendicular) to the face grain, with an interval of 22.5 degrees. The stress-strain relationships, failure modes, strength and elastic properties of birch plywood are highly dependent on the load-to-face-grain angle. The strength and the elastic properties are also predicted by various analytical and empirical models. Parametric analyses are performed to study the influence of the interaction coefficient F-12 in Tsai-Wu failure criterion and the Poisson's ratio v(xy) in the transformation model on the predicted strength and modulus respectively. Lastly, the possibilities of predicting the on-axis shear modulus based on the off-axis uniaxial tests are discussed in this paper.

Place, publisher, year, edition, pages
Elsevier BV, 2022
Keywords
Birch plywood, Mechanical properties, Load-to-face-grain angle, Failure criteria, Analytical models
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-316024 (URN)10.1016/j.conbuildmat.2022.127852 (DOI)000829923300002 ()2-s2.0-85130880201 (Scopus ID)
Note

QC 20220809

Available from: 2022-08-09 Created: 2022-08-09 Last updated: 2024-03-12Bibliographically approved
2. Influence of face grain angle, size, and moisture content on the edgewise bending strength and stiffness of birch plywood
Open this publication in new window or tab >>Influence of face grain angle, size, and moisture content on the edgewise bending strength and stiffness of birch plywood
2022 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 223, p. 111227-, article id 111227Article in journal (Refereed) Published
Abstract [en]

Birch plywood exhibits outstanding mechanical properties with regards to tensile, compressive, and shear behaviors, making this engineered wood product promising in timber connection applications. However, the edgewise bending strength and stiffness, which are often critical for the design of gusset plates, have not been investigated thoroughly. Moreover, in engineering applications, the size and moisture content of the plywood plate are in general very different from those adopted in laboratory testing according to current standards. This paper aims to investigate the influence of face grain angle, size, and moisture content on the edgewise bending strength and stiffness of birch plywood. In total, 288 birch plywood specimens were tested in three-point bending at five different face grain angles to the beam longitudinal axis (from 0 degrees (parallel) to 90 degrees (perpendicular), with an angle step of 22.5 degrees), with four different sizes (with the nominal depth of 20 mm, 30 mm, 40 mm, and 50 mm) and three different moisture contents (7.2 %, 11.9 %, and 21.8 %). Analytical and numerical models, both taking non-linear elasto-plastic compressive behaviors into account, were developed for the prediction of the ultimate moment capacity based on different failure definitions. Lastly, the relationships between the bending strength and elastic modulus were analyzed.

Place, publisher, year, edition, pages
Elsevier BV, 2022
Keywords
Birch plywood, Edgewise bending, Face grain angle, Size effect, Property -moisture relationships
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-321008 (URN)10.1016/j.matdes.2022.111227 (DOI)000868979400002 ()2-s2.0-85139590700 (Scopus ID)
Note

QC 20221104

Available from: 2022-11-04 Created: 2022-11-04 Last updated: 2024-03-12Bibliographically approved
3. Adhesively bonded joints between spruce glulam and birch plywood for structural applications: experimental studies by using different adhesives and pressing methods
Open this publication in new window or tab >>Adhesively bonded joints between spruce glulam and birch plywood for structural applications: experimental studies by using different adhesives and pressing methods
Show others...
2023 (English)In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 18, no 3, p. 1141-1150Article in journal (Refereed) Published
Abstract [en]

Due to the increasing interest in reducing the usage of metals in the construction sector, birch plywood has shown good potential in structural engineering and timber connections as a substitute for the more conventional slotted-in metal plate solutions. However, a proper way of bonding birch plywood with other timber parts that results in adequate connection strength has not been fully investigated. In this paper, experimental studies were carried out to investigate the proper workflow when manufacturing bonded connections made of birch plywood and spruce glulam beams. Tensile shear mechanical tests were then conducted on produced bonded joints to evaluate the bond strength. Three adhesives, i.e. melamine-urea-formaldehyde (MUF), phenol–resorcinol–formaldehyde (PRF), and two-component polyurethane (2C PUR) were evaluated in dry and moist conditions. The influence of three pressing methods, i.e. (a) screw-gluing, (b) clamping by means of clamps, and (c) clamping by application of weight loads, on the bonding strength was investigated. The bonding strength was thereafter compared with the shear strength of spruce glulam and the wood failure percentage was also examined in this study.

Place, publisher, year, edition, pages
Informa UK Limited, 2023
Keywords
Birch plywood, bonding strength, tensile shear tests, wood failure percentage
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-338463 (URN)10.1080/17480272.2023.2201577 (DOI)000982422900001 ()2-s2.0-85153583741 (Scopus ID)
Note

QC 20231116

Available from: 2023-11-16 Created: 2023-11-16 Last updated: 2024-03-12Bibliographically approved
4. Bonding strength between spruce glulam and birch plywood at different load-to-plywood face grain angles
Open this publication in new window or tab >>Bonding strength between spruce glulam and birch plywood at different load-to-plywood face grain angles
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-344301 (URN)
Note

QC 20240313

Available from: 2024-03-12 Created: 2024-03-12 Last updated: 2024-03-13Bibliographically approved
5. Glulam beams adhesively bonded by birch plywood plates in moment-resisting beam-to-beam connections
Open this publication in new window or tab >>Glulam beams adhesively bonded by birch plywood plates in moment-resisting beam-to-beam connections
Show others...
2024 (English)In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 302, p. 117471-, article id 117471Article in journal (Refereed) Published
Abstract [en]

The design of timber connections is of vital importance in timber structures. Bonded connections exhibit the advantages of lower cost, higher load-bearing capacity, and higher stiffness compared to conventional mechanical connections. However, the potential of the bonded connections has yet to be fully exploited, not only due to their sensitivity to the adhesive types and process-related parameters but also due to the lack of studies regarding the structural performance of the bonded connection in various loading conditions. In this paper, birch plywood plates were utilized to adhesively connect two glulam beam halves to create a longer span. Plywood made of birch was chosen because birch is highly resourced on the Eurasian continent, with its mechanical properties better than most softwoods. Specifically, glulam beams were connected by birch plywood plates at mid-span and then loaded in four-point bending. Four test series with two different bonding areas and birch plywood face grain orientations were carried out. The bonded region was designed as the weakest part to investigate the failure modes, moment capacity, bending stiffness, and moment-rotation angle relationships. Furthermore, numerical models were developed to predict the structural behaviors in the linear elastic stage, while analytical models were proposed and subsequently modified to predict the moment-carrying capacities. Both numerical and analytical models displayed satisfactory agreement with the test results.

Place, publisher, year, edition, pages
Elsevier BV, 2024
Keywords
Bonded joints, Birch plywood, Moment capacity, Bending stiffness, Timber connections, Wood failure, Bond shear strength
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-343874 (URN)10.1016/j.engstruct.2024.117471 (DOI)001155080400001 ()2-s2.0-85182021874 (Scopus ID)
Note

QC 20240227

Available from: 2024-02-27 Created: 2024-02-27 Last updated: 2024-03-12Bibliographically approved
6. Spreading angle analysis on the tensile capacities of birch plywood plates in adhesively bonded timber connections
Open this publication in new window or tab >>Spreading angle analysis on the tensile capacities of birch plywood plates in adhesively bonded timber connections
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-344303 (URN)
Note

QC 20240313

Available from: 2024-03-12 Created: 2024-03-12 Last updated: 2024-03-13Bibliographically approved

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