Structure and Performance of Timber for Construction Purposes - Research Paper Example

RUNNING HEAD: Structure and Performance of Timber for Construction Purposes Experiment on Structure and Performance of Timber: Course Code: Lecture’s Name: Date of presentation Contents Contents 2 Abstract 3 Introduction 4 Objectives 6 Hypotheses 6 Static Bending Tests 6 Methods and Materials 6 Results 8 Discussion 10 Conclusion 15 References 16 Abstract Timber can be divided into two botanical classes based on their particular cell structure; namely: Angiosperms, and Gymnosperms. Angiosperms are mostly hardwood and include mostly the deciduous trees while Gymnosperms are generally softwood and mainly consists of the coniferous trees. The flexural deflection was carried out by using two hardwood and softwood specimens. Results showed that the properties of wood do no necessarily depend on the tree category but also the effect of other parameters such as age, and moisture content. Introduction All over the world, timber has been traditional construction material for both external and external uses. The choice of timber for construction is usually determined by the following factors; Strength of the timber type, appearance of the final product, dimensional stability which is influenced by the moisture content of timber, sizes, species and sections available in the market, ease of preservation and durability, and finally cost. Although the distinction does not depend on the particular physical properties of each group, it is worth noting that hardwoods in large cases tend to be more durable and stronger than softwoods. Softwoods on the other had are easily available, easier to work on, and are relatively cheaper when compared to the hardwood timber (Parsapajouh, 1998). The tree trunk performs three main physical functions while the tree is growing. Firstly, the trunk offers support to the upper part of the tree i.e. the crown; it contains xylem and phloem vessels that conduct water and mineral salts from the roots to the crown, and lastly it is responsible for both manufacturing and storage of food necessary for growth. The inner layers of the trunk, the heart wood is generally dormant thus providing only the role of structural support. The peripheral layer i.e. the sapwood performs mainly the role of material transportation. The outer layer i.e. the bark produces resin and gum material that help to protect the inner reproductive layers from both insect and fungal attack. Each layer of the tree trunk consists of cells which develop on top of each other annually. These annular cells consist of layers of cellulose polymers- complex hydrocarbon chains consisting of hydrogen, carbon and oxygen. A more complex polymer forms about 10 to 35 percent helps to boost strength (Erdogan, 2002). Softwoods consist of two different types of cells. Tracheids cells form the bulk share of the cells. The length of tracheid cells is between 2 to 4mm, with a ratio of length: diameter of 100:1. They are aligned vertically along the trunk to offer both support and acting as a transport medium for both food and water. Parenchyma cells are block shaped cells positioned along horizontal fibers called rays. Parenchyma cells which measure approximately 30 by 200µm help to store food as well as providing of lateral support. Hardwoods have four types of cell. Tracheids and parenchyma cells in hardwood serve similar factions as to those in softwood. However, parenchyma cells are both horizontally and longitudinally arranged along the tree trunk. Additional lateral support is provided in the hardwood by long thin cells called fibers. The fiber cells also have a large length: length ratio of 100:1and a length of between 1 to 2 mm. The fibre cells also act as transport medium by using of pores or vessels of between 0.2 and 1.2mm length and o.5mm diameter. Growth generally occurs mainly during the spring season. At this time conduction of material is very crucial as a lot of cell growth tend to happen at this time. Reduced growth rate characterized by dense tinier cells is witnessed during summer season, while autumn season and through winter season; growth is characterized by a rather dormant phase. The created alternating series of summer and springwood layers, give rise to the annular growth rings which characterized the tree trunk. This annular growth layers are known as cambium and are located between woody part and the protective layer of stem trunks and branches. The experiment seeks to examine the properties of timber as used in construction purposes. In order to carry out the experiment, tests will be carried out on the provided samples of both softwood and hardwood and the results will be compared. Tests include; examination of the cellular structure of the thin sections of the samples and flexural load frame tests to compare Objectives The experiment below seeks to determine the structural difference of hardwood material and softwood materials by subjection then to deflection though subjection of forces perpendicular to grain direction until they fail. To achieve the above, two hardwood specimens its Ashwood and Meranti wood samples while softwood specimen used were pine and redwood samples. Hypotheses H0- Hardwood trees have low deflection and high failure force than softwoods H1- Hardwood trees have high deflection and high failure force than softwoods Static Bending Tests Methods and Materials In order to perform the tests samples of timber, hand lens, Deflection test gauge and Flexural load flame together with a load ring were used. The distance (L) between the two supports on the load frame was measure and recorded. By use of a hand lens, the grain structure at the ends of two softwood timber specimens and two hardwood specimens were examine. Samples of Pine, Meranti, Ash and Red wood timber specimen were used. Based on the observed grain structure, the orientation that yielded greater deflection was decided. NOTE: Maximum resistance is achieved when force is applied perpendicularly to the grain. The specimen was first secured firmly on flexural load frame. The deflection gauge was centrally placed and ensured that it stayed free from the sample; the dials were adjusted to zero. An initial load of 50N was applied to the sample and the resultant deflection as a result of load applied was recorded on the first column of the provided table. The load increments of 100N were added i.e. 150N, 250N, 350N and 450N, and their respective deflection values measured and recorded in the results on the same first column. The load was then released. The same procedure was then repeated again with the same sample and starting again with a 50N load with 100N cumulative increments. The deflection as a result of the loading was recorded on the second column of the same table. The flexural load frame was then relieved off the load. The process was then repeated by loading the same specimen for the third time. The deflection readings are then recorded on the third column of the same table. The load was however not released. The load was then increased cumulatively by 250N at loads of 700N and 950N and so on until the specimen failed. The load at failure and the deflection readings of the additional loading were also recorded on the third column of the Static Bending Test table. Results The results were recorded in the tables below Length between supports (L) = …285(mm) specimen dimensions 300mm x 50mm Specimen pine ash Load (N) Deflection (mm) Deflection (mm) Load (N) 1 2 3 Average 1 2 3 Average Deflection 50 0.01 0.014 0.014 0.013 0.008 0.011 0.011 0.010 150 0.028 0.032 0.033 0.031 0.026 0.027 0.028 0.027 250 0.046 0.05 0.051 0.049 0.042 0.044 0.044 0.043 350 0.064 0.068 0.069 0.067 0.059 0.06 0.061 0.060 450 0.083 0.086 0.087 0.085 0.076 0.077 0.077 0.077                                     700     0.16       0.134   950     Fail       0.28   1200             Fail   1450                 1700                 1950                 2200                 2450                 2700                 Failure load (N)     800       1100                     Specimen Merratti Red wood Load (N) Deflection (mm) Deflection (mm)   1 2 3 Average 1 2 3 Average 50 0.008 0.011 0.011 0.010 0.003 0.008 0.008 0.006 150 0.021 0.023 0.023 0.022 0.015 0.019 0.019 0.018 250 0.033 0.034 0.034 0.034 0.027 0.029 0.03 0.029 350 0.044 0.045 0.045 0.045 0.036 0.038 0.038 0.037 450 0.055 0.055 0.055 0.055 0.046 0.047 0.047 0.047                                     700     0.084       0.07   950     0.113       0.092   1200     0.147       0.111   1450     0.193       0.133   1700     0.28       0.157   1950     Fail       0.195   2200             0.252   2450             0.35   2700             Fail   Failure load (N)     1800       2650   Discussion The graph of Load (N) against Deflection (mm) for Pine Tree Sample The graph of Load (N) against Deflection (mm) for Pine Tree Sample The graph of Load (N) against Deflection (mm) for Meranti Tree Sample The graph of Load (N) against Deflection (mm) for Red wood Tree Sample The slope of the graph give the respective modulus of elasticity. From the graph, the modulus of elasticity for 9.7N/M for Ash tree, 8.8N/M for Pine tree, 5.97N/M for Red wood tree and finally 4.65N/M for Meranti. According to (Dinwoodie, (2001), the tensile strength exihibited by a timber sample with straight and clear-grains is far much higher its compressive strength when it is measured parallel to the grain direction. This is due to the fact that compressive forces causes plastic crushing and buckling of the grain fibres. However a structural timber sample with discontionous and distorted grains, knots and dead tissue is more likely to fail at this weak points even before the yield point of grains is attained. Failure as a result of tension is mainly due to to the resultant shear stresses across the fibres and cells. Bending stresses are very common in structural timber application and, therefore flexural strength and modulus of failure is often between compressive and tensile strength ((Dinwoodie, (2001). Bending test seeks to measure slowly the deflection as a result of gradually applied load. Bending stresses are subjected to the specimen perpendicular to the wood grains. This is because in most situations where wood is applied as structural beam. This type of experiment is known as a 3-point flexural test. Since equivalent sectional sizes i.e. 30cm X5cm, were used in all the four samples; their relative flexural strength can be obtained by plotting Load (N) against particular deflection in mm obtained from the various samples. The maximum flexural strength i.e. maximum load before failure; was attained from Ash wood specimen at 2650N. This was then followed by Pine at 1800N, Red wood sample at 1100N and finally Pine tree sample at 800N.The results are mixed up and do not follow the broad categories of hardwood and softwood. Schematic representation of Sheering Forces and Resultant Moment in a 3-point flexural test Theoretically, deflection is caused by both shear and bending forces. However, where the length to height ratio is greater than 15, the stress related contribution is often ignored. The ration for this case of specimen selected is 295/5=57. There the percentage contribution of shear to deflection is negligible when compared to that of bending forces (Taylor, 2002). From the stiffness attributes of timber recorded above we can justify the various application of the tree species in building and construction. Ash for instance has the highest modulus of elasticity; thus making it to be the stiffest among the specimen tested. Based on the above, we realize that due to its strength and durability, ash wood is mainly used in beam and tie construction on areas where it will be subjected to considerable loading. It is often though, substituted in favor of oak. According to (Dumont, 1992), ash tree as a hardwood is very strong hard and dense wood material, for instance it has a density of 670kg/m2 at 20 percent moisture content. Due to its high elasticity, ash wood is widely used for making bows, baseball and cricket bats, and tool handles due to its resilience, toughness, and high strength. In the old days ash tree was used in the manufacture of rigid structural members of car bodies and carriages. However, this material due to its stiffness it tends to be brittle thus making it difficult to work on. In fact even today, Morgan Motor Company is still using the Ash wood for manufacturing suspension frames as part of suspension system thus simplifying the manufacturing process (Taylor, 2002). Meranti timber on the other hand shows completely opposite characteristics. The timber possesses the highest defection characteristics with the lowest modulus of deflection. This characteristics makes the timber is least applicable in areas which are expected to be frequently loaded. Meranti timber also shows the least failure load at 800N; thus making it of no much use as a structural member in construction industry. It is therefore mostly application in furniture making and in processing of fiber boards for light job application. The rest of the specimens fall in between. Conclusion The results do not support any of the stated hypotheses. This means that apart from the broad categories of hardwood and softwood it is important to include other factors that can affect the performance of the sample for instance age, moisture content and seasoning process carried out on the sample. The moisture content should be as close to each other as possible. Also the specimen age should be considered prior to carrying out the tests for instance a softwood will most likely posses better properties than hardwood of the same age. References Dumont, D J. (1992). "The Ash Tree In Indo-European Culture". Mankind Quarterly 32 Dinwoodie, J.M., (2001) “Construction Materials-Their Nature and Behavior”, Edited by Illston J.M, and Domone P.L.J. Spon Press, Taylor & Francis Group, London and New York, Erdogan, T.Y. (, 2002), “Materials of Construction”, METU Press, Ankara Lab Report and Lecture Notes Parsapajouh, D., 1998. Wood Technology. 4th Edition University of Tehran,. Iran Taylor, G.D., (2002 )“Materials in Construction Principles,Practice and Performance”, Pearson Education Limited, Malaysia. Read More