What is capacity design in seismic steel connections?

Capacity design ensures the designated energy-dissipating fuse yields before the connection or column fails. The connection is designed for the expected strength of the yielding member (using Ry × Fy), not the calculated seismic force from analysis. For example, an SMF beam-to-column connection must develop the expected plastic moment of the beam, Mp = Ry × Fy × Zx, rather than the reduced design moment from the R-factor. The expected yield strength factor Ry accounts for actual material overstrength: A36 has Ry=1.50, A992 Gr 50 has Ry=1.10, A500 Gr B has Ry=1.40. This philosophy prevents brittle failure modes and ensures energy is dissipated through controlled yielding in the beam plastic hinge region.

What are the prequalified SMF connections per AISC 358?

AISC 358 prequalifies six SMF connection types for use without project-specific cyclic testing. The Reduced Beam Section (RBS or dog-bone) connection is the most popular, trimming beam flanges in a circular arc to force the plastic hinge away from the column face, and is prequalified up to W36. Bolted Unstiffened End Plate (BUEP) connections use 4 bolts per flange and are prequalified to W24. Bolted Stiffened End Plate (BSEP) uses 8 bolts with stiffeners and is prequalified to W36. Welded Unreinforced Flange (WUF-W) uses CJP welds with web bolts, prequalified to W36. Kaiser Bolted Bracket (KBB) and SidePlate are proprietary systems prequalified to W24 and W36 respectively. All prequalified connections have strict beam and column limitations including Fy ≤ 55 ksi and flange thickness ≤ 1.75 inches.

What is the strong-column/weak-beam (SCWB) requirement?

The SCWB requirement in AISC 341 Section E3.4a mandates that the sum of column expected flexural strengths at the panel zone must equal or exceed the sum of beam expected flexural strengths: ΣM*pc / ΣM*pb ≥ 1.0. M*pc is the column expected flexural strength considering axial load interaction, and M*pb is the beam expected flexural strength at the column face using Ry × Fy × Zx. This requirement prevents story mechanisms (soft-story collapse) by ensuring plastic hinges form in beams rather than columns. When columns are too weak, all hinges concentrate in a single story, concentrating inelastic demand and leading to collapse. The SCWB check is required at every beam-column joint in SMF and IMF systems.

How are SCBF gusset plate connections designed for seismic?

SCBF gusset plate connections must develop the expected brace strength in both tension (Te = Ry × Fy × Ag) and compression (Ce, the expected compressive strength including Ry). The connection must be designed for the larger of Te and Ce. Key design requirements include: the 2t linear offset rule, where the brace end is held 2 times the gusset thickness away from the restraint line to allow gusset plate buckling and brace end rotation without fracture; the balanced force method (Thornton method), which distributes forces to beam and column interfaces based on gusset geometry; Whitmore section checks for tension yielding, tension rupture, and compression buckling at the effective width defined by 30° projection lines; and block shear checks at all bolt groups. Gusset plates must accommodate brace buckling without fracturing the connection.

What weld and bolt requirements apply to seismic connections?

AISC 341 imposes strict requirements on welds and bolts in seismic force-resisting systems. Complete joint penetration (CJP) groove welds in SMF connections are designated demand-critical and require ultrasonic testing (UT) inspection per AWS D1.1. Weld electrodes must meet toughness requirements — FCAW or GMAW processes with appropriate filler metals. For bolts: A325 and A490 high-strength bolts are permitted in SMF, IMF, SCBF, and EBF connections, but A307 bolts are never permitted in any seismic force-resisting system. Slip-critical bolted connections must use Class A or B faying surfaces. The maximum bolt hole size is 1/16 inch larger than the bolt diameter. Bolts must be pretensioned per RCSC specifications. Panel zone doubler plates require specific weld detailing to prevent lamellar tearing. Continuity plates are required per AISC 358 for each prequalified connection type.

Seismic Connections — AISC 341 Design Requirements

Seismic connections in steel buildings must dissipate energy through controlled yielding while preventing brittle fracture. AISC 341 (Seismic Provisions for Structural Steel Buildings) establishes the requirements. This page covers the connection types, ductility demands, capacity design principles, and prequalification requirements.

Seismic Design Categories

The seismic design category (SDC) determines the level of seismic detailing required:

SDC	Seismic Risk	AISC 341 Applicability	Connection Requirements
A, B	Low	Not required	Standard AISC 360 design
C	Moderate	Partial (selected systems)	Some special detailing
D	High	Full AISC 341	All special systems required
E, F	Very high	Full AISC 341 + additional	Most stringent requirements

SDC is determined from ASCE 7 based on spectral acceleration, site class, and risk category.

Steel Seismic Force-Resisting Systems

System	R	Ω₀	Cd	Detailing
Special Moment Frame (SMF)	8	3	5.5	AISC 341, AISC 358
Intermediate Moment Frame (IMF)	4.5	3	4	AISC 341
Ordinary Moment Frame (OMF)	3.5	3	3	Limited requirements
Special Concentric Braced Frame	6	2	5	AISC 341
Ordinary Concentric Braced Frame	3.25	2	3.25	Limited requirements
Eccentrically Braced Frame	8	2	4	AISC 341
Buckling-Restrained Braced Frame	8	2.5	5	AISC 341
Special Plate Shear Wall	7	2.5	6	AISC 341

R = response modification coefficient (higher = more ductility expected, lower design force).

Capacity Design Philosophy

Seismic connections use capacity design: the connection is designed for the expected strength of the yielding member, not the calculated seismic force.

Principle: Ensure the designated fuse (beam, brace, or link) yields before the connection or column fails.

Expected yield strength: Fye = Ry × Fy

where Ry is the ratio of expected to specified yield strength:

ASTM Spec	Grade	Ry
A36	—	1.50
A572	Gr 50	1.10
A992	50	1.10
A500	Gr B	1.40
A500	Gr C	1.30
A1085	—	1.25

Special Moment Frame (SMF) Connections

SMF connections must sustain a story drift angle of at least 0.04 rad (4%) with minimal strength degradation. Only prequalified connections per AISC 358 may be used without project-specific testing.

Prequalified SMF Connections

Connection	Description	Max Depth	Popular?
Reduced Beam Section (RBS)	Flanges trimmed in circular arc	W36	Yes
Bolted Unstiffened End Plate	End plate with 4 bolts per flange	W24	Moderate
Bolted Stiffened End Plate	End plate with stiffeners and 8 bolts	W36	Moderate
Welded Unreinforced Flange	CJP welds, web bolts	W36	Moderate
Kaiser Bolted Bracket	Proprietary bracket connection	W24	Limited
SidePlate	Proprietary side plates	W36	Limited

SMF Connection Requirements

Beam limitations: Fy ≤ 55 ksi, depth ≤ W36, flange thickness ≤ 1.75 in
Column limitations: Must satisfy strong-column/weak-beam ratio
Panel zone: Must develop the expected beam moment
Continuity plates: Required per AISC 358 for each connection
Weld quality: CJP welds must be demand-critical, UT inspected
Bolt grade: A325 or A490 (A307 not permitted in SMF)
Weld electrodes: Must meet toughness requirements (FCAW or GMAW)

Strong-Column/Weak-Beam (SCWB)

AISC 341 requires that the column flexural strength exceed the beam flexural strength at every connection:

ΣM*pc / ΣM*pb ≥ 1.0

where M*pc = column expected flexural strength (at panel zone), M*pb = beam expected flexural strength (at column face).

This prevents story mechanisms (soft stories) and ensures distributed yielding.

Concentric Braced Frame (CBF) Connections

Special Concentric Braced Frame (SCBF)

SCBF connections must develop the expected strength of the brace in both tension and compression.

Design Requirements

Brace expected tension: T_e = Ry × Fy × Ag
Brace expected compression: C_e = expected compressive strength (including Ry)
Connection capacity: Must develop min(T_e, C_e)
Gusset plate: Designed for expected brace force + balanced interface forces
Whitmore section: Check effective width at gusset end
Block shear: Check at all bolt groups

SCBF Gusset Plate Design

Gusset plates must allow brace buckling without fracturing the connection:

2t linear offset rule: The gusset is detailed so the brace can buckle by bending the gusset plate. The end of the brace is held 2t (twice the gusset thickness) away from the restraint line.
Balanced force method (Thornton): Distributes forces to beam and column interfaces based on geometry.

Brace Connection at Mid-Span

When diagonal braces connect to the beam at a point away from the column, the beam must be designed for the unbalanced force from brace tension (after compression brace buckles).

Brace-to-Gusset Weld Requirements

Requirement	Specification
Fillet weld	Minimum leg size per AISC Table J2.4
Groove weld	CJP required for heavy braces
Weld inspection	UT or MT for CJP; VT for fillet
Demand-critical	CJP welds in SCBF are demand-critical
Electrode toughness	Per AISC 341 Section A3.4

Eccentrically Braced Frame (EBF) Connections

EBF connections must develop the expected shear and axial force in the active link beam.

Active Link Requirements

Link Type	e (link length)	Yielding Mode
Short link	e ≤ 1.6 Ms/Vs	Shear yielding
Long link	e ≥ 2.6 Ms/Vs	Flexural yielding
Intermediate	1.6 Ms/Vs < e < 2.6 Ms/Vs	Combined

where Ms = expected plastic moment, Vs = expected plastic shear.

EBF Connection Design

The link-to-column and link-to-brace connections must develop the full expected link strength:

Link shear capacity: Vs = expected shear yield = 0.6 × Fy × d × tw
Link axial interaction: When axial load > 0.15 × Py, reduce shear capacity
Link stiffeners: Full-depth web stiffeners required at link ends and intermediate points
Link lateral bracing: Top and bottom flange bracing at link ends

Column Splice Requirements (Seismic)

Frame Type	Splice Requirement
SMF	CJP groove weld or bolted for 2% story drift
IMF	Bolted for flexural demand
SCBF	Designed for expected brace forces
EBF	Designed for expected link forces

Column splices in SMF must develop the full column flexural strength. Partial-joint-penetration (PJP) welds are NOT permitted for column splices in SMF unless the splice is located in the middle third of the column height.

Protected Zones

In seismic design, certain regions are designated as protected zones where no attachments, openings, or reductions are permitted:

System	Protected Zone
SMF	Beam plastic hinge region (near column face)
RBS	Reduced section plus 6 in each side
SCBF	Mid-length of brace (buckling region)
EBF	Active link length plus 6 in each side

No holes, welded attachments, or reinforcement may be placed in protected zones without analysis demonstrating no adverse effect.

Frequently Asked Questions

What is the difference between SMF, IMF, and OMF? SMF (Special Moment Frame) requires the most ductile connections with 4% drift capacity and prequalified connections per AISC 358. IMF (Intermediate) requires 2% drift capacity. OMF (Ordinary) has minimal ductility requirements. SMF has the highest R value (8) meaning lower design forces but more stringent detailing.

Why is capacity design used for seismic connections? Capacity design ensures the connection is stronger than the yielding member (beam or brace). This forces yielding into a controlled, ductile location rather than the connection, which could fail in a brittle manner. The connection is designed for the maximum force the yielding member can deliver (its expected strength), not the calculated seismic force.

What is a demand-critical weld? Demand-critical welds are CJP groove welds in seismic force-resisting systems that must meet enhanced toughness requirements and 100% UT inspection. They include beam flange-to-column welds in SMF, and certain brace-to-gusset welds in SCBF.

Can I use fillet welds instead of CJP in seismic connections? Fillet welds may be used for some components (web connections, gusset plates) but NOT for critical moment transfer locations like beam flange-to-column connections in SMF. These require CJP groove welds.

Moment Connection Design — AISC moment connections
Connection Types Explained — All connection types
Welded Connections — Weld capacity calculator
Bolted Connections — Bolt capacity calculator
Wind Load Calculator — ASCE 7 lateral loads

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.