What is the grade of timber as per timber grading (IS 399), if the modulus of elasticity in bending is between 5.6 to 9.8 kN/mm²?
Timber Grading by Modulus of Elasticity
For structural applications, it's crucial to know the strength and stiffness of timber. The Modulus of Elasticity (E), also known as Young's Modulus, is a measure of a material's stiffness or its resistance to being deformed elastically when a force is applied. Indian Standard IS 399 classifies timber into three groups (or grades) based on this property to help engineers select the appropriate material for structural design.
📝 Detailed Analysis of the Options
(a) Grade B
This grade corresponds to a Modulus of Elasticity between 9.8 and 12.5 kN/mm². It represents timber with medium stiffness.
(b) Grade A
This is the highest grade, for timber with a Modulus of Elasticity greater than 12.5 kN/mm². This timber is the stiffest and strongest.
(d) Grade D
There is no "Grade D" in this classification system as per IS 399.
(c) Grade C
This is the correct answer. Grade C is the classification for timber with a Modulus of Elasticity in the range of 5.6 to 9.8 kN/mm². This represents timber with the lowest stiffness among the three structural grades.
📊 IS 399: Timber Grades by Modulus of Elasticity (E)
| Grade | Modulus of Elasticity (E) in kN/mm² | Stiffness |
|---|---|---|
| A | > 12.5 | High |
| B | 9.8 – 12.5 | Medium |
| C | 5.6 – 9.8 | Low |
💡 Study Tips
- A is Best: Remember that Grade A is the highest quality with the highest stiffness (E > 12.5).
- Memorize the Boundaries: The key numbers to remember for the boundaries are 9.8 and 5.6.
- C is the Lowest Range: Grade C covers the lowest range of stiffness (5.6 - 9.8 kN/mm²), which is what the question asks for.