Internal forces
The panel a soft joint empties into the span
Under a load on one bay, an interior column's web panel carries the difference between the two beams' moments. Make the beam-to-column joints semi-rigid and the panel is relieved — by 5 per cent at the stiffness EN 1993-1-8 still calls rigid, by a third at three times the beam's EI/L — because a softer joint takes less moment. The moment it does not take lands at mid-span, where the beam was not designed for it, and in sway it lands in the columns. And soften only some of a frame's joints, and the sway goes looking for the ones still stiff: their panels carry up to a quarter more than they would in an all-rigid frame.
