Properties of wood & Strength of wood
- After stone, wood is the second material widely used in construction industry since a ages.
- Wood is material which used in traditional construction, nad has rish history in construction world.
- The chemical properties of wood are complexed in general, but still human beings have successfully make a total use the unique characteristics of wood to build a unique and unrestricted variety of structures.
- This is a exceptionally versatile material is commonly used to build houses, shelters and boats, but it is also extensively used in the furniture and home décor industry as well.
- Wood is an underestimated Structural Material in the Modern World.
- Wood is an organic and material.
Absolute dry weight of wood is an index of the cell wall substance present in wood. Which represents the characteristics strength of wood. The density of wood determines the changes in the moisture content.
Timber is low cost materials, hardwood is costlier than softwood. But of some varieties of Timber is preferred for
- Temporary Structures
Properties affecting Strength of Timber:
The Strength of structural wood (Timber) depends on natural characteristics wood, some them mentioned below,
- Checks and Shakes
- Slopes of grain
- Specific Gravity
- Moisture Content
Knots are caused by the trunk of a tree growing around a limb or Branch.
- If the knots is there on wood, it doesn’t mean that the wood is inferior or not able to use as construction material.
- Knots affect the strength of wood because the deviation of grains from regular direction in passing around them and the weakening effect is felt more in tension than in compression
- Knots does not have any effect on the Stiffness.
- In a simply supported beam, a Knot on the lower side will affect the load carrying capacity of beam.
- In short or Intermediate columns Knots will reduce the strength of wood in proportion to their sizes
- In Long Columns whose load carrying capacity is dependent on Stiffness, the knots have no effect on load carrying capacity of column
2. Checks and Shakes:
- Separation of wood along the length is known as checks, it mainly occusrs at rings of annual growth.
- Separation along grain between annual growth rings is known as a Shake.
- Checks and Shakes reduce the resistance to Shear depending on the shape and size of shakes and checks and it is directly proportional to each other.
3. Slope of Grain:
- Slope of grain means the grains is not parallel to each other, and this concept is known as cross grains
- Wood is weaker across the grains and hence slope of grain is important in Structural Timber
- Compressive Strength is not affected upto the slope of 1 in 10
- Modulus of Elasticity is affected at a slope of 1 in 15 shock Resistance decreases rapidly at a slope of 1 in 15
4. Rate of Growth:
- The cross section of a tree shows concentric rings and the number of rings per unit
width is an indication of the growth of the tree.
- For Structural Timber 5 rings per 20 mm width may be specified
5. Specific Gravity:
- The wood is natural material and the well walls are composed of wood substances with specific gravity of 0.64. and it is different for different type of trees and as per their age.
- The Strength of wood depends on its Specific Gravity
6. Moisture Content:
- Wood start to begins to lose water till equilibrium, after it get cut down upto which the surrounding atmosphere is reached.
- Moisture content is reduced if it get dried after cutting, and strength of wood can be increases with decrease in moisture content
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Terminologies associated with wood and their importance
1. Shrinkage: is due to loss of moisture content and shrinkage along longitudinal direction can be negligible
2. Heartwood (central portion of the Log) consists of inactive tissues which gives strength to the tree trunk and Sapwood (portion between Heartwood and Bark) is made up of Living Cells. Sapwood is liquid permeable and should be treated to increase the resistance against
3. Live and Dead Trees: Wood cut from trees killed by insects, fungi, wind or fire is good structurally compared to “Cut Live Trees “. The portion free of insects, fungi or is in good condition should be used.
4. Seasoning of Timber: To determine the Working Stresses in Timber the effect of seasoning and use conditions is important to consider. The moisture content of Structural Timber is completely depend on seasoned condition and unseasoned condition of wood. The values of working stresses are arrived at on the basis of unseasoned Timber.
5. Destructive Agencies: Engineering Materials are not absolutely permanent under all conditions of service
and use. Wood is also vulnerable to
- Marine Borers
- Mechanical Wear
- Weathering and
- Chemical Resistance
Decay: This generally happens due to fungi and insects, they constantly try to increase decay. In presence of water, the decay is more. To protect wood preservative treatment using Oil Preservatives or Oil Borne Preservatives or Water borne preservatives should be used either with a pressure process or a non pressure process.
Fire Resistance: Wood is fire prone and it can’t be fire proof in any way. In construction, wood can be used and there are lost of methods to decrease the fire like firefighting systems. Fire resistance can also be improved by chemicals paints or coating the wood with a non-combustible paint.
Chemical Resistance: Wood is resistant to chemicals like Organic Materials, Hot and Cold Solutions of Acids, Neutral Salts and Dilute Acids and hence sometimes considered superior to other construction material. Impregnation of Wood with Paraffin (for resistance to chemical solutions) and Resin forming Material is widely acceptable.