Analysis of Longitudinal Timber Beam Joints Loaded with Simple Bending

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. gradually. The sampleThe sample was loaded was loaded with vertical with vertical force applied in thirds of the span. An optimal force rate was chosen for the press. Failure verticalforce applied force applied in thirds in of thirds the span. of the An span. optimal An op forcetimal rate force was rate chosen was chosen for the press.for the Failurepress. Failure among among all tested samples appeared in a time boundary of 300 ± 120 s which corresponds to the amongall tested all samplestested samples appeared appeared in a time in boundarya time boundary of 300 of120 300 s ± which 120 s correspondswhich corresponds to the interval to the interval of laboratory tests for short-time strength according± to the current European standards for intervalof laboratory of laboratory tests for tests short-time for short-time strength streng accordingth according to the currentto the current European European standards standards for timber for timber structure capacity [14,15]. timberstructure structure capacity capacity [14,15]. [14,15]. During testing, the force (maximum joint force) was recorded for each test sample (accuracy of DuringDuring testing, testing, the the force force (maximum (maximum joint joint force) force) was was recorded recorded for for each each test test sample sample (accuracy (accuracy of of 0.01 kN), and the corresponding deformation of the joint was measured in the supports and in the 0.010.01 kN), kN), and and the the corresponding corresponding deformation deformation of of the the joint joint was was measured measured in in the the supports supports and and in in the the middle of the span at half the height of the cross-section of the test sample (accuracy of 0.01 mm). middlemiddle of of the the span span at at half half the the height height of of the the cross- cross-sectionsection of of the the test test sample sample (accuracy of 0.01 mm). 2.2. Calculation According to Standards 2.2.2.2. Calculation Calculation According According to to Standards Standards

2.2.1.2.2.1. GluedGlued JointsJoints 2.2.1. Glued Joints AccordingAccording toto previousprevious laboratorylaboratory teststests andand numericalnumerical calculations,calculations, thisthis typetype ofof jointjoint hashas twotwo According to previous laboratory tests and numerical calculations, this type of joint has two basicbasic typestypes of failure: failure: failure failure along along the the glue glue line line (Rbc1 ()R orbc1 failure) or failure of splice of spliceveneers veneers (LVL or (LVL solid or timber). solid basic types of failure: failure along the glue line (Rbc1) or failure of splice veneers (LVL or solid timber). timber).Failure Failurealong the along glue the line glue lineis calculated is calculated with with two two variants: variants: uniform uniform ( Rbc22) )and unequalunequal (R(Rbcbc33)) Failure along the glue line is calculated with two variants: uniform (Rbc2) and unequal (Rbc3) distributiondistribution of of shear shear stress. stress. distribution of shear stress. CalculationCalculation of of the the maximum maximum joint joint force forceR Rbcbcis is basedbased onon thethe momentmoment transferredtransferred by by the the glued glued joint joint Calculation of the maximum joint force Rbc is based on the moment transferred by the glued joint forfor both both central central elements elements as as well well as as the the designed designed load-carrying load-carrying capacity capacity of of the the joint joint (Figure (Figure3). 3). for both central elements as well as the designed load-carrying capacity of the joint (Figure 3).

FigureFigure 3. 3.Maximum Maximum jointjoint force—gluedforce—glued joint. joint. Figure 3. Maximum joint force—glued joint.

TheThe maximum maximum joint joint force forceR Rbcbc isis determineddetermined fromfrom thethe followingfollowing expression: expression: The maximum joint force Rbc is determined from the following expression: 2𝑀 2Mb 𝑅= 2𝑀[ kN] (1) R𝑅bc 𝑟 kNkN (1)(1) r𝑟 where Mb is the moment transferred by the glued joint, and r is the lever arm of the test sample. wherewhere MMbb isis the the moment moment transferred transferred by by the the glued glued joint, joint, and and rr isis the the lever lever arm arm of of the the test test sample. sample. The moment transferred by the glued joint is determined from the following expression: TheThe moment moment transferred transferred by by the the glued glued joint joint is is determined determined from from the following expression: 𝑀 𝐹 .2.𝑅 kNm (2) 𝑀 𝐹 .2.𝑅 kNm (2) M =Ftc 2 Rcc [kNm] (2) b × × Sustainability 2020, 12, 9288 4 of 15

Sustainability 2020, 12, x FOR PEER REVIEW 4 of 15 where Ftc is the designed load-carrying capacity of the joint determined from expression (3) for shear strengthwhere F oftc is the the splice designed material load-carrying and (4) for capacity strength of of the the joint glue determined line. Rcc is from the lever expression arm of (3) forces for actingshear onstrength the joint. of the splice material and (4) for strength of the glue line. Rcc is the lever arm of forces acting on theRgarding joint. the load-carrying capacity of the joint, the shear strength of the splice material is determinedRegarding as follows: the load-carrying capacity of the joint, the shear strength of the splice material is fv,k kmod Atc determined as follows: Ftc = × × [kN] (3) 2 γM 𝑓 ×.𝑘 . 𝐴 , where Atc is the active glued area of the joint,𝐹 and fv,k is the shearkN strength of the splice material. (3) 2. 𝛾 Regarding the load-carrying capacity of the joint, the glue line strength is determined as where Atc is the active glued area of the joint, and fv,k is the shear strength of the splice material. Regarding the load-carrying capacity offk thek modjoint, Athetc glue line strength is determined as Ftc = × × [kN] (4) 2 γM 𝑓.𝑘× . 𝐴 𝐹 kN (4) 2. 𝛾 where Atc is the active glued area of the joint, and fk is the strength of the glued surface for uniform andwhere unequal Atc is distributionthe active glued of shear areastress. of the joint, and fk is the strength of the glued surface for uniform and unequal distribution of shear stress. 2.2.2. Screw Joints 2.2.2. Screw Joints The maximum joint force Rbc4 (see Figure4) is based on the load-carrying capacity of the single shear fastenerThe maximum and is determinedjoint force Rbc from4 (see the Figure following 4) is based expression: on the load-carrying capacity of the single shear fastener and is determined from the following expression: 2M = b [ ] Rbc 2𝑀kN (5) 𝑅 r kN (5) 𝑟 where Mb is the moment transferred by the glued joint, and r is the lever arm of the test sample. where Mb is the moment transferred by the glued joint, and r is the lever arm of the test sample.

FigureFigure 4.4. MaximumMaximum joint force—screw joint.

TheThe moment moment transferred transferred by theby jointthe joint onto theonto beams the isbeams determined is determined using the using following the expression:following expression:

Mb = Fv,Rd 2 2 Rcc [kNm] (6) 𝑀 𝐹,.2.2.𝑅× × × kNm (6) wherewhereF vF,Rdv,Rd isis thethe designeddesigned load-carryingload-carrying capacitycapacity for the fastener per shear plane, and and Rcccc isis the the lever lever armarm of of forces forces acting acting on on the the joint. joint. DesignDesign load-carrying load-carrying capacity capacity for for fastener fastener per per shearshearF Fvv,,Rd isis determined determined according according to to expressions expressions givengiven in in [ 14[14].].

2.2.3.2.2.3. Combination Combination ofof GluingGluing andand ScrewsScrews InIn the the applicable applicable standards, standards, thethe calculationcalculation ofof load-carryingload-carrying capacity for for combined combined joints joints with with gluedglued and and mechanical mechanical fastenersfasteners isis notnot described;described; thus,thus, the maximum strength is is not not designated designated for for thesethese joints. joints. Essentially, Essentially, joints joints of of varying varying rigidityrigidity shouldshould notnot be combined, and if if they they are, are, they they should should havehave at at least least a a similar similar load-carrying load-carrying capacity.capacity.

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3. Results 3. Results 3.1. Laboratory Testing 3.1. Laboratory Testing Based on laboratory test results, statistical variables were determined for each set of samples for maximumBased joint on laboratory force (failure test force) results, and statistical vertical variables deformation were of determined the joint. for each set of samples for maximum joint force (failure force) and vertical deformation of the joint. 3.1.1. Glued Joints 3.1.1. Glued Joints The mean value of the maximum force acting on the joint with glued LVL splices was Rbc = 28.86 The mean value of the maximum force acting on the joint with glued LVLsplices was Rbc = 28.86 kN, kN, and with solid timber splices it was Rbc = 30.80 kN. The mean value of vertical deformation at and with solid timber splices it was R = 30.80 kN. The mean value of vertical deformation at maximal maximal force with glued LVL splicesbc was 8.38 mm, and with solid timber splices it was 9.03 mm force with glued LVL splices was 8.38 mm, and with solid timber splices it was 9.03 mm (Table1). (Table 1).

Table 1. Glued joints—laboratory results. Table 1. Glued joints—laboratory results. Force [kN] Vertical Deformation [mm] Force [kN] Vertical Deformation [mm] MeanMean 28.86 28.86 8.38 8.38 LVL SD 4.24 0.39 LVL SD 4.24 0.39 COV 0.15 0.05 COV 0.15 0.05 Mean 30.80 9.03 Solid timber MeanSD 30.80 6.72 9.03 1.24 Solid timber COVSD 6.72 0.22 1.24 0.14 COV 0.22 0.14 Figure5 shows that the deformation curves of glued joints with both types of splices were partially Figure 5 shows that the deformation curves of glued joints with both types of splices were linear; only in the ﬁnal phase of loading did the joints start to show plastic deformation. When the partially linear; only in the final phase of loading did the joints start to show plastic deformation. maximum load-carrying capacity of these joints was reached, there was a sudden failure of the joint by When the maximum load-carrying capacity of these joints was reached, there was a sudden failure brittle fracture. of the joint by brittle fracture.

GLUED JOINTS 35 Solid timber 30.80 kN 30 Solid timber - failure 25 LVL - failure 28.86 kN 20 LVL 15 10 Force [kN] Force 5 0 012345678910 Vertical deformation [mm]

Figure 5. Glued joints—deformation curves.

Joints withwith LVLLVL splices splices su sufferedﬀered primarily primarily from from shear shear failure failure of of the the ﬁrst first or secondor second veneer veneer from from the gluethe glue line line (Figure (Figure6). 6). Sustainability 2020, 12, 9288 6 of 15 Sustainability 2020,, 12,, xx FORFOR PEERPEER REVIEWREVIEW 6 of 15

(a) (b)

FigureFigure 6. 6.Glued Glued jointsjoints withwith LVLLVL splice.splice. ( (aa)) Typical Typical failure; failure; (b (b) )Detailed Detailed image. image.

JointsJoints with with solid solid timber timber splices splices primarilyprimarily suffe suﬀeredred splice splice failure failure in in combination combination with with shear shear and and tensiontension perpendicular perpendicular to to the the grain. grain. ThisThis failurefailure was characterized characterized by by cracks cracks forming forming in in the the area area of ofthe the glueglue line. line. The The joint joint predominately predominately failedfailed duedue to tension perpendicular perpendicular to to the the grain. grain. (Figure (Figure 7).7 ).

FigureFigure 7. 7.Glued Glued joints joints with with solidsolid timbertimber splice: typical typical failure failure of of solid solid timber timber splice. splice.

3.1.2.3.1.2. Screw Screw Joints Joints TheThe mean mean value value of of the the maximum maximum jointjoint force for the the set set of of samples samples with with screws screws and and LVL LVL splices splices waswasRbc Rbc= =16.32 16.32 kN, kN, and andfor forsolid solid timbertimber splicessplices it was Rbcbc == 18.9818.98 kN. kN. The The mean mean value value of ofvertical vertical deformationdeformation at at the the maximal maximal forceforce withwith screwedscrewed LVL splices was was 53.61 53.61 mm, mm, and and with with solid solid timber timber splicessplices it wasit was 54.12 54.12 mm mm (Table (Table2). 2).

TableTable 2. 2. ScrewScrew joints—laboratoryjoints—laboratory results. results. Force [kN] Vertical Deformation [mm] ForceForce [kN] [kN] Vertical Vertical Deformation Deformation [mm] [mm] Mean 16.32 53.61 Mean 16.32 53.61 LVL SD 1.94 12.78 LVL SD 1.94 12.78 COVCOV 0.12 0.12 0.25 0.25 Mean 18.98 54.12 Mean 18.98 54.12 Solid timber SD 1.73 13.27 Solid timber SD 1.73 13.27 COVCOV 0.09 0.09 0.25 0.25 Sustainability 2020, 12, 9288 7 of 15

SustainabilitySustainability 20202020,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW 77 ofof 1515 Figure8 shows the deformation curves of this type of joint were partially linear, and then the joint hadFigureFigure ductile 88 showsshows behavior. thethe deformationdeformation Ductility is curves typicalcurves of forof thisthis mechanical typetype ofof jointjoint fasteners, werewere especiallypartiallypartially linear,linear, for small-diameter andand thenthen thethe fastenersjointjoint hadhad that ductileductile can behavior.behavior. bend. When DuctilityDuctility the maximum isis typicaltypical load-carryingforfor mechanicalmechanical capacity fasteners,fasteners, of especially thisespecially joint was forfor small-diametersmall-diameter reached, there wasfastenersfasteners a sudden thatthat failurecancan bend.bend. of the WhenWhen joint; thethe however, maximummaximum due load-caload-ca to therrying userrying of mechanicalcapacitycapacity ofof this fastenersthis jointjoint was (i.e.,was reached,reached, screws), there totalthere failurewaswas aa sudden wassudden postponed. failurefailure ofof The thethe joint joint;joint; displayed however,however, ductile duedue toto behavior, ththee useuse ofof and mechanicalmechanical its load-carrying fastenersfasteners capacity (i.e.,(i.e., screws),screws), was limited totaltotal byfailurefailure the mechanical waswas postponed.postponed. joint. In TheThe the ﬁnaljointjoint phase,displayeddisplayed excessive ductileductile bending behavior,behavior, of the andand outer itsits screws, load-carryingload-carrying closer to capacitycapacity the center waswas of thelimitedlimited joint, byby simultaneously thethe mechanicalmechanical occurred joint.joint. InIn as thethe the finalfinal splice phase,phase, split excessiveexcessive perpendicular bendingbending to ofof its thethe longitudinal outerouter screws,screws, axis. closercloser toto thethe centercenter ofof thethe joint,joint, simultaneouslysimultaneously occurredoccurred asas thethe splicesplice splitsplit perpendicularperpendicular toto itsits longitudinallongitudinal axis.axis.

18.9818.98 kNkN 2020 SCREWSCREW JOINTSJOINTS

1515 16.3216.32 kNkN

1010 SolidSolid timbertimber SolidSolid timbertimber -- failurefailure 55 Force [kN] Force [kN] LVLLVL LVLLVL -- failurefailure 00 00 102030405060 102030405060 Vertical deformation [mm] Vertical deformation [mm] FigureFigure 8.8. ScrewScrew joints—deformationjoints—deformation curves.curves. Figure 8. Screw joints—deformation curves.

TheTheThe most mostmost common commoncommon type typetype of ofof joint jointjoint failure failurefailure with withwith screwed screscrewedwed LVL LVLLVL splices splicessplices was waswas splitting splittingsplitting perpendicular perpendicularperpendicular to toto veneersveneersveneers at atat the thethe screw screwscrew level levellevel (see (see(see Figure FigureFigure9 ). 9).9).

FigureFigureFigure 9. 9.9. Screw ScrewScrew joints jointsjoints with withwith LVL LVLLVL splice: splice:splice: typical typicaltypical failures. failures.failures.

FailureFailureFailure forforfor screwed screwedscrewed solidsolidsolid timber timbertimber splicessplicessplices waswaswas the thethe same samesame as asas that thatthat with withwith LVL LVLLVL splices: splices:splices: splitting splittingsplitting waswaswas perpendicularperpendicularperpendicular to toto veneers veneersveneers at atat the thethe screw screwscrew level levellevel (see (see(see Figure FigureFigure 10 10).10).). Sustainability 2020, 12, x FOR PEER REVIEW 8 of 15

Sustainability 2020, 12, 9288 8 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 8 of 15

Figure 10. Screw joints with solid timber splice: typical failure of solid timber splice.

3.1.3. Combination of Gluing and Screws The mean value of the maximum joint force for the set of samples with a combination of gluing and screws was Rbc = 30.40 kN for LVL splices, and for solid timber splices it was Rbc = 35.16 kN. The mean value of vertical deformation at the maximal force was 7.68 mm for LVL splices, and Figure 10. Screw joints with solid timber splice: typical failure of solid timber splice. for solid timberFigure splices 10. Screw it was joints 10.90 with mm solid (Table timber 3). splice: typical failure of solid timber splice. 3.1.3.3.1.3. Combination of Gluing andand ScrewsScrews Table 3. Combination of gluing and screw joints—laboratory results. TheThe meanmean valuevalue ofof thethe maximummaximum jointjoint forceforce forfor thethe setset ofof samplessamples withwith aa combinationcombination ofof gluinggluing Force [kN] Vertical Deformation [mm] andand screwsscrews waswas RRbcbc = 30.4030.40 kN kN for for LVL LVL splices, and for solid timber splices it was Rbc == 35.1635.16 kN. kN. TheThe meanmean valuevalue of of vertical vertical deformation deformationMean 30.04 at at the th maximale maximal force force was 37.78 was 7.68 7.68 mm mm for for LVL LVL splices, splices, and and for solidfor solid timber timber splices splices itLVL was it was 10.90 10.90 mmSD mm (Table (Table3).5.21 3). 6.22 COV 0.18 0.17 Table 3.3. CombinationCombinationMean ofof gluinggluing35.16 andand screwscrew joints—laboratoryjoints—laboratory 34.53 results.results. Solid timber SD 2.67 9.19 ForceForce [kN] [kN] Vertical Vertical Deformation Deformation [mm] [mm] COV 0.08 0.28 MeanMean 30.04 30.04 37.78 37.78 LVLLVL SDSD 5.21 5.21 6.22 6.22 Vertical deformation increasedCOV even after 0.18reaching the maximal joint 0.17 force. The mean value of the maximal vertical deformation COVwas 37.78 mm0.18 for LVL splices, and 0.17 with solid timber splices it was Mean 35.16 34.53 34.56 mm (Figure 11). Mean 35.16 34.53 Solid timber SD 2.67 9.19 Solid timber SD 2.67 9.19 Figure 11 shows that the deformationCOV curves 0.08 of this type of joint were 0.28 partially linear. When the maximum load-bearing capacity ofCOV this joint 0.08was reached, there was 0.28 a sudden failure of the joint by brittle fracture, but total failure was delayed as a result of using mechanical fasteners (i.e., screws). Vertical deformation increased even after reaching the maximal joint force. The mean value of The jointVertical displayed deformation ductile increased behavior, even and afterits load-b reachiearingng the capacity maximal was joint limited force. byThe the mean mechanical value of the maximal vertical deformation was 37.78 mm for LVL splices, and with solid timber splices it was joint.the maximal After the vertical mechanical deformation joint’s maximum was 37.78 load-carryingmm for LVL splices, capacity and was with reached, solid timber the splice splices was it split was 34.56 mm (Figure 11). by34.56 tensile mm force(Figure perpendicular 11). to the fibers (brittle behavior of timber). Figure 11 shows that the deformation curves of this type of joint were partially linear. When the maximum load-bearing capacity of this joint was reached, there was a sudden failure of the joint by brittle fracture,40 butCOMBINATION total failure was delayed OF as GLUINGa result of using AND mechanical SCREWS fasteners (i.e., screws). The joint35 displayed ductile behavior, and its load-bearing capacity was limited by the mechanical joint. After30 the mechanical30.04 kN joint’s maximum load-carrying capacity was reached, the splice was split by tensile25 force perpendicular to the fibers (brittle behavior of timber). 20 Solid timber 1540 COMBINATION OF GLUING AND SCREWS Solid timber - failure 1035 LVL Force [kN] Force 5 30 30.04 kN LVL - failure 250 0 5 10 15 20 25 30 35 40 20 Vertical deformation [mm] Solid timber

15 Solid timber - failure Figure 11. Combination of gluing and screw joints—deformation curves. 10 Figure 11. Combination of gluing and screw joints—deformation LVL curves. Force [kN] Force 5 Figure 11 shows that the deformation curves of this type of joint were LVL partially- failure linear. When the 0 maximum load-bearing0 5 capacity 10 of this joint 15 was reached, 20 there 25 was a 30 sudden failure 35 of the 40 joint by brittle fracture, but total failure was delayedVertical as deformation a result of using [mm] mechanical fasteners (i.e., screws).

Figure 11. Combination of gluing and screw joints—deformation curves. Sustainability 2020, 12, 9288 9 of 15

TheSustainability joint displayed 2020, 12, x ductileFOR PEER behavior, REVIEW and its load-bearing capacity was limited by the mechanical9 joint. of 15 SustainabilityAfter the mechanical2020, 12, x FOR joint’s PEER REVIEW maximum load-carrying capacity was reached, the splice was split9 of by15 tensileThe force first perpendicular or second veneer to the primarily fibers (brittle suffered behavior shear failure of timber). in this type of joint with LVL splicing. The first or second veneer primarily suffered shear failure in this type of joint with LVL splicing. Thereafter,The first splitting or second of the veneer upper primarily pressed su andffered botto shearm tensional failure in thisparts type of the of jointsplice with occurred LVL splicing. in the Thereafter, splitting of the upper pressed and bottom tensional parts of the splice occurred in the Thereafter,plane of screws splitting (Figure of the 12). upper pressed and bottom tensional parts of the splice occurred in the plane plane of screws (Figure 12). of screws (Figure 12).

Figure 12. Combination of gluing and screws with LVL splice: typical failures of LVL splice. FigureFigure 12. 12. CombinationCombination of of gluing gluing and and screws screws with with LV LVLL splice: splice: typical typical failures failures of of LVL LVL splice. splice. This type of joint with solid timber splices primarily suffered splitting in the upper pressed part ThisThis type type of of joint joint with with solid solid timber timber splices splices primarily primarily suffered suﬀered splitting splitting in in the the upper upper pressed pressed part part of the splice in the plane of screws, while it simultaneously developed a crack from the middle ofof the splicesplice inin the the plane plane of of screws, screws, while while it simultaneously it simultaneously developed developed a crack a fromcrack the from middle the elementmiddle element (beam) parallel or perpendicular to the annual rings, which was the same as for solid timber element(beam) parallel(beam) parallel or perpendicular or perpendicular to the annual to the rings,annual which rings, waswhich the was same the as same for solid as for timber solid timber splices splices (Figure 13). splices(Figure (Figure 13). 13).

Figure 13. CombinationCombination of ofgluing gluing and and screws screws with with solid solid timber timber splice: splice: typical typical failures failures of solid of timber solid Figure 13. Combination of gluing and screws with solid timber splice: typical failures of solid timber timbersplice. splice. splice. 3.2. Calculation According to Standards 3.2. Calculation According to Standards 3.2. Calculation According to Standards 3.2.1. Glued Joints 3.2.1. Glued Joints 3.2.1. Glued Joints The maximum force of the glued joint Rbc is designated using Formulas (1) to (4), as described in The maximum force of the glued joint Rbc is designated using Formulas (1) to (4), as described in SectionThe 2.2.1 maximum. force of the glued joint Rbc is designated using Formulas (1) to (4), as described in Section 2.2.1. SectionThe 2.2.1. resulting values for the maximum glued joint force (Rbc1) at the maximum load-carrying The resulting values for the maximum glued joint force (Rbc1) at the maximum load-carrying capacityThe ofresulting the splice values in shear for the for LVLmaximum and solid glued timber joint are force shown (Rbc in1) at Table the4 .maximum load-carrying capacity of the splice in shear for LVL and solid timber are shown in Table 4. capacity of the splice in shear for LVL and solid timber are shown in Table 4. Table 4. Load-carrying capacity of splices in shear. Table 4. Load-carrying capacity of splices in shear. Quantity Unit Quantity Unit Ftc1 20.30 kN LVL Ftc1 20.30 kN LVL Mbd1 4.06 kNm Mbd1 4.06 kNm Sustainability 2020, 12, 9288 10 of 15

Table 4. Load-carrying capacity of splices in shear.

Quantity Unit

Ftc1 20.30 kN LVL Mbd1 4.06 kNm Rbc1 23.88 kN

Ftc1 18.83 kN Solid timber Mbd1 4.02 kNm Rbc1 23.63 kN

The resulting values for the maximum glued force at the maximum load-carrying capacity of the glue line for LVL and solid timber are shown in Table5 for a uniform distribution of shear stress ( Rbc2) and in Table6 for an unequal distribution of shear stress ( Rbc3).

Table 5. Load-carrying capacity of the glue-line: uniform distribution of shear.

Quantity Unit

Ftc1 6.62 kN LVL Mbd1 1.32 kNm Rbc1 7.79 kN

Ftc1 7.06 kN Solid timber Mbd1 1.51 kNm Rbc1 8.86 kN

Table 6. Load-carrying capacity of the glue-line: unequal distribution of shear.

Quantity Unit

Ftc1 3.31 kN LVL Mbd1 0.66 kNm Rbc1 3.89 kN

Ftc1 3.53 kN Solid timber Mbd1 0.75 kNm Rbc1 4.43 kN

3.2.2. Screw Joints

The maximum joint force for screwed joint Rbc4 (Table7) is designated using formulas (5) and (6) and Johansen formulas according to [14] for determining the load-carrying capacity of one fastener per shear plane.

Table 7. Load-carrying capacity of screwed joints.

Quantity Unit

Ftc1 3.73 kN LVL Mbd1 1.34 kNm Rbc1 7.89 kN

Ftc1 4.10 kN Solid timber Mbd1 1.64 kNm Rbc1 9.64 kN

4. Summary Figure 14 shows a comparison of the load-carrying capacity of glued joints with LVL splices determined using laboratory tests and calculations according to European standards. Results are Sustainability 2020, 12, 9288 11 of 15 displayed as deformation curves, with a maximum joint force of 28.26 kN. The load-carrying capacity of this joint type, determined using calculations according to valid standards, had three values for LVL splices: the load-carrying capacity of splices in shear was Rbc1 = 23.88 kN, of the glue-line with uniform distribution of shear was Rbc2 = 7.79 kN, and of the glue-line with unequal distribution of shear was Rbc3 = 3.89 kN. Sustainability 2020, 12, x FOR PEER REVIEW 11 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 11 of 15 GLUED JOINTS - LVL 30 GLUED JOINTS - LVL 30 LVL 28.86 kN 25 LVL 28.86 kN 25 LVL - failure 23.88 20 LVL - failure 23.88 20 Rbc1 Rbc1 15 Rbc2 15 Rbc2 10 Rbc3 10 Rbc3 Force [kN] Force 5 7.79 Force [kN] Force 7.79 5 3.89 0 3.89 0 012345678910 012345678910Vertical deformation [mm] Vertical deformation [mm]

Figure 14. Joints with LVL splices—comparison of laboratory and calculation results. Figure 14. Joints with LVL splices—comparison of laboratory and calculation results. Figure 14. Joints with LVL splices—comparison of laboratory and calculation results. Figure 1515 compares compares the the load-carrying load-carrying capacity capacity of glued of jointsglued with joints solid with timber solid splices timber determined splices Figure 15 compares the load-carrying capacity of glued joints with solid timber splices determinedusing laboratory using tests laboratory and calculations tests and accordingcalculations to according European to standards. European Results standards. are displayed Results are as determined using laboratory tests and calculations according to European standards. Results are displayeddeformation as deformation curves, with curves, a maximum with a joint maximum force of joint 30.80 force kN. of The 30.80 load-carrying kN. The load-carrying capacity of thiscapacity joint displayed as deformation curves, with a maximum joint force of 30.80 kN. The load-carrying capacity oftype, this determined joint type, determined using calculations using calculations according to ac validcording standards, to valid had standards, three values had three for solid values timber for solidof this timber joint type,splices: determined the load-carrying using calculations capacity of acsplicescording in shearto valid was standards, Rbc1 = 23.63 had kN, three of the values glue-line for splices: the load-carrying capacity of splices in shear was Rbc1 = 23.63 kN, of the glue-line with uniform bc1 withsolid uniformtimber splices: distribution the load-carrying of shear was capacity Rbc2 = 8.86 of spliceskN, and in of shear the glue-linewas R = with 23.63 unequal kN, of the distribution glue-line distribution of shear was Rbc2 = 8.86 kN, and of the glue-line with unequal distribution of shear was bc2 ofwith shear uniform was R distributionbc3 = 4.43 kN. of shear was R = 8.86 kN, and of the glue-line with unequal distribution Rbc3 = 4.43 kN. of shear was Rbc3 = 4.43 kN. GLUED JOINTS - SOLID TIMBER 35 GLUED JOINTS - SOLID TIMBER 35 30.80 kN 30 Solid timber 30.80 kN Solid timber 30 Solid timber - failure 25 Solid timber - failure 25 Rbc1 23.63 20 Rbc1 20 Rbc2 23.63 15 Rbc2 15 Rbc3 10 Rbc3 Force [kN] Force 10 8.86

Force [kN] Force 5 8.86 5 4.43 0 4.43 0 012345678910 012345678910Vertical deformation [mm] Vertical deformation [mm]

Figure 15. Screw joints with solid timber splices—comparison of laboratory and calculation results. FigureFigure 15. 15. ScrewScrew joints joints with with solid solid timber timber splices—compar splices—comparisonison of of laboratory laboratory and and calculation calculation results. results. Figure 16 compares the load-carrying capacity of screwed joints with LVL splices determined usingFigure laboratory 16 compares tests and the calculations load-carrying according capacity to of ofEuropean screwed standards. joints with Results LVL splices are displayed determined as deformationusing laboratory curves, tests with and maximum calculations joint according force of 16 to .32 European kN. The standards. load-carrying Results Results capacity are displayed of this joint as typedeformation was Rbc4 curves, curves,= 7.89 kN, with with calculated maximum maximum according joint joint force force to validof of 16 16.32 .32standards. kN. The load-carrying capacity of this joint type was Rbc44 == 7.897.89 kN, kN, calculated calculated according according to to valid valid standards. standards. Sustainability 2020, 12, 9288 12 of 15

Sustainability 2020, 12, x FOR PEER REVIEW 12 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 12 of 15 SCREW JOINTS - LVL 20 SCREW JOINTS - LVL 20 LVL LVL 15 LVL - failure 15 LVL - failure 16.32 kN Rbc4 16.32 kN 10 Rbc4 10 7.89 5

Force [kN] Force 7.89 5 Force [kN] Force 0 0 0 102030405060 0 102030405060Vertical deformation [mm]

Vertical deformation [mm] Figure 16. Screw joints with LVL splices—comparison of laboratory and calculation results. Figure 16. Screw joints with LVL splices—comparison of laboratory and calculation results. Figure 16. Screw joints with LVL splices—comparison of laboratory and calculation results. Figure 17 compares the load-carrying capacity of screwed joints with solid timber splices Figure 17 compares the load-carrying capacity of screwed joints with solid timber splices determined using laboratory tests and calculations according to standards. Results are displayed as determined using laboratory tests and calculations according to standards. Results are displayed as deformation curves, with maximum joint force of 118.98 kN. The load-carrying capacity of this joint deformationtype was Rbc4 curves, = 9.64 kN, with calculated maximum according joint force to ofvalid 118.98 standards. kN. The load-carrying capacity of this joint type was Rbc4 = 9.64 kN, calculated according to valid standards. type was Rbc4 = 9.64 kN, calculated according to valid standards. SCREW JOINTS - SOLID TIMBER 20 SCREW JOINTS - SOLID TIMBER 20 18.98 kN 15 18.98 kN 15 9.64 10 9.64 10 Solid timber Solid timber 5 Solid timber - failure Force [kN] Force 5 Solid timber - failure Force [kN] Force Rbc4 0 Rbc4 0 0 102030405060 0 102030405060Vertical deformation [mm]

Vertical deformation [mm] Figure 17. Screw joints with solid timber splices—comparison of laboratory and calculation results. Figure 17. Screw joints with solid timber splices—comparison of laboratory and calculation results. Figure 17. Screw joints with solid timber splices—comparison of laboratory and calculation results. Figure 18 shows the deformation curves of all six tested sample sets—LVL and solid timber splicesFigureFigure in combination 1818 showsshows the thewith deformation deformation three connection curves curves of types: allof sixall gluing, testedsix tested samplescrews, sample sets—LVLand sets—LVLcombination. and solid and timbersolid timber splices splicesin combinationFigure in combination 18 withshows three withthe connection threedeformation connection types: curves gluing,types: of gluing, screws,glued screws,joints and combination. were and combination.partially linear. When the maximumFigureFigure load-carrying 18 shows the capacity deformation of these curves joints was of gluedgluedreached, jointsjoints there werewere was partiallyapartially sudden linear.linear.failure ofWhen When the joint the maximumbymaximum brittle fracture. load-carrying load-carrying capacity capacity of of these these joints joints was was reached, reached, there there was was a sudden a sudden failure failure of the of joint the byjoint brittleDeformation by brittle fracture. fracture. curves of screwed joints were partially linear, and then the joint had a ductile behavior.DeformationDeformation Ductility curves curvesis typical of of forscrewed screwed mechanical joints joints fasteners,were were part partially especiallyially linear, linear, for and andsmall-diameter then then the the joint joint fasteners had had a a thatductile can behavior.bend.behavior. When Ductility Ductility the maximum is is typical typical forload-carrying for mechanical mechanical capacityfasteners, fasteners, of especially especially this joint for for was small-diameter small-diameter reached, there fasteners fasteners was a that thatsudden can can bend.failurebend. When of the thethe joint; maximummaximum however, load-carrying load-carrying due to the capacity capacity mechanical of thisof thisjoint fasteners joint was was reached, (i.e., reached, screws), there there was total a was sudden failure a sudden failure was failure of the joint; however, due to the mechanical fasteners (i.e., screws), total failure was postponed.of the joint; however, due to the mechanical fasteners (i.e., screws), total failure was postponed. postponed. Figure 1111 shows shows the the deformation deformation curves curves of this of jointthis typejoint were type partially were partially linear. When linear. the When maximum the Figure 11 shows the deformation curves of this joint type were partially linear. When the maximumload-bearing load-bearing capacity of thiscapacity joint wasof this reached, joint was there re wasached, a sudden there was failure a sudden of the jointfailure by brittleof the fracture,joint by maximum load-bearing capacity of this joint was reached, there was a sudden failure of the joint by brittlebut total fracture, failure but was total delayed failure as awas result delayed of using as a screws. result of The using joint screws. displayed The ductile joint displayed behavior, ductile and its brittle fracture, but total failure was delayed as a result of using screws. The joint displayed ductile behavior,load-bearing and capacity its load-bearing was limited capacity by the was mechanical limited by joint. the mechanical joint. behavior, and its load-bearing capacity was limited by the mechanical joint. Sustainability 2020, 12, 9288 13 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 13 of 15

COMPARISON OF ALL DEFORMATION CURVES 40 35 30 25 20 15 10 5

Force [kN] 0 0 102030405060 Vertical deformation [mm] Solid timber Solid timber - failure LVL LVL - failure Solid timber Solid timber - failure LVL - failure LVL Solid timber Solid timber - failure LVL LVL - failure

Figure 18. Screw joints with solid timber splices—comparison of laboratory and calculation results. Figure 18. Screw joints with solid timber splices—comparison of laboratory and calculation results. 5. Discussion 5. Discussion Based on the results from our laboratory measurements, one can state that glued and combined jointsBased displayed on the a results higher from values our of laboratory potential load meas actingurements, on the one joint can compared state that glued to that and of joints combined with onlyjoints mechanical displayed fasteners.a higher values Glued of joints potential alone load (i.e., notacting combined on the joint with compared screws) su fftoered that sudden of joints failure with (brittleonly mechanical fracture) whenfasteners. the maximumGlued joints load alone was (i.e., reached. not combined with screws) suffered sudden failure (brittleFor fracture) this reason, when combined the maximum joints appear load was to bereached. beneficial and include both the rigidity and strength of gluedFor jointsthis reason, as well combined as the good joints properties appear ofto mechanical be beneficial fasteners and include due to both their the plastic rigidity behavior and (ductility)strength of when glued the joints joint’s as load-bearingwell as the good capacity proper is reached.ties of mechanical fasteners due to their plastic behaviorDuctility (ductility) is typical when for the mechanical joint’s load-bearing fasteners, especially capacity foris reached. small-diameter fasteners that can bend. TheirDuctility use is suitable is typical in building for mechanical structures fasteners, because, beforeespecially failure, for the small-diameter joints indicate fasteners impending that failure can bybend. their Their increased use is suitable deformations. in building structures because, before failure, the joints indicate impending failure by their increased deformations. 6. Conclusions 6. ConclusionsBased on our calculations and laboratory tests, information can be acquired regarding glued, screw, as wellBased as combined on our calculations connections and of beamslaboratory using tests, double-sided information slices can made be acquired of wood regarding and wood-based glued, materialsscrew, as well stressed as combined by pure connections bending without of beams the influenceusing double-sided of shear force. slices Gluedmade of splices wood hadand higherwood- rigiditybased materials in comparison stressed to screwed.by pure Brittlebending failure without of the the joint influence occurred of by shear shear force. force Glued in a layer splices of wood; had inhigher the case rigidity of LVL in comparison splices, failure to occurredscrewed. inBrittle the veneer failure closest of the tojoint the occurred glued layer by shear (this is force the dominantin a layer combinedof wood; in failure the case seen of by LVL rolling splice shears, failure force occurred and tension in the parallel veneer to cl theosest grain). to the Thus, glued increasing layer (this the is splicethe dominant thickness combined does not failure necessarily seen by lead rolling to increased shear force rigidity andor tension bearing parallel capacity to the of the grain). joint. Thus, The load-bearingincreasing the capacity splice thickness of the joint does is, therefore,not necessarily proportional lead to increased to the dimensions rigidity or of bearing splices, thecapacity quality of ofthe the joint. adhesive The load-bearing and actual capacity application of the of joint theadhesive, is, therefore, surface proportional coverage, to andthe dimensions creation of theof splices, active surfacethe quality (epoxy of the two-component adhesive and resin).actual application of the adhesive, surface coverage, and creation of the activeIt is advisable surface (epoxy to use adhesivestwo-component that do resin). not show brittle failure and do not form a rigid boundary layer,It which is advisable could cause to use local adhesives extremes that in do stress not dueshow to brittle different failure wood and behaviors do not form and thea rigid adhesive boundary film itselflayer, (especially which could changes cause inlocal humidity extremes perpendicular in stress due to to the different grain). wood behaviors and the adhesive film itselfIn contrast, (especially splices changes mechanically in humidity fastened perpendicular with screws to have the lower grain). sti ffness, and joint failure occurs in theIn wood contrast, splices splices via tension mechanically perpendicular fastened to with the grain.screws The have bearing lower capacity stiffness, of and a joint joint depends failure onoccurs many in factors.the wood This splices is especially via tension true perpendicular for the tensile strengthto the grain. of wood The perpendicularbearing capacity to theof a grain; joint depends on many factors. This is especially true for the tensile strength of wood perpendicular to the grain; the number of natural imperfections (grown wood has more imperfections than LVL boards); Sustainability 2020, 12, 9288 14 of 15 the number of natural imperfections (grown wood has more imperfections than LVL boards); overall dimensions and, in particular, dimensions of splices; number, arrangement, size, and quality of the screws. The joint with mechanical fasteners shows a higher ductility than that of glued joints, which is significant from the point of view of safety. The joint shows noticeable deformations before failure occurs and can acoustically warn of the approaching maximum load-bearing capacity. This leaves space for possible repair or replacement of the joint. Combined joints (gluing and screws, mechanical fasteners) include the advantages of both methods of joining, when screws can additionally create sufficient pressure for the glued joint in the phase of its solidification and hardening. This can lead to a more even distribution of the adhesive and the formation of a uniform surface, which must be checked to avoid an uneven film adhesive. This phase of jointing is very sensitive and can largely affect the load-bearing capacity. The number of tests was limited due to time and capacity reasons, so it will be necessary to continue verifications of the above hypotheses with a larger series of tests, including the creation of relevant numerical models verified by standard design approaches and literature, and validated by new physical tests.

Author Contributions: Conceptualization, K.V. and A.L.; methodology, D.M., A.L. and K.V.; formal analysis, D.M. and A.L.; test preparation and processing, K.V., D.M. and O.S.; data evaluation, K.V. and D.M.; writing—original draft preparation, K.V., D.M. and A.L.; writing—review and editing, K.V. All authors have read and agreed on the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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