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?
Try it first: pick an option, then read the explanation.
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.