Test: Timber - Civil Engineering (CE) MCQ

The terminology used in Timber:
Rake angle:

• The angle between the face of the tool and a plane parallel to its base. If this inclination is towards the shank, it is known as the back rake angle and if measured along with the side is known as the side rake angle.

Lip angle:

• The angle between the face and flank of the tool. As the lip angle increases, the cutting edge will go stronger.

Relief angle:

• The angle formed between the flank of the tool and a perpendicular line drawn from the cutting point to the base of the tool.

Cutting Angle:

• The angle between the face of the cutter and the line joining the cutting edge to the center of the block.

Bastard Grain:

• Timber or veneer, so saw that the annual growth rings make angles of 30o to 60o with the surface of the piece.

Chipped Grain:

• A defect caused by the breaking of the wood below the finished surface by the action of a tool or cutter.

Bow is a defect that occurs in the seasoning process of wood and is indicated by the curvature which forms in the direction of the length of timber. Blue stain, sap stain and brown rot are the defects caused in timber by fungi.

• Solution of barium hydroxide: Barium hydroxide is not commonly used for the preservation of timber as it is not an effective preservative.


• Creosote: Creosote is a commonly used preservative for timber, as it helps protect against decay and insect damage.


• Chemical salt: Chemical salts are often used in timber preservation to prevent rot and deterioration.


• Solignum salt: Solignum salt is also a popular timber preservative that helps protect against fungal decay and insect damage.

• Barium hydroxide is not an effective preservative for timber because it does not have the necessary properties to protect the wood from decay and insect damage.


• Creosote, chemical salts, and Solignum salt are all commonly used in timber preservation because they have properties that help prevent deterioration and prolong the life of the wood.


• When choosing a preservative for timber, it is important to select one that is effective and will provide long-lasting protection against decay and insect damage.

IS 287-1993

• The permissible moisture content for timber used for different purposes recommendations.

Important Points
Some important IS codes are following

• IS: 456-2000 – Plain reinforced concrete
• IS: 800-2007 – General construction of steel
• IS: 1343- 1980 – code for practice for prestressed
• IS: 383- 1970 – Coarse and fine aggregate for concrete-specification
• IS: 875- 1987 – Practice for the design of dead load (Part 1) , live load (Part 2) , wind load (Part 3) , snow load (Part 4) , special load & load combination (Part 5)
• IS: 1893- 2002 – Earthquake resistance design of the structure
• IS 10262- 2009 – Guidelines for concrete mix design proportioning
• IS: 269 – 2013 – Specification for 33 grade OPC
• IS: 8112 – 2013 – Specification for 43 grade OPC
• IS: 12269 – 2013 – Specification for 53 grade OPC
• IS 1237 - Cement Concrete Flooring Tile
• ​IS 893 - Criteria for Earthquake Resistant Design of Structure

Dry rot is a defect caused in timber in which wood is converted into dry powder form by fungi. It usually occurs at those places where the free circulation of air is absent. It is found that unseasoned softwoods are easily attacked by dry rot.

• Sapwood: Sapwood is the outermost layer of the tree and is generally weaker compared to heartwood. It is less dense and more susceptible to decay and insect damage, which can affect the shear strength of timber.



• Heartwood: Heartwood is the inner, darker part of the tree that is generally stronger and more durable than sapwood. It contains more lignin and other structural components that contribute to the shear strength of timber.



• Medullary Rays: Medullary rays are structures within the wood that radiate out from the center of the tree. While they play a role in the overall strength and structure of timber, they do not directly affect the shear strength of timber.



• Lignin with Fibers: Lignin is a complex organic polymer that binds the cellulose fibers together in wood. The presence of lignin, along with strong fibers, contributes significantly to the shear strength of timber. The arrangement and quality of fibers within the wood also affect its shear strength.

Different type of strength of timber:
Compressive strength:

• The compressive strength is found to be the highest when acting parallel to the axis of growth.
• The compressive strength perpendicular to the fibers of wood is much lower than that parallel to fibers of the wood. 

Tensile strength:​

• Tensile strength along a direction parallel to the grains is found to have the greatest strength that can be developed under any kind of stress.
• Tensile strength parallel to fibers is of the order 80.0 to 190.0 N/cm².

Shearing strength:

• Resistance to shear in across direction is found 3 to 4 times greater than that along fibers.
• The shear strength along the fiber is found of the order 6.5 to 14.5 N/mm². 

Explanation:
The strength of timber is the highest parallel to the grains and minimum perpendicular to grains.

Additional Information
Timber: 
The wood that is going to use for the building. The structure of the wood is:

Pith:

• The innermost central portion or core of the tree is called the pith or medulla.
• As the plant becomes old, the pith dies up and decays.

Sap Wood:

• Outer annual rings between the heartwood and cambium layer are the sapwood.
• It is light in color and weight.
• It takes an active part in the growth of the trees.
• It does not impart any strength.

Cambium Layer:

• A thin layer of sap in between the sapwood and inner bark is referred to as the cambium layer.
• It indicates the portion of the sap which is yet to be converted into the sapwood.

Bark:
The Outer protective layer or covering provided around the cambium layer is referred as bark.

When a portion of the timber is affected by dry rot, the left out portion which is not affected should be painted with copper sulphate solution. While that portion of the timber which is affected and damaged due to dry rot should be completely taken out.

• Given: The natural heartwood timber used for structural purposes and of high durability would have an average life of minimum M months.


• To find: The value of M in months.

• Option A (90 months): If the average life of the timber is 90 months, it would exceed the minimum average life requirement mentioned in the question. Therefore, this option is not correct.


• Option B (120 months): If the average life of the timber is 120 months, it meets and exceeds the minimum average life requirement mentioned in the question. Hence, this option is correct.


• Option C (75 months): If the average life of the timber is 75 months, it would fall short of the minimum average life requirement mentioned in the question. Therefore, this option is not correct.


• Option D (60 months): If the average life of the timber is 60 months, it would significantly fall short of the minimum average life requirement mentioned in the question. Therefore, this option is not correct.

Therefore, the correct answer is Option B (120 months), as it meets and exceeds the minimum average life requirement of the natural heartwood timber used for structural purposes with high durability.

Timber that is scientifically prepared in a factory is called industrial timber. There are different varieties of industrial timber. These are veneers, plywood, fibreboard, impreg timbers and compreg timbers.