I6Load transfer

I6.1 General Requirements

When external forces are applied to an axially loaded encased or filled composite member, the introduction of force to the member and the transfer of longitudinal shear within the member shall be assessed in accordance with the requirements for force allocation presented in this section.

The available strength of the applicable force transfer mechanisms as determined in accordance with Section I6.3 shall equal or exceed the required shear force to be transferred, $V_{r}^{\prime}$, as determined in accordance with Section I6.2. Force transfer mechanisms shall be located within the load transfer region as determined in accordance with Section I6.4.

I6.2 Force Allocation

Force allocation shall be determined based upon the distribution of external force in accordance with the following requirements.

User Note: Bearing strength provisions for externally applied forces are provided in Section J8. For filled composite members, the term $\sqrt{A_{2} / A_{1}}$ in Equation J8-2 may be taken equal to 2.0 due to confinement effects.

I6.2a External Force Applied to Steel Section

When the entire external force is applied directly to the steel section, the force required to be transferred to the concrete, $V_{r}^{\prime}$, shall be determined as

where

$P_{n o}=$ nominal axial compressive strength without consideration of length effects, determined by Equation I2-7 for encased composite members, and Equation I2-9a or Equation I2-9c, as applicable, for compact composite or noncompact composite filled composite members, kips (N)

$P_{r}=$ required external force applied to the composite member, kips (N)

User Note: Equation I6-1 does not apply to slender filled composite members for which the external force is applied directly to the concrete fill in accordance with Section I6.2b, or concurrently to the steel and concrete, in accordance with Section I6.2c.

I6.2b External Force Applied to Concrete

When the entire external force is applied directly to the concrete encasement or concrete fill, the force required to be transferred to the steel, $V_{r}^{\prime}$, shall be determined as follows:

• (a) For encased or filled composite members that are compact composite or non-compact composite

(16-2a)

(b) For slender filled composite members

(16-2b)

where

$F_{n}=$ critical buckling stress for structural steel sections of filled composite members determined using Equation I2-10 or Equation I2-11, as applicable, ksi (MPa)

$P_{n o}=$ nominal axial compressive strength without consideration of length effects, determined by Equation I2-7 for encased composite members, and Equation I2-9a, Equation I2-9c, or Equation I2-9e for filled composite members, kips (N)

I6.2c External Force Applied Concurrently to Steel and Concrete

When the external force is applied concurrently to the steel section and concrete encasement or concrete fill, $V_{r}^{\prime}$ shall be determined as the force required to establish equilibrium of the cross section.

User Note: The Commentary provides an acceptable method of determining the longitudinal shear force required for equilibrium of the cross section.

I6.3 Force Transfer Mechanisms

The available strength of the force transfer mechanisms of direct bond interaction, shear connection, and direct bearing shall be determined in accordance with this section. Use of the force transfer mechanism providing the largest nominal strength is permitted. Force transfer mechanisms shall not be superimposed.

The force transfer mechanism of direct bond interaction shall not be used for encased composite members or for filled composite members where bond failure would result in uncontrolled slip.

I6.3a Direct Bearing

Where force is transferred in an encased or filled composite member by direct bearing from internal bearing mechanisms, the available bearing strength of the concrete for the limit state of concrete crushing shall be determined as

(I6-3)

$\phi_{B}=0.65$ (LRFD) $\quad \Omega_{B}=2.31$ (ASD)

where

$A_{1}=$ loaded area of concrete, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$

User Note: An example of force transfer via an internal bearing mechanism is the use of internal steel plates within a filled composite member.

I6.3b Shear Connection

Where force is transferred in an encased or filled composite member by shear connectors, the available shear strength of steel headed stud or steel channel anchors shall be determined as

(I6-4)

where

$\Sigma Q_{c v}=$ sum of available shear strengths, $\phi_{v} Q_{n v}$ (LRFD) or $Q_{n v} / \Omega_{v}$ (ASD), as applicable, of steel headed stud or steel channel anchors, determined in accordance with Section I8.3a or Section I8.3d, respectively, placed within the load introduction length as defined in Section I6.4, kips (N)

I6.3c Direct Bond Interaction

Where force is transferred in a filled composite member by direct bond interaction, the available bond strength between the steel and concrete shall be determined as follows:

R₁ =PLF (16-5)

$\phi_{d}=0.50$ (LRFD) $\quad \Omega_{d}=3.00$ (ASD)

where

I6.4 Detailing Requirements

I6.4a Encased Composite Members

Force transfer mechanisms shall be distributed within the load introduction length, which shall not exceed a distance of two times the minimum transverse dimension of the encased composite member above and below the load transfer region. Anchors utilized to transfer shear shall be placed on at least two faces of the structural steel shape in a generally symmetric configuration about the steel shape axes.

Steel anchor spacing, both within and outside of the load introduction length, shall conform to Section I8.3e.

I6.4b Filled Composite Members

Force transfer mechanisms shall be distributed within the load introduction length, which shall not exceed a distance of two times the minimum transverse dimension of a rectangular steel member or two times the diameter of a round steel member both above and below the load transfer region. For the specific case of load applied to the concrete of a filled composite member containing no internal longitudinal reinforcement, the load introduction length shall extend beyond the load transfer region in only the direction of the applied force. Steel anchor spacing within the load introduction length shall conform to Section I8.3e.