A uniform rise in temperature on a portal frame will cause no sway.

Explanation:

Portal frames are structural frames typically used in buildings and bridges to provide support and stability. They consist of two vertical columns connected by a horizontal beam at the top and bottom, forming a rectangular shape. The frame is designed to withstand various loads and forces, including the effects of temperature changes.

When there is a uniform rise in temperature, the entire frame, including the columns and beam, will expand uniformly. This expansion is due to the thermal expansion of the material from which the frame is made.

Here's why a uniform rise in temperature will cause no sway in a portal frame:

1. Symmetry of the frame:
- Portal frames are designed to be symmetrical, with equal length columns and a horizontal beam connecting them.
- The symmetry of the frame ensures that the expansion due to temperature change is uniform throughout the structure.
- As a result, there will be no differential expansion or contraction between the columns and the beam, maintaining the overall stability of the frame.

2. Balanced forces:
- When the frame expands uniformly, the forces acting on the columns and the beam remain balanced.
- The expansion of the columns and the beam will exert equal and opposite forces on each other, preventing any significant movement or sway.
- The balanced forces maintain the structural integrity of the frame and prevent it from deflecting or deforming under the influence of temperature changes.

3. Structural design:
- Portal frames are designed to resist various loads, including the effects of temperature changes.
- The material properties, dimensions, and connections of the frame members are carefully considered during the design process to ensure stability and minimize deflection.
- The design accounts for the expected thermal expansion and incorporates measures to mitigate its effects, such as providing expansion joints or allowances for movement.

In conclusion, a uniform rise in temperature on a portal frame will not cause any sway because of the frame's symmetry, balanced forces, and structural design. The uniform expansion of the frame maintains its stability and prevents any significant movement or deformation.