Default modeling procedures locate object boundary conditions along the centroid of the cross section. When a simply supported (pin-pin) frame object is modeled with a top-center or bottom-center insertion point, internal constraints are generated which have the effect of a vertical offset. A kinematic result of this modeling technique is the generation of longitudinal forces which act on an arm about the neutral axis. If this behavior is not desired, one support may be changed from a pin to a roller condition such that horizontal reactions are released. This behavior may be exhibited when bridge objects are supported by bearing links located at the bottom of the bridge girders.
Presented in Figure 1 are visuals which depict how standard and offset objects:
Figure 1 - Simply-supported beam with center and bottom-center insertion points
The images from Figure 1, as they appear in SAP2000, are given in Figure 2:
Figure 2 - Images as they appear in SAP2000