In
structural engineering
Structural engineering is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and muscles' that create the form and shape of man-made structures. Structural engineers also must understand and cal ...
, a rigid frame is the load-resisting skeleton constructed with straight or curved members interconnected by mostly rigid connections, which resist movements induced at the joints of members. Its members can take bending moment,
shear, and axial loads.
The two common assumptions as to the behavior of a building frame are (1) that its beams are free to rotate at their connections or (2) that its members are so connected that the angles they make with each other do not change under load. Frameworks with connections of intermediate stiffness will be intermediate between these two extremes. Frameworks with connections of intermediate stiffness are commonly called semirigid frames. The
AISC
The American Institute of Steel Construction (AISC) is a not-for-profit technical institute and trade association for the use of structural steel in the construction industry of the United States.
AISC publishes the Steel Construction Manual ...
specifications recognize three basic frame types: Rigid Frame, Simple Frame, and Partially Restrained Frame.
AISC standard
The AISC Steel Specification Commentary on Section B3 provides guidance for the classification of a connection in terms of its
rigidity. The
secant stiffness of the connection K
s is taken as an index property of connection stiffness. Specifically,
Ks = Ms/θs
where
Ms = moment at service loads, kip-in (N-mm)
θs = rotation at service loads, rads
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.
Notes
References
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Structural system
Construction
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