I5Combined flexure and axial force

The interaction between flexure and axial forces in composite members shall account for stability as required by Chapter C. The available compressive strength and the available flexural strength shall be determined as defined in Sections I2 and I3, respectively. To account for the influence of length effects on the axial strength of the member, the nominal axial strength of the member shall be determined in accordance with Section I2.

• (a) For encased composite members and for filled composite members with compact composite sections, the interaction between axial force and flexure shall be based on the interaction equations of Section H1.1 or one of the methods defined in Section I1.2.
• (b) For filled composite members with noncompact composite or slender-element composite sections, the interaction between axial force and flexure shall be based either on the interaction equations of Section H1.1, the method defined in Section II.2d, or Equations I5-1a and b.
• (1) When $\frac{P_{r}}{P_{c}} \geq c_{p}$

(2) When $\frac{P_{r}}{P_{c}}<c_{p}$

where

For design according to Section B3.1 (LRFD)

• $\begin{aligned} M_{r} & =\text { required flexural strength, determined in accordance with Section } \\ & 11.5 \text {, using LRFD load combinations, kip-in. (N-mm) }\end{aligned}$
• $\begin{aligned} M_{c} & =\text { design flexural strength determined in accordance with Section I3, } \\ & \text { kip-in. (N-mm) }\end{aligned}$
• $=\phi_{b} M_{n}$
• $\begin{aligned} P_{r} & =\text { required axial strength, determined in accordance with Section II.5 } \\ & \text { using LRFD load combinations, kips (N) }\end{aligned}$
• $\begin{aligned} P_{c} & =\text { design axial strength, determined in accordance with Section I2, } \\ & \text { kips (N) }\end{aligned}$
• $=\phi_{c} P_{r}$
• $\phi_{c}=$ resistance factor for compression
• = 0.75
• $\phi_{b}=$ resistance factor for flexure
• = 0.90

For design according to Section B3.2 (ASD)

• $M_{r}=$ required flexural strength, determined in accordance with Section 11.5, using ASD load combinations, kip-in. (N-mm)
• $\begin{aligned} M_{c} & =\text { allowable flexural strength, determined in accordance with Section I3, } \\ & \text { kip-in. (N-mm) }\end{aligned}$
• $=M_{n} / \Omega_{b}$
• $\begin{aligned} P_{r} & =\text { required axial strength, determined in accordance with Section II.5 } \\ & \text { using ASD load combinations, kips (N) }\end{aligned}$
• $\begin{aligned} P_{c} & =\text { allowable axial strength, determined in accordance with Section I2, } \\ & \text { kips (N) } \\ & =P_{n} / \Omega_{c}\end{aligned}$
• $\Omega_{b}=$ safety factor for flexure
• = 1.67
• = safety factor for compression = 2.00

$c_{m}$ and $c_{p}$ are determined from Table I5.1

$c_{s r}=\frac{A_{s} F_{y}+A_{s r} F_{y r}}{A_{c} f_{c}^{\prime}}$

(15-2)

• (c) For composite plate shear walls, the interaction between axial force and flexure shall be based on the methods defined in Section II.2.