Plastic design of eccentrically braced frames, I: Moment–shear interaction

With reference to eccentrically braced frames (EBFs), the issues regarding moment–shear interaction in plastic design are examined. In particular, a procedure for computing the ultimate shear forces and end moments of intermediate links for a given collapse mechanism is presented. The requirements to be fulfilled are the yielding condition, the flow rule, the kinematic compatibility condition depending on the examined collapse mechanism and, finally, the minimisation of the internal work according to the kinematic theorem of plastic collapse.Moreover, through the definition of an equivalent moment, the link is modelled as an element with plastic hinges in simple bending. This is obtained by imposing the equivalence between the internal work developed by the actual link and the internal work corresponding to the simplified theoretical model. Therefore, moment–shear interaction in plastic design can be accounted for by means of formulations which are formally coincident with those commonly used for plastic hinges in simple bending.Finally, the kinematic theorem of plastic collapse is used to define the design requirements to be fulfilled to prevent yielding of the beam part outside of the link and yielding of the bracing element. The additional design rules to be satisfied for failure mode control of multi-storey EBFs are presented in a companion paper.