What are condition in which sway can occurs in portal frames ?

Situations where sway can occur in portal frames:

Asymmetrical or Unsymmetrical Loading: 

Sway can occur when a portal frame is subjected to asymmetric or unsymmetrical loads, such as uneven wind pressure or non-uniformly distributed loads. This imbalance creates lateral forces that can cause the frame to sway.

Non-Uniform Section of the Members: 

If the members of a portal frame have non-uniform cross-sections or varying strengths, it can lead to differential stiffness and load distribution. This non-uniformity can cause uneven deformation and sway, especially under lateral loads.

Insufficient Bracing: 

Inadequate or improperly designed bracing systems can lead to insufficient lateral support. When exposed to external forces like wind or seismic activity, the frame may sway due to the lack of adequate stability.

These factors contribute to the instability and lateral movement of portal frames, necessitating careful design and bracing to ensure stability and reduce sway.