What are the main steel seismic force-resisting systems?

Per ASCE 7-22 Table 12.2-1, steel seismic systems fall into three families: (1) Moment frames — SMF (R=8, most ductile), IMF (R=4.5), OMF (R=3.5, least ductile). SMF requires the most stringent detailing but provides the highest R factor = lowest seismic forces. (2) Concentrically braced frames — SCBF (R=6), OCBF (R=3.25). SCBF uses capacity design: braces yield in tension and buckle in compression; connections and columns are protected. (3) Eccentrically braced frames — EBF (R=7-8). Uses a link beam that yields in shear or flexure as the energy dissipation fuse. Selection depends on seismic design category (SDC), building height, architectural constraints, and drift limits.

What is capacity design in seismic steel structures?

Capacity design (AISC 341 Chapter D) ensures ductile failure modes occur before brittle ones. The principle: (1) Identify the yielding element (fuse) — e.g., beam plastic hinge in SMF, brace buckling in SCBF, link yielding in EBF. (2) Calculate the maximum force the yielding element can deliver using expected yield stress Ry x Fy (not minimum Fy). (3) Design all non-yielding elements (connections, columns, foundations) to resist this amplified force. For SMF: the probable maximum moment at the plastic hinge Mpr = Cpr x Ry x Zx x Fy. The connection and column must resist Mpr, not the code-level design moment. This prevents brittle connection failure before ductile beam yielding — the lesson of the 1994 Northridge earthquake.

What is the protected zone in seismic steel connections?

Per AISC 341-22, the protected zone is the region where inelastic yielding is expected to occur — typically within 1/2 beam depth from the column face for SMF moment connections. Within the protected zone: NO welded, bolted, screwed, or shot-in attachments of any kind (decking, edge angle, conduit supports, sprinkler lines). NO thermal cutting or tack welds. NO gouges, notches, or re-entrant corners from fabrication. The reasoning: any discontinuity in the yielding region creates a stress concentration that can initiate fracture. The protected zone must be clearly marked on shop drawings and verified during inspection. Violations require repair procedures approved by the EOR.

How does R factor affect seismic design forces?

The response modification coefficient R reduces the elastic seismic base shear to account for ductility and energy dissipation. Higher R = lower design force but more stringent detailing. For a building with Cs_elastic = 1.0g (elastic response): SMF (R=8) → Cs = 1.0/8 = 0.125g design base shear. IMF (R=4.5) → Cs = 1.0/4.5 = 0.222g (78% higher than SMF). OMF (R=3.5) → Cs = 0.286g (129% higher than SMF). The trade-off: SMF requires PQR-tested moment connections, CVN-qualified materials, continuity plates, and 100% UT of demand-critical welds. IMF and OMF have relaxed detailing requirements but must resist higher forces. In low-seismic regions, OMF (lowest detailing cost) often wins despite higher member sizes.

What are the CVN toughness requirements for seismic steel?

Per AISC 341-22 A3.3 and A3.4: Heavy shapes (flange thickness >= 1.5 in) used in SMF/IMF must have CVN = 20 ft-lb at 70 deg F. Weld filler metal for demand-critical welds in SMF: CVN = 20 ft-lb at -20 deg F per AWS D1.8 Annex A. Column splice welds in SMF: same as demand-critical welds. These requirements ensure that thick sections (which cool slower during rolling and have lower toughness) and welds (which have as-cast microstructure) remain ductile at ambient and low temperatures. The -20 deg F requirement for weld metal is more stringent than the +70 deg F requirement for base metal because weld metal toughness is inherently lower.

Steel Seismic Design — AISC 341, AS 1170.4, EN 1998 & CSA S16

AISC 341 steel seismic design: SFRS selection, R-factors, capacity design hierarchy, protected zones, expected material strength, and base shear distribution.

Seismic design philosophy for steel

Steel seismic design relies on capacity design — designating specific members or regions as ductile fuses that yield and dissipate energy while protecting the rest of the structure from inelastic demand. The fuse location depends on the seismic force resisting system (SFRS):

Moment frames — plastic hinges form in beams near the column face (strong-column-weak-beam principle).
Concentrically braced frames — braces yield in tension and buckle inelastically in compression.
Eccentrically braced frames — link beams yield in shear or flexure between brace connections.

The connections, columns, and non-fuse members must be designed for the expected (not nominal) capacity of the fuse members, using overstrength factors.

SFRS types and R-factors (ASCE 7-22 Table 12.2-1)

System	R	Omega_0	Cd	Height limit SDC D (ft)
SMF (Special Moment Frame)	8	3	5.5	No limit
IMF (Intermediate Moment Frame)	4.5	3	4	No limit (SDC C); not permitted (SDC D-F)
OMF (Ordinary Moment Frame)	3.5	3	3	65 ft (SDC D)
SCBF (Special Concentrically Braced)	6	2	5	No limit
OCBF (Ordinary Concentrically Braced)	3.25	2	3.25	35 ft (SDC D)
EBF (Eccentrically Braced Frame)	8	2.5	4	No limit
BRBF (Buckling-Restrained Braced)	8	2.5	5	No limit
Dual: SMF + SCBF	7	2.5	5.5	No limit
Steel plate shear wall (SPSW)	7	2	5.5	No limit

Higher R-factors mean lower design base shear but more stringent detailing requirements. The overstrength factor Omega_0 is used for designing connections and collector elements.

Expected material strength (Ry values)

AISC 341 Table A3.2 provides Ry values that account for actual mill yield strengths exceeding minimum specified values:

Steel Specification	Fy (ksi)	Ry	Expected Fy = Ry x Fy (ksi)
A36 (plates)	36	1.5	54
A572 Gr 50 (plates)	50	1.3	65
A992 (W-shapes)	50	1.1	55
A500 Gr B (HSS)	42	1.4	59
A500 Gr C (HSS)	46	1.4	64
A913 Gr 50 (shapes)	50	1.1	55
A913 Gr 65 (shapes)	65	1.1	72

Connections and capacity-protected members must be designed for Ry x Fy, not the nominal Fy. For A500 Gr C HSS braces, this means designing connections for 64 ksi, not 46 ksi — a 39% increase.

Seismic Design Category triggers (ASCE 7)

SDC	S_DS Range	S_D1 Range	Description	Steel SFRS Required
A	< 0.167	< 0.067	Minimal	Any system
B	0.167-0.33	0.067-0.133	Moderate	Any system
C	0.33-0.50	0.133-0.20	Significant	SMF, SCBF, EBF, BRBF, OMF
D	>= 0.50	>= 0.20	High	SMF, SCBF, EBF, BRBF (no OMF > 65 ft)
E	>= 0.50	>= 0.20	Very High	Same as D, more restrictive
F	>= 0.50	>= 0.20	Extreme	Same as D, most restrictive

Most steel buildings in SDC D and higher must use special systems (SMF, SCBF, EBF, BRBF).

Brace slenderness limits by system

System	Brace Type	KL/r Limit	Rationale
SCBF	Any	<= 200	General AISC limit
OCBF	Any	<= 200	General AISC limit
BRBF	BRB (yielding core)	N/A (controlled yielding)	BRB manufacturer specifies
EBF	Link beam	N/A (length/rotation controls)	e <= 1.6 Mp/Vp for shear link

Preferred brace slenderness for SCBF

KL/r Range	Post-Buckling Behavior	Energy Dissipation	Recommended?
< 60	Very high post-buckling force	Low (abrupt)	No (fracture risk)
60-80	Moderate	Good	Marginal
80-120	Controlled	Best	Yes (preferred)
120-160	Lower force	Good	Yes
160-200	Low force	Moderate	Acceptable but check fracture

Mid-range slenderness (KL/r = 80-120) provides the best combination of energy dissipation and controlled post-buckling behavior.

Worked example — SCBF brace design

Building data: 4-story office, SDC D, 13 ft story heights, tributary seismic weight per brace = 280 kips. Design base shear coefficient Cs = 0.12 (from ASCE 7 Equivalent Lateral Force procedure).

Factored brace axial force at first story: V_base = 0.12 x 280 = 33.6 kips distributed over stories. By ELF distribution, the first-story brace force is approximately 1.4 x V / cos(theta), where theta = arctan(13/15) = 41 degrees. Brace force P_u = 1.4 x 33.6 / cos(41) = 62.3 kips.

For SCBF, AISC 341-22 Section F2.5a limits brace slenderness to KL/r <= 200. Brace length = sqrt(13^2 + 15^2) = 19.8 ft = 238 in. Using HSS5x5x3/8 (r = 1.84 in, A = 6.18 in^2): KL/r = 1.0 x 238 / 1.84 = 129 < 200. OK.

Compression capacity: phi*Pn = 0.9 x Fcr x A. With KL/r = 129 and Fy = 46 ksi (A500 Gr. C), Fe = pi^2 x 29000 / 129^2 = 17.2 ksi. Since Fe < 0.44Fy, Fcr = 0.877 x Fe = 15.1 ksi. phi*Pn = 0.9 x 15.1 x 6.18 = 84.0 kips > 62.3 kips. OK.

Expected tensile yield for capacity design of connections: Ry x Fy x Ag = 1.4 x 46 x 6.18 = 398 kips. The gusset plate and welds must resist 398 kips (not 62.3 kips).

Worked example — SMF column-beam check

Given: W14x176 column, W24x76 beams each side. A992 steel. Axial load Pu = 800 kips.

Step 1 — Expected beam moment: M_pr = C_pr x Ry x Fy x Z_RBS = 1.15 x 1.1 x 50 x 143 = 9,067 kip-in = 756 kip-ft (each beam)

Step 2 — Column moment capacity (reduced for axial): W14x176: Zx = 342 in^3, Ag = 51.7 in^2 M_pc = Zx x (1 - Pu/(Ag x Fy)) = 342 x (1 - 800/(51.7 x 50)) = 342 x 0.69 = 236 kip-in... Wait, use Ry x Fy for column too: M_pc = Zx x Ry x Fy x (1 - Pu/(Ag x Ry x Fy)) = 342 x 1.1 x 50 x (1 - 800/(51.7 x 55)) = 18,810 x (1 - 0.281) = 13,528 kip-in = 1,127 kip-ft

Step 3 — SCWB ratio: Sum M_pc / Sum M_pr = 2 x 1,127 / 2 x 756 = 2,254 / 1,512 = 1.49 >= 1.0. Pass.

If the column were W14x120: Zx = 228 in^3, Ag = 35.2 in^2. M_pc = 228 x 55 x (1 - 800/(35.2 x 55)) = 12,540 x (1 - 0.413) = 7,361 kip-in = 613 kip-ft Ratio = 2 x 613 / 2 x 756 = 0.81 < 1.0. Fails — need heavier column.

Protected zones

AISC 341 designates protected zones — regions of expected plastic hinging where no attachments, connections, or penetrations are permitted. For moment frames, the protected zone extends from the column face to one beam depth past the plastic hinge location. For braced frames, the entire brace length and gusset plate region are protected zones.

Protected zone extent by system

System	Protected Zone	Prohibited Actions
SMF (RBS)	Column face to end of RBS cut + d_b beyond	Welding, bolting, drilling, shear studs
SMF (other)	Per AISC 358 for each prequalified connection	All attachments
SCBF	Entire brace length + gusset yield zone	Attachments to brace body
EBF	Entire link beam	Any penetrations or attachments
BRBF	BRB yielding core per manufacturer	Confined to manufacturer's restrictions

Welding shear studs, attaching decking, or drilling holes in protected zones is prohibited.

Seismic drift limits by SDC

SDC	Max. Story Drift Ratio	For Steel SMF (Cd=5.5)	For Steel SCBF (Cd=5)
C	0.020 hs	0.020 hs	0.020 hs
D	0.020 hs	0.020 hs	0.020 hs
E	0.015 hs	0.015 hs	0.015 hs
F	0.015 hs	0.015 hs	0.015 hs

The amplified drift is delta_x = Cd x delta_xe / I_e. For a 13 ft story height in SDC D: max drift = 0.020 x 13 x 12 = 3.12 in.

Code comparison — seismic steel design

Aspect	AISC 341-22	AS 4100 + AS 1170.4	EN 1998-1	CSA S16-19
System factor	R (response modification)	Sp x mu	q (behaviour factor)	Rd x Ro
Expected strength	Ry x Fy (Table A3.2)	Not explicitly used	gamma_ov x f_y (1.25 typical)	Ry x Fy (Table 3)
Brace slenderness	KL/r <= 200 (F2.5a)	Cl. 6.3.3 (KL/r limit varies)	EN 1998-1 Cl. 6.7.3	Cl. 27.5.3.2 (KL/r <= 200)
Strong column-weak beam	E3.4a: sum(Mpc) > sum(Mpr)	Not explicitly required	Cl. 4.4.2.3: sum(MRc) >= 1.3 sum(MRb)	Cl. 27.2.3.2
Protected zones	Section I2: no attachments	Not defined	Not explicitly defined	Cl. 27.2.6
Column splice min.	50% RyFyAg (SMF)	Design actions	Full capacity design	50% factored resistance

Common pitfalls

Designing connections for the code-level force, not the expected member capacity. In an SCBF, the gusset plate must resist Ry x Fy x Ag of the brace (often 3-6 times the design force), not just the factored load. This is the most frequent seismic steel design error.
Ignoring the strong-column-weak-beam check. AISC 341 E3.4a requires sum of column plastic moments (reduced for axial load) to exceed 1.0 x sum of beam expected plastic moments at every joint.
Using Fy instead of Ry*Fy for capacity design. Expected material strength Ry*Fy accounts for actual mill yield strengths exceeding minimum specified values. For A992 steel, Ry = 1.1, so expected yield is 55 ksi vs nominal 50 ksi. For A500 Gr. C HSS, Ry = 1.4.
Neglecting brace post-buckling behavior. SCBF braces must sustain cyclic buckling without fracture. Short, stocky braces (KL/r < 60) develop very high post-buckling demands at the plastic hinge. Mid-range slenderness (KL/r = 80-120) is preferred.
Not amplifying drift for code check. Code drift limits apply to Cd x delta_xe / I_e, not the raw elastic displacement from analysis.

Frequently asked questions

What is the difference between R, Omega_0, and Cd? R reduces the design base shear (higher R = lower force). Omega_0 amplifies forces for capacity design of connections and collectors. Cd amplifies elastic drift for code drift limit checks.

When do I use Ry x Fy instead of Fy? For capacity-protected elements: column design in moment frames, gusset plate design in braced frames, connection design, and any member that must remain elastic while the fuse yields.

Which seismic system should I choose? For ductility: SMF or EBF (R=8). For economy: SCBF (R=6). For low seismicity: OMF or OCBF. For new construction with budget: BRBF (R=8, predictable behavior).

What is a protected zone? A region where no attachments, penetrations, or welding is permitted because plastic hinging is expected there during an earthquake. Defined per AISC 341 and AISC 358 for each connection type.

Do I need special inspections for seismic steel? Yes. AISC 341 requires special inspection for all demand-critical welds (100% UT or MT), all bolt installation in SFRS connections, and all steel framing in the SFRS.

What is the minimum steel grade for seismic systems? AISC 341 requires a maximum Fy/Fu ratio of 0.80 for SFRS members. A992 (Fy=50, Fu=65, ratio=0.77) meets this. A36 (Fy=36, Fu=58, ratio=0.62) also meets it but is rarely used for SFRS.

AISC 341 seismic force resisting systems — detailed overview

AISC 341-22 defines eight primary steel seismic force resisting systems, each with specific ductility expectations, detailing requirements, and capacity design provisions.

System types and characteristics

System	Acronym	Ductility	Fuse Mechanism	R	Key AISC 341 Section
Special Moment Frame	SMF	High	Beam plastic hinges	8	Chapter E (E3)
Intermediate Moment Frame	IMF	Moderate	Beam plastic hinges	4.5	Chapter E (E2)
Ordinary Moment Frame	OMF	Low	Limited ductility	3.5	Chapter E (E1)
Special Concentrically Braced Frame	SCBF	High	Brace tension yield + compression buckling	6	Chapter F (F2)
Ordinary Concentrically Braced Frame	OCBF	Low	Brace yielding	3.25	Chapter F (F1)
Eccentrically Braced Frame	EBF	High	Shear or flexural link yielding	8	Chapter F (F3)
Buckling-Restrained Braced Frame	BRBF	High	BRB core yielding (no buckling)	8	Chapter F (F4)
Steel Plate Shear Wall	SPSW	High	Web plate tension field yielding	7	Chapter G

Response modification coefficients (R values) explained

The R factor reduces the elastic seismic base shear to a design level, accounting for ductility and overstrength. Higher R means lower design force but requires more stringent detailing.

Cs = SDs / (R / Ie)     (ASCE 7-22 Eq. 12.8-2, simplified)

R Factor Range	Design Force Reduction	System Examples	Detailing Cost
3.0 - 3.5	Low reduction (3x)	OMF, OCBF	Minimal
4.5 - 5.0	Moderate (4-5x)	IMF, dual systems	Moderate
6.0	Significant (6x)	SCBF	Significant
7.0 - 8.0	Large (7-8x)	SMF, EBF, BRBF, SPSW	Substantial

Capacity design hierarchy

Capacity design ensures that ductile fuses yield before brittle elements fail. The hierarchy from most ductile to capacity-protected:

Fuse elements (beams in SMF, braces in SCBF, links in EBF) -- designed for code-level forces, permitted to yield
Connections (beam-to-column joints, gusset plates, link-to-column connections) -- designed for RyFyAg of the fuse member
Columns -- designed for Omega_0 amplified forces plus gravity, with strong-column-weak-beam check
Foundation -- designed for overstrength forces (Omega_0 x seismic effect + dead + live)

Detailing requirements by system

Requirement	SMF	IMF	SCBF	EBF	BRBF
Beam flange continuity plates	Required	Required	N/A	Required	N/A
Panel zone strength	Ry*Fy demand	Ry*Fy demand	N/A	Per EBF spec	N/A
Strong-column weak-beam	Section E3.4a	Not required	N/A	Section F3.5b	Section F4.5b
Protected zone	Per AISC 358	Reduced scope	Full brace + gusset	Full link beam	BRB core zone
Demand-critical welds	All CJP welds in SFRS	Flange CJP welds	Gusset-to-brace	Link-to-column	BRB connections
Max. brace KL/r	N/A	N/A	200	N/A	N/A
Link length ratio	N/A	N/A	N/A	e <= 1.6 Mp/Vp	N/A
Column splice min.	50% RyFyAg	50% RyFyAg	Per gravity	Per gravity	Per gravity

Prequalified SMF connections per AISC 358-22

Connection Type	Key Feature	Max. Beam Depth	Reference
Reduced Beam Section (RBS)	Radius cuts in flanges	W36	AISC 358 Sec. 5.4
Bolted Unstiffened End Plate (BUEP)	End plate with bolts	W24	AISC 358 Sec. 5.5
Bolted Stiffened End Plate (BSEP)	Stiffened end plate	W33	AISC 358 Sec. 5.6
Welded Flange Plate (WFP)	Flange plates welded to column	W36	AISC 358 Sec. 5.7
Kaiser Bolted Bracket (KBB)	Bolted bracket	W24	Proprietary
SidePlate	Side plates welded to column	W33	Proprietary
Slotted Web (SLW)	Web slots at flanges	W27	AISC 358 Sec. 5.8

AISC 358 prequalification eliminates the need for physical testing, provided the connection parameters fall within the prequalified range (beam depth, span, flange thickness).

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