D2.1 General
Connections, joints and fasteners that are part of the seismic force-resisting system (SFRS) shall comply with Specification Chapter J, and with the additional requirements of this section.
Splices and bases of columns that are not designated as part of the SFRS shall satisfy the requirements of Sections D2.5a, D2.5c, and D2.6.
Where protected zones are designated in connection elements by these Provisions or ANSI/AISC 358, they shall satisfy the requirements of Sections D1.3 and I2.1.
D2.2 Bolted Joints
Bolted joints shall satisfy the following requirements:
- (a) The available shear strength of bolted joints using standard holes or short-slotted holes perpendicular to the applied load shall be calculated as that for bearing-type joints in accordance with Specification Sections J3.7 and J3.11. The nominal bolt bearing and tearout equations per Section J3.10 of the Specification where deformation at the bolt hole at service load is a design consideration shall be used.
Exception: Where the required strength of a connection is based upon the expected strength of a member or element, it is permitted to use the bolt bearing and tearout equations in accordance with Specification Section J3.11 where deformation is not a design consideration.
- (b) Bolts and welds shall not be designed to share force in a joint or the same force component in a connection.
User Note: A member force, such as a diagonal brace axial force, must be resisted at the connection entirely by one type of joint (in other words, either entirely by bolts or entirely by welds). A connection in which bolts resist a force that is normal to the force resisted by welds, such as a moment connection in which welded flanges transmit flexure and a bolted web transmits shear, is not considered to be sharing the force.
- (c) Bolt holes shall be standard holes or short-slotted holes perpendicular to the applied load in bolted joints where the seismic load effects are transferred by shear in the bolts. Oversized (OVS) holes or short-slotted holes are permitted in connections where the seismic load effects are transferred by tension in the bolts but not by shear in the bolts.
Exception:
- (1) For diagonal braces, OVS holes are permitted in one connection ply only when the connection is designed as a slip-critical joint.
- (2) Alternative hole types are permitted if designated in ANSI/AISC 358, or if otherwise determined in a connection prequalification in accordance with Section K1, or if determined in a program of qualification testing in accordance with Section K2 or Section K3.
User Note: Diagonal brace connections with OVS holes must also satisfy other limit states, including bolt bearing and bolt shear for the required strength of the connection as defined in Sections F1, F2, F3, and F4.
- (d) All bolts shall be installed as pretensioned high-strength bolts. Faying surfaces shall satisfy the requirements for slip-critical connections in accordance with Specification Section J3.9 with a faying surface with a Class A slip coefficient or higher.
Exceptions: Connection surfaces are permitted to have coatings with a slip coefficient less than that of a Class A faying surface for the following.
- (1) End-plate moment connections conforming to the requirements of Section E1 or ANSI/AISC 358
- (2) Bolted joints where the seismic load effects are transferred either by tension in bolts or by compression bearing but not by shear in bolts
D2.3 Welded Joints
Welded joints shall be designed in accordance with Specification Chapter J.
D2.4 Continuity Plates and Stiffeners
The design of continuity plates and stiffeners located in the webs of rolled shapes shall allow for the reduced contact lengths to the member flanges and web based on the corner clip sizes in Section I2.4.
D2.5 Column Splices
D2.5a Location of Splices
For all building columns, including those not designated as part of the SFRS, column splices shall be located 4 ft (1.2 m) or more away from the beam-to-column flange connections.
Exceptions:
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(a) When the column clear height between beam-to-column flange connections is less than 8 ft (2.4 m), splices shall be at half the clear height.
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(b) Column splices with webs and flanges joined by complete-joint-penetration groove welds are permitted to be located closer to the beam-to-column flange connections, but not less than the depth of the column.
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(c) Splices in composite columns.
User Note: Where possible, splices should be located at least 4 ft (1.2 m) above the finished floor elevation to permit installation of perimeter safety cables prior to erection of the next tier and to improve accessibility. Refer to 1926.756(e)(1) of OSHA Safety and Health Regulations for Construction, Standards—29 CFR 1926, Subpart R—Steel Erection.
D2.5b Required Strength
- (a) The required strength of column splices in the SFRS shall be the greater of the following:
- (1) The required strength of the columns, including that determined from Chapters E, F, G, and H and Section D1.4a, or
- (2) The required strength determined using the overstrength seismic load.
- (b) In addition, welded column splices in which any portion of the column is subject to a calculated net tensile load effect determined using the overstrength seismic load shall satisfy the following requirements:
- (1) The available strength of partial-joint-penetration (PJP) groove welded joints, if used, shall be at least equal to 200% of the required strength. Exception: PJP groove welds are excluded from this requirement if the Exceptions in Sections E2.6g, E3.6g, or E4.6c are invoked.
- (2) The available strength for each flange splice shall be at least equal to , where
- specified minimum yield stress, ksi (MPa)
- width of flange of the smaller column connected, in. (mm)
- thickness of flange of the smaller column connected, in. (mm)
- (3) Where butt joints in column splices are made with complete-joint-penetration groove welds and when tension stress at any location in the smaller flange exceeds , tapered transitions are required between flanges of unequal thickness or width. Such transitions shall be in accordance with AWS D1.8/D1.8M, clause 4.2.
D2.5c Required Shear Strength
For all building columns, including those not designated as part of the SFRS, the required shear strength of column splices with respect to both orthogonal axes of the column shall be , where is the lesser plastic moment of the column sections for the direction in question and is the height of the story, which is permitted to be taken as the distance between the centerline of floor framing at each of the levels above and below or the distance between the top of floor slabs at each of the levels above and below.
The required shear strength of splices of columns in the SFRS shall be the greater of the foregoing requirement or the required shear strength determined per Section D2.5b(a).
D2.5d Structural Steel Splice Configurations
Structural steel column splices are permitted to be either bolted or welded, or welded to one column and bolted to the other. Splice configurations shall meet all specific requirements in Chapters E, F, G, or H.
Splice plates or channels used for making web splices in SFRS columns shall be placed on both sides of the column web.
For welded butt-joint splices made with groove welds, weld tabs shall be removed in accordance with AWS D1.8/D1.8M, clause 6.16. Steel backing of groove welds need not be removed.
D2.5e Splices in Encased Composite Columns
For encased composite columns, column splices shall conform to Section D1.4b and ACI 318, Section 18.7.4.4.
D2.6 Column Bases
The required strength of column bases, including those that are not designated as part of the SFRS, shall be determined in accordance with this section.
The available strength of steel elements at the column base, including base plates, anchor rods, stiffening plates, and shear lug elements, shall be in accordance with the Specification.
Where columns are welded to base plates with groove welds, weld tabs and weld backing shall be removed, except that weld backing located on the inside of flanges and weld backing on the web of I-shaped sections need not be removed if backing is attached to the column base plate with a continuous 5/16 in. (8 mm) fillet weld. Fillet welds of backing to the inside of column flanges are prohibited. Weld backing located on the inside of HSS and box-section columns need not be removed.
The available strength of concrete elements and longitudinal reinforcement at the column base shall be in accordance with ACI 318. When the design of anchor rods assumes that the ductility demand is provided for by deformations in the anchor rods and anchorage into reinforced concrete, the design shall meet the requirements of ACI 318, Chapter 17. Alternatively, when the ductility demand is provided for elsewhere, the anchor rods and anchorage into reinforced concrete are permitted to be designed for the maximum loads resulting from the deformations occurring elsewhere, including the effects of material overstrength and strain hardening.
User Note: When using concrete steel reinforcement as part of the anchorage embedment design, it is important to consider the anchor failure modes and provide reinforcement that meets the development length requirements on both sides of the expected failure surface. See ACI 318, Chapter 17, including Commentary.
D2.6a Required Axial Strength
The required axial strength of column bases that are designated as part of the SFRS, including their attachment to the foundation, shall be the summation of the vertical components of the required connection strengths of the steel elements that are connected to the column base, but not less than the greater of the following:
- (a) The column axial load calculated using the overstrength seismic load
- (b) The required axial strength for column splices, as prescribed in Section D2.5
User Note: The vertical components can include both the axial load from columns and the vertical component of the axial load from diagonal members framing into the column base. Section D2.5 includes references to Section D1.4a and Chapters E, F, G, and H. Where diagonal braces frame to both sides of a column, the effects of compression brace buckling should be considered in the summation of vertical components. See Section F2.3.
D2.6b Required Shear Strength
The required shear strength of column bases, including those not designated as part of the SFRS and their attachments to the foundations, shall be the summation of the horizontal component of the required connection strengths of the steel elements that are connected to the column base as follows:
- (a) For diagonal braces, the horizontal component shall be determined from the required strength of diagonal brace connections for the SFRS.
- (b) For columns, the horizontal component shall be equal to the lesser of the following:
- (1) of the column
- (2) The shear calculated using the overstrength seismic load
- (c) The summation of the required strengths of the horizontal components shall not be less than of the column,
where
- height of the story, in. (mm)
- plastic section modulus about the axis of bending, in.
Exceptions:
- (a) Single-story columns with simple connections at both ends need not comply with Sections D2.6b(b) or D2.6b(c).
- (b) Columns that are part of the systems defined in Sections E1, F1, G1, H1, H4, or combinations thereof need not comply with Section D2.6b(c).
- (c) The minimum required shear strength per Section D2.6b(c) need not exceed the maximum load effect that can be transferred from the column to the foundation as determined by either a nonlinear analysis per Section C3, or an analysis that includes the effects of inelastic behavior resulting in 0.025H story drift at either the first or second story, but not both concurrently.
User Note: The horizontal components can include the shear load from columns and the horizontal component of the axial load from diagonal members framing into the column base. Horizontal forces for columns that are not part of the SFRS determined in accordance with this section typically will not govern over those determined according to Section D2.6b(c).
D2.6c Required Flexural Strength
Where column bases are designed as moment connections to the foundation, the required flexural strength of column bases that are designated as part of the SFRS, including their attachment to the foundation, shall be the summation of the required connection strengths of the steel elements that are connected to the column base as follows:
- (a) For diagonal braces, the required flexural strength shall be at least equal to the required flexural strength of diagonal brace connections.
- (b) For columns, the required flexural strength shall be at least equal to the lesser of the following:
- (1) of the column or
- (2) The moment calculated using the overstrength seismic load, provided that a ductile limit state in either the column base or the foundation controls the design.
User Note: Moments at column-to-column base connections designed as simple connections may be ignored.
D2.7 Composite Connections
This section applies to connections in buildings that utilize composite steel and concrete systems wherein seismic load is transferred between structural steel and reinforced concrete components. Methods for calculating the connection strength shall satisfy the requirements in this section. Unless the connection strength is determined by analysis or testing, the models used for design of connections shall satisfy the following requirements.
- (a) Force shall be transferred between structural steel and reinforced concrete through the following:
- (1) Direct bearing from internal bearing mechanisms
- (2) Shear connection
- (3) Shear friction with the necessary clamping force provided by reinforcement normal to the plane of shear transfer
- (4) A combination of these means
The contribution of different mechanisms is permitted to be combined only if the stiffness and deformation capacity of the mechanisms are compatible. Any potential bond strength between structural steel and reinforced concrete shall be ignored for the purpose of the connection force transfer mechanism.
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(b) The nominal bearing and shear-friction strengths shall meet the requirements of ACI 318. Unless a higher strength is substantiated by cyclic testing, the nominal bearing and shear-friction strengths shall be reduced by 25% for the composite seismic systems described in Sections G3, H2, H3, H5, and H6.
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(c) Face bearing plates consisting of stiffeners between the flanges of steel beams shall be provided when beams are embedded in reinforced concrete columns or walls.
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(d) The nominal shear strength of concrete-encased steel panel zones in beam-to-column connections shall be calculated as the sum of the nominal strengths of the structural steel and confined reinforced concrete section as determined in Section E3.6e and ACI 318, Section 18.8, respectively.
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(e) Reinforcement shall be provided to resist all tensile forces in reinforced concrete components of the connections. Additionally, the concrete shall be confined with transverse reinforcement. All reinforcement shall meet the development length requirements in ACI 318 in tension or compression, as applicable, beyond the point at which it is no longer required to resist the forces. Development lengths shall be determined in accordance with ACI 318, Chapter 25. Additionally, development lengths for the systems described in Sections G3, H2, H3, H5, and H6 shall satisfy the requirements of ACI 318, Section 18.8.5.
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(f) Composite connections shall satisfy the following additional requirements:
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(1) When the slab transfers horizontal diaphragm forces, the slab reinforcement shall be designed and anchored to carry the in-plane tensile forces at all critical sections in the slab, including connections to collector beams, columns, diagonal braces, and walls.
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(2) For connections between structural steel or composite beams and reinforced concrete or encased composite columns, transverse reinforcement shall be provided in the connection region of the column to satisfy the requirements of ACI 318, Section 18.8, except for the following modifications:
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(i) Structural steel sections framing into the connections are considered to provide confinement over a width equal to that of face bearing plates welded to the beams between the flanges.
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(ii) Lap splices are permitted for perimeter transverse reinforcement when confinement of the lap splice is provided by face bearing plates or other means that prevents spalling of the concrete cover in the systems described in Sections G1, G2, H1, and H4.
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(iii) The longitudinal bar sizes and layout in reinforced concrete and composite columns shall be detailed to minimize slippage of the bars through the beam-to-column connection due to high force transfer associated with the change in column moments over the height of the connection.
User Note: The commentary provides guidance for determining panel-zone shear strength.
D2.8 Steel Anchors
Where steel headed stud anchors or welded reinforcing bar anchors are part of the intermediate or special SFRS of Sections G2, G3, G4, H2, H3, H5, and H6, their shear and tensile strength shall be reduced by 25% from the specified strengths given in Specification Chapter I. The diameter of steel headed stud anchors shall be limited to 3/4 in. (19 mm).
User Note: The 25% reduction is not necessary for gravity and collector components in structures with intermediate or special seismic force-resisting systems designed for the overstrength seismic load.