Modified seismic design of concentrically braced frames considering flexural demand on columns

Abstract

Summary Special concentrically braced frames (SCBFs) are considered as one of the most economical and effective lateral force‐resisting systems in structures located in the regions of high seismicity. Steel braces in a braced frame undergo large axial deformations in tension and compression to dissipate the seismic energy. However, past studies have shown that SCBFs exhibit the soft‐story hinge mechanisms and unpredictable failure patterns under earthquake loading conditions. These inelastic responses along with the use of continuous structural sections as columns over consecutive floors induce flexural demand that is not considered in the current design practice. In this study, the evaluation of seismic performance of nine SCBFs designed as per the current practice has been carried out for three different story heights (i.e., three‐story, six‐story, and nine‐story) and three types of brace configurations (namely, chevron, split X, and single X). Three additional design techniques are also explored based on (i) the inclusion of column moments in the design; (ii) the theory of formation of plastic hinges; and (iii) the design of braces considering the forces computed at their post‐buckled stages. Nonlinear dynamic analyses of these study frames have been evaluated numerically using a computer software Perform‐3D for a suite of 40 ground motions representing the design basis earthquake and maximum considered earthquake hazard levels. Analyses results showed that the SCBFs designed as per the modified procedures achieved the desired performance objectives without the formation of soft‐story mechanism. Copyright © 2017 John Wiley & Sons, Ltd.

Numerical and experimental investigations on inelastic cyclic performance of mid‐span gusset plate connections

“…Nine cyclic loading steps are applied until a maximum brace deformation corresponding to a story drift of 5%. This limiting value of story drift has been fixed considering the maximum inter‐story drift noted in SCBFs under the seismic loading and the probable connection failure beyond this drift level …”

“…This limiting value of story drift has been fixed considering the maximum inter-story drift noted in SCBFs under the seismic loading 37 and the probable connection failure beyond this drift level. 38…”

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

“…suggest that the design axial forces in columns should be evaluated assuming that braces have yielded at all storeys, whereas Bosco et al 15 . and Kumar et al 16 . point out that the design procedures reported in seismic codes or in literature generally neglect or significantly underestimate the bending moments of columns.…”

“…Based on the results of previous experimental tests and numerical studies indicating that a limited yielding beam is not detrimental to nonlinear response, 10 a different approach with yielding beams is suggested by Tan et al 11 Several research studies have also proved that the rules for the application of the capacity design principles are often not effective in preventing yielding or buckling of the columns of the braced frame. 7,[12][13][14][15][16] In this regard, Longo et al 12 suggest that the design axial forces in columns should be evaluated assuming that braces have yielded at all storeys, whereas Bosco et al 15 and Kumar et al 16 point out that the design procedures reported in seismic codes or in literature generally neglect or significantly underestimate the bending moments of columns. Indeed, these bending moments are negligible when braces are elastic, but they can be significant when braces are in their inelastic range of behaviour and damage concentrates at a few storeys.…”

Seismic design study of concentrically braced frames with and without buckling-controlled braces

“…Because pin connection performance in actual structures closely resembles that of rigid connections, it can be assumed that the additional demand in the connections will be transferred to the columns as well. Kumar, et al, (2017) performed dynamic analysis in Perform-3D to study the seismic response of steel braced frames and they consequently proposed a new design approach taking into account the flexural demand on columns. They proposed three different limit states (P1 through P3) for braced-frame design, but it seems difficult to utilize this design approach in practice.…”