This is not intuitive, because the first story braces did not yield in tension while the second story braces did. The curves in Figure 2b show that there was little difference in the strength and stiffness of the weak- and strong-beam chevron frames. Equation 1 shows V bb as a function of the buckling capacity of the brace, P cr, and the brace angle, θ. The base shear is normalized by the base shear force corresponding to buckling, V bb. The frames were loaded quasistatically under a fully reversed increasing amplitude cyclic protocol. While the member sizes of these frames are different than those shown in Figure 1, the relative strengths of the various members are similar. The braces in all three frames were identical (HSS5×5×3/8). Figures 2b and 2c present the experimental cyclic and backbone curves of the three frames. These tested frames include a chevron braced frame with a weak beam (left column of Figure 2), a chevron braced frame with a strong beam (center column of Figure ), and a multi-story X-braced frame (right column of Figure 2 Lumpkin 2009). The authors have conducted tests that reflect the three categories of braced frames considered in the November article, and these experiments are discussed here. Comparison of Weak-Beam Chevron, Strong-Beam Chevron, and Multi-Story X Frames. The consequences of these assumptions are clear in the experimental results described below, which vary drastically from the results in the previous work.įigure 2. While the observations from these analyses are logical based on the results presented, this article contends those results are dependent on numerous assumptions that mischaracterize CBFs. This assumed story height aligns with dimensions common in practice and corresponds to approximately 0.3% average story drift at brace buckling, which is typical of braced frames in experiments. The author did not provide dimensions for the frames reproduced in Figure 1a, but assuming a story height of 12 feet, this deformation corresponds to 4.0% to 4.8% average story drift. Further, the analysis suggested that Frame 2, with a strong beam and HSS braces meeting current AISC SCBF slenderness limits, may achieve an inelastic deformation of about 12 times the buckling deformation. The 2014 article thereby concludes that a frame with an undersized beam results in poor frame performance. As seen in the Figures, the predicted response of each braced frame system is characterized by sudden drops in strength, which is not characteristic of braced frame tests with adequate connections. Frames 2 and 3 were found to perform adequately, but Frame 1 exhibited a significant loss in strength and stiffness after brace buckling. The analyzed frames included a weak-beam chevron braced frame (Frame 1), a strong-beam chevron braced frame (Frame 2), and a multi-story X-braced frame (Frame 3) the results are repeated in this article as Figure 1b. The author analyzed three two-story frames using a nonlinear pushover analysis, shown in Figure 1a. The article How Big is that Beam? (STRUCTURE magazine, November 2014) addresses this beam strength issue. A research project to address the performance and potential retrofit of these older braced frames is currently in progress.įigure 1b. However, many braced frames built prior to the mid-1990s do not meet this requirement and therefore are generally considered to be “weak” and substandard. In 1997, a requirement was introduced that the strength of the beam in a frame with braces in a chevron configuration must be able to withstand the post-brace buckling unbalanced forces, based on the assumption that the braces in tension yield and the braces in compression resist 30% of their critical buckling loads (AISC 2010). Since 1990, AISC has focused on improving seismic resistance of CBFs by introducing detailing requirements for the connection, geometric limits of the brace, and capacity-design-type strength requirements for the gusset plate and the framing members. Concentrically braced frames (CBFs) resist large lateral forces due to wind and earthquake loading, and their ductility is largely derived from tension yielding and compressive buckling of the braces.
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