Support Restraint

Pinned and fixed supports create very different structural behaviour.

A pinned support restrains translation while allowing rotation, so it does not carry an idealized support moment. A fixed support restrains translation and rotation, so it can develop moment reaction. That single modelling choice changes bending moment shape, reaction distribution, member end forces, and displacement under load.

Model the detailChoose restraint conditions that match the real connection stiffness you want the analytical model to represent.

Compare sensitivityRun pinned and fixed versions when reactions, drift, or end moments are sensitive to the base assumption.

Pinned Rotation free

Use when the support idealization should carry force but release moment.

Fixed Rotation restrained

Use when the model should develop moment reaction at the supported node.

Pinned support in practice

Pinned supports are often used where the real connection is assumed to transfer force but not meaningful moment. In NextForm, use this for truss-like supports, simple beam seats, pinned portal bases, and comparison models where rotation should be free.

Fixed support in practice

Fixed supports are used when the model assumes strong rotational restraint, such as a built-in cantilever base or a moment-resisting base detail. This can reduce displacement, create support moments, and pull more bending demand into adjacent members.

Restraint Matrix

What each support condition restrains

Behaviour

Pinned

Fixed

Horizontal translation

Restrained

Vertical translation

Restrained

Rotation

Released

Restrained

Moment reaction

Not idealized

Develops

Model Review

Questions to ask before choosing a support model

Does the real detail provide rotational stiffness, or should the node be allowed to rotate in the analytical model?

Would a fixed assumption hide sway or reduce deflection unrealistically for the actual base detail?

Would solving both pinned and fixed versions reveal how sensitive reactions, end moments, and displacement are to the support idealization?

Can a wrong support assumption mislead the analysis?

Yes. It can change moments, reactions, displacement, and the apparent force path enough to alter the engineering conclusion.

Can NextForm help compare both cases?

Yes. You can model one arrangement with pinned restraint and another with fixed restraint to compare how the system response changes.

Try both assumptions in the frame workspace.

Create a simple model, duplicate the support condition, and compare reactions, displacement, shear, and moment diagrams.

Open Frame