Theory of plastic mechanism control for eccentrically braced frames with inverted y-scheme

Abstract The theory of plastic mechanism control is presented with reference to eccentrically braced frames with vertical links, i.e. EB-Frames with inverted Y-scheme. Such theory has been originally developed with reference to moment resisting steel frames and gradually extended to other seismic resistant structural typologies aiming to the assure, in all cases, a collapse mechanism of global type. This result is of primary importance in earthquake resistant design, because partial and local failure modes are responsible for the worsening of the energy dissipation capacity leading to an increased risk of collapse under destructive seismic events. With reference to EB-Frames with inverted Y-scheme, the proposed method is based on the assumption that vertical link elements are preliminarily designed according to the internal actions due to the design seismic forces. Beam and diagonal sections are also assumed to be known quantities, because they need to be designed to fulfil a local hierarchy criteria assuring, at storey level, that yielding occurs in the link element only. Conversely, column sections constitute the unknowns of the design problem. The theory of plastic mechanism control includes also the influence of second order effects by means of the concept of mechanism equilibrium curve. In fact, the design requirements are derived by means of the kinematic theorem of plastic collapse extended to the concept of mechanism equilibrium curve. Column sections are obtained by imposing that the mechanism equilibrium curve corresponding to the global mechanism has to be located below those corresponding to all the undesired mechanisms within a displacement range compatible with the local ductility supply. Aiming at the evaluation of the accuracy of the presented design method, the inelastic response of EB-Frames with inverted Y-scheme designed according to the proposed method are investigated by means of push-over analyses to check the collapse mechanism actually developed.