Structural end connections: rotation and translation

One-line orientation

Every structural end connection is just a choice about two freedoms — rotation and translation — and the exam tests whether you can map the right combination to the right connection type and structural behavior.

Key points

• Two freedoms define everything: an end connection either restrains or releases rotation and translation. Naming the connection = naming which of the two it holds.
• Moment connection — rotation fixed + translation fixed. A rigid joint that transfers bending moment plus axial and shear forces. Used where rigidity and continuity are needed for stability (rigid/moment frames).
• Shear (simple) connection — rotation free + translation fixed. Designed for shear; the member can pivot (rotate) slightly under load. The typical beam-to-column steel connection. Not intended to carry significant moment.
• Base isolation: base isolators allow controlled horizontal movement. Rotational restraint varies by device.
• Roller support (statics): releases rotation and one direction of translation while still providing vertical support. Do not mix this with a seismic-isolation bearing, which is better read as a sliding movement detail rather than a generic roller.
• Link to columns: these same end conditions set a column’s effective-length factor K (fixed ends → lower K → more buckling capacity; pinned/free ends → higher K). See the buckling card.

End connections: rotation × translation set the joint type

Two independent choices set every joint — only the moment cell (both restrained) transfers bending.

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Confusions / comparison

Connection	Rotation	Translation	Transfers	Where used
Moment	Fixed	Fixed	Bending + shear/axial	Rigid/moment frames needing continuity
Shear / simple	Free	Fixed	Shear (no real moment)	Typical beam-to-column joints
Base isolation	Device-dependent	Horizontal movement allowed	Reduces earthquake force transferred to the building	Flexible, yielding, or sliding devices
Roller support	Free	Free in one direction	Vertical reaction only	Thermal movement / simple support idealization

Related

→ Column buckling & K-values (this topic): end conditions set the K-factor, so these connections drive buckling capacity · Structural system selection (this topic): which frames need moment vs simple connections · Steel connections (Steel): how shear vs moment joints are physically detailed in steel.