The primary difference between pin-jointed and rigid-jointed plane frames lies in the moment transfer capability at the joints: pin joints cannot transfer moments, while rigid joints can. This fundamentally alters the behavior and load-carrying capacity of the structure.
Here's a more detailed breakdown:
Joint Behavior
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Pin Jointed Frames: Also known as hinged frames, pin joints allow rotation between connected members. This means no bending moment can be transmitted from one member to another at the joint. Consequently, members primarily experience axial forces (tension or compression). These frames rely on triangulation for stability.
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Rigid Jointed Frames: Also known as moment-resisting frames, rigid joints prevent relative rotation between connected members. This allows bending moments, shear forces, and axial forces to be transferred between members. This significantly enhances the frame's stiffness and load-carrying capacity.
Structural Implications
The difference in joint behavior leads to distinct structural characteristics:
| Feature | Pin Jointed Frame | Rigid Jointed Frame |
|---|---|---|
| Moment Transfer | No moment transfer. | Moment transfer is possible. |
| Primary Forces | Axial forces (tension/compression) only. | Axial forces, shear forces, and bending moments. |
| Deflection | Generally larger deflections. | Generally smaller deflections. |
| Stability | Relies on triangulation for stability. | Stability derived from joint rigidity. |
| Joint Rotation | Allows rotation between members at the joint. | Prevents rotation between members at the joint. |
| Load Capacity | Generally lower load capacity for same material. | Generally higher load capacity. |
| Design Complexity | Simpler design calculations. | More complex design calculations. |
Examples
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Pin Jointed Frame Example: A typical truss bridge is a good example of a pin-jointed frame. The members are designed to primarily experience axial forces, and the joints are designed to allow rotation.
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Rigid Jointed Frame Example: A multi-story building frame where the columns and beams are rigidly connected is an example of a rigid-jointed frame. The rigid connections allow the frame to resist lateral loads (e.g., wind or seismic forces) through bending action.
Advantages and Disadvantages
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Pin Jointed Frames:
- Advantages: Simpler analysis and construction, lighter members can be used for same span.
- Disadvantages: Larger deflections, requires triangulation for stability, lower load capacity compared to rigid frames for the same material usage.
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Rigid Jointed Frames:
- Advantages: Higher load-carrying capacity, smaller deflections, greater stability without relying solely on triangulation.
- Disadvantages: More complex analysis and construction, requires stronger joint connections, can be more expensive.
In summary, the ability or inability to transfer moments at the joints defines the key difference between pin-jointed and rigid-jointed plane frames, impacting their behavior, stability, load-carrying capacity, and design complexity.
