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Answer: Red springs are typically generated for solid objects with edge constraints to restrain their joints from rotation.
Solid objects activate only three translational See the Solid elements and rotational degrees of freedom (DOF) at each joint location. Edge-constraint implementation then activates the three rotational DOF. Since solid objects do not provide rotational stiffness, internal rotational springs are generated at affected joints to provide rotational stiffness such that local numerical instabilities do not occur.Without these internal rotational springs, warning messages in the output log would report zero stiffness at affected DOF. These warnings may either be ignored or eliminated by providing rotational springs.at their joints article for additional information on why the joints need to be restrained from rotation.