Behaviour of tensile strength and displacement concerning Big Screw Joint with Cross Laminated Panel Keiichi Tsubouchi Graduate student Meiji University Kawasaki, Japan (Hideyuki Nasu, Hiroyuki Noguchi, Göran Forsberg, Anders Gustafsson and Carl-Johan Johansson) Summary The purpose of this research is to improve performance of the joints used in Big Frame construction system, see Fig.1 and Fig.2. If the Big Screw is embedded into parallel direction to wood fiber direction, the joint performance has very high rigidity and high strength, however, it seems to be brittle. On the other hand, if the screw is embedded into cross direction, joint doesn t have so high rigidity. However, it has very high ductility. If we adopt cross laminated panel to this joint system, the system might have a character of both high rigidity and high ductility. To prove the performance of the joints made with cross laminated panels, we have done tensile experiments. By the results of tensile experiments, we prove that if we use cross laminated panel for this joint, we get characteristic with high performance. Then, we got the possibility that can control both rigidity and ductility by combinations of lamina. 1. Introduction The ductility of timber structures is depending on their joints rigidity and strengths. Big Screw joint system is a system with high rigidity and strength. Big Screw joint system is made of steel parts as a connecter between timber pillars and beams. Special for this system is the screw that is installed in the pillars and beams, called Big Screw. Big Screw has a big diameter and its thread is wide and thin. No glue is used in this joint system, the timber at each joint have predrilled holes with the same shape as the screws. In this joint system tensile load transfers from the steel screw to the timber by the wide and thin threads. Big Frame is a timber moment resistant structure that uses Big Screw joint. The Big Screw joint is the most important part in Big Frame. If the performance of the joints can be improved, Big Frame system can be used for houses with bigger spans. Fig.1 Big Frame Construction System Fig.2 Big Screw joint system
From earlier studies we know following about Big Screw joints. If the Big Screw is used in parallel direction of the wood fiber direction, the behaviour of the joint has high rigidity, high strength but brittle failure, see Fig.3. On the other hand, if the Big Screw is used in cross direction, behaviour of the joint doesn t have so high rigidity but it has higher ductility, see Fig.4. Therefore, it seems that wood fiber direction have a high influence on the behaviour of the joint parts. From now on, we describe parallel as the direction when the fiber direction is parallel to Big Screw, cross when the fiber direction is orthogonal to Big Screw. 2 RP_P_37_36 2 1 RP_P_37_36_1 RP_P_37_36_2 RP_P_37_36_3 RP_P_37_36_4 RP_P_37_36_5 RP_P_37_36_6 2 4 6 8 1 Fig.3 Load displacement and behaviour of Big Screw parallel to fibre direction 2 RP_C_37_36 2 1 RP_C_37_36_1 RP_C_37_36_2 RP_C_37_36_3 RP_C_37_36_4 RP_C_37_36_5 RP_C_37_36_6 2 4 6 8 1 Fig.4 Load displacement and behaviour of Big Screw cross to fiber direction The hypothesis is that if the Big Screw is screwed into laminated wood that combined both parallel fiber direction and cross fiber direction, it can be possible to get joint parts that have both characteristic performance of parallel and cross, with high rigidity and high ductility. In this research we adopted Cross Laminated Panels as a material for new joint systems. Fig.5 Big Screw and the other parts Fig.6 Cross laminated panel
We thought that the wood fiber direction around Big Screw influence much to the joint behaviour and determined the Area ratio as a parameter of this research. The Area ratio is the ratio of the parallel and cross fibers area that is affected by sheer force. Fig.8 indicates the image of area. We defined Area ratio (Rp) as expression (1). Area ratio can be expressed by circumference ratio of parallel and cross fiber directions that have contact with the external diameter of the Big Screw. Fig.9 shows each circumference. Ap C p R p = = (1) AP + AC C P + CC It is important to find out the best Area ratio to achieve best balance of rigidity and ductility. To verify equation 1 we performed a series of tests with three different Types of spices. We also studied the way to get out more ductility using cross laminated panels. Fig.7 Parallel and Cross Lamina around Big Screw Fig.8 Area image Cc : Circumference that Cross Lamina contact with φ31 Cp : Circumference that Parallel Lamina contact with φ37 Fig.9 Circumference image 2. Experiment 2.1 Purpose The main purpose of this research is to gain knowledge about the performance of Big Screw used in cross laminated panel. For this purpose we carried out tensile experiments with Type A, see Fig.1. We anticipated that the joint need to have both parallel and cross fibers characteristic to receive best performance. The second purpose is to study a way to get more ductility by using cross laminated panel. We studied therefore a way to join each lamina section using test pieces Type B and Type C to avoid cracks that arise in our first tests. 2.2 Test pieces Fig.1 Research flow In order to obtain practical behaviour of the joints three different test pieces was used, Type A, B and C. There were three test pieces for each Type, totally 9 test pieces. The Area ratio of Type A was 64%, Type B and Type C the ratio was 34%. They were prepared considering not only by the balance of rigidity and ductility but also supply situation from factory. The dimensions of the timber cross sections is presented in Fig.12. The dimensional species of Type A was thickness width 21 length 12mm. Type B and C were thickness width 21 length 117mm. The wood pieces tested in Type A, were made of Red Pine, Pinus Resinosa, and was tested at Meiji University in Japan. The wood pieces tested in Type B and Type C were made of Norway spruce, Picea Abies, and was tested at SP Technical Research Institute of Sweden, SP Trätek, in Sweden. All test pieces were combined with parallel and cross direction s lamina alternately. All the holes in test pieces were predrilled in Japanese and Swedish factories before threading the Big Screw. The length of Big Screw is 36mm, the external diameter was 37 mm and core diameter is 31 mm. The pitch of the wings is 12 mm. The thickness of the wings is 2 mm. Type B and C were made of five layers cross laminated panel with the center board parallel to the Big Screw. To receive better ductility and prevent cracks Type C was reinforced with double small screws, see Fig.13 and Fig.14.
9 22 2 2.5 22 2.5 37 37 21 21 Rp = 64% Rp = 34% Rp = 34% A type B type C type Fig.11 Lamina combination drawing of each Type 21 12 117 Fig.13 Small screw 36 36 6 21 φ37 21 φ37 75 6 75 A type B type C type Fig.12 Dimensional drawing of each Type 2.3 Experimental method The experiment for Type A was performed in Japan. Type B and C were performed in Sweden, but the both experiments were performed in the similar method except loading speed (Fig.15). The loading speed of Type A in Japan was around.5 kn/second, Type B and C in Sweden were around 5 kn/second. Displacement gauges were fixed to the end of each wood piece directory. And these were measured relative displacement between the end of wood and the plate that is fixed on the Big Screw. Tensile load was measured by load cell that was fixed between Big Screw and the loading block. Load meter Fig.14 Position of small screw Loading block Displacement gauge Big Screwφ37 Joint device Test pieces Fig.15 Experimental set-up 2.4 Result of Experiment Fig.16, 17 and 18 show the behaviours of Type A, B and C. Graph.1, 2 and 3 show load displacement curves. Table.1 shows Maximum loads, Rigidities, Displacements (.8Pmax). Each value is average of three test pieces. Displacement (.8Pmax) is the displacement when the load came down until 8% of maximum load. Displacement (.8Pmax) is a value that is used for indicate ductility.
2 2 1 A-1 A-2 A-3 Fig.16 Behaviour of Type A 2 4 6 8 1 Graph.1 Load displacement curve of Type A 2 2 1 B-1 B-2 B-3 Fig.17 Behaviour of Type B 2 4 6 8 1 Graph.2 Type B 2 2 1 C-1 C-2 C-3 Fig.18 Behaviour of Type C 2 4 6 8 1 Graph.3 Type C
Type A (Graph.1) is almost the same load displacement curve compared with parallel test pieces of earlier studies (Fig.3) until maximum load. The failure is not as brittle comparing with only using wood parallel to the Big Screw. So it seemed that cross lamina worked and the result was increased ductility. However, splits occurred to all Type A test pieces like Fig.16. The splits of all test pieces occurred in the thickness direction. It seems to be important to prevent these splits by cross direction lamina. To reinforce the section, we changed the combination of lamina positions to Type B. As for Type B, the splits in thickness direction like Type A didn t happen to Type B. So it seemed that two lines of cross direction laminas around Big Screw could hold effectively in width direction. However, another splits occurred in width direction, Fig 17. This result indicates the need to restrain in thickness direction. As a next step, we used small screws for Type C. Type C didn t have splits in both width and thickness directions, Fig.18. Displacement (.8Pmax) of Type C was 6.81 mm. The ductility was much better comparing with Type A and Type B. The small screws that were screwed in thickness direction seemed to be able to hold the section together without any cracks occurs. As a result, cross direction lamina could work with enough ductility. The rigidity of Type C was 157.2 kn/mm. This was higher value and 1.13 times standard parallel test made by all parallel lamina test pieces, Area ratio %. The rigidity of Type B was 123.9 kn/mm and.89 times lower than all parallel lamina test pieces. 3. Discussion Test pieces Name Maximum Rigidity (kn/mm) Displacement (.8Pmax) (mm) Type A 172.2 152.7 1.47 Type B 163.3 123.9 3.71 Type C 159.1 157.2 6.81 Using wood parallel to Big Screw you will have a joint with high rigidity and high maximum load but a brittle failure. Using wood in right angel to the Big Screw you will have a joint with low rigidity and a lower maximum load but high ductility. Designing timber frame systems it is important to have a system with high rigidity but it is also important to have systems that have high ductility to avoid brittle failures. To control and anticipate the behaviour of the joints can be difficult. In these experiments, we knew fundamental behaviour and performance of Big Screw joint that adopts Cross Laminated Panel. The possibility that the joint has high rigidity and high ductility was confirmed. As a next stage of this research, we are going to get the performance about Big Frame column and beam joint frame system that reflects this research results. And we will study the influence of the Area ratio. Finally, Big Frame system can be used for even longer spans. 4. Conclusion Table.1 Test result of each Type. When Big Screw joint utilities adopts the material that combines parallel and cross fiber laminas, the joint could have the ideal performance from both parallel and cross direction. In this study, we had confirmed this by using Cross Laminated Panels. The performance of this joint is caused by the combination of parallel fiber and cross fiber direction laminas. This is influenced by Area ratio of fiber directions around Big Screw. Improving the effectiveness of cross fiber direction by reducing the risk of splitting the effectiveness of cross laminated panel is improved. Based on experimental test results following conclusions can be made -Using Big Screw in combination with wood fibers parallel and cross direction you can receive a joint with both high rigidity and ductility -Using Area ratio method for designing combinations of parallel and cross direction can be a reliable method. - Reinforcement with small screws is a good method for increase the capacity for five layer boards. - It is possible to control and anticipate the behaviour of the joints made of Big Screw using different combination of cross laminated boards.
5. References [1] Hideyuki NASU., Hiroki ISHIYAMA., Norihito YAMAMOTO., Mayuko TAKAOKA., Tatsuya MIYAKE., and Hiroyuki NOGUCHI., Shaking table test of full scale 3-story timber frame, AIJ JOURNAL OF STRUCTUAL AND CONSTRUCTION ENGINEERING, No. 617, July 27, pp. 129-135. [2] Hideyuki NASU., Hiroki ISHIYAMA., Norihito YAMAMOTO., Mayuko TAKAOKA., Tatsuya MIYAKE., and Hiroyuki NOGUCHI., DEVEROPMENT OF TIMBER FRAME STRUCTURES WITH THROUGH BEAM TYPE JOINT USING LARGE DIAMETER BOLTS, AIJ JOURNAL OF TECHNOLOGY AND DESIGIN, No.22, Dec. 25, pp. 3-24. [3] Hideyuki NASU., Hiroki ISHIYAMA., Tatsuya MIYAKE., Norihito YAMAMOTO., Mayuko TAKAOKA., and Hiroyuki NOGUCHI., Development of Timber Frame Structure Part 1 ~ Part 5, AIJ Summaries of Technical Papers of Annual Meeting 25 C-1 StructuresⅢ, September 25, pp. 177-186. [4] Hideyuki NASU., Hiroki ISHIYAMA., Tatsuya MIYAKE., Norihito YAMAMOTO., Mayuko TAKAOKA., and Hiroyuki NOGUCHI., Development of Timber Frame Structure Part 6 ~ Part 8 Shaking Table Test of Full Scale 3 story Timber Frame, AIJ Summaries of Technical Papers of Annual Meeting 26 C-1 StructuresⅢ, September 26, pp. 163-168. [5] Keiichi TSUBOUCHI., Hideyuki NASU., and Hiroyuki NOGUCHI., Experimental Study on Moment Joint Using Large Diameter Bolt with Double Screw and Cross Laminated Panel, AIJ Summaries of Technical Papers of Annual Meeting 27 C-1 StructuresⅢ, September 27, pp. 65-66.