Plastic Moment Capacity (Mp = Z fy)

Once every fibre is at yield, the section is carrying all it can in bending and the moment stops rising: that plateau is $M_p = Z f_y$. A section with $Z = 1.5 \times 10^6$ mm³ in 350 MPa steel reaches $M_p = 525$ kN·m. In LRFD steel design this is the nominal flexural strength $M_n$ of a compact, adequately braced beam, and the design strength is $\phi M_n$ with $\phi = 0.90$ — so 472 kN·m goes up against the factored demand.

What makes plastic design more than an accounting change is what happens next in a statically indeterminate structure. The fully yielded section becomes a plastic hinge: it holds $M_p$ while it rotates, shedding further load to the rest of the frame. A fixed-ended beam that reaches $M_p$ at its supports does not fail, it redistributes and keeps going until enough hinges form to make a mechanism — which for that beam takes three, and delivers a collapse load well above the load that first yielded it. That reserve is real, it was demonstrated by John Baker's team at Cambridge in the 1930s and 40s, and it is why plastic design produces lighter frames than elastic design of the same members.

The conditions attached are strict, and they are all about being able to get there. The section must be compact, so its flange and web slenderness stay below code limits and no local buckling intervenes. The compression flange must be braced closely enough to prevent lateral-torsional buckling. The steel must have the ductility to sustain the rotation, which rules out high-strength low-ductility grades and cold-formed sections. And $M_p$ says nothing about serviceability — a beam sized on plastic capacity may still deflect far more than anyone will accept.