Shorten a concrete beam

Consider a design where two concrete beams are placed in an L shaped configuration.

Select (Modify Member End) from the ribbon.
The Modify Member End tool settings window opens.
Set the Modify Type menu to Extend end to point, which will lengthen the member by moving the end point to the new location that you define with a data point.
Move your cursor to the end of the bottom concrete beam. You will see the target highlight. Accept the target, then move your pointer to shorten the beam along the X-axis.
The member shortens to the new length that you defined.