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Support reactions prevent both translational and rotational motion of structures. They apply equal and opposite forces to resist translation and apply couples of moments in the opposite direction to prevent rotation. These reactions are essential for maintaining equilibrium and structural stability under applied loads.

Roller support allows translation in only one direction and provides a single reaction force normal to its surface, making it ideal for structures like bridges that expand or contract. Pin support restricts translational motion but permits rotation, producing reaction forces in the plane perpendicular to its axis. Pin supports suit structures with cantilevers or uniform loading conditions.

Fixed support provides two reaction forces and a moment to any applied force. The horizontal force Fx acts along the support axis, while Fy acts normal to the support. This comprehensive restraint prevents both translation and rotation in any direction, making fixed supports ideal for buildings requiring high structural stability.

Roller support allows structures to translate in one direction while restricting motion in others. This flexibility accommodates thermal expansion and contraction without creating excessive internal stresses. The single normal reaction force simplifies load analysis while maintaining structural integrity during temperature-induced dimensional changes.

Pin support reaction forces act in the plane perpendicular to the pin axis and can be resolved into rectangular components. These components are typically analyzed using equations of equilibrium in three dimensions to determine their magnitudes and directions. This resolution allows engineers to evaluate structural behavior under complex loading conditions.

Support selection depends on the structure's design requirements and loading conditions. Fixed supports provide maximum restraint for buildings requiring high stability. Pin supports suit structures allowing rotation with restricted translation. Roller supports accommodate directional movement, like thermal expansion in bridges. Each type balances structural safety with functional requirements.

Support reactions provide constraints that determine whether a structure is statically determinate or indeterminate. The number and type of reactions from roller, pin, and fixed supports affect how many equilibrium equations can be satisfied. Understanding these relationships through constraints and statical determinacy helps engineers design stable, analyzable structures.