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SEAOC Blue Book ­ Seismic Design Recommendations

Concentrically Braced Frames

Table 2. Recent code revisions for SCBF systems.

AISC SEISMIC 341-02 Braces shall have KL/r 5.87

AISC SEISMIC 341-05 Braces shall have KL/r 4 E/Fy , unless columns have available strength for brace capacities, RyFyAg. Width-thickness ratios per table I-8-1 (no changes), and AISC 360-02, table B4.1 (changes to numerous section types, including HSS). Alternative requirement of gusset plate to accommodate inelastic post-buckling deformations (typical condition detailed in commentary). Beam bracing (at V and inverted-V config.'s) must meet max. spacing criteria, and axial demand shall satisfy Pbr = 0.02MrCd/ho, where Mr is the required flexural strength, Cd is a curvature factor, and ho is the distance between flange centroids Exception for penthouses, ones story structures, and top stories of buildings removed for special beam design requirements for V and inverted-V conifig's. Column splices shall develop shear strength, Mpc/H, where Mpc is the sum of nominal plastic flexural column strengths. No K brace systems allowed.

E/Fy

Width-thickness ratios per table I-8-1, and AISC 360-99, table B5.1. Alternative requirement of gusset plate to accommodate inelastic postbuckling rotation (no guidelines given as to how). Beam bracing axial demand (at V and inverted-V config.'s) shall satisfy 2% of Fybftbf. Column splices shall develop nominal shear strength of smaller connecting member. No K brace systems allowed.

UBC 1997 R = 6.4; = 2.2 (Uses exact provisions of AISC Seismic 341-92, as follows) Braces shall have L/r 720 /

Fy .

Brace compressive strength shall not exceed 0.8cPn. Width-thickness ratios based on AISC 360-86. Limit net section area to: Ae/Ag (1.2Pu*)/tPn, where Pu = brace axial demand, = fraction of demand across net section, and tPn = brace tension capacity. Gusset plates required to have a 2tgp offset of brace from the theoretical line of bending. No unbalanced brace force design requirement for V and inverted-V braced frame beams. Beam bracing axial demand (at V and inverted-V config.'s) shall satisfy 1.5% of Fybftbf. K-brace systems allowed (see UBC 1997 for specifications).

IBC 2006 R = 6; = 2 Steel Design per applicable codes, including AISC 34105 and AISC 36005.

Article 8.03.050

Page 3 of 19

www.seaoc.org/bluebook

August 2008

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