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A structure is statically indeterminate when it has more supports than necessary for equilibrium, creating more unknown reactions than available equilibrium equations. In the example of two cylindrical rods joined at point B with rigid supports at points A and C, the extra support makes the structure statically indeterminate, requiring additional analysis beyond basic statics to solve.

The superposition method determines deformation in each section of the rod structure separately, then combines these individual deformations to find total deformation. By treating the redundant reaction at point C as an additional load and applying equilibrium conditions where total deformation equals zero, unknown reaction forces can be calculated for the entire structure.

A redundant reaction is an excess support force that exceeds what is needed for equilibrium. In the two-rod example, the reaction at point C is considered redundant. By releasing this support and treating the reaction as an additional load, the problem becomes solvable using superposition and equilibrium equations.

Deflection at point B is calculated by summing the deformations in all rod sections preceding point B. Each section's deformation is determined separately using material properties and applied loads, then combined to find the total deflection at the intermediate point where the steel and brass rods are joined.

Total deformation must equal zero because the rod structure is restrained by rigid supports at both ends. Since the supports prevent any net movement, the combined deformations from all sections must sum to zero. This constraint, combined with force equilibrium, allows determination of unknown reaction forces.

Material properties such as modulus of elasticity determine how each section deforms under load. Since the steel and brass rods have different elastic properties, their individual deformations differ. These material-dependent deformations are essential inputs for calculating total deformation and ultimately determining the unknown reaction forces.

Releasing the redundant support at point C converts the statically indeterminate structure into a determinate one. The released reaction is then treated as an unknown external load. This transformation allows equilibrium equations and deformation compatibility conditions to work together, making the system solvable through superposition.