JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II BUILDING MATERIALS, CONSTRUCTION AND PLANNING Previous Question Paper (March - 2021 - R16)

Timber is a versatile natural material with distinct physical and mechanical properties that dictate its suitability for structural applications. These properties are categorized as follows:

• Physical Properties:
  • Density and Specific Gravity: Timber density varies based on moisture content and species. It is a critical indicator of strength; generally, higher density implies higher strength.
  • Moisture Content: Timber is hygroscopic, meaning it absorbs and releases moisture. The Fiber Saturation Point (FSP), typically around 25-30%, is the moisture level where cell walls are saturated but cell cavities are empty.
  • Shrinkage and Swelling: As timber loses moisture below the FSP, it shrinks. Shrinkage is anisotropic, occurring most in the tangential direction, followed by radial, and least in the longitudinal direction.
  • Durability: Resistance to decay, fungi, and insect attack, often influenced by the presence of natural extractives in heartwood.
• Mechanical Properties:
  • Compressive Strength: Timber is strongest when loaded parallel to the grain and weakest when loaded perpendicular to the grain.
  • Tensile Strength: High parallel to the grain but significantly lower perpendicular to the grain due to the ease of splitting along fibers.
  • Flexural Strength (Bending): Crucial for beams and joists, determined by the Modulus of Rupture (MOR) and Modulus of Elasticity (MOE).
  • Shear Strength: Resistance to sliding of fibers past one another, which is critical in short beams and joints.

Seasoning of timber is the controlled process of reducing the moisture content of freshly cut timber to a level suitable for its intended use, typically between 10% and 15%. This process minimizes shrinkage, warping, and decay while enhancing strength and durability.

The primary methods of seasoning are:

• Natural (Air) Seasoning: Timber logs or planks are stacked in open sheds with adequate air circulation. Spacers (stickers) are placed between layers to ensure uniform drying. While economical, it is a slow process and cannot reduce moisture below the equilibrium moisture content of the environment.
• Kiln Seasoning: A rapid, controlled process conducted in a closed chamber where temperature, relative humidity, and air circulation are regulated. It is highly efficient and allows for precise moisture control, though it is more expensive than air seasoning.
• Water Seasoning: Logs are submerged in running water. This leaches out sap and soluble sugars, which are food for fungi and insects, thereby increasing durability. After water immersion, the timber is usually air-dried.
• Chemical (Salt) Seasoning: Timber is immersed in a solution of chemicals like urea or sodium chloride. These chemicals draw moisture from the interior to the surface, accelerating the drying process and reducing surface checking.
• Electrical Seasoning: High-frequency alternating currents are passed through the timber. The internal resistance generates heat, causing rapid evaporation of moisture. This is the fastest method but is restricted to small-scale or specialized applications due to high costs.