§9 Concentrically Braced Frames

§9.2. Bracing Members

To perform the lateral force distribution checks on braced frames per Section 9.2c, the engineer is required to assign the same frame number to all the braces in the same line of bracing. Also, the Use Frame Nos. to Designate a Frame Line option must be selected in the Seismic Code Dialog obtained by selecting the Criteria-Codes command.

Note: The force distribution requirement for §9.2c is performed on a per story basis. If the distribution on a single story exceeds the lateral distribution limit, then the braces of that story are checks for compliance with the exception. Note that the full strength of the braces (excluding phi factor) is used to check compliance with the exception.

The web of a HSS (Tube) section for special concentric braced frame is checked in accordance with the Stiffened Webs in Combined Axial and Bending provision of LRFD Table B5.1. This is true even if no bending exists on the member as the tube is required to be Compact per 12.2d (for SCBF) and pure compression does not have a compact limit in Table B5.1.

In §9.2d, the h/t and b/t limits are calculated in accordance with the AISC-LRFD specification. For HSS sections b = Flange Width – 3 x Web Thickness and h = Depth – 2 x Flange Thickness.

§9.4. Special Bracing Configuration Requirements

Per Section 9.4.a.1. The chevron braces are checked for all the loads from standard provision load combinations that include a seismic load case.

§9.5 Low Buildings

The low building exception is applied if the user indicates to apply the one and two story exceptions (refer to Criteria–Codes command), or if the member is at the top most lateral story.