In a seismic frame you do not design for strength alone - you design where it yields. An earthquake delivers energy, not just force, and a structure survives by absorbing that energy through controlled, ductile yielding rather than resisting it elastically. The discipline that makes this deliberate is capacity design, and it changes how you think about every member and joint.
Energy, not just force
The key shift from gravity design is that a seismic event is an energy problem. You cannot economically make a building strong enough to ride out a large earthquake elastically; instead you let it deform inelastically and dissipate the input energy through repeated yielding. A ductile frame is one that can do this many times without losing its ability to carry gravity load - it bends, absorbs, and stays standing.
That only works if the yielding happens in the right places, in the right order. Left to chance, a frame might form its weak link somewhere catastrophic. Capacity design removes the chance.
The weak-beam, strong-column principle
The central idea is to choose the failure mechanism in advance:
You deliberately make the beams yield before the columns, so plastic hinges form in the beams where they can absorb energy without collapsing the building. A column hinge can drop a storey; a beam hinge just bends a beam. Choose the failure.
A mechanism with hinges in the beams spreads the energy dissipation across many ductile locations and keeps the columns - the members holding the building up - essentially elastic. A mechanism with hinges in the columns can concentrate all the deformation into a single storey, which then loses its capacity to carry gravity and collapses. The columns are therefore deliberately made stronger than the beams framing into them, so the beams are guaranteed to go first.
Detailing the hinge so it can actually be ductile
Choosing where the hinge forms is only half the job. The hinge then has to behave ductilely under reversing cycles, and that is a detailing problem:
- Close stirrup (link) spacing in the hinge zone confines the core concrete, so it can keep carrying load even as it cracks and the cover spalls.
- That same confinement stops the longitudinal bars from buckling when they go into compression on the reversing cycle - an unconfined bar buckles and the hinge fails brittlely.
- Adequate anchorage and continuity ensure the bars can develop their yield force through the cycles without pulling out.
Without that confinement, ductility evaporates exactly where you need it - the hinge you planned to be the building's shock absorber becomes its point of failure.
This is why seismic detailing looks so different from gravity detailing: the tight links in beam-column zones are not arbitrary, they are what turns a planned hinge into a working energy dissipator.
Carrying it into design
- Decide the mechanism, then enforce it. Strong columns relative to beams is not an outcome to check at the end; it is a rule to design to from the start.
- Detail the dissipative zones for ductility, not just strength - confinement, anti-buckling links, and anchorage are the difference between a ductile hinge and a brittle one.
- Protect the elements that must stay elastic - columns, joints and foundations are designed for the forces the yielding beams can actually deliver, not the lower analysis forces.
The related robustness article covers the alternative-load-path thinking that complements ductile design, and the seismic provisions sit alongside the other Eurocodes in the standards reference.