Plastic Design of Seismic Resistant Steel Frames

Abstract

SUMMARYA new method for designing moment resisting steel frames failing in a global mode is presented in this paper. Starting from the analysis of the typical collapse mechanisms of frames subjected to horizontal forces, the method is based on the application of the kinematic theorem of plastic collapse. The beam section properties are assumed to be known quantities, because they are designed to resist vertical loads. As a consequence, the unknowns of the design problem are the column sections. They are determined by means of design conditions expressing that the kinematically admissible multiplier of the horizontal forces corresponding to the global mechanism has to be the smallest among all kinematically admissible multipliers.In addition, the proposed design method includes both the influence of distributed loads acting on the beams and the influence of second-order effects. In particular, the influence of second-order effects, which can play an important role in the seismic design of steel frames, is accounted for by the mechanism equilibrium curves of the analysed collapse mechanisms.Moreover, in order to show the practical application of the proposed design procedure, a worked example is presented. Finally, the inelastic behaviour of the designed frame is compared to that obtained when the simple member hierarchy criterion or a similar rule is applied.

On the Use of Genetic Algorithms to Assess the Seismic Resistance of Planar Frame Structures

“…In this case either the external or the internal work for each beam mechanism must be calculated considering that, as shown in Figure 3, a plastic hinge can be opened at a position x ij from the left end of the beam dependent on the magnitude of the uniform load acting on the beam. The superimposition of the uniformly distributed vertical loads and of the horizontal forces implies that, by increasing the horizontal forces the first plastic hinge opens at the right end of the beam, while the second hinge occurs at the left end when the distributed load is equal to the limit value [10]:…”

“…In the described seismic load scenario the plastic hinges may occur at the two ends of the columns and also in any section along the beam. In the paper the exact location of the plastic hinges along the beam is calculated by means of the strategy proposed in [10].…”

Failure Mode and Drift Control of MRF-CBF Dual Systems

“…The unknowns of the design problem are the column sections whose plastic modulus has to be defined so that the mechanism equilibrium curve ( -) corresponding to the global mechanism has to lie below the equilibrium curves corresponding to all the other undesired mechanisms within a displacement range compatible with the local ductility supply of dissipative elements. This statement corresponds to the extension of the upper bound theorem to the concept of mechanism equilibrium curve [1,2] and it allows to take into account also second order effects. Therefore, according to such kinematic approach, the global failure mode is the mechanism actually developed in the whole displacement range compatible with the local ductility supply.…”

“…In particular D v,jk = L j x jk / 2 when the beam participates to the collapse mechanism and D v,jk = 0 in the opposite case [2].…”

“…Therefore, in order to design structures able to assure the development of a collapse mechanism of global type under destructive seismic actions, a rigorous application *Address correspondence to this author at the Department of Civil Engineering, University of Salerno, Italy; Tel: +39+089+964108; Fax: +39+089+964108; E-mail: v.piluso@unisa.it of capacity design principles is needed, requiring more sophisticated design procedures. In particular, sophisticated design procedures assuring a collapse mechanism of global type have been already successfully developed with reference to moment-resisting frames (MRFs) [1,2], concentrically braced frames (CBFs) [3,4], eccentrically braced frames (EBFs) [5,6] and knee braced frames (KBFs) [7].…”