UNIT 2 Truss: Classification and Characteristics

Truss Structure

• A truss is a structure composed of straight members connected at their ends by joints, typically slender and arranged in a triangular or polygonal pattern.
• Trusses are widely used in civil and structural engineering to support loads and resist forces.

Classification of Trusses

• Planar Trusses: All members and joints lie within a single plane, the most common type of truss.
• Space Trusses: Members and joints are three-dimensional, extending into space rather than lying entirely within a plane.
• Simple Trusses: Made up of linear elements, statically determinate, and the equilibrium equations alone are sufficient to solve for all member forces.
• Compound Trusses: Composed of simple trusses connected to form a more complex structure, statically determinate or indeterminate.
• Plane Trusses: Members and joints are all in one plane, analyzed using two-dimensional methods.
• Multi-Storey Trusses: Used in multi-storey buildings, supporting vertical loads over several levels.
• Howe Truss, Pratt Truss, and Warren Truss: Examples of different geometric configurations, named after their respective designers.

Assumptions in Truss Analysis

• Joint Rigidity: Joints are assumed to be perfectly rigid, allowing forces to be transmitted only along the members.
• Straight Members: Truss members are assumed to be straight, with any deformations neglected.
• Pin Joints: Joints are considered frictionless and hinged, allowing rotation but no translation.
• No Thermal Effects: Thermal effects are neglected in truss analysis.

Method of Sections

• Advantageous for selective analysis of specific sections or portions of a truss.
• Simpler and more efficient when dealing with isolated members or sections.
• Enables elimination of redundant members, focusing on relevant ones.
• Applicable to complex trusses, simplifying analysis and focusing on critical areas.
• Convenient for handling external loads applied away from joints.

Friction

• A force that opposes the relative motion or tendency of motion between two surfaces in contact.
• Types of friction:
  • Static Friction: Opposes initiation of motion between two surfaces.
  • Kinetic Friction: Opposes motion of two objects sliding past each other.
  • Rolling Friction: Occurs when an object rolls over a surface, smaller than sliding friction.
  • Fluid Friction (Lubricated Friction): Occurs when an object moves through a fluid, influenced by viscosity.
  • Internal Friction: Resistance to motion within a material, contributes to vibration damping.
  • Skin Friction (Shear or Traction): Resistance to motion at the surface of contact between two materials.

Cone of Friction

• A concept in soil mechanics and geotechnical engineering, visualizing the distribution of soil forces around a foundation or object.
• Key points:
  • Geometry: A three-dimensional shape extending outward and downward from the base of the foundation.
  • Frictional Resistance: Lateral soil resistance is mobilized along the surface of the cone.
  • Angle of Internal Friction: Related to the angle at which the cone slopes outward, a property of the soil that describes its resistance to shearing.
  • Stability: The stability of a foundation or structure subjected to lateral loads depends on the interaction between the applied forces and the cone of friction.
  • Design Considerations: Engineers use the cone of friction concept in designing foundations and retaining structures to ensure stability against horizontal forces.