By Payam Khashaee, PhD, PE, Thornton Tomasetti, Inc.  /  This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

In January 2008, the California Building Code 2007 edition, which is based on the International Building Code 2006 edition, became effective, and consequently ASCE 7-05 for loading, and AISC 360-05 Specifications and AISC 341-05 Seismic Provisions for steel design. The prior California Building Code was approved in 2001, which was based on the 1997 edition of Uniform Building Code. Therefore, there are major changes in the building code including the seismic design parameters for SCBF systems. For example, Response Modification Coefficient R has changed from 6.4 to 6.0 and Over Strength Factor  W  from 2.2 to 2.0. The width-thickness ratio requirements (local buckling) for braces have not changed in the code. However, the slenderness limit (member buckling) for an SCBF brace in AISC 2005 is over 30% more stringent than the former editions of the code.

 CBC 2001 §2210 Division IV (based on AISC 1992) and AISC 341-02 Section 13.2a limit the slenderness (kl/r) of an SCBF bracing member to 5.87    Es/Fy), whereas the AISC 341-05 (same section) limits slenderness to 4.0   Es/Fy. The new code, however, is more lenient on the upper limit of brace slenderness (200 vs 5.87   Es/Fy) if a capacity-based design is used for SCBF columns. That is, the columns should be checked for the maximum brace load transfers using the lesser of Ry (the ratio of the expected yield stress to the specified minimum yield stress, ‘AISC Table I-6-1’) times the nominal strengths of the connecting braces or the maximum load effect which can be determined through a number of ways such as pushover analysis, nonlinear time-history analysis, or other reasonable collapse mechanisms, see Section C13.3a.
For a rectangular HSS section with ASTM A500 Grade B, Fy=46 ksi, the slenderness limit previously was 147, whereas in the new code it is only 100. In long span SCBF's, the slenderness of the bracing most likely governs the design. Therefore, in many cases even strength does not govern, and one brace size will be used through the whole height of frame. The effective unbraced length of the brace (kl) can be determined between the two yield lines at gusset plate since SCBF connections are detailed to make the gusset buckle at the plastic hinges in the event of a major earthquake.

Figure 1. Maximum square HSS brace effective length per SCBF brace slenderness limits in AISC 341-05

Conservatively, slenderness checks by RAM Structural software (version 11.2.1) are based on the center-to-center brace length, and unconservatively the upper limit for slenderness in the software are not updated to the current code yet.

Based on the section compactness criteria in CBC 2007 edition, 32 square sections can be used for SCBF bracing, see AISC Seismic Design Manual Table 1-2. The smallest square HSS section available for seismic bracing is HSS 2x2x1/8 and the largest is HSS 10x10x5/8, see Figure 1. If the effective unbraced length of a brace (yield-line to yield-line) exceeds 31.8 ft or due to strength requirements a larger section is needed, one may need to use a different shape, e.g., W sections. The flaw of W sections as bracing members is their relatively more difficult connection to gusset.
Structural Detailing around the  Brace
CBC 2001 does not prohibit attachments to bracing member or gusset. Therefore, it has been common in California to provide only one row metal stud inside the frame and connect it to the brace. CBC 2007 (AISC 341-05), however, defines the Protected Zones as area of members in which limitations apply to fabrication and attachments.  The AISC commentary section C12.1, see Figure 2, illustrates the protected zone for inverted V and X braced frames.  It is permitted to connect attachments to the beam or column. Framing any metal stud in the plane of these walls is difficult. It is feasible though to place one row of metal stud each side of the braces with no connection to the brace or gusset plates.  In practice, when SCBF is used, almost there is no room for any attachment of the metal stud infill to the brace within the frame. Alternatively, when the architecture allows one may expose the braces and avoid the two layers of the metal stud framing.

AISC Fig. C-I-13.6 Protected zone of inverted - V braced frame - AISC Fig. C-I-13.7 Protected zone of X- braced frame

Figure 2. Protected zones of inverted V and X brace (After AISC 341-05)