What is a rigid frame, and how does it behave under lateral loads?

• A. A frame with moment-resisting connections that transfer bending moments; it resists lateral loads with stiffness and deforms accordingly.
• B. A frame with pinned connections and no lateral stiffness.
• C. A frame that cannot resist lateral loads.
• D. A frame made of rigid materials only.

Answer: (A)

Rigid frames resist lateral loads through moment-resisting connections that transfer bending moments between beams and columns, creating a continuous frame action. Because the joints resist rotation, the frame acts as a single structural system rather than a collection of unconnected members. When a lateral force such as wind or seismic ground motion is applied, the frame develops internal bending moments in the connections and along its members, distributing the load through the stiffness of the entire frame. The frame’s lateral stiffness—set by member sizes, material properties, and how rigid the connections are—determines how much it will sway and how the moments are redistributed throughout the structure. This behavior is what gives a rigid frame its ability to resist lateral loads effectively.

If the connections were pinned and could not transfer moments, the frame would rely only on shear transfer at joints and would have much less lateral stiffness, which is not characteristic of a rigid frame. A frame cannot resist lateral loads simply by being made of rigid materials; resistance comes from the ability to transfer bending moments across joints.