In structural engineering, a rigid frame is the load-resisting skeleton constructed with straight or curved members interconnected by predominantly rigid connections, which resist movements induced at the joints of members.
[1] The AISC Steel Specification Commentary on Section B3 provides guidance for the classification of a connection in terms of its rigidity.
Specifically, Ks = Ms/θs where Ms = moment at service loads, kip-in (N-mm) θs = rotation at service loads, rads [2] The secant stiffness of the connection is compared to the rotational stiffness of the connected member as follows, in which L and EI are the length and bending rigidity, respectively, of the beam.
If KsL/EI ≥ 20, it is acceptable to consider the connection to be fully restrained (in other words, able to maintain the angles between members).
If KsL/EI ≤ 2, it is acceptable to consider the connection to be simple (in other words, it rotates without developing moment).