Moment vs shear connections — what a release actually costs

Build the fixed-base portal exactly as before:

N 0,0 N 0,4 N 6,4 N 6,0 M 1 2 M 2 3 M 4 3 S 1 F S 4 F L 2 -10

Now release the rafter’s ends. Select the rafter, then set its Connection to Shear — or right-click either end and switch that end alone. A released end carries shear and axial force but no moment, and the drawing marks it with a small open circle just inside the member end.

That single change is the whole experiment. Everything else — geometry, section, load, supports — is identical to the rigid version, so every difference below is caused by the connection.

Try this with pinned bases instead and the solver refuses to run, reporting that the frame is a mechanism. It is right. With pinned feet and both rafter ends released, every column end is a pin — nothing anywhere resists the frame simply folding sideways.

That refusal is worth dwelling on. A moment connection is not a refinement you can delete wherever you like: in many frames it is the only thing making the structure stable. The bases are fixed here so that the frame stands up and the cost of the release can actually be measured.

With both ends released, the rafter can no longer hand moment to the columns. It carries its load exactly as a simply supported beam would:

The solver returns 45 kN·m — the textbook value to the decimal, in a frame that is otherwise indeterminate.

And the columns now carry 0 kN·m of moment. Zero. They have become props: pure axial members holding the beam up.

Compare the rigid version: rafter 22.55 kN·m, columns 22.45 kN·m. The moment connections were sharing the work. Release them and the rafter absorbs everything the columns were helping with.