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2D Frame Analysis with Rigid Joints and Hinges

A 2D frame combines beams and columns that carry axial force, shear and bending. The distinction between a rigid joint, a pin support and a member-end moment release is fundamental to getting a meaningful moment diagram.

Rigid joint, hinge and support: what changes?

Rigid frame joint: connected frame elements share translational and rotational degrees of freedom at their meeting node. The joint can transmit bending moment between connected members.

Moment release (hinge): connected member ends can rotate without transmitting an end bending moment at that release, while the connection can still transmit axial and shear forces. Removing bending continuity can change deflections, moments and stability.

Pin support: restrains horizontal and vertical displacement but leaves rotation unrestrained at the support node. A pin support does not automatically release the bending moment between two rigidly connected members meeting at that same node.

Try the portal frame template

1. Launch FramePad and select Portal frame.
2. Inspect the two columns, connecting beam, base restraints and applied loads.
3. Use the Moment and Deflected tabs to see frame action.
4. Select a top corner node. In the node inspector, change Connection from Rigid joint to Hinge / moment release; alternatively choose the Hinge toolbar tool.
5. Compare the moment distribution and displacement. Restore the rigid joint and compare again.

Why moment releases change the solution

In conventional planar frame analysis, each rigid node can have three degrees of freedom: horizontal displacement, vertical displacement and rotation. A standard frame element has axial and flexural stiffness. A moment release removes the associated end-moment restraint; the remaining active degrees of freedom must still form a stable system.

A structure that is stable with rigid corners can become a mechanism after multiple hinges are introduced. FramePad flags some singular or under-restrained systems, but automated warnings cannot replace an engineer's stability assessment.

How to check frame analysis results

• Confirm global equilibrium of vertical and horizontal support reactions.
• Check whether internal bending moments become zero at fully released member ends, absent a directly applied nodal moment.
• Compare deformed shape with expected sway and bending behavior.
• Check stiffness magnitudes and units: EA in force units, EI in force × length².
• Validate a representative simple case independently before applying the tool to more complicated frames.