This paper presents the experimental investigation of two brace middle connections: 1) using a slotted tube knife‐plate connection (STKPC), and 2) a bolted sandwich plate connection (BSPC). The STKPC consists of a gusset plate slotted through the continuous brace and welded to discontinuous braces. The BSPC consists of two cover plates and four U‐shaped plates connecting the discontinuous braces using bolts. The two cover plates pass on either side of the continuous brace and are bolted to the U‐shaped plates. These connections are used to join the discontinuous braces in steel concentrically braced frames (CBFs) configured with X‐bracing. These connections were tested as part of a full‐scale CBF using quasi‐static testing. In both specimens, seismic energy dissipated through yielding and buckling of the braces. However, in the specimen with STKPC, axial deformation is concentrated in a single discontinuous brace, which led to local buckling at mid‐length and subsequent fracture. In contrast, the BSPC is able to better distribute axial deformation between the two discontinuous braces delaying local buckling. Moreover, the measured buckling capacity of braces when using the BSPC matches well the expected compressive resistance calculated using an effective length factor K = 0.45.

This paper investigates the possibility of using the chevron bracing configuration for multi-tiered concentrically braced frames subjected to seismic excitations. A prototype two-tiered braced frame part of a single-storey building structure was designed using three different brace force scenarios for the roof beam and the intermediate strut. Columns were designed to resist the bending expected at the maximum anticipated storey drift. The lateral response of the frame was examined through nonlinear static and dynamic analyses. For all cases studied, frame lateral deformations tend to concentrate in the first tier, where brace buckling initiated first, due to the reduced tier lateral stiffness in the brace post-buckling range. The flexural action in the intermediate struts was engaged when a reduced force was used for tension-acting braces in design, limiting nonlinear response in braces. Finally, the frames exhibited stable inelastic response with limited residual deformations, as a result of the re-centring capacity provided by the strut acting in flexure.

This paper investigates the possibility of using multi-tiered concentrically braced frames in two adjacent column bays to resist seismic loads. Three prototype frames part of a single-storey building were chosen and designed using current knowledge of multi-tiered behaviour. The columns were selected to resist in-plane bending and axial loads arising from tensile yielding and compression buckling of braces in critical tiers. The lateral response of the frame was then examined using the nonlinear response history analyses under ground motion accelerations. The analyses confirmed that all frames exhibited nonuniform brace tensile yielding between tiers, which resulted in the concentration of inelastic drifts in the uppermost tiers. Peak storey drift values remained under 2.5%, although higher than the design predictions, which influenced the prediction of column in-plane bending.

Steel multi-tiered concentrically braced frames (MT-CBFs) are commonly used in North America as a lateral load resisting system of tall single-story buildings. Past studies show that MT-CBF columns designed in accordance with the 2010 AISC Seismic Provisions are prone to buckling due to a high axial compression force combined with in-plane bending moments caused by the nonuniform distribution of inelastic brace deformations along the frame height. Special design provisions have been introduced in the 2016 AISC Seismic Provisions to address flexural demands imposed on MT-CBF columns and prevent column instability. In this paper, the seismic design methods for multi-tiered special concentrically braced frames are evaluated using the nonlinear finite element analysis method. A two-tiered special concentrically braced frame was then created, and nonlinear static and dynamic analyses were performed to evaluate the seismic performance of both frames. Analysis results confirmed that the inelastic deformations in the frame designed using the 2010 requirements are not uniformly distributed but rather concentrated in one of the tiers and cause column instability under large story drifts, whereas, the 2016 design method significantly improves the distribution of inelastic deformation along the height of the frame and prevents column instability. Furthermore, it was found that the 2016 AISC Seismic Provisions accurately estimate the axial load but overestimate the in-plane flexural demands and underestimates the out-of-plane flexural demand. Nonetheless, the overestimation of in-plane flexure demands results in acceptable strength capacity even though out-of-plane flexural demands is underestimated.

Steel multi-tiered concentrically braced frames (MT-CBFs) are commonly used in North America as lateral load-resisting systems of tall single-story buildings. MT-CBFs are composed of multiple tiers of diagonal braces stacked on top of each other along the height of a story. Past research showed that MT-CBF columns designed in accordance with the 2010 U.S. Seismic Provisions are prone to column buckling due to non-uniform distribution of inelastic seismic demands along the frame height. Special design provisions have been introduced in the current AISC Seismic Provisions to address limit states and ensure columns remain stable under seismic load effects. Nevertheless, the recent improvements lack full-scale experimental testing and comprehensive finite element simulations to validate the proposed design requirements further and improve design provisions. In this paper, the current seismic design provisions for multi-tiered special concentrically braced frames (MT-SCBFs) are evaluated using the finite element method. A two-tiered SCBF was first designed in accordance with the 2010 and 2016 AISC Seismic Provisions. A detailed finite element model of the frame was then created using the Abaqus program. The model was used to perform nonlinear history response analyses. The analysis results showed that the inelastic deformations in the frame, designed as per the 2010 AISC Seismic Provisions, are not uniformly distributed but rather concentrated in one of the tiers, which leads to column yielding and buckling. Whereas, the current design method led to distributing the frame's inelastic deformation along the frame height. Furthermore, it was found that column in-plane flexural demands are overestimated when the current seismic provisions are employed; however, the out-of-plane flexural demand of columns exceeded the code-specified demands.