I1General provisions

In determining load effects in members and connections of a structure that includes composite members, consideration shall be given to the effective cross sections at the time each increment of load is applied.

I1.1 Concrete and Steel Reinforcement

The design, detailing, and material properties related to the concrete and reinforcing steel portions of composite construction shall comply with the reinforced concrete design specifications stipulated by the applicable building code. Additionally, the provisions in the Building Code Requirements for Structural Concrete (ACI 318) and the Metric Building Code Requirements for Structural Concrete (ACI 318M), subsequently referred to in Chapter I collectively as ACI 318, shall apply with the following exceptions and limitations:

• (a) Concrete and steel reinforcement material limitations shall be as specified in Section II.3.
• (b) Longitudinal and transverse reinforcement requirements shall be as specified in Sections I2 and I3 in addition to those specified in ACI 318.

Concrete and steel reinforcement components designed in accordance with ACI 318 shall be based on a level of loading corresponding to LRFD load combinations.

User Note: It is the intent of this Specification that the concrete and reinforcing steel portions of composite concrete members are designed and detailed utilizing the provisions of ACI 318 as modified by this Specification. All requirements specific to composite members are covered in this Specification.

Note that the design basis for ACI 318 is strength design. Designers using ASD for steel must be conscious of the different load factors.

I1.2 Nominal Strength of Composite Sections

The nominal strength of composite sections shall be determined in accordance with either the plastic stress distribution method, the strain compatibility method, the elastic stress distribution method, or the effective stress-strain method, as defined in this section.

The tensile strength of the concrete shall be neglected in the determination of the nominal strength of composite members.

Local buckling effects shall be evaluated for filled composite members, as defined in Section II.4. Local buckling effects need not be evaluated for encased composite members or composite plate shear walls meeting the requirements of this chapter.

I1.2a Plastic Stress Distribution Method

For the plastic stress distribution method, the nominal strength shall be computed assuming that steel components have reached a stress of $F_{y}$ in either tension or compression, and concrete components in compression due to axial force and/or flexure have reached a stress of $0.85 f_{c}^{\prime}$, where $f_{c}^{\prime}$ is the specified compressive strength of concrete, ksi (MPa). For round HSS filled with concrete, a stress of $0.95 f_{c}^{\prime}$ is permitted to be used for concrete components in compression due to axial force and/or flexure to account for the effects of concrete confinement.

I1.2b Strain Compatibility Method

For the strain compatibility method, a linear distribution of strains across the section shall be assumed, with the maximum concrete compressive strain equal to 0.003 in./in. (mm/mm). The stress-strain relationships for steel and concrete shall be obtained from tests or from published results.

User Note: The strain compatibility method can be used to determine nominal strength for irregular sections and for cases where the steel does not exhibit elasto-plastic behavior. General guidelines for the strain compatibility method for encased members subjected to axial load, flexure, or both are given in AISC Design Guide 6, Load and Resistance Factor Design of W-Shapes Encased in Concrete.

I1.2c Elastic Stress Distribution Method

For the elastic stress distribution method, the nominal strength shall be determined from the superposition of elastic stresses for the limit state of yielding or concrete crushing.

I1.2d Effective Stress-Strain Method

For the effective stress-strain method, the nominal strength shall be computed assuming strain compatibility and effective stress-strain relationships for structural steel, reinforcing steel, and concrete components accounting for the effects of local buckling, yielding, interaction, and concrete confinement.

I1.3 Material Limitations

For concrete, structural steel, and reinforcing steel in composite systems, the following limitations shall be met unless the design is based on the requirements of Appendix 2:

• (a) For the determination of the available strength, concrete shall have a specified compressive strength, $f_{c}^{\prime}$, of not less than 3 ksi (21 MPa) nor more than 10 ksi (69 MPa) for normal weight concrete and not less than 3 ksi (21 MPa) nor more than 6 ksi (41 MPa) for lightweight concrete.
• (b) The specified minimum yield stress of structural steel used in calculating the strength of composite members shall not exceed 75 ksi (525 MPa).
• (c) The specified minimum yield stress of reinforcing bars used in calculating the strength of composite members shall not exceed 80 ksi (550 MPa).

The design of filled composite members constructed from materials with strengths above the limits noted in this section shall be in accordance with Appendix 2.

User Note: Appendix 2 includes equations for determining the available strength of rectangular filled composite members with either the specified minimum yield stress of structural steel exceeding 75 ksi (525 MPa) but less than 100 ksi (690 MPa) or specified compressive strength, $f_{c}^{\prime}$, exceeding 10 ksi (69 MPa) but less than 15 ksi (100 MPa).

I1.4 Classification of Filled Composite Sections for Local Buckling

For compression, filled composite sections are classified as compact composite, non-compact composite, or slender-element composite sections. For a section to qualify as compact composite, the maximum width-to-thickness ratio, $\lambda$, of its compression steel elements shall not exceed the limiting width-to-thickness ratio, $\lambda_{p}$, from Table II.1a. If the maximum width-to-thickness ratio of one or more steel compression elements exceeds $\lambda_{p}$, but does not exceed $\lambda_{r}$ from Table II.1a, the filled composite section is noncompact composite. If the maximum width-to-thickness ratio of any compression steel element exceeds $\lambda_{r}$, the section is slender-element composite. The maximum permitted width-to-thickness ratio shall be as specified in Table II.1a.

For flexure, filled composite sections are classified as compact composite, noncompact composite, or slender-element composite sections. For a section to qualify as compact composite, the maximum width-to-thickness ratio of its compression steel elements shall not exceed the limiting width-to-thickness ratio, $\lambda_{p}$, from Table II.1b. If the maximum width-to-thickness ratio of one or more steel compression elements

I1.5 Stiffness for Calculation of Required Strengths

For the direct analysis method of design, the required strengths of encased composite members, filled composite members, and composite plate shear walls shall be determined using the provisions of Section C2 and the following requirements:

• (a) The nominal flexural stiffness of encased and filled composite members subjected to net compression shall be taken as the effective stiffness of the composite section, $(E I)_{e f f}$, as defined in Section I2.
• (b) The nominal axial stiffness of encased and filled composite members subjected to net compression shall be taken as the summation of the elastic axial stiffnesses of each component.
• (c) The stiffness of encased and filled composite members subjected to net tension shall be taken as the stiffness of the bare steel members in accordance with Chapter C.
• (d) The stiffness reduction parameter, $\tau_{b}$, shall be taken as 0.8 for encased and filled composite members.

User Note: Taken together, the stiffness reduction factors require the use of 0.64(EI)eff for the flexural stiffness and 0.8 times the nominal axial stiffness of encased composite members and filled composite members subjected to net com- pression in the analysis.

Stiffness values appropriate for the calculation of deflections and for use with the effective length method are discussed in the Commentary.

• (e) The flexural stiffness, (EI)eff, axial stiffness, (EA)eff, and shear stiffness, (GA)eff, of composite plate shear walls shall account for the extent of concrete cracking under LRFD load combinations or 1.6 times the ASD load combinations. It is permitted to use the following to estimate effective stiffness:

where

• $A_{c}=\operatorname{area}$ of concrete, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$
• $A_{s}=\text { area of steel section, in. }^{2}\left(\mathrm{~mm}^{2}\right)$
• $A_{s w}=$ area of steel plates in the direction of in-plane shear, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$
• $E_{c}=$ modulus of elasticity of concrete
• $w_{c}^{1.5} \sqrt{f_{c}^{\prime}}, \mathrm{ksi}\left(0.043 w_{c}^{1.5} \sqrt{f_{c}^{\prime}}, \mathrm{MPa}\right)$
• $E_{s}=$ modulus of elasticity of steel
• = 29,000 ksi (200 000 MPa)
• $G_{c}=$ shear modulus of concrete
• $=0.4 E_{c}$
• $G_{s}=$ shear modulus of steel
• = 11,200 ksi (77 200 MPa)
• $\begin{aligned} I_{c} & =\text { moment of inertia of the concrete section about the elastic neutral axis } \\ & \text { of the composite section, in. }^{4}\left(\mathrm{~mm}^{4}\right)\end{aligned}$
• $I_{s}$ = moment of inertia of steel shape about the elastic neutral axis of the composite section, in. ${ }^{4}\left(\mathrm{~mm}^{4}\right)$
• $\begin{aligned} w_{c} & =\text { weight of concrete per unit volume }\left(90 \leq w_{c} \leq 155 \mathrm{lb} / \mathrm{ft}^{3} \text { or } 1500 \leq w_{c} \right. \\ & \left.\leq 2500 \mathrm{~kg} / \mathrm{m}^{3}\right) \end{aligned}$
• (f) The stiffness reduction parameter, $\tau_{b}$, shall be taken as 1.0 for composite plate shear walls.

I1.6 Requirements for Composite Plate Shear Walls

The steel plates shall comprise at least 1% but no more than 10% of the total composite cross-sectional area. The opposing steel plates shall be connected to each other using ties consisting of bars, structural shapes, or built-up members. For filled composite plate shear walls, the steel plates shall be anchored to the concrete using ties or a combination of ties and steel anchors. Walls without flange (closure) plates or boundary elements are not permitted.

I1.6a Slenderness Requirement

The slenderness ratio of the plates, $b / t$, shall be limited as follows:

(II-4)

where

$b=$ largest clear distance between rows of steel anchors or ties, in. (mm)

$t=$ thickness of plate, in. (mm)

I1.6b Tie Bar Requirement

Tie bars shall have spacing no greater than 1.0 times the wall thickness, $t_{s c}$. The tie bar spacing to plate thickness ratio, $s_{t} / t$, shall be limited as follows:

(II-5M)

(II-6)

where

$d_{t i e}=$ effective diameter of the tie bar, in. (mm)

$s_{t}=$ largest clear spacing of the ties, in. (mm)

• $t \quad$ = thickness of plate, in. (mm)

$t_{s c}=$ thickness of composite plate shear wall, in. (mm)