I8Steel anchors

I8.1 General

The diameter of a steel headed stud anchor, dsa, shall be w in. (19 mm) or less, except where anchors are utilized solely for shear transfer in solid slabs, in which case, d-in.- (22-mm-) and 1-in.- (25-mm-) diameter anchors are permitted. Additionally, dsa shall not be greater than 2.5 times the thickness of the base metal to which it is welded, unless it is welded to a flange directly over a web.

Section I8.2 applies to a composite flexural member where steel anchors are embedded in a solid concrete slab or in a slab cast on a formed steel deck. Section I8.3 applies to all other cases.

I8.2 Steel Anchors in Composite Beams

The length of steel headed stud anchors shall not be less than four stud diameters from the base of the steel headed stud anchor to the top of the stud head after installation.

I8.2a Strength of Steel Headed Stud Anchors

The nominal shear strength of one steel headed stud anchor embedded in a solid concrete slab or in a composite slab with decking shall be determined as follows:

(I8-1)

where

• $\begin{aligned} A_{s a} & =\text { cross-sectional area of steel headed stud anchor, in. }^{2}\left(\mathrm{~mm}^{2}\right)\end{aligned}$
• $E_{c}$ = modulus of elasticity of concrete

$w_{c}^{1.5} \sqrt{f_{c}^{\prime}}$, ksi $\left(0.043 w_{c}^{1.5} \sqrt{f_{c}^{\prime}}, \mathrm{MPa}\right)$

• $\begin{aligned} F_{u} & =\text { specified minimum tensile strength of a steel headed stud anchor, ksi } \\ & \text { (MPa) }\end{aligned}$

• $\begin{aligned} R_{g} & =1.0 \text { for the following: }\end{aligned}$

• (a) One steel headed stud anchor welded in a steel deck rib with the deck oriented perpendicular to the steel shape

• (b) Any number of steel headed stud anchors welded in a row directly to the steel shape

• (c) Any number of steel headed stud anchors welded in a row through steel deck with the deck oriented parallel to the steel shape and the ratio of the average rib width to rib depth ≥ 1.5

• 0.85 for the following:

• (a) Two steel headed stud anchors welded in a steel deck rib with the deck oriented perpendicular to the steel shape

• (b) One steel headed stud anchor welded through steel deck with the deck oriented parallel to the steel shape and the ratio of the average rib width to rib depth < 1.5

• = 0.7 for three or more steel headed stud anchors welded in a steel deck rib with the deck oriented perpendicular to the steel shape

• $R_{p}=0.75$ for the following:

• (a) Steel headed stud anchors welded directly to the steel shape

• (b) Steel headed stud anchors welded in a composite slab with the deck oriented perpendicular to the beam and emid-ht ≥ 2 in. (50 mm)

• (c) Steel headed stud anchors welded through steel deck, or steel sheet used as girder filler material and embedded in a composite slab with the deck oriented parallel to the beam

• = 0.6 for steel headed stud anchors welded in a composite slab with deck oriented perpendicular to the beam and $e_{\text {mid-ht }}<2$ in. (50 mm)

$e_{\text {mid-ht }}=$ distance from the edge of steel headed stud anchor shank to the steel deck web, measured at mid-height of the deck rib, and in the load bearing direction of the steel headed stud anchor (in other words, in the direction of maximum moment for a simply supported beam), in. (mm)

User Note: The table below presents values for $R_{g}$ and $R_{p}$ for several cases. Available strengths for steel headed stud anchors can be found in the AISC Steel Construction Manual.

I8.2b Strength of Steel Channel Anchors

The nominal shear strength of one hot-rolled channel anchor embedded in a solid concrete slab shall be determined as

where

• $l_{a}=$ length of channel anchor, in. (mm)
• tf = thickness of channel anchor flange, in. (mm)
• $t_{w}=$ thickness of channel anchor web, in. (mm)

The strength of the channel anchor shall be developed by welding the channel to the beam flange for a force equal to $Q_{n}$, considering eccentricity on the anchor.

I8.2c Required Number of Steel Anchors

The number of anchors required between the section of maximum bending moment, positive or negative, and the adjacent section of zero moment shall be equal to the horizontal shear as determined in Sections I3.2d.1 and I3.2d.2 divided by the nomi- nal shear strength of one steel anchor as determined from Section I8.2a or Section I8.2b. The number of steel anchors required between any concentrated load and the nearest point of zero moment shall be sufficient to develop the maximum moment required at the concentrated load point.

I8.2d Detailing Requirements

Steel anchors in composite beams shall meet the following requirements:

• (a) Steel anchors required on each side of the point of maximum bending moment, positive or negative, shall be distributed uniformly between that point and the adjacent points of zero moment, unless specified otherwise on the design documents and specifications issued for construction.
• (b) Steel anchors shall have at least 1 in. (25 mm) of lateral concrete cover in the direction perpendicular to the shear force, except for anchors installed in the ribs of formed steel decks.
• (c) The minimum distance from the center of a steel anchor to a free edge in the direction of the shear force shall be 8 in. (200 mm) if normal weight concrete is used and 10 in. (250 mm) if lightweight concrete is used. The provisions of ACI 318, Chapter 17, are permitted to be used in lieu of these values.
• (d) Minimum center-to-center spacing of steel headed stud anchors shall be four diameters in any direction. For composite beams that do not contain anchors located within formed steel deck oriented perpendicular to the beam span, an additional minimum spacing limit of six diameters along the longitudinal axis of the beam shall apply.
• (e) The maximum center-to-center spacing of steel anchors shall not exceed eight times the total slab thickness or 36 in. (900 mm).

I8.3 Steel Anchors in Composite Components

This section shall apply to the design of cast-in-place steel headed stud anchors and steel channel anchors in composite components.

The provisions of the applicable building code or ACI 318, Chapter 17, are permitted to be used in lieu of the provisions in this section.

User Note: The steel headed stud anchor strength provisions in this section are applicable to anchors located primarily in the load transfer (connection) region of composite columns and beam-columns, concrete-encased and filled composite beams, composite coupling beams, and composite walls, where the steel and concrete are working compositely within a member. They are not intended for hybrid construction where the steel and concrete are not working compositely, such as with embed plates.

Section I8.2 specifies the strength of steel anchors embedded in a solid concrete slab or in a concrete slab with formed steel deck in a composite beam.

Limit states for the steel shank of the anchor and for concrete breakout in shear are covered directly in this section. Additionally, the spacing and dimensional limitations provided in these provisions preclude the limit states of concrete pryout for anchors loaded in shear and concrete breakout for anchors loaded in tension as defined by ACI 318, Chapter 17.

For normal weight concrete: Steel headed stud anchors subjected to shear only shall not be less than five stud diameters in length from the base of the steel headed stud to the top of the stud head after installation. Steel headed stud anchors subjected to tension or interaction of shear and tension shall not be less than eight stud diameters in length from the base of the stud to the top of the stud head after installation.

For lightweight concrete: Steel headed stud anchors subjected to shear only shall not be less than seven stud diameters in length from the base of the steel headed stud to the top of the stud head after installation. Steel headed stud anchors subjected to tension shall not be less than ten stud diameters in length from the base of the stud to the top of the stud head after installation. The nominal strength of steel headed stud anchors subjected to interaction of shear and tension for lightweight concrete shall be determined as stipulated by the applicable building code or ACI 318, Chapter 17.

Steel headed stud anchors subjected to tension or interaction of shear and tension shall have a diameter of the head greater than or equal to 1.6 times the diameter of the shank.

User Note: The following table presents values of minimum steel headed stud anchor h d ratios for each condition covered in this Specification.

$h / d_{\mathrm{sa}}=$ ratio of steel headed stud anchor shank length to the top of the stud head-to-shank diameter.

[a]Refer to ACI 318, Chapter 17, for the calculation of interaction effects of anchors embedded in lightweight concrete.

I8.3a Shear Strength of Steel Headed Stud Anchors in Composite Components

Where concrete breakout strength in shear is not an applicable limit state, the design shear strength, $\phi_{v} Q_{n v}$, and allowable shear strength, $Q_{n v} / \Omega_{v}$, of one steel headed stud anchor shall be determined as

$Q_{n v}=F_{u} A_{s a}$

(I8-3)

O₁ = 0.65 (LRFD) 22.31 (ASD)

where

• $A_{s a}=$ cross-sectional area of a steel headed stud anchor, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$
• $F_{u}=$ specified minimum tensile strength of a steel headed stud anchor, ksi (MPa)

$Q_{n v}=$ nominal shear strength of a steel headed stud anchor, kips (N)

Where concrete breakout strength in shear is an applicable limit state, the available shear strength of one steel headed stud anchor shall be determined by one of the following:

• (a) Where anchor reinforcement is developed in accordance with ACI 318 on both sides of the concrete breakout surface for the steel headed stud anchor, the minimum of the steel nominal shear strength from Equation I8-3 and the nominal strength of the anchor reinforcement shall be used for the nominal shear strength, $Q_{n v}$, of the steel headed stud anchor.
• (b) As stipulated by the applicable building code or ACI 318, Chapter 17.

User Note: If concrete breakout strength in shear is an applicable limit state (for example, where the breakout prism is not restrained by an adjacent steel plate, flange, or web), appropriate anchor reinforcement is required for the provisions of this section to be used. Alternatively, the provisions of the applicable building code or ACI 318, Chapter 17, may be used.

I8.3b Tensile Strength of Steel Headed Stud Anchors in Composite Components

Where the distance from the center of an anchor to a free edge of concrete in the direction perpendicular to the height of the steel headed stud anchor is greater than or equal to 1.5 times the height of the steel headed stud anchor measured to the top of the stud head, and where the center-to-center spacing of steel headed stud anchors is greater than or equal to three times the height of the steel headed stud anchor measured to the top of the stud head, the available tensile strength of one steel headed stud anchor shall be determined as

(I8-4)

$\phi_{t}=0.75$ (LRFD) $\quad \Omega_{t}=2.00$ (ASD)

where

$Q_{n t}=$ nominal tensile strength of steel headed stud anchor, kips (N)

Where the distance from the center of an anchor to a free edge of concrete in the direction perpendicular to the height of the steel headed stud anchor is less than 1.5 times the height of the steel headed stud anchor measured to the top of the stud head, or where the center-to-center spacing of steel headed stud anchors is less than three times the height of the steel headed stud anchor measured to the top of the stud head, the nominal tensile strength of one steel headed stud anchor shall be determined by one of the following:

• (a) Where anchor reinforcement is developed in accordance with ACI 318 on both sides of the concrete breakout surface for the steel headed stud anchor, the minimum of the steel nominal tensile strength from Equation I8-4 and the nomi- nal strength of the anchor reinforcement shall be used for the nominal tensile strength, Qnt, of the steel headed stud anchor.
• (b) As stipulated by the applicable building code or ACI 318, Chapter 17.

User Note: Supplemental confining reinforcement is recommended around the anchors for steel headed stud anchors subjected to tension or interaction of shear and tension to avoid edge effects or effects from closely spaced anchors. See the Commentary and ACI 318 for guidelines.

I8.3c Strength of Steel Headed Stud Anchors for Interaction of Shear and Tension in Composite Components

Where concrete breakout strength in shear is not a governing limit state, and where the distance from the center of an anchor to a free edge of concrete in the direction perpendicular to the height of the steel headed stud anchor is greater than or equal to 1.5 times the height of the steel headed stud anchor measured to the top of the stud head, and where the center-to-center spacing of steel headed stud anchors is greater than or equal to three times the height of the steel headed stud anchor measured to the top of the stud head, the nominal strength for interaction of shear and tension of one steel headed stud anchor shall be determined as

(I8-5)

where

• $\begin{aligned} Q_{c t} & =\text { available tensile strength, determined in accordance with Section I8.3b, } \\ & \text { kips (N) }\end{aligned}$
• $\begin{aligned} Q_{c v} & =\text { available shear strength, determined in accordance with Section } 18.3 \mathrm{a}, \\ & \text { kips (N) }\end{aligned}$
• $Q_{r t}=$ required tensile strength, kips (N)
• $Q_{r v}=$ required shear strength, kips (N)

Where concrete breakout strength in shear is a governing limit state, or where the distance from the center of an anchor to a free edge of concrete in the direction perpendicular to the height of the steel headed stud anchor is less than 1.5 times the height of the steel headed stud anchor measured to the top of the stud head, or where the center-to-center spacing of steel headed stud anchors is less than three times the height of the steel headed stud anchor measured to the top of the stud head, the nominal strength for interaction of shear and tension of one steel headed stud anchor shall be determined by one of the following:

• (a) Where anchor reinforcement is developed in accordance with ACI 318 on both sides of the concrete breakout surface for the steel headed stud anchor, the minimum of the steel nominal shear strength from Equation I8-3 and the nominal

strength of the anchor reinforcement shall be used for the nominal shear strength, $Q_{n v}$, of the steel headed stud anchor, and the minimum of the steel nominal tensile strength from Equation I8-4 and the nominal strength of the anchor reinforcement shall be used for the nominal tensile strength, $Q_{n t}$, of the steel headed stud anchor for use in Equation I8-5.

(b) As stipulated by the applicable building code or ACI 318, Chapter 17.

I8.3d Shear Strength of Steel Channel Anchors in Composite Components

The available shear strength of steel channel anchors shall be based on the provisions of Section I8.2b with the following resistance factor and safety factor:

fv = 0.75 (LRFD) Wv = 2.00 (ASD)

I8.3e Detailing Requirements in Composite Components

Steel anchors in composite components shall meet the following requirements:

• (a) Minimum concrete cover to steel anchors shall be in accordance with ACI 318 provisions for concrete protection of headed shear stud reinforcement.
• (b) Minimum center-to-center spacing of steel headed stud anchors shall be four diameters in any direction.
• (c) The maximum center-to-center spacing of steel headed stud anchors shall not exceed 32 times the shank diameter.
• (d) The maximum center-to-center spacing of steel channel anchors shall be 24 in. (600 mm).

User Note: Detailing requirements provided in this section are absolute limits. See Sections I8.3a, I8.3b, and I8.3c for additional limitations required to preclude edge and group effect considerations.

I8.4 Performance-Based Alternative for the Design of Shear Connection

In lieu of shear connection prescribed by, and the corresponding strength determined in accordance with, Sections I8.1 and I8.2, it is permitted to use an alternate form of shear connection and determine its strength through testing, provided its performance requirements are established in accordance with Sections I8.4a through I8.4d and satisfy the approval requirements of the authority having jurisdiction. The geometric limitations of Sections I3.2c, I8.1, and I8.2 do not apply to the performance evaluated by Section I8.4.

I8.4a Test Standard

Shear connection strength, slip capacity, and stiffness shall be established in accordance with AISI S923. An alternative test protocol may be used in the evaluation when approved by the authority having jurisdiction.

I8.4b Nominal and Available Strength

When determining available strength of a flexural member, the nominal tested strength of shear connection, Qne, shall be taken as 0.85 times the mean tested strength determined in accordance with Section I8.4a. When required, the design shear strength, φvQne, and the allowable shear strength, Qne Ω, shall be determined v in accordance with Section I8.3a. Alternatively, it is permitted to take Qne as the mean tested strength provided that φvQne or Qne Ω, as applicable, is determined on the v basis of a reliability analysis.

User Note: An approach for establishing available strength using test data is provided in Chapter K of AISI North American Specification for the Design of Cold-Formed Steel Structural Members, with Supplement 2.

I8.4c Shear Connection Slip Capacity

The nominal shear connection slip capacity shall be taken as the average shear connection slip corresponding to each specific tested shear connection configuration. Shear connection slip capacity shall be measured at no less than 95% of the post-peak strength.

I8.4d Acceptance Criteria

The design using tested properties of the shear connection per Section I8.4a through Section I8.4c shall be limited to the geometric and material properties tested. The nominal performance characteristics are permitted to be used in design provided either conditions (1), (2), and (3) are satisfied, or condition (4) is met.

• (1) The maximum permitted coefficient of variation corresponding to each tested configuration of shear connection does not exceed 0.09 established over four replicate tests, or 0.15 established over nine replicate tests. It is permitted, for this purpose, to establish the number of tests using all tests of the same type of shear connection that exhibit the same failure mode.
• (2) The nominal shear connection slip capacity is at least 0.25 in. (6 mm).
• (3) The minimum shear elastic stiffness of the shear connection shall not be less than 2,000 kip/in. (180 N/mm).
• (4) Shear connections corresponding to the values of coefficient of variation, shear connection slip capacity, and elastic stiffness, other than those stipulated in conditions (1), (2), and (3), shall be deemed acceptable, provided their effect is captured in the design. In lieu of using in an analysis the shear connection elastic stiffness determined per this section, it is permitted to establish the stiffness of a composite section, incorporating shear connection evaluated by this section, directly through testing in accordance with AISI S924. When stiffness of a composite section is established in accordance with AISI S924, it shall be a mean tested value established based on at least three tests.