sustainability Article Analysis of Longitudinal Timber Beam Joints Loaded with Simple Bending Kristyna Vavrusova 1,* , Antonin Lokaj 1 , David Mikolasek 1 and Oldrich Sucharda 2 1 Department of Structures, Faculty of Civil Engineering, VSB—Technical University of Ostrava, 708 00 Ostrava-Poruba, Czech Republic; [email protected] (A.L.); [email protected] (D.M.) 2 Department of Building Materials and Diagnostics, Faculty of Civil Engineering, VSB—Technical University of Ostrava, 708 00 Ostrava-Poruba, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.: +420-599-321-375 Received: 6 October 2020; Accepted: 4 November 2020; Published: 9 November 2020 Abstract: The joints in timber structures are often the decisive factor in determining the load-bearing capacity, rigidity, sustainability, and durability of timber structures. Compared with the fasteners used for steel and concrete structures, fasteners for timber structures generally have a lower load-bearing capacity and rigidity, with the exception of glued joints. Glued joints in timber structures constitute a diverse group of rigid joints which are distinguished by sudden failure when the joint’s load-bearing capacity is reached. In this contribution, the load-bearing capacity of a longitudinal joint for a beam under simple flexural stress is analyzed using glued, double-sided splices. Joints with double-sided splices and connecting screws were also tested to compare the load-bearing capacity and rigidity. A third series of tests was carried out on joints made using glued double-sided splices augmented with screws. The aim of this combined joint was to ensure greater ductility after the load-bearing capacity of the glued splice joint had been reached. Keywords: timber; joint; screw; glued; adhesive; bending strength; sustainability 1. Introduction Timber use in the building industry has grown because of its sustainability, great material properties, and renewability. This has brought new trends, not only in the field of innovative wood-based materials but also the joining of the timber structure elements. The joints in timber structures are often the decisive factor in determining the load-bearing capacity, rigidity, and durability of timber structures. Besides commonly used connections in the building industry for joining timber elements, the second largest group consists of connections used for the reconstruction of timber structure elements—for its strengthening or for the replacement of damaged sections of wood. Replacement of damaged sections is typical for beams loaded mostly with bending. For these joints it is possible to use either glued joints or joints with steel fasteners. Glued-in steel rods or plates are also commonly used in glued joints. Some specialists from all around the world [1–3], including the Czech Republic [4–6], are dedicated to improving the capacity and performance of joints in timber structures using glued-in steel rods and plates [7]. A second option is joints with glued outer splices (wood, wood-based, and steel). The load-bearing capacity and deformation of these joints are influenced by considerably more factors than in the case of glued-in steel rod or plate joints [8]. Factors mainly include the type of wood species, adhesive properties, glued line thickness, moisture, and geometry. Worldwide, research inquiries and the testing of these joints, focusing on various influences and their combinations affecting their bearing capacity, are already in progress. For example, authors in [9] focus on the mechanical behavior of these joints. Sustainability 2020, 12, 9288; doi:10.3390/su12219288 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 9288 2 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 2 of 15 behavior of these joints. Other works are mainly devoted to the carrying capacity of adhesives in Other works are mainly devoted to the carrying capacity of adhesives in combination with various combination with various aspects [10–12] and the thickness of the glued lines [13]. aspects [10–12] and the thickness of the glued lines [13]. When the maximum load-carrying capacity of these joints is reached, there is a sudden failure When the maximum load-carrying capacity of these joints is reached, there is a sudden failure of of the joint by brittle fracture. Sudden failure without large deformation is very dangerous and affects the joint by brittle fracture. Sudden failure without large deformation is very dangerous and affects the reliability of structures. Therefore, we decided to add another fastener with plastic deformation the reliability of structures. Therefore, we decided to add another fastener with plastic deformation properties to a brittle bonded joint in order to ensure greater ductility and thereby increase its safety, properties to a brittle bonded joint in order to ensure greater ductility and thereby increase its safety, even at the cost of large deformations. The occurrence of these deformations highlights the even at the cost of large deformations. The occurrence of these deformations highlights the overloading overloading of the joint and allows for corrective action. This is the reason we tested both glued and of the joint and allows for corrective action. This is the reason we tested both glued and screw joints screw joints themselves as well as their combination. themselves as well as their combination. 2.2. Materials Materials and and Methods Methods 2.1.2.1. Laboratory Laboratory Testing Testing ToTo analyze analyze the the load-bearing load-bearing capacity capacity of of a a longitudinal longitudinal joint joint subjected subjected to to simple simple flexural flexural stress, stress, destructivedestructive laboratory laboratory testing testing was was performed performed on on sa samplemple joints joints designed designed with with three three basic basic types types of of splicessplices in in a a central central beam: beam: by by gluing, gluing, by by gluing gluing in in combination combination with with mechanical mechanical fasteners, fasteners, and and using using onlyonly mechanical mechanical fasteners. fasteners. Two Two material material variants variants of of splices splices were were selected: selected: from from mature mature and and laminated laminated veneerveneer lumber. lumber. In In total, total, six six test test sets sets were were created created for for two two material material variants variants of of splices splices in in combination combination withwith three three methods methods of of their their fastening. fastening. Each Each test test set set consisted consisted of of five five samples. samples. For For the the testing, testing, samples samples werewere assembled assembled consisting consisting of a central beam with dimensions 110 × 180180 × 12201220 mm mm and and made made of of solid solid × × timbertimber with with the the use of laminatelaminate veneerveneer lumberlumber splicessplices andand splices splices made made of of solid solid timber timber (see (see Figure Figure1). 1). Figure 1. Laboratory testing scheme. Figure 1. Laboratory testing scheme. TheThe central central beam beam and and solid solid timber timber splices splices were were made made of of solid solid spruce spruce ( (PiceaPicea abies abies),), which which has has a a strength class of C24 and average density of 370 kg m−3.3 Laminated veneer lumber (LVL) splices were strength class of C24 and average density of 370 kg m− . Laminated veneer lumber (LVL) splices were made of the R type softwood veneers (spruce/pine) with average density of 510 kg m−33. made of the R type softwood veneers (spruce/pine) with average density of 510 kg m− . Two-componentTwo-component epoxy epoxy adhesive adhesive was was used used for for structural structural gluing, gluing, which which was was applied applied in in a a 2 2 mm mm thickthick layer layer on on the the entire entire contact contact surfac surfacee between between the the splice splice and and central central element. element. TheThe testtest samplessamples were were conditioned conditioned prior prior to destructive to destructive testing attesting a standard at a ambientstandard temperature ambient temperatureof 20 2 C andof 20 relative ± 2 °C humidity and relative of 65 humidity5%. To determine of 65 ± 5%. the To moisture determine in the the test moisture samples, in a moisturethe test ± ◦ ± samples,detector wasa moisture used. Thedetector average was moisture used. The content average of moisture tested elements content was of tested 10.2%. elements was 10.2%. CountersunkCountersunk self-tapping self-tapping screws screws were were used used as as the the mechanical mechanical fasteners: fasteners: Ø8/80 Ø8/80 for for joints joints with with splicessplices with with laminated laminated veneer veneer lumber lumber and and Ø8/100 Ø8/100 for for joints joints with with splices splices made made of of solid solid timber. timber. Screws Screws were made of carbon steel with white galvanic zinc coating and had a yield strength fyk = 1000 N2 were made of carbon steel with white galvanic zinc coating and had a yield strength fyk = 1000 N mm− . mmFor− the2. For joining the joining of each of splice, each splice, eight screwseight screws were usedwere (see used Figure (see Figure2). 2). Sustainability 2020, 12, x FOR PEER REVIEW 3 of 15 SustainabilitySustainability 20202020,, 1212,, x 9288 FOR PEER REVIEW 33 of of 15 15 (a) (b) (a) (b) Figure 2. Layout char of screws. (a) Laminated veneer lumber (LVL) splice; (b) Solid timber splice. FigureFigure 2. 2. LayoutLayout char char of of screws. screws. (a (a) )Laminated Laminated veneer veneer lumber lumber (LVL) (LVL) splice; splice; ( (bb)) Solid Solid timber timber splice. splice. Testing proceeded on a hydraulic pressure machine at the laboratories of the Faculty of Civil TestingTesting proceeded proceeded on on a a hydraulic hydraulic pressure pressure machine machine at at the the laboratories laboratories of of the the Faculty Faculty of of Civil Civil Engineering, VSB-TU Ostrava, and force was increased gradually. The sample was loaded with Engineering,Engineering, VSB-TU Ostrava,Ostrava, and and force force was was increased increased gradually.