Example

Consider a beam that has pin supports at both of its end. The beam is loaded by a point load in gravity direction applied at the midspan of the beam. The following 2 cases are being considered:

• Case 1: The beam is created with default (centroid, 10) insertion point.

• Case 2: The beam is created with top-center (8) insertion point.

It was observed that there is a larger midspan deflection for case 1 than for case 2. This is caused by different boundary conditions, which are a consequence of using different insertion points. The stiffness of the beam is the same for the two cases.

Since you are using pinned support at both ends of the girder, the girder is essentially restrained in longitudinal direction at its both ends. For case 2, the longitudinal restraint is not located at the centroid of the cross-section, but at the top-center (8) insertion point. This restraint prevents the top fibers from shortening and it introduces a tensile longitudinal force acting on an arm about the neutral axis of the section. This eccentric tensile reaction creates negative moment about the centroid of the cross-section, which reduces the positive moment due to the applied loads and thus results in smaller midspan deflection. Reviewing the screenshots below (mainly the moment diagram, axial force diagram and reactions) should make this clear.

If you change one support from pin to roller, you would see exactly the same response for the two cases, because no longitudinal forces is being created. You may also find the attached verification Problem 1-011 useful to understand insertion points. This problem is also accessible from "Help > Documentation > Analysis Verification > Frames" menu.

Composite Section