Rjeas Research Journal in Engineering and Applied Sciences 1(3) Rjeas

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1 Rjeas Research Journal in Engineering and Applied Sciences 1() Rjeas Emerging Academy Resources (2012) (ISSN: ) COMPARATIVE OVERVIEW OF TIMBER AND STEEL ROOF TRUSS SYSTEMS 1 Ezeagu C. A ; 2 Umenwaliri S.N., Aginam C. H and Joseph C.A. Department of Civil Engineering, Faculty of Engineering, Nnamdi Azikiwe University, P.M.B. 5025, Awka. 2 Department of Civil Engineering, Faculty of Engineering, Nnamdi Azikiwe University. P.M.B. 5025, Awka. Department of Civil Engineering, Nnamdi Azikiwe University, P.M.B. 5025, Awka. Corresponding Author: Ezeagu C.A ABSTRACT In this study, a 1.6m span, twelve web kingpost model trusses were fabricated using timber and steel, also fabricated were twelve monochord and double chord trusses with different connection technique in timber and in steel and then eight truss models in timber and steel with four different shapes namely: - Howe truss system, Hip-stop down truss system, Dual pitch truss system, and Parallel chord truss system. These truss systems were fabricated in timber and steel of 2 (0.08 inch) thickness and 25 (0.98 inch) width. Each of the chords (bottom) had a length of 1600 (6 inches) and height of 200 (7.9 inches). During the course of the experiment, the trusses systems were loaded and the deflections at the nodes along the bottom chord of the trusses were recorded. It was observed that the maximum deflection occurred at the mid-span for all the trusses tested and of different shapes and configuration. The results show that double chord trusses resist more load than the monochord truss system of both timber and steel. And also the cost of fabrication timber trusses is cheaper than steel trusses. However all the systems satisifies the deflection requirement both of short and long spans.at the end of this study, it was obvious that the type of material, chord model system, configuration of the shape and connection techniques had a direct or indirect effect on the load bearing capacity, deflection of truss systems and cost of the truss fabrication. Emerging Academy Resources KEYWORDS: Configuration, Cost, Deflection, Shape, Trusses INTRODUCTION It is a popular adage that people usually say I want to put a roof over my head However; this popular statement involves a lot. To a structural engineer, the statement means to analyze, design and construct with aesthetic, economy and safety a roof system. To over a long span a lot of material may be wasted supporting only the beams self weight. This is because; the bending moment capacity is most efficiently governed by the depth of the section. According to Ezeagu et al (2009); for simply supported beams; In steel structures; talk about aesthetic is to talk about shape and m= wl 2 1 configuration, to talk about economy is to talk about the cost of the roof and to talk of safety brings us to 8 z xx = m 2 deflection as one of the serviceability requirements. In Nigeria, many of these roofs are fabricated with f y And z xx = bd 2 either steel or timber. This research work focuses on 6 several research works on cost comparison of modeled timber and steel trusses and the comparative effects of shape configurations on the deflections of Where; m=bending moment; F y = max. Shear stress of the section; d = effective depth of the section; Z xx = section modulus; timber/steel trusses. In general, a truss system can be described as a triangular frame-work consisting of essentially, axially loaded members {Lau Wei Theing; 2005}. Trusses can also be described as braced frame-works consisting of members (or bars) connected together at joints {or nodes} {Weniyatra 2004}. Trusses act like deep beams a beam becomes stronger and stiffer as its depth increases {Sattish et al; 2009}. But, when a deep beam carry a light load It is obvious that the effective depth d; governs the bending moment capacity. The main objective of this research is to determine the effect of shape and configuration on steel trusses. It is geared towards the determination of the most suitable truss section in terms of failure strength (deflection) and load bearing capacity. The scope of this research involves the analysis, design and fabrication of four (4) individual 177

2 types of general truss system with different shapes and configuration. This research is to be carried out using all plane two-dimensional trusses with supports at both ends of the span and assumed to be pinned. All the trusses are to bear the concentrated loads at the nodes only. The structural analysis is carried out manually using code BS 5950, A formula is generated, relating actual truss sizes to modeled truss shapes. The modeled shapes are to be loaded and their deflections noted respectively. The loads will be gradually increased {using a sequence} until the truss system deflects. LITERATURE REVIEW Ohahuna (2008) in a work titled Short term and long term comparative cost analysis of timber and steel roof trusses carried out a cost comparison study between timber and steel truss frame used in residential and coercial buildings in Benin city to show the cost effectiveness of each material and connection type within the short term and long term periods. Ohahuna (2008) fabricated samples of models of trusses and prepared the Bill of Engineering Measurement and Evaluation (BEME) and used it for the cost comparison. Four samples of 1.6m span, six web king post trusses were designed in accordance with the standard truss method, all fabrications were single unit types and the fabrications were carried out in two categories for both nail connection and bolts and nuts connection. The four fabricated trusses can be described as follows: Timber roof trusses pitch with nail connections Timber roof trusses pitch with bolts and nuts connections Steel roof trusses pitch with arc welded connections Steel roof trusses pitch with bolts and nuts connections. For the steel fabrication work, the 8.1 (1½ inch) width, flat bar was used for both connection types and for the timber fabrication work, the Bafia wood was used for both connection types at 8.1 x 25.4 (1½ inches by 1 inch) dimensions. Below is the sketch of the dimensional layout for the fabricated roof trusses in both timber and steel. In Nov 2008, a further research arose to verify Ohahuna 2008; Uwaya (2008) researched further on cost comparison of timber roof truss and steel roof truss of residential buildings Uwaya adopted Ohahuna research idea and concluded that the difference in cost between the modeled timber trusses and the modeled steel trusses is also approximately 50 percent. Ezeagu and Eze(2009) also confirm the percentage ratio. In a work titled Deflection of monochord and double chord roof truss system using timber and steel Onoyivbeta (2010) carried out a destructive experimental test on six modeled roof trusses, namely monochord steel truss with welded connection, double chord steel truss with welded connection, monochord timber truss with nail connection, double chord timber truss with nail connection, monochord timber truss with bolt connection and double chord timber truss with bolt connection. Onoyivbeta (2010) was seen as the expansion of the earlier work done by Ayodele (2009) on the topic Deflection of monochord and double chord timber truss system Ayodele fabricated eight trusses as described below; Monochord timber truss with nail connections Double chord timber truss with nail connections Monochord timber truss with bolt connections Double chord timber truss with bolt connections Ayodele used an empirical scaled equation in the modeling considering the choice of scale and properties of the prototype to those of the model. The tests by Ayodele and Onoyivweta were to note the failure loads and maximum deflections. Offor (2011) in a work titled Effect of shape configuration on the deflection of trusses carried out a non destructive experimental test on twelve number of four modeled monochord trusses namely; Howe trusses, Hip step down trusses, parallel chord trusses and dual pitch trusses. The trusses were constructed using soft wood (Obeche) of 25 x 25 (1inch by 1 inch). The objective of the test was to obtain the deflections of the aforementioned truss configurations when subjected to known load values. Offor equally adopted Ohahuna model size but fabricated different shapes of the trusses METHODOLOGY This experiment is carried out on twelve samples of four individual truss shapes and configurations. This includes; Dimensions The steel bar used has a width of 25 and thickness of 2. All the models have span of 1600 {1.6m}. The rise is 200 {0.2m}. The vertical web members which form the nodal points are placed at 200 {0.2m} centres. The diagonal webs run inbetween adjacent joints of top chord and vertical web, forming a series of interrelated triangles. Consequently, Ayodele (2009) and Onoyivbeta Elizabeth (2010) adopted Ohahuna (2008) models of timber and steel trusses, but considered the deflections of the models. 178

3 Fig.. A loaded truss system on a simply supported platform Table1: BEME suary result table S/No Item Description Amount 1. Fabrication cost of single unit timber truss,440 with nail connection 2. Fabrication cost of single unit timber truss 4,790 with bolted connection. Fabrication cost of single unit steel truss 5,800 with arc welded connection 4 Fabrication cost of single unit steel truss with bolted connection 9,980 Source: (Ohahuna 2008) Table 2: Suary of test result one S/No Item Description Max. Deflection 1. Monochord timber truss with 20kN nail connections 2. Monochord timber truss with 0kN bolted connections. Double chord timber truss with 0kN nail connections 4. Double chord timber truss with bolted connections 40kN Source: (Ayodele 2009) Table : Suary of test result two S/No Item Description Max. Deflection 1. Monochord timber truss 20kN with nail connections 2. Monochord timber truss 0kN with bolted connections. Double chord timber truss 0kN with nail connections 4. Double chord timber truss 40kN with bolted connections 5. Monochord steel truss with 10kN bolted connections 6. Monochord steel truss with 10kN welded connections 7. Double chord steel truss 0kN with bolted connections 8. Double chord steel truss with welded connections 70kN (Source: Onoyivweta,2010) Table 4: Suary of Deflections S/No Truss shapes Average Maximum deflections 1. Howe truss 50kN 2. Hip Step down 50kN. Parallel 50kN 4. Dual Pitch 50kN Source: Offor (2011), Ezeagu and Offor (2011). After concluding the experiment on the four different truss shapes and configuration, the results obtained for all truss models is presented in a tabular form as shown below. It was observed that the maximum deflections occurred at the mid-span for all truss models except for the pitch. However, when loaded with a load of mid-span, maximum deflection occurred at the mid-span for all truss models. Below are tables showing average maximum deflections for each truss models and their respective loadings. This is obtained by taking the average of each truss model. Table 5. Average maximum deflection for Howe truss model Truss 1 Truss 2 Truss mid-span Table 6. Average maximum deflection for hip-step down truss model Truss 1 Truss 2 Truss mid-span Source: Joseph, 2011 Table 7. Average maximum deflection for Dual-pitch truss model Truss 1 Truss 2 Truss mid-span Source: Joseph, 2011 Table 8 Average maximum deflection for parallel chord truss model Truss 1 Truss 2 Truss mid-span Source: Joseph, 2011 Since the experiment was performed on three samples of each truss shape and configuration namely: - Howe truss system, Hip-step down, Dualpitch and parallel chord system there is every tendency that the results might differ from each other. Therefore, it is of utmost importance to verify how different these results are from each other.the following table shows average nodal deflections for the four truss shapes and configurations. Table 9 Average results for Howe truss system mid- span

4 Table 10 Average results for Hip-step down truss mid- span Source: (Joseph, 2011) Table 11. Average results for Dual-pitch truss system mid- span (Source: Joseph, 2011) Table 12. Average results for Parallel Chord truss system mid- span (Source: Joseph, 2011) DISCUSSION From table 1, the total amount for steel fabrication is ( 5, ,980 = 15, 780), and the total amount for timber fabrication work is (, ,790 = 8,20). The difference in cost between steel and timber fabrications is 7,550 amounting to approximately 48% extra cost to timber truss for both material and labour lumped together. From tables 2 and, it was inferred that the deflection of timber trusses using bafia species was less than 50 for all models fabricated and tested, and that the deflection of steel trusses were less than 50 for every model considered, showing that the loading condition generated within the deflection required (L/100). Howe trusses, Hip-step down trusses and dual-pitch trusses are types of trusses which are frequently used to support roofs while the parallel chord truss systems is mostly used to support floors and also in bridges. From tables 4.1a, b and c, the acceptable deflection limits for roof support is given as, Span/60 = 1600/60=4.44 Span/00 = 1600/00 = 5. or 15 for maximum consideration From tables 5 and 6, it is observed that the maximum deflections for both Howe and Hip-step down truss configurations satisfy the deflection limits for all loading condition applied. For the Dual-pitched truss system, table 7, shows that only loadings which are 10kg and less placed at 200 centres satisfy the above condition. For parallel chord truss systems which as earlier stated are mostly used to support floors and in bridges, the acceptable limit from tables 8 and 9 include. Span/60 = 1600/60=4.44 Span/00 = 1600/00 = 5. Or 10 In this case, results from table 10 shows that only loadings of 5kg and less placed at 200 centres satisfy the deflection limit. In suary, Howe and Hip-step down truss configurations can carry loads up to midspan whereas Dual-pitch should not be loaded with more than 10kg loads placed at 200 centres. Likewise, parallel chord should not be loaded with more than 5kg loads placed at 200 centres. Finally, it can be deduced from the results of the experiment that Howe truss configuration is the best of the four truss configurations in terms of strength. Hip-step down, Dual-pitch and parallel chord follows in that order respectively. Graphs of deflections against loadings are then plotted as shown below; GRAPH 1a; Howe truss 1: Graphs of deflections against loadings 180

5 GRAPH 4.4.2a; Hip-step down truss 1: Graphs of deflections against loadings CONCLUSION AND RECOMMENDATION After thorough observations of the graphs obtained from each of the twelve truss systems and comparing the results of each shape and configuration to its average result; it could be deduced that the difference in the results were so small and therefore negligible. This shows that the level of accuracy observed during the experiment is arguably high and therefore the results is valid and can be deemed accurate. Also, comparing results from the graphs showing average nodal deflection results for the four truss systems, it can vividly be noted that Howe and Hipstep down truss systems gave similar curves and carries same range of loadings effectively and efficiently. Effect of shape and configuration on steel trusses which is aimed at determining if the shape and configuration of a truss system actually has effect on its load bearing capacity was carried out on four of the most coonly used truss shapes and configurations, namely; Howe, Hip-step down, Dualpitch and parallel chord truss systems. This study provided us with the following:- From the results recorded in table 5 to 10; with the maximum deflections being our utmost concern, it was observed that Howe truss system out of the four can be regarded as the best in terms of its load bearing capacity. During the course of the experiment, average maximum deflection values were recorded with respect to varying loading condition (table 5,6,7,8) at the end of which the Howe truss configuration was found to withstand all of such loads effectively and efficiently without exceeding the deflection limits. GRAPH 4.4.a; Dual-pitch truss 1: Graphs of deflections against loadings GRAPH 4.4.4a; Parallel chord truss 1: Graphs of deflections against loadings Hip-step down truss configuration was also found to be effective and efficient under these loading conditions (as in Howe truss systems) except that its deflection values were slightly higher than those of Howe (table 6) However, it satisfied the deflection limits for all loading condition. It can therefore be rated as the second best in terms of its load bearing capacity. Using same loading conditions for the Dual-pitch and Parallel chord, their respective deflections were noted and recorded accordingly (tables 7 and 8). It was observed that the parallel chord had the least load bearing capacity of the four truss shapes and configurations. After conducting the experiment, the deflection values obtained were compared with acceptable defection limits as stated by well known and established bodies and institution which include. British standard codes (BS 8110, 1997 and BS 5940, 2000), Department of civil Engineering, Monash University, Timber Research Association and Development of America (TRADA). It was found that:- Howe truss configuration satisfied the acceptable deflection limit of 15 for roof trusses 181

6 under long term effect for all the loading system used. Also, the Hip-step down configuration satisfied the same acceptable deflection limit (15) for loading system used. The Dual-pitch truss system on the other hand when compared against the acceptable deflection limit of 15 was found that only under loadings of 10kg and less placed at 200 centers satisfy this condition. Finally, the Parallel chord truss system when compared with the acceptable deflection limit of 10 for floors, it was noted that it will only be satisfactory under loadings of 5kg placed at 200 centres and 10kg placed at 400 centres and less. Maximum serviceability will be derived from the above truss configuration provided no alterations are made on the shape, configurations and dimension of the truss models designed by researcher (The dimensions of the model can be related to real life prototypes using the model equations and physical prototype.this calls for research funding. OBSERVATIONS Consequent upon the results of these experiment the maximum deflection for all the truss configurations except that of the Dual-pitch truss configuration occurred at the mid-span for all loading condition. However, with 50kg Load placed at the mid-span, the maximum deflection occurred at the mid-span for the four truss configurations. CONTRIBUTION TO KNOWLEDGE It has been evidently been shown that these variables i.e shape configuration, chord system, connection techniques and the type of material had a direct and indirect relationship with the deflection and cost of fabrication of the trusses. REFERENCES Ayodele (2009) Deflection of Monochord and Double chord Timber Roof Truss Systems An unpublished B.Eng work at the Department of Civil Engineering, University of Benin, supervised by Engr. Dr. C.A. Ezeagu. Ezeagu C.A and Eze A. B. K (2009) '' Comparative cost Analysis of timber and steel roof trusses'' Journal of Management and Enterprise Development. Vol. 6, No.1 pp 0-8 ISSN Ezeagu C. A. and Offor N. I (2011) ''Effect of Shape Configuration on the deflection of timber trusses'' journal of Emerging trends in Engineering and applied science. Vol2. No, pp Ezeagu. C.A and Nwokoye D.N (2009) Design in structural Timber published by Mufti books, Nigeria. 1 st edition. ISBN: Joseph C. (2011) Effect of Shape Configuration on the Deflection of Steel Trusses An unpublished B.Eng work at the Department of Civil Engineering, Nnamdi Azikiwe University Awka, supervised by Engr. Dr. C.A. Ezeagu Offor N.I. (2011) Effect of Shape Configuration on the Deflection of Trusses An unpublished B.Eng work at the Department of Civil Engineering, University of Benin, supervised by Eng. Dr. C.A. Ezeagu. Ohahuna U (2008), Short term and Long term Comparative Cost Analysis of Timber and Steel Roof Trusses An unpublished B.Eng work at the department of Civil Engineering, General Abdulsalami Abubakar College of Engineering, Igbinedion University Okada, Edo state, supervised by Engr. Dr. C. A. Ezeagu, July Onoyivweta E. (2010) Deflection of Monochord and Double chord Timber Roof Truss Systems using timber and steel An unpublished B.Eng work at the Department of Civil Engineering, University of Benin, supervised by Engr. Dr. C.A. Ezeagu. Uwaya P.I. (2008), Cost Comparison of Timber Roof Truss and Steel Roof Truss of Residential Buildings An unpublished B.Eng work at the Department of Civil Engineering, University of Benin, supervised by Engr. Dr. C.A. Ezeagu, Nov

7 Rjeas Research Journal in Engineering and Applied Sciences 1() Rjeas Emerging Academy Resources (2012) (ISSN: ) Fig 1: dimension layout Fig.2. four individual truss shapes and configurations 18