Proposal and validation of a design procedure for concentrically braced frames in the chevron configuration

Abstract

In the recent past, several research studies have highlighted that the prescriptions reported in codes for the design of structures with concentric braces in the chevron configuration are often not effective in preventing yielding or buckling of non‐dissipative members and in ensuring collapse mechanisms characterised by uniform damage of braces. To investigate the reasons of these deficiencies, in this paper the seismic response of concentrically braced structures designed according to procedures reported in the literature and in the European seismic code is first examined. Then, a new design procedure is proposed, in which the innovative aspects are mainly related to the estimation of the bending moments in columns and to the formulation of requirements on the stiffness of braced beams and columns. The impact of the proposed procedure on the structural costs is computed on a large number of buildings characterised by different occupancy types and geometric properties. The seismic performance of these structures is evaluated by incremental nonlinear dynamic analysis and discussed at the achievement of the significant damage and near‐collapse limit states.

An Alternative Approach for the Design of Chevron-Braced Frames

“…Finally, to promote a more dissipative behaviour of braces, a beam stiffness requirement is proposed, suggesting that the beam-to-brace stiffness ratio should be greater than 0.2. Barbagallo et al [19] showed that the lateral torsional buckling of columns was the predominant failure mode of concentrically braced frames designed by the abovementioned procedures because of the underestimation of the bending moment in columns. Based on this, the authors proposed a new design procedure where an analytical equation was introduced to predict the bending moments in columns.…”

“…To evaluate the fulfilment of the SD and NC limit states, the response of the dissipative members (braces and braced beams) was expressed in terms of ductility demand-tocapacity ratios. To this end, the damage index DI of the braces is defined in [19] as follows:…”

“…Specifically, the ductility demand was calculated as the ratio of the sum of the absolute values of maximum elongation and shortening of the brace over the elongation corresponding to yielding of the brace. In keeping with [19], with regard to the Near Collapse (NC) limit state, a limit value of the DI equal to 1.0 was considered, whereas a limit value of DI equal to 0.75 was considered to be representative of the achievement of the Significant Damage (SD) limit state. The damage index of the beams was calculated as the ratio of the demanded plastic rotation to the plastic rotation capacity, defined as reported in Eurocode 8 part 3 [34].…”

“…Based on an analysis of features and the shortcomings of these approaches, a new design method is formulated, proposed and investigated in the paper. The most common approach for the design of chevron-braced frames assumes that the inelastic response should be limited to the braces [16][17][18][19]. Based on this concept, braces are designed to sustain the internal forces determined by the selected elastic method of analysis, so as to provide an almost uniform distribution of overstrength along the height of the building, whereas beams and columns are designed according to the capacity design principle.…”