Connecting Frame with Shell

The shell element formulation in SAP2000 combines membrane and plate behavior and the element has six degrees of freedom at each of its joints. Please refer to CSI Analysis Reference Manual, Chapter "The Shell Element", Section "Degrees of Freedom" for additional detailed information about the degrees of freedom for shell elements. Since frame elements have also six degrees of freedom at each of their joint, frame elements can be connected directly to the joints of the shell elements.

However, the stiffness for the drilling DOF (about the normal axis) for the shell is not reliable for connecting frame elements. The connection becomes increasingly flexible as the mesh is refined. Depending on the orientation of the frame element relative to the shell element, the following two scenarios are possible:

• If the frame element is in the 1-2 local plane of the shell, we recommend running the frame element into the shell mesh by at least one shell element length so the it connects to two joints of the shell, and releasing the moment at the far end of the frame element.
• If the frame element is normal to the 1-2 local plane of the shell, a small frame element could similarly be added in the plane of the shell, again releasing the moment at the far end of the new frame element.

Connecting Solid with Frame or Shell

Connecting frame or shell element to a solid element is a slightly different problem, but the basic idea to model this connection is similar. The main difference is that the solid elements have only translational degrees of freedom at their joints, therefore they would not provide any rotational resistance to connected frame or shell elements. The idea here is to resist the moments at the joints of frame or shell elements by a force couple at joints of the solid element. This can be done by using body constraints or rigid links to connect end joint of the frame element with the tributary joints of the solid elements as shown in the figure below:

Remember that when you are using frame elements, you are assuming that the cross section is rigid. Therefore it is a reasonable assumption to connect to a rigid plane at the face of the solid elements using constraints. For the most accurate results, the size of the solid elements at the connection point should be such that the constrained joints have dimensions about the same size as the cross section of the frame element.