Theory of plastic mechanism control for eccentrically braced frames: Closed form solution

Abstract

Collapse mechanism control is universally recognized as one of the primary goals of the structural design process. The aim is to avoid partial collapse mechanisms, such as soft storey mechanisms, which are unsatisfactory in terms of energy dissipation capacity. The optimization of the seismic structural response is, conversely, obtained when a collapse mechanism of global type is developed, because, in such case, all the dissipative zones are involved in the corresponding pattern of yielding, leaving all the other structural parts in elastic range. These are the basis of the so called “capacity design” principles, which state that dissipative zones have to be designed according to the internal actions arising from the design seismic forces, while the non-dissipative zones have to be proportioned on the basis of the maximum internal actions which dissipative ones are able to transmit in the fully yielded and strain-hardened state. In order to decrease the probability of plastic hinge formation in columns, MR-Frames must be designed to have strong columns and weak beams. To this scope, different simplified design criteria have been proposed and the so-called beam-column hierarchy criterion has been introduced in Eurocode. Even though studies on this topic started several decades ago mainly with reference to reinforced concrete structures and, in particular, in New Zealand where the capacity design procedure found its codification since 1982, codified design rules included in Eurocode 8 as well as similar procedures adopted by other codes cannot achieve the design goal, i.e. the development of a global type mechanism. There are a number of reasons why the beam-column hierarchy criterion cannot achieve the above mentioned design goal and these have been widely discussed both with reference to reinforced concrete frames and to steel frames. In fact, it is well known that such hierarchy criterion is able to prevent soft-storey mechanisms, but is not adequate to assure a collapse mechanism of global type. Among the different reasons leading the beam-column hierarchy criterion to fail in the achievement of the design goal, probably the most important, and difficult to be accounted for in a simplified design approach, is the shifting of the contraflexure point in columns during the seismic excitation. This considerable shifting leads to a bending moment distribution substantially different from that resulting from code-prescribed design rules. The shift of the contraflexure point is caused by the formation of hinges in beams adjacent to the column and even in part of the columns. All these factors alter the stiffness of beam-column subassemblage, hence the moment distribution.... [edited by Author]