Effect of insertion point on beam reactions presents additional information on this topic.
The insertion point relates the actual position of an object to the line drawn to represent that object in a model. By default, prismatic objects are positioned such that their centroid and analytical properties align with the line shown in the computational model. Curvilinear objects are positioned such that their midspan centroid is in alignment. Sometimes, however, an insertion point is specified such that an object is positioned relative to this line. For example, if a girder should be drawn such that its nodes are at each end of the top flange, the top-center insertion point should be specified before drawing the object. This will position the girder below the line which represents its location.
To demonstrate, an example considers a simply supported beam with pin supports at either end. A point load, oriented in the gravity direction, is applied to the beam midspan. The two cases considered for beam location include:
- Case 1: Default insertion point at the object centroid (object 10)
- Case 2: Top-center insertion point (object 8)
Upon completion of analysis, it is observed that the midspan deflection for Case 1 is larger than that for Case 2. While beam stiffness is the same for each model, this discrepancy may be attributed to the difference in boundary conditions which results from variable insertion-point location.
The pinned-support configuration restrains each beam against longitudinal displacement. For Case 2, this longitudinal restraint is not at the centroid of the cross-section (10), but at the top-center insertion point (8). This prevents the top fibers from shortening, and introduces a longitudinal tension force which acts on an arm about the neutral axis. Eccentricity creates a negative moment which reduces the positive moment induced by applied loading. This also reduces midspan displacement. Figure 1 displays beam geometry and deflection, and Figure 2 presents moment- and axial-force diagrams.
Figure 1 - Beam geometry and deflection
Figure 2 - Moment- and axial-force diagrams
The insertion of a roller support at one of the previous pin locations would free the beam from longitudinal response. This is shown in Figure 3:
Figure 3 - Longitudinal release from roller support
- Effect of insertion point on beam reactions test problem
- Verification Problem 1-011, available in Context Help through the Help > Documentation > Analysis Verification > Frames menu