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Connecting frames to shells

Shell formulation in SAP2000 combines membrane and plate behavior. Each joint within a shell object has six degrees of freedom (DOF). Since joints within frame objects also have six DOF, frames may connect directly to the joints of shell objects. However, the stiffness properties of drilling DOF, which are normal to shell surfaces, are not reliable for the connection of frame objects. As the mesh is refined, the connection becomes increasingly flexible.

Depending upon the orientation of a frame object, recommendations for connecting a frame to a shell include:

  • When the frame is in the 1-2 local plane of the shell, model the frame such that it extends the length of the shell object and connects to two shell joints. All forces but shear (axial, torsion, and moments) should be released at the far end of the frame object.
  • Similarly, when the frame is normal to the 1-2 local plane of the shell, model a small frame object within the plane of the shell, and release the moment at the far end of the frame object.

Additional information is available on shell DOF in the CSI Analysis Reference Manual (Chapter X The Shell Element, Degrees of Freedom).

Connecting frames or shells to solids

Joints within solid objects have only translational DOF, therefore they provide no rotational resistance to interconnected frame and shell objects. A body constraint or rigid link should connect the end joint of a frame to the tributary joints of a solid such that a force couple is available to resist moment within the frame joint, as shown in Figure 1:



Figure 1 - Connecting frames to solids

It is reasonable to use constraints to connect frames to a rigid plane at the face of a solid because frame cross sections are assumed to be rigid. Results are most accurate when constrained joints, located around the point of connection, have dimensions similar to that of the frame cross section.