The type of timber recommended for constructional purpose, whose modulus of elasticity in bending is above 12.5 kN/mm², is classified as ______ timber.
뻣 Timber Classification for Structural Use
For structural engineering purposes, timber is classified into groups based on its mechanical properties. One of the most important properties is the Modulus of Elasticity (E), which measures the timber's stiffness or its resistance to bending. A higher Modulus of Elasticity indicates a stiffer and stronger wood, suitable for more demanding structural applications like long-span beams and columns.
📝 Detailed Analysis of the Options
(a) Group C
This group is for timber with a Modulus of Elasticity in bending that is more than 5.6 kN/mm² but less than 9.8 kN/mm². This is considered a lower-strength group, suitable for light construction and temporary works.
(b) Group D
This is not a standard classification in this context. The standard structural grouping for timber in India is A, B, and C.
(c) Group B
This group includes timber with a Modulus of Elasticity in bending that is more than 9.8 kN/mm² but less than 12.5 kN/mm². This is a medium-strength group, widely used for common structural elements like joists, rafters, and posts.
(d) Group A
This is the correct answer. Group A is the highest classification, reserved for timbers with a Modulus of Elasticity in bending above 12.5 kN/mm². These are the strongest and stiffest timbers, recommended for heavy structural purposes where high loads and long spans are involved.
📊 Structural Timber Classification by Modulus of Elasticity (E)
| Group | Modulus of Elasticity in Bending (E) in kN/mm² | Structural Suitability |
|---|---|---|
| Group A | E > 12.5 | Heavy construction (e.g., long-span beams, trusses) |
| Group B | 9.8 < E ≤ 12.5 | Common construction (e.g., joists, posts, rafters) |
| Group C | 5.6 < E ≤ 9.8 | Light construction (e.g., temporary structures, packing) |
💡 Study Tips
- A is for "Above 12.5": This is the key value to remember. Group A is the top tier, for timbers with E above 12.5 kN/mm².
- Follow the Alphabet Down: As you go down the alphabet from A to C, the strength and stiffness requirements decrease.
- Memorize the Thresholds: The key numbers that separate the groups are 12.5 and 9.8 kN/mm². Knowing these values will help you answer any question on this topic.