Solution:

Endogenous Trees:

These trees grow inwards. Fresh fibrous mass is in the inner most portion. Examples of endogenous trees are bamboo and cane.

They are not useful for structural works.

(i) Classification Based on Modulus of Elasticity:

Young’s modulus is determined by conducting bending test.

On this basis timber is classified as:

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.

(ii) Classification Based on Durability:

Durability tests are conducted by the forest research establishment.

They bury test specimen of size 600 × 50 × 50 mm in the ground to half their length and observe their conditions regularly over several years.

Then timbers are classified as:

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

Moderate durability: Average life between 5 to 10 years.

Low durability: Average life less than 5 years.

(iii) Classification Based on 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.

(iv) Classification Based on Availability:

Forest departments classify timbers based on the availability as

X—Most common. 1415 m3 or more per year

Y—Common. 355 m3 to 1415 m3 per year

Z—Less common. Less than 355 m3 per year.