Transform stiffness involves an important distinction where the line which represents a frame element in a model may be different from the line which represents the element centroid and physical attributes. When the [insertion point] is at the centroid, these lines coincide. In this case, the Do not Transform Stiffness for Offsets from Centroid option generates identical results regardless of whether or not it is selected. In all other cases, these lines are different, and some notable points include the following:

The default setting leaves the Do not Transform Stiffness for Offsets from Centroid option unchecked. This does transform stiffness to account for element offset from the line drawn to represent that element in the model. As a result, the analytical model accounts for the actual offset location of a frame element.

By selecting the Do not Transform Stiffness for Offsets from Centroid option, users locate the centroid of the frame element along the line drawn to represent the element in the model.

It may be best to implement the default setting by leaving the Do not Transform Stiffness for Offsets from Centroid option unchecked, since this coordinates stiffness with the actual physical location of the frame element.

Figure 1 illustrates an example where a frame element is modeled using a top center insertion point. As seen in the first image, the element is actually located entirely beneath the line which represents its location in SAP2000. By following the default setting, stiffness is automatically transformed to account for the offset location of the element centroid. This is shown in the second image, where the analytical line extends downward to align with the centroid. Selecting the Do not Transform Stiffness for Offsets from Centroid option has the effect shown in the third image. Here, the default transformation is not made, and the analytical model aligns element attributes with the projection of the insertion point (top center), though the element is actually entirely beneath this line. Different stiffness properties will result.

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Figure 1 - Caption

An instance in which this topic may be relevant is the modeling of a bridge superstructure. A line may be drawn to represent the location of a composite bridge deck. In relation to this line, girders may be specified with a top-center insertion point such that they are positioned below the line, which will then align with the plane of their top flanges. Then the deck may be specified with a bottom-center insertion point such that it is above the line, which will then align with its bottom surface. This would then form a composite bridge deck section.

Additional diagrams and a discussion on how insertion point affects horizontal reaction may be found in the Effect of insertion point on beam reactions test problem.

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