Remarks on Seismic Design Rules of EC8 for Inverted-V CBFs

Abstract

Chevron concentrically braced frames (C-CBFs) are expected to provide limited ductility in the framework of Eurocode 8: differently from North American codes, lower values of behavior factors are recommended by EN 1998 for C-CBFs than for other concentric bracing configurations (namely diagonal and cross bracings). The research presented in this paper is aimed at revising the design rules and requirements provided for by EN 1998-1 for C-CBFs in order to improve the ductility and the dissipative capacity of this structural system. The proposed design criteria are validated by means of nonlinear dynamic analyses performed on a study case. The results confirm the effectiveness of developed design procedure.

Seismic Behaviour of EC8-Compliant Moment Resisting and Concentrically Braced Frames

“…As highlighted by previous research [17][18][19], the force transfer mechanism in the post-buckling range differs significantly from the elastic behavior and the columns are likely be subjected to a force significantly larger than that calculated according to Equation (11).…”

“…The seismic performance of chevron concentric bracings depends significantly on the behaviour of the brace-intercepted beam [17,29,30]. Indeed, after the buckling of the compression diagonal, an unbalanced vertical force is applied at the brace-intercepting section, inducing high bending demand.…”

“…These detrimental features are notably juxtaposed to the philosophy behind the second design concept, and numerous research [11][12][13][14][15]18,23] have been devoted to investigate and propose new design criteria to improve the seismic performance of steel structural systems. In these framework, special attention has been put on both moment resisting frames (MRFs) [2][3][4][5][6][7][8][9]21,22] and chevron concentrically braced frames (CCBFs) [11][12][13][14][15][16][17][18][19][20][21][22][23]; indeed, the design rules provided by EC8 for these structural typologies are affected by the largest number of criticisms and fallacies.…”

Seismic Design Criteria to Improve the Performance of X-CBFS

“…Numerous researchers [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] critically discussed and revised the design rules codified within EN 1998-1 and even identified some issues needing upgrading and/or amendment.…”

“…Another critical aspect of EC8-compliant design is related to the requirement on the capacity-to-demand ratios Ωi= Npl,br,Rd,i/NEd,br,i (being Npl,br,Rd,i the brace yield strength and NEd,br,i the design forces due to the seismic actions) which should vary in the range (Ω; 1.25Ω) to guarantee uniform distribution of plasticity along the building height and thus mitigating the tendency of CBFs to soft-story mechanisms. Several Authors [11][12][13][14][15][16][17][18] showed that this requirement is not adequate to guarantee an overall plastic engagement of the dissipative elements and it also leads to oversize the diagonals at lower and intermediate stories. Indeed, to contemporarily satisfy the requirements of slenderness limits and the overstrength variation often leads to impractical solutions with significant difficulties to select the proper member for braces, being both requirements closely interrelated and juxtaposed [4-9, 11-12, 14-15].…”