THE BENDING BEHAVIOR OF COMPOSITE TIMBER-STEEL BEAMS

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 4, April 2018, pp , Article ID: IJCIET_09_04_088 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed THE BENDING BEHAVIOR OF COMPOSITE TIMBER-STEEL BEAMS Ashraf A. M. R. Hiswa and Oday M. Albuthbahak Faculty of Engineering, University of Kufa, Najaf, Iraq ABSTRACT The rising need for utilizing alternative construction materials has promoted use timber as a construction material. Composite elements from available timber and other strengthening materials can be produced in order to find elements have strength similar to those of bigger size. Six timber beams have been strengthened with two steel channels and then tested. A finite element method was undertaken besides experimental tests as investigation methods. This study is conducted to investigate the bending behavior of composite timber-steel beams and also to enrich the research field regarding the investigation of such beams. According to the experimental tests, it was evident that both of the timber cracks and steel channels bond arrangement have strongly affected specimens behavior. The increases in the bond between the two channels have increased the load carrying capacity and the ductility for the specimens. Finite element method has showed a good tool in investigating the composite timber-steel beams behavior. Keywords: Bending behavior; Composite elements; Timber-steel beam; Finite Element Method Cite this Article: Ashraf A. M. R. Hiswa and Oday M. Albuthbahak, The Bending Behavior of Composite Timber-Steel Beams, International Journal of Civil Engineering and Technology, 9(4), 2018, pp INTRODUCTION The rising need for utilizing alternative construction materials has promoted use timber as a construction material. Timber is one of the new building materials has recently utilized in construction process in Iraq. It is considered as a natural cheap material in comparison with other construction materials where solar energy is capable of producing it and wood can be obtained from forest which needs low amounts of power for timber production (Isopescu etc., 2012). The wood growth is influenced by the environmental conditions which widely affect the mechanical and physical properties of timber. The physical and mechanical properties of timber vary according to tangential, radial and longitudinal axes of its element, therefore it is considered as an orthotropic material (Isopescu etc., 2012). In ancient times, timber was the editor@iaeme.com

2 The Bending Behavior of Composite Timber-Steel Beams main structural elements in many structures due to its aesthetic properties. Besides, it remains beneficial building material in limited resources countries. Timber has less strength in comparison with other construction materials such as steel and reinforced concrete, therefore bigger size of timber elements, which are difficult to be obtained, are required if better strength is obtained (Yusof, 2010). Alternatively, composite elements from available timber and other strengthening materials can be produced in order to find elements have strength similar to those of bigger size. (Bravo etc.), (Bravo etc., 2012) and (Tavoussi etc.) have discussed the enhancement of load carrying capacity for timber beams reinforced with steel of different forms. This paper was conducted to study the bending behavior of timber beams strengthened with steel channels at both top and bottom of them and also to enrich the research field regarding the investigation of such beams. The available timber in Iraqi construction section was utilized in this research which was the red and white timber. Eight beams, with two meters long, have been tested in the scientific laboratories of the university of kufa. One sample of each timber types was tested to obtain the pure timber load carrying capacity. The other six beams have been strengthened with two steel channels and then tested. Four point load technique has been utilized to study the bending behavior of the beams. Mid-span displacements have been taken during the tests to draw load displacement diagrams. 2. BEAMS SPECIMENS AND GEOMETRY The workshops of University of Kufa were utilized to build up the test specimens. The lengths of all specimens were two meters and they all have 145 mm by 75 mm cross section for timber. The type of timber was red and white timber which is the available imported timber to Iraq. The wood beams were strengthened with steel channels at both top and bottom of them, all channels were C8, having 80*40 mm for outer dimensions with 8 mm thick for webs and 8 mm for flange. Both top and bottom channels were connected together by utilizing tying steel plates, of (4) mm thick, welded to them. The measurements and the arrangements of tying plates were different from beam to another in order to observe the effect of channels bond on the beams' load carrying capacity. Besides the tying plates, the channels have been connected with the timber by utilizing steel bolts of 11 mm in diameter spaced at 300 mm. Table (1) illustrates the properties of the tying plates, bolts arrangements and the timber type for each beam. The channels have been set in way that the total depth of the combined beam is 225 mm including two channels of 40 mm in depth for each and timber beam of 145 mm in depth. Figure (1) illustrates a cross section of the combined beam. Additional plates, of 4 mm thick, have been added to the combined beams at the load concentrations points which are the positions of applying loads and the reactions. The width of theses plates were 80 mm to fit the channel width and the length was 100 mm. The purpose of theses added plates was to spread the stress on the channels and prevent the possible local buckling. Figure (2) illustrates additional plates arrangements editor@iaeme.com

3 Ashraf A. M. R. Hiswa and Oday M. Albuthbahak Figure 1 Cross Section Figure 2 Additional Plates Arrangements Table 1 Beams Details editor@iaeme.com

4 The Bending Behavior of Composite Timber-Steel Beams 3. EXPERIMENTAL TESTS The specimens had been built up at the laboratories of university of Kufa before tested. The tests have been performed via applying loads by a hydraulic pump through a steel frame fabricated to work with the pump. The hydraulic pump is also equipped with loading gauge and can apply load up to 500 kn. Figure (3) illustrates the machine test. Figure 3 Test Machine The specimens have been set on the base plates and the load applied on the loading plates in order to prevent the local buckling as mentioned previously. During the test, the vertical movement for the specimens was prevented by the machine structure and the machine test applies loads vertically upwards. A stiff steel tool was between the loading point of the machine test and the loading plates of the specimens in order to transform the load from the machine test to the two loading points of the specimens. The loading plates have been set at L/3 from the specimens ends in order to achieve the four point load technique. During the tests, the mid-span displacements for the specimens were measured by utilizing a gauge with accuracy of (0.01) mm. The gauge was set under the specimens at their mid spans. The purpose from utilizing the gauge was to draw the load-mid-span displacements curves where the displacement data were taken coinciding with the loading gauge. Figure (4) illustrates the displacement gauge editor@iaeme.com

5 Ashraf A. M. R. Hiswa and Oday M. Albuthbahak Figure 4 Displacement Gauge 4. FINITE ELEMENT MODEL Besides experimental tests, finite element models have been made for the combined beams through utilizing ANSYS 12.0 software program. The reasons behind utilizing the finite element method in the analysis were firstly, to compare between its results with the experimental results in order to see whether this method can predict the actual behavior of the specimens or not and secondly, to predict the loading capacity for the specimens as steel only without the timber beams to see the influence of the timber and the steel channels behaviors on each other. Only half of the specimens have been modeled in the ANSYS because of the symmetry of them. According to (Čecháková et al, 2012) in their research, SOLID45 and SHELL63 elements have been utilized to model timber and steel respectively. A regular meshes have been utilized for timber, channels, tying plates and additional plates (Zhu, 2003). Figure (5) illustrates a finite element model. (a) Model of Combined Beams editor@iaeme.com

6 Load (kn) Load (kn) The Bending Behavior of Composite Timber-Steel Beams (b) Only Steel Frame Model Figure 5 Finite Element Model. a) Model of Combined Beams, b) Only Steel Frame Model 5. RESULTS AND DISCUSSION 5.1. Experimental Results The collected data from the experimental tests were the actual load carrying capacities for the specimens and the mid-span displacements values. The actual load capacities were specified for specimens when the applied load decreased suddenly with the mid-span increase. After the tests had been completed, load-displacements curves were drawn for specimens. Figure (6) illustrates the load mid-span displacement for specimens. Figure (7) illustrates the failure mechanism for the tested specimens. B1-Red Wood B2-White Wood Displacement (mm) Displacement (mm) editor@iaeme.com

7 Load (kn) Load (kn) Load (kn) Load (kn) Ashraf A. M. R. Hiswa and Oday M. Albuthbahak B3-Red Wood B4-White Wood Displacement (mm) Displacement (mm) B5- White Wood B6-White Wood Displacement (mm) Displacement (mm) Figure 6 Mid-Span Displacement - Load Curves for Specimens B1 B editor@iaeme.com

8 The Bending Behavior of Composite Timber-Steel Beams B3 B4 B5 B6 Figure 7 Failure Mechanism for Specimens Besides the tested specimens, two timber beams, read and white wood, have been tested to obtain the load carrying capacity for only timber beams. The load carrying capacity for red and white timber beams were 55 kn and 28.5 kn respectively. According to figure (6) it can be seen that the relationship between the load carrying capacity and the mid span displacement is linear for both B1 and B3, which are from red timber, prior to the first yield point. Then the beams resisted loading up to the failure point with an increase in mid span displacement ratio. The load carrying capacity for B3 was more than B1 and this may be attributed to the tying plates of this specimen which are spaced at 200 mm while B1 had only 3 tying plates spaced at 900mm. The rest specimens were from white timber. B2 had the least load carrying capacity and had first yield point at 40 kn then it resisted the applied load with increase in the mid span displacement ratio up to the failure point 60 kn. B4 also changed its behavior after the yield point which was 55 kn then it resisted the applied load more than B2 up to the failure point 72 kn. The difference in the behavior of B2 and B4 may be attributed to the tying plates of these specimens where B2 had tying plates spaced at 300 mm while the tying plates of B4 spaced at 220 mm which is nearly the total depth of the specimens. Both B5 and B6 had showed high load carrying capacity in comparison with other specimens and no sudden failure has occurred in them. B5 had more load carrying capacity than B6 and this may be due to the tying plates of them where B5 had tying plates of 700 m in length and B6 had 300 mm long tying plates at their ends. Local buckling has been noticed in the tying plates for B5 and B6 and this may be due to the high load carrying capacity for them which induced high stress in tying plates resulting in local buckling in them as illustrates in figure (8). Cracks have been noticed in some places of the utilized timber beams. The cracks arrangements have affected the behavior of the specimens where the failure mechanism of the specimens has been controlled by them as illustrated in figure (7). No failure has occurred in timber before steel in B4, B5 and B6, while in B1, B2 and B3 the stresses induced in timber have increased the small cracks, localized at the middle, up to the failure of specimen editor@iaeme.com

9 Ashraf A. M. R. Hiswa and Oday M. Albuthbahak Figure 8 Local Buckling in tying plates 5.2. Finite Element Analysis Results The tested specimens have been modeled in ANSYS 12.0 software to analyze them by finite elements method. Loads have been added to the modeled specimens gradually till the steel channels reached the yield point. The load carrying capacity, for each specimen, was specified at this point. Then, the models have been loaded again with loads similar to failure loads specified in tests. Another models have been performed for specimens excluding timber materials from them. The only steel models have been loaded up to the yield of the steel frame to obtain the loading carrying capacity for them. Specimens No. Load (kn) Tension Table 2 Finite Element Analysis Results Steel Stress Steel + Timber Compression Tension Timber Stress Compression Load (kn) Steel Only Tension Steel Stress Compression B B B B B B By looking at figure (6) which illustrates mid-span displacement - load curves for specimens, it can be seen that there are some changes in curves paths. The points where the curves changed sharply were considered yield point for the specimens. The applied test loads at the yield of specimens and the failure loads for specimens have been used in the finite element method to analyze the specimens. Table (3) explains these results editor@iaeme.com

10 The Bending Behavior of Composite Timber-Steel Beams Table 3 Finite Element Analysis Results for Tests Loads Specimens No. B1 B2 B3 B4 B5 B6 Load (kn) Load Case Tension Steel Stress Compression Tension Timber Stress Compression 55 yield failure yield failure yield failure yield failure yield failure yield failure Results Comparison The results obtained from both experimental and finite elements analysis for specimens are summarized in table (4). Table 4 Finite Element Analysis and Experimental Tests Results Specimens No. Load Case Experimental Tests Finite Element Method Steel + Timber Steel Only Load (kn) Load (kn) Load (kn) yield B1 failure 72 yield B2 failure 60 yield B3 failure 80 yield B4 failure 64 yield B5 failure 194 yield B6 failure 126 By looking at table (4), it can be seen that B1, B2 and B3 have showed yield load in experimental tests less than those predicted by finite element method but the failure load for them are quite similar to the yield load of finite element analysis. This leads to conclude that the specified yield points on curves for these specimens were just an increase in the specimens mid span displacement and the steel channels have not yielded as shown in table (3) where the steel stress, at yield points, were Mpa, 169 Mpa and Mpa for B1, B2 and B3 respectively. The increase in the specimens mid span displacement, for these specimens, may be attributed to the timber cracks at the middle of these specimens, as shown in figure (7), which were magnified with loading increase and triggered changes in the editor@iaeme.com

11 Ashraf A. M. R. Hiswa and Oday M. Albuthbahak specimens behavior. The rest of the specimens, B4, B5 and B6, have showed yield loads greater than finite element analysis results. Although the high stress in channels at yield loads as shown in table (3), no noticeable increase rate in the mid span displacement against load for these specimens. This may be attributed to the strong tying plates utilized in these specimens as previously discussed. Although B3 has tying plates spaced at 200 mm which is similar to B4 which has tying plates spaced at 220 mm, B3 has not resisted extra load beyond yield such as B4. This may be attributed to noticed timber cracks in B3 where it had cracks at the middle which is the position of maximum moments while the timber in B4 has failed as a brittle material and no growing cracks have been noticed during loading. By looking at the load carrying capacity for the specimens, it can be seen that all the timber beams have gained more loading capacity when steel channels were utilized. The ratio of the increase in loading capacity defers for the specimens. The increase ratio for B1 and B3 were 1.31 and 1.45 respectively. It is obvious that the loading capacity has increased when the bond between the channels has enforced by adding extra tying plates. Also, the stress induced in timber due loading has decreased when the bond has enforced as shown in table (3). B2 and B4 have convergent values for the ratio of the increase in load carrying capacity with 2.1 and 2.24 for B2 and B4 respectively. B4 has tying plates space at 220 mm at its extreme thirds and the tying plates at the middle third spaced at 330 mm while B2 has tying plates spaced at 300 mm for all the beam. Although the middle third for B2 and B4 has nearly the similar bond, B4 has showed more loading capacity ratio due to the bond increase between the channels at the beam ends. The bond increase effect is also obvious in B5 and B6 where the tying plates were continuous of length of 700 mm and 300 mm for B5 and B6 respectively which resulted in having the maximum values for the ratio of the increase in load carrying capacity with 6.8 for B5 and 4.42 for B6. Table (2) shows that the only steel models have yielded in compression except for B6. While the models formed from steel and timber have yielded with steel stresses defer from those for only steel models. This may be explained that the steel and timber models have behaved as one beam therefore the stress distribution along the cross section have changed. Also, it can be seen from table (2) that the stress induced in timber under steel yield has decreased with the increase in the bond between the steel channels where the maximum timber stress has noticed in B1 which had the weakest bond while the minimum timber stress was in B5 which had the strongest bond among the specimens. 6. CONCLUSIONS In this research, six composite timber-steel beams were tested by utilizing four-point technique. A finite element method was undertaken besides experimental tests in order to investigate the behavior of these beams. Two steel channels have been utilized at top and bottom of timber cross section in order to reinforce it. Different steel tying plates arrangements between the two channels have been adopted for the specimens. According to the experimental tests, it was evident that both of the timber cracks and steel channels bond arrangement have strongly affected specimens behavior. The relationship between the mid span displacement and the load was liner prior to the yield point for the specimens. The existence of cracks in the middle third of the beams has led the specimens to yield before the steel channels reached yield stresses. While the yield load of the specimens, with no cracks in the middle third, have increased with the increase of the bond between the channel. It was also found that the increase in the bond between the two channels have increased the load carrying capacity and the ductility for the specimens. Therefore, strong bond can be recommended for decreasing the cracks and knots effects on timber loading capacity and enhancing the resistance of structures under seismic. Red timber beams had more loading editor@iaeme.com

12 The Bending Behavior of Composite Timber-Steel Beams capacity than white timber beams and gained less loading capacity compared to those of white timber when steel channels were utilized in reinforcing them. The stronger timber is used, the more steel reinforcing are required to achieve best results in enhancing timber properties. Finite element method has showed a good tool in investigating the composite timber-steel beams behavior. REFERENCES [1] A. Yusof, Bending behavior of timber beams strengthened using fiber reinforced polymer bars and plates, PhD dissertation. Faculty of Civil Eng., Technology University of Malaysia, Malaysia., [2] V. Čecháková, M. Rosmanit, and R. Fojtik, FEM modeling and experimental tests of timber bridge structure: Procedia Engineering, 40, 79-84, [3] C. G. Bravo, F. A. Martitegui and R. D. Barra, Bending reinforcement of wooden beams with steel cross sections, Proc. tenth World Conf. on Timber Engineering. Miyazaki,. Japan [4] C. G. Bravo, J. Claver, R. Alvarez, and R. Domingo, Improvement of the Mechanical Properties of Formed Steel Cross Section Profile for Timber Upgrading. In Advanced pp , Materials Research [5] D. Isopescu, O. Stanila, and I. Asatanei, Analysis of Wood Bending Properties on Standardized Samples and Structural Size Beams Tests Buletinul Insititutului PolitehnicDIN Din Iasi, Publicat de Universitatea Tehnică Gheorghe Asachi din Iaşi., [6] K. Tavoussi, W. Winter, T. Pixner, and M. Kist, Steel reinforced timber structures for. engineering multi storey buildings In World conference on timber [7] E. Zhu, Modelling the structural behaviour of OSB webbed timber I-beams, PhD dissertation. University of Brighton, editor@iaeme.com