The line constraint is difficult to use. It has one slave joint interpolated between two master joints. In the example below, the two joints at the tops of the two intermediate columns are each slaves. The master joints are the corners of the corresponding shells. If the joints circled below are numbered 1, 2, 3, 4, 5, then 2 is slaved to masters 1 and 3, and 4 is slaved to masters 3 and 5.

To use the line constraint, select the slave joint first and assign it to the line constraint. Then select the two master joints and assign them to the same line constraint. Each line constraint can only have 3 joints. You would need two line constraints. At the corners, use an Equal constraint to connect the top of the column to the corner shell. You will have four constraints in all.

A better way to model this may be:

• Divide the two shells at the intermediate columns, and use edge constraints to transition the mesh further in.
• Disconnect the tops of the columns from the shells, yielding 8 joints
• Select the 8 joints, and assign them to a single weld constraint.

Example