name the four major classifications of structural lumbr

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Expert Solution

Timber is mainly classified on the basis of:

Mode of Growth
Durability
Modulus of Elasticity
Grading

1. Mode of Growth: On the basis of mode of growth lumber is classified into two types:

(i) Exogeneous Trees: These trees grow outward by adding distinct consecutive rings every year, these rings are known as annual rings. Hence we can find the age of timber by counting these annual rings.

(ii) Endogeneous Trees: These trees grow inwards. Fresh fibrous mass is in the inner most portion. They are not useful for structural works. Eg. Bamboo

2. Durability:

Low durability: Average life less than 5 years.

Moderate durability: Average life between 5 to 10 years.

High durability: If average life is more than 10 years.

3. Modulus of Elasticity:

On the basis of young's modulus to elasticity timber is classified into 3 groups:

Group A: E = 12.5 kN/mm2
Group B: E = 9.8 kN/mm2 to 12.5 kN/mm2
Group C: E = 5.6 kN/mm2 to 9.8 kN/mm2

4. Grading:

IS 883-1970 classifies the structural timber into three grades-select grade, grade I and grade II. The classification is based on permissible stresses, defects etc.

Ally Wells answered 3 months ago

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