2.1rectangular filled composite members

2.1.1 Limitations

For rectangular filled composite members, the following limitations shall be met:

• (a) The area of the steel section shall comprise at least 1% of the total composite cross section.
• (b) Concrete shall be normal weight, and the specified compressive strength of concrete, $f_{c}^{\prime}$, shall not exceed 15 ksi (100 MPa).
• (c) The specified minimum yield stress of steel, $F_{y}$, shall not exceed 100 ksi (690 MPa).
• (d) The maximum permitted width-to-thickness ratio for compression steel elements shall be limited to $5.00 \sqrt{E / F_{y}}$.
• (e) Longitudinal reinforcement is not required. If longitudinal reinforcement is provided, it shall not be considered in the calculation of available strength, and the minimum reinforcement requirements of Sections I2.2a and I3.4a shall apply.

2.1.2 Compressive Strength

The available compressive strength shall be determined in accordance with Section I2.2b with the following modifications:

where

• $A_{c}=\operatorname{area}$ of concrete, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$
• $A_{s}=\operatorname{area}$ of structural steel section, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$
• $F_{n}=$ critical buckling stress for steel section of filled composite members, kips (N)
• $F_{y}=$ specified minimum yield stress, ksi (MPa)
• $\begin{aligned} P_{n o} & =\text { nominal axial compressive strength without consideration of length effects, } \\ & \text { kips (N) }\end{aligned}$
• $\begin{aligned} \lambda & =\text { maximum width-to-thickness ratio of compression steel elements multiplied } \\ & \text { by } \sqrt{F_{y} / E}\end{aligned}$
• $E \quad=$ modulus of elasticity of steel, ksi (MPa)
• = 29,000 ksi (200 000 MPa)

2.1.3 Flexural Strength

The available flexural strength shall be determined as follows:

φb = 0.90 (LRFD) Wb = 1.67 (ASD)

The nominal flexural strength, $M_{n}$, shall be determined as $90 \%$ of the moment corresponding to a stress distribution over the composite cross section assuming that steel components have reached a stress of $F_{y}$ in tension and $F_{n}$ in compression, where $F_{n}$ is calculated using Equation A-2-2, and concrete components in compression have reached a stress of $0.85 f_{c}^{\prime}$, where $f_{c}^{\prime}$ is the specified compressive strength of concrete, ksi (MPa).

2.1.4 Combined Flexure and Axial Force

The interaction of flexure and compression shall be limited by Equations I5-1a and I5-1b, where the term $c_{p}$ is determined using Equation A-2-3 and $c_{m}$ is determined using Equation A-2-4:

(A-2-3)

(A-2-4)

where

• $B \quad$ = flange width of rectangular cross section, in. (mm)
• $\begin{aligned} H & =\text { web depth of rectangular cross section, in. (mm) }\end{aligned}$
• $F_{y, \max }=$ maximum permitted yield stress of steel $=100 \mathrm{ksi}(690 \mathrm{MPa})$
• $\begin{aligned} P_{n} & =\text { nominal axial strength calculated in accordance with Section } 2.1 .2 \\ & \text { kips (N) }\end{aligned}$
• $\begin{aligned} P_{n o} & =\text { nominal axial compressive strength without consideration of length ef- } \\ & \text { fects calculated in accordance with Section 2.1.2, kips (N) }\end{aligned}$