Capacity Design

The capacity design approach explicitly considers the problem of determining the failure mechanism of members. The basic idea is to force the member to fail in a ductile manner by making the capacity of the member in other possible failure modes greater. It involves the simple application of plastic analysis on an element-wise basis as shown below.
In the case of the beam shown at left, the preferred ductile mode of failure is flexure, while brittle shear failure is to be avoided. Therefore the shear corresponding to the plastic moment in the beam is the design shear. The beam is then designed so that its nominal shear strength is greater than this shear.

In order for capacity design to work, it is vital that reasonable, probable capacities for a member be determined. Often, beams have significant overstrength in flexure which must be taken into account in order to determine what the actual plastic moments might be.

This procedure works well for designing the beams in a strong-column/weak beam design and for joints, but doesn't work nearly as well for columns. Also, way the system behaves as a whole is not considered.

Capacity design procedures are implemented in the New Zealand code and in various parts of the concrete sections of the UBC. They are also hidden in some parts of the UBC provisions.