Short Answer:

A frame and a truss are both structural systems made of members connected together, but they differ in how they carry and transfer loads. A truss consists of straight members connected by pin joints and carries only axial forces (tension or compression). A frame, on the other hand, consists of members that are rigidly connected and can carry bending moments, shear forces, and axial forces.

In simple terms, trusses are used where members act only in tension or compression, while frames are used where members need to resist bending, such as in building structures, cranes, and bridges.

Detailed Explanation:

Difference Between Frame and Truss

In structural and mechanical engineering, both frames and trusses are used to support loads and provide stability to structures. However, the way they transfer loads and the types of forces developed in their members are quite different. Understanding the difference between these two systems is important for designing strong and stable structures.

A truss is a structure made of straight and slender members connected at their ends by frictionless pins to form a network of triangles. Trusses are primarily designed to carry axial loads only — that is, the members are either in tension or compression. The load applied at one point of the truss is transferred efficiently through these axial forces to the supports.

A frame, on the other hand, consists of straight or curved members that are connected by rigid joints. These joints can resist bending moments and shear forces in addition to axial forces. Therefore, frames can support not only vertical loads but also lateral (horizontal) loads such as wind or earthquake forces.

Construction and Nature of Members

1. Truss:
   • The members of a truss are connected by pin joints, which allow rotation but no bending.
   • All loads are applied at the joints, and the members carry only axial forces (either tension or compression).
   • Trusses are composed mainly of triangular units, which give them stability and rigidity.
   • Typical examples include roof trusses, bridge trusses, and tower trusses.
2. Frame:
   • The members in a frame are connected by rigid joints or welded joints, which can resist bending and rotation.
   • Loads can act not only at the joints but also at any point along the member.
   • Each member carries axial force, bending moment, and shear force.
   • Frames are used in structures like buildings, cranes, towers, and machines where bending resistance is important.

Types of Forces in Members

• In a Truss:
  The members are subjected only to axial forces. If a load is applied, one member may elongate (tension) while another shortens (compression). There are no bending moments or shear forces in truss members because of the pin-jointed connections and the assumption that loads act only at the joints.
• In a Frame:
  The members experience axial forces, bending moments, and shear forces. The rigid connections between members prevent rotation, so bending moments develop when the frame is loaded. Frames, therefore, can resist both vertical and horizontal loads effectively.

Structural Behavior

• Truss:
  • A truss structure acts as a single rigid unit.
  • When an external load is applied, it gets distributed through the members as axial forces.
  • The members do not bend but may elongate or shorten slightly under tension or compression.
  • Because of the triangular geometry, trusses are very strong and stable for their weight.
• Frame:
  • A frame behaves as a flexible structure capable of resisting bending.
  • Loads cause bending and shear in the members, and the entire structure deflects slightly.
  • Frames are more adaptable for complex building designs and can resist wind and earthquake forces better than trusses.

Geometry and Shape

• Truss:
  Usually built from triangular units because a triangle is a geometrically stable shape.
  Examples include Pratt truss, Warren truss, Howe truss, and King post truss.
• Frame:
  Can have any shape — rectangular, square, curved, or irregular — depending on the design and purpose.
  Examples include portal frames, machine frames, and building frames.

Applications

1. Applications of Trusses:
   • Roof structures in industrial buildings and warehouses.
   • Railway and highway bridges.
   • Transmission towers.
   • Roofs of stadiums and auditoriums.
   • Space frames in aircraft hangars.
2. Applications of Frames:
   • Structural skeletons of buildings and skyscrapers.
   • Cranes and lifting devices.
   • Automobile chassis and machine structures.
   • Towers and pipelines.
   • Rigid frameworks in factories and industrial plants.

Advantages and Limitations

Truss:

• Advantages:
  • Lightweight but very strong.
  • Economical, as it uses less material.
  • Efficient for long-span structures.
• Limitations:
  • Cannot resist bending moments.
  • Suitable only for loads applied at joints.
  • More complex to fabricate and assemble due to numerous members.

Frame:

• Advantages:
  • Can resist bending, shear, and axial forces.
  • Suitable for both vertical and lateral loads.
  • Provides design flexibility for different shapes.
• Limitations:
  • Heavier than trusses due to more material use.
  • Analysis is more complex because of the bending effects.
  • More expensive to construct due to rigid joints.

Key Differences Between Frame and Truss

Aspect	Frame	Truss
Type of Joints	Rigid or welded joints	Pin or hinged joints
Type of Forces	Axial, bending, and shear forces	Only axial forces (tension or compression)
Load Application	Loads act at any point	Loads act only at joints
Member Shape	Can be rectangular or irregular	Usually triangular
Main Use	Buildings, cranes, towers	Bridges, roofs, towers

(Note: The above table is only for clarity; explanation remains textual as per instructions.)

Conclusion

A frame and a truss differ mainly in their structural behavior and load-carrying mechanism. A truss carries only axial loads through its pin-jointed members arranged in triangles, making it lightweight and ideal for long spans. A frame, on the other hand, has rigid joints that resist bending, shear, and axial loads, making it suitable for buildings and structures exposed to lateral forces. Both are essential structural systems in mechanical and civil engineering, each serving its unique purpose based on stability, load conditions, and design requirements.