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 1and Oldrich Sucharda 2 1Department 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; oldrich.suchar[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 Other works are mainly devoted to the carrying capacity of adhesives in combination with various 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 of 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 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 overloading of the joint and allows for corrective action. This is the reason we tested both glued and screw joints themselves as well as their combination. 2. Materials and Methods 2.1. Laboratory Testing To analyze the load-bearing capacity of a longitudinal joint subjected to simple flexural stress, destructive laboratory testing was performed on sample joints designed with three basic types of splices in a central beam: by gluing, by gluing in combination with mechanical fasteners, and using only mechanical fasteners. Two material variants of splices were selected: from mature and laminated veneer lumber. In total, six test sets were created for two material variants of splices in combination with three methods of their fastening. Each test set consisted of five samples. For the testing, samples were assembled consisting of a central beam with dimensions 110 × 180 × 1220 mm and made of solid timber with the use of laminate veneer lumber splices and splices made of solid timber (see Figure 1). 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 combination with various 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 of 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 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 overloading of the joint and allows for corrective action. This is the reason we tested both glued and screw joints themselves as well as their combination. 2. Materials and Methods 2.1. Laboratory Testing To analyze the load-bearing capacity of a longitudinal joint subjected to simple flexural stress, destructive laboratory testing was performed on sample joints designed with three basic types of splices in a central beam: by gluing, by gluing in combination with mechanical fasteners, and using only mechanical fasteners. Two material variants of splices were selected: from mature and laminated veneer lumber. In total, six test sets were created for two material variants of splices in combination with three methods of their fastening. Each test set consisted of five samples. For the testing, samples were assembled consisting of a central beam with dimensions 110 × 180 × 1220 mm and made of solid timber with the use of laminate veneer lumber splices and splices made of solid timber (see Figure 1). Figure 1. Laboratory testing scheme. The central beam and solid timber splices were made of solid spruce (Picea abies), which has a strength class of C24 and average density of 370 kg m−3. Laminated veneer lumber (LVL) splices were made of the R type softwood veneers (spruce/pine) with average density of 510 kg m−3. Two-component epoxy adhesive was used for structural gluing, which was applied in a 2 mm thick layer on the entire contact surface between the splice and central element. The test samples were conditioned prior to destructive testing at a standard ambient temperature of 20 ± 2 °C and relative humidity of 65 ± 5%. To determine the moisture in the test samples, a moisture detector was used. The average moisture content of tested elements was 10.2%. Countersunk self-tapping screws were used as the mechanical fasteners: Ø8/80 for joints with splices with laminated veneer lumber and Ø8/100 for joints with splices made of solid timber. Screws were made of carbon steel with white galvanic zinc coating and had a yield strength fyk = 1000 N mm−2. For the joining of each splice, eight screws were used (see Figure 2). Figure 1. Laboratory testing scheme. The central beam and solid timber splices were made of solid spruce (Picea abies), which has a strength class of C24 and average density of 370 kg m −3 . Laminated veneer lumber (LVL) splices were made of the R type softwood veneers (spruce/pine) with average density of 510 kg m−3. Two-component epoxy adhesive was used for structural gluing, which was applied in a 2 mm thick layer on the entire contact surface between the splice and central element. The test samples were conditioned prior to destructive testing at a standard ambient temperature of 20 ± 2 ◦ C and relative humidity of 65 ± 5%. To determine the moisture in the test samples, a moisture detector was used. The average moisture content of tested elements was 10.2%. Countersunk self-tapping screws were used as the mechanical fasteners: Ø8/80 for joints with splices with laminated veneer lumber and Ø8/100 for joints with splices made of solid timber. Screws were made of carbon steel with white galvanic zinc coating and had a yield strength f yk =1000 N mm −2 . For the joining of each splice, eight screws were used (see Figure 2). Sustainability 2020,12, 9288 3 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 3 of 15 (a) (b) Figure 2. Layout char of screws. (a) Laminated veneer lumber (LVL) splice; (b) Solid timber splice. Testing proceeded on a hydraulic pressure machine at the laboratories of the Faculty of Civil Engineering, VSB-TU Ostrava, and force was increased gradually. The sample was loaded with vertical force applied in thirds of the span. An optimal force rate was chosen for the press. Failure among all tested samples appeared in a time boundary of 300 ± 120 s which corresponds to the interval of laboratory tests for short-time strength according to the current European standards for timber structure capacity [14,15]. During testing, the force (maximum joint force) was recorded for each test sample (accuracy of 0.01 kN), and the corresponding deformation of the joint was measured in the supports and in the middle of the span at half the height of the cross-section of the test sample (accuracy of 0.01 mm). 2.2. Calculation According to Standards 2.2.1. Glued Joints According to previous laboratory tests and numerical calculations, this type of joint has two basic types of failure: failure along the glue line (R bc1 ) or failure of splice veneers (LVL or solid timber). Failure along the glue line is calculated with two variants: uniform (R bc2 ) and unequal (R bc3 ) distribution of shear stress. Calculation of the maximum joint force R bc is based on the moment transferred by the glued joint for both central elements as well as the designed load-carrying capacity of the joint (Figure 3). Figure 3. Maximum joint force—glued joint. The maximum joint force R bc is determined from the following expression: 𝑅 2𝑀 𝑟 󰇟kN󰇠 (1) where M b is the moment transferred by the glued joint, and r is the lever arm of the test sample. The moment transferred by the glued joint is determined from the following expression: 𝑀𝐹 .2.𝑅  󰇟kNm󰇠 (2) Figure 2. Layout char of screws. (a) Laminated veneer lumber (LVL) splice; (b) Solid timber splice. Testing proceeded on a hydraulic pressure machine at the laboratories of the Faculty of Civil Engineering, VSB-TU Ostrava, and force was increased gradually. The sample was loaded with vertical force applied in thirds of the span. An optimal force rate was chosen for the press. Failure among all tested samples appeared in a time boundary of 300 ± 120 s which corresponds to the interval of laboratory tests for short-time strength according to the current European standards for timber structure capacity [14,15]. During testing, the force (maximum joint force) was recorded for each test sample (accuracy of 0.01 kN), and the corresponding deformation of the joint was measured in the supports and in the middle of the span at half the height of the cross-section of the test sample (accuracy of 0.01 mm). 2.2. Calculation According to Standards 2.2.1. Glued Joints According to previous laboratory tests and numerical calculations, this type of joint has two basic types of failure: failure along the glue line (R bc1 ) or failure of splice veneers (LVL or solid timber). Failure along the glue line is calculated with two variants: uniform (R bc2 ) and unequal (R bc3 ) distribution of shear stress. Calculation of the maximum joint force R bc is based on the moment transferred by the glued joint for both central elements as well as the designed load-carrying capacity of the joint (Figure 3). Sustainability 2020, 12, x FOR PEER REVIEW 3 of 15 (a) (b) Figure 2. Layout char of screws. (a) Laminated veneer lumber (LVL) splice; (b) Solid timber splice. Testing proceeded on a hydraulic pressure machine at the laboratories of the Faculty of Civil Engineering, VSB-TU Ostrava, and force was increased gradually. The sample was loaded with vertical force applied in thirds of the span. An optimal force rate was chosen for the press. Failure among all tested samples appeared in a time boundary of 300 ± 120 s which corresponds to the interval of laboratory tests for short-time strength according to the current European standards for timber structure capacity [14,15]. During testing, the force (maximum joint force) was recorded for each test sample (accuracy of 0.01 kN), and the corresponding deformation of the joint was measured in the supports and in the middle of the span at half the height of the cross-section of the test sample (accuracy of 0.01 mm). 2.2. Calculation According to Standards 2.2.1. Glued Joints According to previous laboratory tests and numerical calculations, this type of joint has two basic types of failure: failure along the glue line (R bc1 ) or failure of splice veneers (LVL or solid timber). Failure along the glue line is calculated with two variants: uniform (R bc2 ) and unequal (R bc3 ) distribution of shear stress. Calculation of the maximum joint force R bc is based on the moment transferred by the glued joint for both central elements as well as the designed load-carrying capacity of the joint (Figure 3). Figure 3. Maximum joint force—glued joint. The maximum joint force R bc is determined from the following expression: 𝑅 2𝑀 𝑟 󰇟kN󰇠 (1) where M b is the moment transferred by the glued joint, and r is the lever arm of the test sample. The moment transferred by the glued joint is determined from the following expression: 𝑀𝐹 .2.𝑅  󰇟kNm󰇠 (2) Figure 3. Maximum joint force—glued joint. The maximum joint force Rbc is determined from the following expression: Rbc =2Mb r[kN](1) where Mbis the moment transferred by the glued joint, and ris the lever arm of the test sample. The moment transferred by the glued joint is determined from the following expression: Mb=Ftc ×2×Rcc [kNm](2) Sustainability 2020,12, 9288 4 of 15 where F tc is the designed load-carrying capacity of the joint determined from expression (3) for shear strength of the splice material and (4) for strength of the glue line. R cc is the lever arm of forces acting on the joint. Rgarding the load-carrying capacity of the joint, the shear strength of the splice material is determined as follows: Ftc =fv,k×kmod ×Atc 2×γM [kN](3) where Atc is the active glued area of the joint, and fv,kis the shear strength of the splice material. Regarding the load-carrying capacity of the joint, the glue line strength is determined as Ftc =fk×kmod ×Atc 2×γM [kN](4) where A tc is the active glued area of the joint, and f k is the strength of the glued surface for uniform and unequal distribution of shear stress. 2.2.2. Screw Joints The maximum joint force R bc4 (see Figure 4) is based on the load-carrying capacity of the single shear fastener and is determined from the following expression: Rbc =2Mb r[kN](5) where Mbis the moment transferred by the glued joint, and ris the lever arm of the test sample. Sustainability 2020, 12, x FOR PEER REVIEW 4 of 15 where F tc is the designed load-carrying capacity of the joint determined from expression (3) for shear strength of the splice material and (4) for strength of the glue line. R cc is the lever arm of forces acting on the joint. Regarding the load-carrying capacity of the joint, the shear strength of the splice material is determined as follows: 𝐹  𝑓 ,.𝑘 . 𝐴  2. 𝛾 󰇟kN󰇠 (3) where A tc is the active glued area of the joint, and f v,k is the shear strength of the splice material. Regarding the load-carrying capacity of the joint, the glue line strength is determined as 𝐹  𝑓 .𝑘 . 𝐴  2. 𝛾 󰇟kN󰇠 (4) where A tc is the active glued area of the joint, and f k is the strength of the glued surface for uniform and unequal distribution of shear stress. 2.2.2. Screw Joints The maximum joint force R bc4 (see Figure 4) is based on the load-carrying capacity of the single shear fastener and is determined from the following expression: 𝑅 2𝑀 𝑟 󰇟kN󰇠 (5) where M b is the moment transferred by the glued joint, and r is the lever arm of the test sample. Figure 4. Maximum joint force—screw joint. The moment transferred by the joint onto the beams is determined using the following expression: 𝑀𝐹 ,.2.2.𝑅  󰇟kNm󰇠 (6) where F v,Rd is the designed load-carrying capacity for the fastener per shear plane, and R cc is the lever arm of forces acting on the joint. Design load-carrying capacity for fastener per shear F v,Rd is determined according to expressions given in [14]. 2.2.3. Combination of Gluing and Screws In the applicable standards, the calculation of load-carrying capacity for combined joints with glued and mechanical fasteners is not described; thus, the maximum strength is not designated for these joints. Essentially, joints of varying rigidity should not be combined, and if they are, they should have at least a similar load-carrying capacity. Figure 4. Maximum joint force—screw joint. The moment transferred by the joint onto the beams is determined using the following expression: Mb=Fv,Rd ×2×2×Rcc [kNm](6) where F v,Rd is the designed load-carrying capacity for the fastener per shear plane, and R cc is the lever arm of forces acting on the joint. Design load-carrying capacity for fastener per shear F v,Rd is determined according to expressions given in [14]. 2.2.3. Combination of Gluing and Screws In the applicable standards, the calculation of load-carrying capacity for combined joints with glued and mechanical fasteners is not described; thus, the maximum strength is not designated for these joints. Essentially, joints of varying rigidity should not be combined, and if they are, they should have at least a similar load-carrying capacity. Sustainability 2020,12, 9288 5 of 15 3. Results 3.1. Laboratory Testing Based on laboratory test results, statistical variables were determined for each set of samples for maximum joint force (failure force) and vertical deformation of the joint. 3.1.1. Glued Joints The mean value of the maximum force acting on the joint with glued LVL splices was Rbc =28.86 kN , and with solid timber splices it was R bc =30.80 kN. The mean value of vertical deformation at maximal force with glued LVL splices was 8.38 mm, and with solid timber splices it was 9.03 mm (Table 1). Table 1. Glued joints—laboratory results. Force [kN] Vertical Deformation [mm] LVL Mean 28.86 8.38 SD 4.24 0.39 COV 0.15 0.05 Solid timber Mean 30.80 9.03 SD 6.72 1.24 COV 0.22 0.14 Figure 5shows that the deformation curves of glued joints with both types of splices were partially linear; only in the final phase of loading did the joints start to show plastic deformation. When the maximum load-carrying capacity of these joints was reached, there was a sudden failure of the joint by brittle fracture. Sustainability 2020, 12, x FOR PEER REVIEW 5 of 15 3. Results 3.1. Laboratory Testing Based on laboratory test results, statistical variables were determined for each set of samples for maximum joint force (failure force) and vertical deformation of the joint. 3.1.1. Glued Joints The mean value of the maximum force acting on the joint with glued LVL splices was R bc = 28.86 kN, and with solid timber splices it was R bc = 30.80 kN. The mean value of vertical deformation at maximal force with glued LVL splices was 8.38 mm, and with solid timber splices it was 9.03 mm (Table 1). Table 1. Glued joints—laboratory results. Force [kN] Vertical Deformation [mm] LVL Mean 28.86 8.38 SD 4.24 0.39 COV 0.15 0.05 Solid timber Mean 30.80 9.03 SD 6.72 1.24 COV 0.22 0.14 Figure 5 shows that the deformation curves of glued joints with both types of splices were partially linear; only in the final phase of loading did the joints start to show plastic deformation. When the maximum load-carrying capacity of these joints was reached, there was a sudden failure of the joint by brittle fracture. Figure 5. Glued joints—deformation curves. Joints with LVL splices suffered primarily from shear failure of the first or second veneer from the glue line (Figure 6). 30.80 kN 28.86 kN 0 5 10 15 20 25 30 35 012345678910 Force [kN] Vertical deformation [mm] GLUED JOINTS Solid timber Solid timber - failure LVL - failure LVL Figure 5. Glued joints—deformation curves. Joints with LVL splices suffered primarily from shear failure of the first or second veneer from the glue line (Figure 6). Sustainability 2020,12, 9288 6 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 6 of 15 (a) (b) Figure 6. Glued joints with LVL splice. (a) Typical failure; (b) Detailed image. Joints with solid timber splices primarily suffered splice failure in combination with shear and tension perpendicular to the grain. This failure was characterized by cracks forming in the area of the glue line. The joint predominately failed due to tension perpendicular to the grain. (Figure 7). Figure 7. Glued joints with solid timber splice: typical failure of solid timber splice. 3.1.2. Screw Joints The mean value of the maximum joint force for the set of samples with screws and LVL splices was Rbc = 16.32 kN, and for solid timber splices it was Rbc = 18.98 kN. The mean value of vertical deformation at the maximal force with screwed LVL splices was 53.61 mm, and with solid timber splices it was 54.12 mm (Table 2). Table 2. Screw joints—laboratory results. Force [kN] Vertical Deformation [mm] LVL Mean 16.32 53.61 SD 1.94 12.78 COV 0.12 0.25 Solid timber Mean 18.98 54.12 SD 1.73 13.27 COV 0.09 0.25 Figure 6. Glued joints with LVL splice. (a) Typical failure; (b) Detailed image. Joints with solid timber splices primarily suffered splice failure in combination with shear and tension perpendicular to the grain. This failure was characterized by cracks forming in the area of the glue line. The joint predominately failed due to tension perpendicular to the grain. (Figure 7). Sustainability 2020, 12, x FOR PEER REVIEW 6 of 15 (a) (b) Figure 6. Glued joints with LVL splice. (a) Typical failure; (b) Detailed image. Joints with solid timber splices primarily suffered splice failure in combination with shear and tension perpendicular to the grain. This failure was characterized by cracks forming in the area of the glue line. The joint predominately failed due to tension perpendicular to the grain. (Figure 7). Figure 7. Glued joints with solid timber splice: typical failure of solid timber splice. 3.1.2. Screw Joints The mean value of the maximum joint force for the set of samples with screws and LVL splices was Rbc = 16.32 kN, and for solid timber splices it was Rbc = 18.98 kN. The mean value of vertical deformation at the maximal force with screwed LVL splices was 53.61 mm, and with solid timber splices it was 54.12 mm (Table 2). Table 2. Screw joints—laboratory results. Force [kN] Vertical Deformation [mm] LVL Mean 16.32 53.61 SD 1.94 12.78 COV 0.12 0.25 Solid timber Mean 18.98 54.12 SD 1.73 13.27 COV 0.09 0.25 Figure 7. Glued joints with solid timber splice: typical failure of solid timber splice. 3.1.2. Screw Joints The mean value of the maximum joint force for the set of samples with screws and LVL splices was R bc =16.32 kN, and for solid timber splices it was R bc =18.98 kN. The mean value of vertical deformation at the maximal force with screwed LVL splices was 53.61 mm, and with solid timber splices it was 54.12 mm (Table 2). Table 2. Screw joints—laboratory results. Force [kN] Vertical Deformation [mm] LVL Mean 16.32 53.61 SD 1.94 12.78 COV 0.12 0.25 Solid timber Mean 18.98 54.12 SD 1.73 13.27 COV 0.09 0.25 Sustainability 2020,12, 9288 7 of 15 Figure 8shows the deformation curves of this type of joint were partially linear, and then the joint had ductile behavior. Ductility is typical for mechanical fasteners, especially for small-diameter fasteners that can bend. When the maximum load-carrying capacity of this joint was reached, there was a sudden failure of the joint; however, due to the use of mechanical fasteners (i.e., screws), total failure was postponed. The joint displayed ductile behavior, and its load-carrying capacity was limited by the mechanical joint. In the final phase, excessive bending of the outer screws, closer to the center of the joint, simultaneously occurred as the splice split perpendicular to its longitudinal axis. Sustainability 2020, 12, x FOR PEER REVIEW 7 of 15 Figure 8 shows the deformation curves of this type of joint were partially linear, and then the joint had ductile behavior. Ductility is typical for mechanical fasteners, especially for small-diameter fasteners that can bend. When the maximum load-carrying capacity of this joint was reached, there was a sudden failure of the joint; however, due to the use of mechanical fasteners (i.e., screws), total failure was postponed. The joint displayed ductile behavior, and its load-carrying capacity was limited by the mechanical joint. In the final phase, excessive bending of the outer screws, closer to the center of the joint, simultaneously occurred as the splice split perpendicular to its longitudinal axis. Figure 8. Screw joints—deformation curves. The most common type of joint failure with screwed LVL splices was splitting perpendicular to veneers at the screw level (see Figure 9). Figure 9. Screw joints with LVL splice: typical failures. Failure for screwed solid timber splices was the same as that with LVL splices: splitting was perpendicular to veneers at the screw level (see Figure 10). 18.98 kN 16.32 kN 0 5 10 15 20 0 102030405060 Force [kN] Vertical deformation [mm] SCREW JOINTS Solid timber Solid timber - failure LVL LVL - failure Figure 8. Screw joints—deformation curves. The most common type of joint failure with screwed LVL splices was splitting perpendicular to veneers at the screw level (see Figure 9). Sustainability 2020, 12, x FOR PEER REVIEW 7 of 15 Figure 8 shows the deformation curves of this type of joint were partially linear, and then the joint had ductile behavior. Ductility is typical for mechanical fasteners, especially for small-diameter fasteners that can bend. When the maximum load-carrying capacity of this joint was reached, there was a sudden failure of the joint; however, due to the use of mechanical fasteners (i.e., screws), total failure was postponed. The joint displayed ductile behavior, and its load-carrying capacity was limited by the mechanical joint. In the final phase, excessive bending of the outer screws, closer to the center of the joint, simultaneously occurred as the splice split perpendicular to its longitudinal axis. Figure 8. Screw joints—deformation curves. The most common type of joint failure with screwed LVL splices was splitting perpendicular to veneers at the screw level (see Figure 9). Figure 9. Screw joints with LVL splice: typical failures. Failure for screwed solid timber splices was the same as that with LVL splices: splitting was perpendicular to veneers at the screw level (see Figure 10). 18.98 kN 16.32 kN 0 5 10 15 20 0 102030405060 Force [kN] Vertical deformation [mm] SCREW JOINTS Solid timber Solid timber - failure LVL LVL - failure Figure 9. Screw joints with LVL splice: typical failures. Failure for screwed solid timber splices was the same as that with LVL splices: splitting was perpendicular to veneers at the screw level (see Figure 10). 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 R bc = 30.40 kN for LVL splices, and for solid timber splices it was R bc = 35.16 kN. The mean value of vertical deformation at the maximal force was 7.68 mm for LVL splices, and for solid timber splices it was 10.90 mm (Table 3). Table 3. Combination of gluing and screw joints—laboratory results. Force [kN] Vertical Deformation [mm] LVL Mean 30.04 37.78 SD 5.21 6.22 COV 0.18 0.17 Solid timber Mean 35.16 34.53 SD 2.67 9.19 COV 0.08 0.28 Vertical deformation increased even after reaching the maximal joint force. The mean value of the maximal vertical deformation was 37.78 mm for LVL splices, and with solid timber splices it was 34.56 mm (Figure 11). 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, but total failure was delayed as a result of using mechanical fasteners (i.e., screws). The joint displayed ductile behavior, and its load-bearing capacity was limited by the mechanical joint. After the mechanical joint’s maximum load-carrying capacity was reached, the splice was split by tensile force perpendicular to the fibers (brittle behavior of timber). Figure 11. Combination of gluing and screw joints—deformation curves. 30.04 kN 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Force [kN] Vertical deformation [mm] COMBINATION OF GLUING AND SCREWS Solid timber Solid timber - failure LVL LVL - failure 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 for solid timber splices it was 10.90 mm (Table 3). Table 3. Combination of gluing and screw joints—laboratory results. Force [kN] Vertical Deformation [mm] LVL Mean 30.04 37.78 SD 5.21 6.22 COV 0.18 0.17 Solid timber Mean 35.16 34.53 SD 2.67 9.19 COV 0.08 0.28 Vertical deformation increased even after reaching the maximal joint force. The mean value of the maximal vertical deformation was 37.78 mm for LVL splices, and with solid timber splices it was 34.56 mm (Figure 11). 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 R bc = 30.40 kN for LVL splices, and for solid timber splices it was R bc = 35.16 kN. The mean value of vertical deformation at the maximal force was 7.68 mm for LVL splices, and for solid timber splices it was 10.90 mm (Table 3). Table 3. Combination of gluing and screw joints—laboratory results. Force [kN] Vertical Deformation [mm] LVL Mean 30.04 37.78 SD 5.21 6.22 COV 0.18 0.17 Solid timber Mean 35.16 34.53 SD 2.67 9.19 COV 0.08 0.28 Vertical deformation increased even after reaching the maximal joint force. The mean value of the maximal vertical deformation was 37.78 mm for LVL splices, and with solid timber splices it was 34.56 mm (Figure 11). 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, but total failure was delayed as a result of using mechanical fasteners (i.e., screws). The joint displayed ductile behavior, and its load-bearing capacity was limited by the mechanical joint. After the mechanical joint’s maximum load-carrying capacity was reached, the splice was split by tensile force perpendicular to the fibers (brittle behavior of timber). Figure 11. Combination of gluing and screw joints—deformation curves. 30.04 kN 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Force [kN] Vertical deformation [mm] COMBINATION OF GLUING AND SCREWS Solid timber Solid timber - failure LVL LVL - failure Figure 11. Combination of gluing and screw joints—deformation curves. 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, but total failure was delayed as a result of using mechanical fasteners (i.e., screws). Sustainability 2020,12, 9288 9 of 15 The joint displayed ductile behavior, and its load-bearing capacity was limited by the mechanical joint. After the mechanical joint’s maximum load-carrying capacity was reached, the splice was split by tensile force perpendicular to the fibers (brittle behavior of timber). The first or second veneer primarily suffered shear failure in this type of joint with LVL splicing. Thereafter, splitting of the upper pressed and bottom tensional parts of the splice occurred in the plane of screws (Figure 12). Sustainability 2020, 12, x FOR PEER REVIEW 9 of 15 The first or second veneer primarily suffered shear failure in this type of joint with LVL splicing. Thereafter, splitting of the upper pressed and bottom tensional parts of the splice occurred in the plane of screws (Figure 12). Figure 12. Combination of gluing and screws with LVL splice: typical failures of LVL splice. This type of joint with solid timber splices primarily suffered splitting in the upper pressed part of the splice in the plane of screws, while it simultaneously developed a crack from the middle element (beam) parallel or perpendicular to the annual rings, which was the same as for solid timber splices (Figure 13). Figure 13. Combination of gluing and screws with solid timber splice: typical failures of solid timber splice. 3.2. Calculation According to Standards 3.2.1. Glued Joints The maximum force of the glued joint Rbc is designated using Formulas (1) to (4), as described in Section 2.2.1. The resulting values for the maximum glued joint force (Rbc1) at the maximum load-carrying 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. Quantity Unit LVL Ftc1 20.30 kN Mbd1 4.06 kNm Figure 12. Combination of gluing and screws with LVL splice: typical failures of LVL splice. This type of joint with solid timber splices primarily suffered splitting in the upper pressed part of the splice in the plane of screws, while it simultaneously developed a crack from the middle element (beam) parallel or perpendicular to the annual rings, which was the same as for solid timber splices (Figure 13). Sustainability 2020, 12, x FOR PEER REVIEW 9 of 15 The first or second veneer primarily suffered shear failure in this type of joint with LVL splicing. Thereafter, splitting of the upper pressed and bottom tensional parts of the splice occurred in the plane of screws (Figure 12). Figure 12. Combination of gluing and screws with LVL splice: typical failures of LVL splice. This type of joint with solid timber splices primarily suffered splitting in the upper pressed part of the splice in the plane of screws, while it simultaneously developed a crack from the middle element (beam) parallel or perpendicular to the annual rings, which was the same as for solid timber splices (Figure 13). Figure 13. Combination of gluing and screws with solid timber splice: typical failures of solid timber splice. 3.2. Calculation According to Standards 3.2.1. Glued Joints The maximum force of the glued joint Rbc is designated using Formulas (1) to (4), as described in Section 2.2.1. The resulting values for the maximum glued joint force (Rbc1) at the maximum load-carrying 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. Quantity Unit LVL Ftc1 20.30 kN Mbd1 4.06 kNm Figure 13. Combination of gluing and screws with solid timber splice: typical failures of solid timber splice. 3.2. Calculation According to Standards 3.2.1. Glued Joints The maximum force of the glued joint R bc is designated using Formulas (1) to (4), as described in Section 2.2.1. The resulting values for the maximum glued joint force (R bc1 ) at the maximum load-carrying capacity of the splice in shear for LVL and solid timber are shown in Table 4.