Structure and Performance of Timbers for Use in Construction - Term Paper Example

Abstract This begins by giving a brief introduction where the importance of the test is highlighted. The aim of this test was to compare properties of softwood and hardwood timber. The highest deflection of 0.35mm was for ash while the lowest was 0.16mm for meranti. Ash failed at the highest load of 2650N, while the failure loads for pine, redwood and meranti were 1800, 1100 and 800 respectively. Introduction and objectives Timber is a popular engineering material as it is used in many countries as a traditional building material. There are several factors that are considered when selecting timber that include strength, appearance, the availability of species size and sections; the durability and the ease of preservation and the cost of the timber. Timber can be broadly be classified as hardwood (Angiosperms) or softwood (Gymnosperms). The hardwoods are mainly deciduous trees while the softwoods are the coniferous trees. The classification does not put into consideration the physical properties but is based on the cell structure of the timber. However, in general it is found that hardwoods will tend be more durable and have high strength as compared to the softwoods however there are exceptions in some cases (Keith F. et al 1999). In the UK about three quarters of the timber used is softwood due to its lower cost, its availability in addition to its ease of working. Structure of Timber The trunk of the tree plays the role of supporting the crown, which is the part that performs the role of food and seed production. The trunk accomplishes the task of conducting minerals ferom the roots to the crown and acts as the store for the manufactured food. Therefore as a whole unit the trunk supports the tree structurally. The outer layer of the trunk is known a the sapwood and is responsible for conducting and storing of food while the inner part which is know as heartwood is dormant as it no longer plays any role. There is production of resin or gum in the heartwood which gives protection to the reproductive layers of the tree against insect and fungal attack. Microscopic Structure The microstructure of the trunk can be used as a basis of understanding its function. In every layer of the trunk there are cells that grow upon each other in the reproductive season of the year. The tree cells are composed of cellulose polymers chemically being made of carbon, hydrogen and oxygen linked together into long chains. In addition there is also 10 to 35% ligni-pectin which is more complex and its function is to strengthen the cell. There are two types of cell which are found in softwood timber. Most of the cells are tracheids with a length of 2-4mm and the ratio of the length to the diameter being 100:1. There is vertical alignment of the cells in the trunk and they provide support allowing food to be conducted through. In the trunk there are other cells called the parenchyma which are smaller and block like with the size of 200x30 mm. the parenchyma are located in rays which are horizontal fibres. The major function of these cells are to store food in addition of providing lateral support. There are four types of cells in hardwood, with tracheids and parenchyma being present and having similar function as those in softwood. There are also the fibre cells which are long and thin and have tapered ends and this give additional support. The fibre cells are 1-2mm lengthwise with the ratio of length to diameter being 100:1. In addition there are other cells with dissolved ends acting in conducting role and they are known as vessels or pores. The cells have a length in the range of 0.2 to 1.2mm with a width of up to 0.5mm (The Encyclopedia of Wood,1989) . The objective of this experiment was to investigate properties of timber. I the experiment there was comparison of a hardwood sample and a softwood sample. The experiment involved analyzing samples on flexural load frames to compare strength. Methodology In this experiment the apparatus used were the flexural load frame and load ring and a deflection gauge. The distance between the two supports of the load frame (L) were measured. The end grain structure of four specimens which had been selected were examined and a decision was made on the orientation that could provide the highest resistance to deflection. This was done bearing in mind that there will be greater resistance when the force is applied normally to the grain. In this experiment there was need of wearing safety goggles. The specimen was placed in the flexural load and secured in position; the deflection gauge was then placed under the sample as central as possible in a way that it could touch the bottom of the sample. The dials were adjusted to zero. The loads were then applied in turn starting with 50N with increments of 100N adding up to 450N with the deflection being recorded in the first column of the recording for every increment. The load was then released and the same procedure was repeated with the initial load of 50N and 100N increments up to 450N and another set of values were recorded in second column of the recording sheet. After the second set of recordings the same procedure was repeated so as to obtain the values recorded in the third columns of the recording sheet but for this third time the load were not released. The load was then increased in steps of 250N so as to have the deflection at 700N, 950N--- till the point of breakage of the specimen. Results Table 1. Meranti test results Length between supports (L) = 285 (mm) Specimen dimensions 30cm x 5 cm Specimens Meranti Load Deflection (mm) 1 2 3 Average 50 0.01 0.014 0.014 0.013 150 0.028 0.032 0.033 0.031 250 0.046 0.05 0.051 0.049 350 0.064 0.068 0.069 0.067 450 0.083 0.086 0.087 0.085 700 0.16 950 Fail Failure load(N) 800 Table 2 Redwood test results Length between supports (L) = 285 (mm) Specimen dimensions 30cm x 5 cm Specimen Redwood Load Deflection (mm) 50 0.008 0.011 0.011 0.01 150 0.026 0.027 0.028 0.027 250 0.042 0.044 0.044 0.043 350 0.059 0.06 0.061 0.06 450 0.076 0.077 0.077 0.077 700 0.134 950 0.28 Fail Failure load(N) 1100 Table 3 Pine test results Length between supports (L) = 285 (mm) Specimen dimensions 30cm x 5 cm Specimen Pine Load Deflection (mm) 1 2 3 Average 50 0.008 0.011 0.011 0.01 150 0.021 0.023 0.022 0.022 250 0.033 0.034 0.034 0.034 350 0.044 0.045 0.045 0.045 450 0.055 0.055 0.055 0.055 700 0.084 950 0.0113 1200 0.147 1450 0.193 1700 0.28 1950 Fail 2200 Failure load(N) 1800 Table 4 Ash test results Length between supports (L) = 285 (mm) Specimen dimensions 30cm x 5 cm Specimen Ash Load Deflection(mm) 1 2 3 Average 50 0.003 0.008 0.008 0.006 150 0.015 0.019 0.019 0.018 250 0.027 0.029 0.03 0.029 350 0.036 0.038 0.038 0.037 450 0.046 0.047 0.047 0.047 700 0.07 950 0.092 1200 0.111 1450 0.133 1700 0.157 1950 0.195 2200 0.252 2450 0.35 Fail Failure load (N) 2650 From figure 1 it can be observed that the highest deflection was recorded in ash where the maximum deflection was 0.35mm. The other timber types had lower maximum deflect of 0.28mm for both pine and redwood while for had a value of 0.16mm. From the figure it is also clear that the ash is the most stiff even though it failed with the highest deflection. On the other hand and redwood had almost the same stiffness with meranti as their graphs are almost coinciding even though the later failed at a much lower load. Comparing the redwood and pine graphs it is observed that even though the two specimens failed after achieving the same deflection of 0.28 the later is stiffer than the former. Figure 1: Deflection load line graphs From figure 2 it can be seen that meranti was the weakest specimen as it failed at the lowest load of 800N. The largest load was sustained by the ash specimen with a failure load of 2650N. In terms of failure load it is observed that red wood can with stand a much higher load of 1800N as compared to 1100N which was the failure load of redwood even though they failed with the same deflection of 0.28mm (figure1) Figure 2: Failure load bar graphs Discussion From the test result it had been observed that ash is the stronger of the two specimens as it failed at the highest load of 2650. It is also observed that ash was the stiffest specimen. This can be very important aspect in engineering work where structural components are supposed to sustain high loads with little deflection. This may be applicable where a beam is required to span a certain length without using any column to support it like in halls where column may not be desired as they may be regarded as barriers. It is also important to note that the area, shape and the orientation with respect to the applied force also play a major role in determining the deflection and the maximum load that can be sustained. Conclusion From the experiment it can be seen that hardwoods are stronger than soft woods as the can withstand considerably higher loads. Hardwoods are seen to be more stiff and therefore and are desirable for use in long span beams. It can be concluded that there was accuracy in this test as the results were in agreement with theory. References BS 4978 (1988). Visual Strength Grading of Softwood. London. The Encyclopedia of Wood. Keith F. et al (1999).Wood Engineering and Construction Handbook, New York, McGran-Hill, Inc. LST EN 518 (2000) Structural Timber Grading – Requirements for Visual Strength Grading Standards. The Encyclopedia of Wood(1989) . New York, Sterling Publishing Co., Inc. Read More