F1Ordinary concentrically braced frames (OCBF)

F1.4a V-Braced and Inverted V-Braced Frames

Beams in V- and inverted V-braced frames shall be continuous at brace connections away from the beam-column connection and shall satisfy the following requirements:

• (a) The required strength of the beam shall be determined assuming that the braces provide no support of dead and live loads. For load combinations that include earthquake effects, the seismic load effect, E, on the beam shall be determined as follows:
• (1) The forces in braces in tension shall be taken as the least of the following:
• (i) The load effect based upon the overstrength seismic load
• (ii) The maximum force that can be developed by the system
• (2) The forces in braces in compression shall be taken as a maximum of $0.3 P_{n}$, where

$P_{n}=$ nominal axial compressive strength, kips (N)

• (b) As a minimum, one set of lateral braces is required at the point of intersection of the braces, unless the member has sufficient out-of-plane strength and stiffness to ensure stability between adjacent brace points.

F1.4b K-Braced Frames

K-braced frames shall not be used for OCBF.

F1.4c Multi-Tiered Braced Frames

An ordinary concentrically braced frame is permitted to be configured as a multitered ordinary concentrically braced frame (MT-OCBF) when the following requirements are met.

• (a) Braces shall be used in opposing pairs at every tier level.
• (b) Braced frames shall be configured with in-plane struts at each tier level.
• (c) Columns shall be torsionally braced at every strut-to-column connection location.

User Note: The requirements for torsional bracing are typically satisfied by connecting the strut to the column to restrain torsional movement of the column. The strut must have adequate flexural strength and stiffness and an appropriate connection to the column to perform this function.

• (d) The required strength of brace connections shall be determined from the load combinations of the applicable building code, including the horizontal seismic load effect including overstrength, $E_{m h}$, multiplied by a factor of 1.5 .

• (e) The required axial strength of the struts shall be determined from the load combinations of the applicable building code, including the horizontal seismic load effect including overstrength, $E_{m h}$, multiplied by a factor of 1.5 . In tensioncompression X-bracing, these forces shall be determined in the absence of compression braces.

• (f) The required axial strengths of the columns shall be determined from the load combinations of the applicable building code, including the horizontal seismic load effect including overstrength, $E_{m h}$, multiplied by a factor of 1.5 .

• (g) For all load combinations, columns subjected to axial compression shall be designed to resist bending moments due to second-order and geometric imperfection effects. As a minimum, geometric imperfection effects are permitted to be represented by an out-of-plane horizontal notional load applied at every tier level and equal to 0.006 times the vertical load resulting in compression in the column and contributed by the compression or tension brace connecting the column at the tier level.

• (h) When tension-only bracing is used, requirements (d), (e), and (f) need not be satisfied if

• (1) All braces have a controlling slenderness ratio of 200 or more.

• (2) The braced frame columns are designed to resist additional in-plane bending moments due to the unbalanced lateral forces determined at every tier level using the capacity-limited seismic load based on expected brace strengths. The expected brace strength in tension is $R_{y} F_{y} A_{g}$, where

• $A_{g}=$ gross area of brace, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$

• $F_{y}=$ specified minimum yield stress of the brace, ksi (MPa)

• $R_{y}$ = ratio of the expected yield stress to the specified minimum yield stress of the brace, $F_{y}$

The unbalanced lateral force at any tier level shall not be less than 5% of the larger horizontal brace component resisted by the braces below and above the tier level.

F1.5a Basic Requirements

Braces shall satisfy the requirements of Section D1.1 for moderately ductile members.

Exception: Braces in tension-only frames with slenderness ratios greater than 200 need not comply with this requirement.

F1.5b Slenderness

Braces in V or inverted-V configurations shall have

(F1-1)

where $E =$ modulus of elasticity of steel $= 29,000 \text{ ksi } (200\ 000 \text{ MPa})$ $L_c =$ effective length of brace, in. (mm) $= KL$

$K=$ effective length factor

• $L=$ length of brace, in. (mm)
• $r$ = governing radius of gyration, in. (mm)

F1.5c Beams

The required strength of beams and their connections shall be determined using the overstrength seismic load.

F1.6a Brace Connections

The required strength of diagonal brace connections shall be determined using the overstrength seismic load.

Exception: The required strength of the brace connection need not exceed the following.

• (a) In tension, the expected yield strength divided by $\alpha_{s}$, which shall be determined as $R_{y} F_{y} A_{g} / \alpha_{s}$, where $\alpha_{s}=\mathrm{LRFD}-\mathrm{ASD}$ force level adjustment factor $=1.0$ for LRFD and 1.5 for ASD.
• (b) In compression, the expected brace strength in compression divided by $\alpha_{s}$, which is permitted to be taken as the lesser of $R_{y} F_{y} A_{g} / \alpha_{s}$ and $F_{n e} A_{g} / \alpha_{s}$, where $F_{n e}$ is the nominal stress calculated from Specification Chapter E using expected yield stress, $R_{y} F_{y}$, in lieu of $F_{y}$. The brace length used for the determination of $F_{n e}$ shall not exceed the distance from brace end to brace end.
• (c) When oversized holes are used, the required strength for the limit state of bolt slip need not exceed the seismic load effect based upon the load combinations without overstrength as stipulated by the applicable building code.