Steel Connection Design (Bolted + Welded, Detailed) — Engineering Reference

1. At a glance

Steel connections are the joints that transfer force between members in a steel structure. They are simultaneously the highest-cost detail (20–30 % of total steel fabrication and erection cost on a typical building), the most error-prone detail (the Hyatt Regency 1981 walkway collapse, the Northridge 1994 moment-frame fractures, and the WTC 7 floor-system failures are all connection failures, not member failures), and the least standardised detail (the same beam may be supported by twenty different connection types and any of them might be code-compliant).

The governing US code is AISC 360-22 Chapter J — Design of Connections, supplemented by:

• AISC 341-22 seismic provisions (Chapters D–F member detailing, Chapter E moment frames, Chapter F braced frames).
• AISC 358-22 prequalified moment connections for SMF/IMF (RBS, BFP, BUEP, BSEP, ConXL, SidePlate, Kaiser, Simpson Yield-Link, etc.).
• RCSC 2020 Specification for Structural Joints Using High-Strength Bolts (snug-tight, pretensioned, slip-critical installation).
• AWS D1.1/D1.1M:2024 Structural Welding Code – Steel (procedures, welder qualification, weld geometry).
• AISC Steel Construction Manual, 16th edition (2023) — the practitioner’s lookup, with selection tables (Parts 7–15), worked design examples, and connection geometry data.

AISC 360 Section B3.4 classifies connections into three families by moment-rotation behaviour:

Class	Rotation	Moment	Typical example
Simple shear	free rotation, M ≈ 0	shear only	Shear tab, double-angle, end-plate, seated
FR (fully-restrained) moment	rotates with the member, transfers full M_p	M and V	RBS, BFP, BSEP, WUF-W
PR (partially-restrained) moment	intermediate stiffness, partial M	M and V	T-stub, top-and-seat angle, flush end-plate

Beyond these, AISC Manual catalogues bracing/gusset, splice, HSS truss, and column-base connections separately.

Where it sits in the design stack: connections come after member design (steel-design) and consume the demands generated there. The two halves are not independent — the AISC 358-22 prequalified moment connections dictate beam-flange geometry and panel-zone strength, and AISC 341-22 capacity design sizes connections off expected member strength (R_y · F_y) rather than design demand, often driving column and continuity-plate sizing back up.

The three governing physics are: (1) bolts transfer load by shear, friction, and bearing; (2) welds transfer load by metallurgical fusion across the throat plane; (3) plate / member local elements (web, flange, gusset, end-plate) yield, buckle, or tear in modes that the bolt/weld math does not predict — those need separate limit-state checks (yielding, block shear, prying, web crippling, panel-zone shear, Whitmore buckling).

2. First principles

2.1 The four force-transfer mechanisms

A steel connection moves force from one piece of steel to another by some combination of:

1. Bearing — bolt shank presses on the side of its hole; the connected ply transfers force as compression at the contact zone (AISC J3.10, the 2.4 d t F_u equation governs ply bearing strength).
2. Friction — pretensioned bolts clamp the faying surfaces; load is transferred by static friction (μ · N) without slip (RCSC 2020 + AISC J3.8, slip-critical joints).
3. Welding — molten weld metal forms a metallurgically continuous bridge across the throat plane; force flows through the weld throat (AISC J2, fillet + groove + plug + slot).
4. Mechanical interlock — bearing pin in a pin-connected eye-bar (rare), or shear keys in column splices, or shear lugs at the base plate (AISC Design Guide 1).

Most connections combine two of these. A snug-tight bolted shear tab is bearing. A pretensioned slip-critical joint is friction at service and bearing at ultimate (it slips, then bears). A welded moment connection with bolted shear plate is welding for flanges + bearing for web.

2.2 Bolt limit states (AISC J3 + RCSC 2020)

For ASTM F3125 high-strength structural bolts — Grade A325 (now F3125 Gr A325) F_u = 825 MPa (120 ksi) and Grade A490 (now F3125 Gr A490) F_u = 1035–1210 MPa (150 ksi):

Limit state	AISC eqn	φ	Nominal strength
Bolt shear	J3-1	0.75	R_n = F_nv · A_b (threads incl./excl.)
Bolt tension	J3-1	0.75	R_n = F_nt · A_b
Combined shear + tension	J3-3a/b	0.75	F_nt′ = 1.3 F_nt − (F_nt / φ F_nv) f_rv ≤ F_nt
Bearing on connected ply	J3-6a	0.75	R_n = 2.4 d t F_u (deformation-considered)
Tearout on connected ply	J3-6c	0.75	R_n = 1.2 L_c t F_u
Block shear on member/plate	J4-5	0.75	R_n = 0.6 F_u A_nv + U_bs F_u A_nt ≤ 0.6 F_y A_gv + U_bs F_u A_nt
Slip resistance (Cl A surf.)	J3-4	1.00 (service) / 0.85 (ult.)	R_n = μ · D_u · h_f · T_b · n_s

F_nv (nominal shear stress) and F_nt (nominal tensile stress) per AISC Table J3.2:

Grade	F_nt (MPa / ksi)	F_nv threads excl. N (MPa / ksi)	F_nv threads incl. X (MPa / ksi)
A325 / F1852	620 / 90	372 / 54	457 / 68
A490 / F2280	780 / 113	457 / 68	579 / 84

Three installation classes (RCSC 2020 § 4):

• Snug-tight (ST) — bolt brought to firm contact by the full effort of an ironworker with a spud wrench, or a few impacts of an impact wrench. No defined pretension. Used for bearing-type joints not subject to fatigue or reversal.
• Pretensioned (PT) — bolt tightened to ≥ 70 % of minimum tensile strength (Table 8.1: e.g. 7/8” A325 = 39 kips / 173 kN pretension; 7/8” A490 = 49 kips / 218 kN). Installed by turn-of-nut (1/3 turn from snug for L ≤ 4 d), calibrated wrench, twist-off-type tension control bolt (F1852/F2280), or direct tension indicator (DTI, Squirter washer, ASTM F959). Required for slip-critical joints, fatigue joints, joints with reversal, and joints in seismic systems.
• Slip-critical (SC) — pretensioned plus the faying surface is certified for slip resistance (Class A blast-cleaned uncoated steel μ = 0.30; Class B blast-cleaned with Class B paint μ = 0.50; Class C galvanised μ = 0.30). Service-level slip checked at service loads; bearing checked at LRFD ultimate.

2.3 Weld limit states (AISC J2 + AWS D1.1)

Fillet weld (most common in shop and field). Effective throat t_e = 0.707 · w where w is leg size, for the standard equal-leg fillet. Design strength:

φR_n = 0.75 · 0.60 · F_EXX · t_e · L

with F_EXX the weld-electrode tensile strength (E70XX → 70 ksi = 483 MPa; E80, E90, E100 series for higher matching). The 0.60 factor converts tensile to shear strength; the 0.75 is the LRFD resistance factor for welds. AISC J2.4 allows a directional-strength increase: φR_n = 0.75 · 0.60 · F_EXX · (1.0 + 0.50 sin¹·⁵ θ) · t_e · L where θ is the angle between weld axis and load. Transverse fillets (θ = 90°) are 1.5× stronger than longitudinal (θ = 0°), but the increase is usually neglected for redundancy.

Groove weld: complete-joint-penetration (CJP) groove weld develops base-metal strength of the thinner connected part — no separate weld check needed if matching filler is used (AISC J2.4 + Table J2.5). Partial-joint-penetration (PJP) requires an explicit weld-throat check with the same φR_n equation, using the effective throat E measured from the AWS prequalified groove geometry (Table J2.1).

Base-metal check: at every weld, also check base-metal yielding φR_n = 0.90 · F_y · t · L and base-metal rupture φR_n = 0.75 · F_u · t · L (AISC J2.4). For thin plates with E70 fillet welds, the base-metal rupture limit governs at fillet leg ≈ 0.7 · t for A36 and ≈ 0.5 · t for A992.

Minimum fillet size (AISC J2.2b, governed by HAZ cooling rate, not strength): 3 mm (1/8”) for material ≤ 6 mm thick, 5 mm (3/16”) for 6–13 mm, 6 mm (1/4”) for 13–19 mm, 8 mm (5/16”) for > 19 mm. Maximum fillet size: along an edge, the leg ≤ thickness of part ≤ 6 mm (1/4”), or leg ≤ thickness − 2 mm (1/16”) for thicker plates.

2.4 Resistance factors summary

Limit state class	φ (LRFD)	Ω (ASD)
Bolt shear, tension, bearing	0.75	2.00
Weld (filler metal & base-metal rupture)	0.75	2.00
Base-metal yielding	0.90	1.67
Tension yielding (gross section)	0.90	1.67
Tension rupture (net section)	0.75	2.00
Block shear	0.75	2.00
Compression (Whitmore on gusset)	0.90	1.67

3. Connection families

3.1 Simple shear connections (AISC Manual Part 10)

Carry beam-end reaction with negligible moment. Allow free rotation. Five common topologies:

Type	AISC Manual table	Typical use	Notes
Single-plate “shear tab”	10-9, 10-10	Beam to column flange or girder web	Cheapest; one row of bolts in beam web + fillet weld of plate to support
Double-angle “clip”	10-1 to 10-3	Beam to girder web, beam to column	Two L-angles bolted/welded; classic field-bolted detail
Single-angle	10-12	Light beam to column	Like clip with one angle removed; light loads
End-plate (shear)	10-4	Beam to column flange, shop-welded	Plate shop-welded to beam end, field-bolted to support
Seated (unstiffened or stiffened)	10-6 to 10-8	Beam on column flange, light loads	Seat angle carries reaction in bearing; top angle is stability only
Tee / split-T	10-5	Heavy reactions	Web of T bolted to beam web, flange of T welded/bolted to support

Simple-shear design checks for any topology: (1) bolt shear, (2) bearing/tearout in beam web, (3) bearing/tearout in plate, (4) plate shear yielding, (5) plate shear rupture, (6) plate block shear, (7) weld (or bolt) to support, (8) coped-beam flexural rupture if beam top flange is coped.

3.2 Fully-restrained moment connections (AISC Manual Part 12 + AISC 358 prequalified)

Transfer the full plastic moment of the beam. Eight prequalified for SMF/IMF in AISC 358-22:

Designation	Section in AISC 358	Mechanism
RBS Reduced Beam Section (“dogbone”)	§ 5	Beam flange trimmed; plastic hinge forced away from column face
BUEP Bolted Unstiffened Extended End-Plate	§ 6.9	End-plate yield-line + bolt tension (4-bolt 4E, 4ES configurations)
BSEP Bolted Stiffened Extended End-Plate	§ 6.10	Same with vertical rib stiffener; 8 ES or 8ES configurations
BFP Bolted Flange Plate	§ 7	Beam flanges to flange plates by bolts; plates CJP/fillet welded to column
WUF-W Welded Unreinforced Flange–Welded Web	§ 8	CJP flange welds + supplemental web weld; post-Northridge fix to “WUF-B”
KBB Kaiser Bolted Bracket	§ 9	Cast steel bracket bolted to beam and column flange
ConXL	§ 10	Square HSS column collar with cast/welded beam connector
SidePlate	§ 11	Beam intercepts two parallel side-plates field-welded to column
Simpson Yield-Link	§ 12	Replaceable steel link bolted between beam stub and column flange

PR (partially-restrained) connections — top-and-seat angle, flush end-plate, T-stub — were the standard in pre-1960 frames and survive in seismic-low-demand applications and in lightly framed mezzanines. They require explicit M–θ curve modelling in analysis (AISC 360 B3.4b commentary).

3.3 Brace and truss connections (AISC Manual Part 13)

Carry axial load between a brace (HSS, double-angle, WT, single-angle, BRB casing) and the beam–column work-point. The connection element is the gusset plate (AISC Design Guide 29 Vertical Bracing Connections). Two design methods:

• Uniform Force Method (UFM) — Thornton 1991, the AISC default. Apportions brace force to beam-gusset and column-gusset interfaces by geometry so that no moments arise at the interfaces under the brace axial.
• Modified UFM with corner clip, work-point offset, or beam-shear release for special geometry.

Gusset-plate limit states: (a) Whitmore section tension yielding/rupture (effective width = 30° spread from first to last bolt or weld); (b) Whitmore compression buckling (K·L_eff / r); (c) block shear; (d) edge buckling; (e) interface welds/bolts to beam and column.

3.4 Splices (AISC Manual Part 14)

Column splices in moment frames: AISC 341-22 § E3.6g requires the splice to develop the lesser of expected flexural strength of the smaller column or 50 % of the larger. Two topologies: bolted lap splice with flange plates and web plates (preferred for site flexibility); welded butt splice with CJP groove welds in both flanges + bolted/welded web (preferred for clean column lines, slightly more costly).

Beam splices: rare except for transfer-girder shipping breaks or HSS chord splices in trusses. CJP groove with backing bar (or backing bar removed in seismic).

3.5 HSS / tubular connections (AISC 360 Chapter K)

HSS-to-HSS K-, N-, T-, Y-, and X- joints have their own limit-state equations (AISC 360 K2/K3 tables, derived from CIDECT and ICC-ES research). The chord-face plastification, chord-side-wall, chord-shear, brace-effective-width, and punching-shear limit states all need to be checked, and the β = b_brace/B_chord and γ = B/2t ratios drive which limit state controls.

3.6 Column base plates (AISC Design Guide 1)

The interface between the steel superstructure and concrete foundation. ASTM F1554 anchor rods (Gr 36, 55, or 105) embedded in the concrete (with hook, plate-washer, or headed end for pull-out capacity per ACI 318 Chapter 17). Base plate sized for concrete bearing (φ = 0.65 · 0.85 · f′_c on A1/A2 effective area), and plate-bending checks per the AISC cantilever method (m, n, n′ dimensions). Anchor rods take uplift, shear (via shear lug or oversized hole + grout pad friction), and overturning moment. Tilt-up “pinned” base plates take only shear and uplift; “fixed” base plates with stiffeners and oversized footprint take base moment for moment-frame columns.

4. Worked examples

All examples use SI as primary, US-customary in parentheses, units carried throughout.

Example A — Single-plate shear connection (W18×35 to W12×96 flange)

Demand: factored beam-end reaction R_u = 200 kN (45 kip).

Geometry: shear tab 13 mm × 100 mm × 150 mm (½” × 4” × 6”) A36 plate, fillet-welded to column flange. 4× 19 mm (¾”) A325-N bolts in single shear, single vertical row at 75 mm (3”) pitch, 38 mm (1.5”) edge distance. Standard 21 mm (13/16”) holes.

Check 1 — Bolt shear, F_nv = 372 MPa (N condition, threads in shear plane), A_b = π·(19)²/4 = 284 mm²:

φR_n,bolt = 0.75 · 4 · 372 MPa · 284 mm² = 317 600 N = 318 kN ✓ (> 200 kN)

Check 2 — Bearing/tearout on beam web (t_w = 7.4 mm for W18×35, F_u = 450 MPa):

φR_n,bearing = 0.75 · 4 · 2.4 · 19 · 7.4 · 450 = 228 000 N = 228 kN ✓

Check 3 — Bearing/tearout on shear tab (t_p = 13 mm, F_u = 400 MPa for A36):

φR_n,bearing = 0.75 · 4 · 2.4 · 19 · 13 · 400 = 356 000 N = 356 kN ✓

Check 4 — Plate shear yielding (A_gv = 13 · 150 = 1950 mm²):

φR_n = 1.00 · 0.60 · 250 · 1950 = 292 500 N = 293 kN ✓

Check 5 — Plate block shear (single line, L_c = 38 mm tearout governs in practice):

φR_n = 0.75 · (0.6 · 400 · A_nv + 1.0 · 400 · A_nt) ≈ 285 kN ✓

Check 6 — Plate-to-column fillet weld (6 mm / ¼” two-sided, L = 150 mm both sides):

φR_n = 0.75 · 0.60 · 483 · 0.707 · 6 · (2 · 150) = 277 000 N = 277 kN ✓

All six checks pass. Bolt shear controls at 318 kN nominal; DCR ≈ 0.63. Conservative, robust shear-tab connection.

Example B — Bolted extended end-plate moment (AISC 358 BUEP, 4E-1¼)

Demand: probable maximum moment M_pr at column face for W24×84 beam (A992, F_y = 345 MPa, F_u = 450 MPa, Z_x = 3.36 × 10⁶ mm³):

M_pr = C_pr · R_y · F_y · Z_x = 1.15 · 1.1 · 345 · 3.36 × 10⁶ = 1467 kN·m (1082 kip·ft) V_u at column face ≈ M_pr / (L_h) where L_h = clear span between hinges ≈ 6 m → V_u ≈ 489 kN (110 kip)

End-plate selection — AISC 358 Table 6.1: 4-bolt unstiffened (4E) configuration with 1¼” (32 mm) A490 pretensioned bolts. Bolt pitch p_f = 70 mm (2.75”), gauge g = 140 mm (5.5”).

Check 1 — End-plate thickness by yield-line analysis (AISC 358 Eqn 6.9-9, four-bolt unstiffened):

t_p,req = √(1.11 · γ_r · M_pr / (φ_d · F_yp · Y_p)) where Y_p is the end-plate yield-line parameter (AISC 358 Table 6.2) For 4E with d_b = 24” beam, b_p = 230 mm, g = 140 mm, p_f,i = p_f,o = 70 mm → Y_p ≈ 3500 mm t_p,req = √(1.11 · 1.2 · 1467 × 10⁶ / (1.00 · 345 · 3500)) ≈ 41 mm — use t_p = 44 mm (1¾”) A572 Gr 50

Check 2 — Bolt tension with prying (AISC 358 Eqn 6.9-3 + Manual Chapter 9):

Required bolt force F_t,req = M_pr / (2 · (d − t_f − p_f)) per row of 2 bolts = 1467×10⁶ / (2 · (612 − 18 − 70)) = 1.40 × 10⁶ N / 4 bolts = 350 kN per bolt φR_n,A490 1¼” = 0.75 · 780 · 794 mm² = 464 kN ✓ Prying force Q ≈ 0 because t_p (44 mm) ≫ thick-plate threshold; “thick-plate” behaviour suppresses prying.

Check 3 — Column-side limit states (W14×370 column):

• Web local yielding (AISC J10.2): φR_n = 1.0 · F_yw · t_w · (5k + l_b) ≥ flange-force demand
• Web local crippling (J10.3): φR_n per J10-4 or J10-5
• Web compression buckling (J10.5): φR_n = 0.90 · 24 t_w³ √(EF_yw)/h
• Continuity plates (AISC 341 E3.6f) required if column web/flange fails any check
• Doubler plate (AISC 341 E3.6e): panel-zone shear demand 0.8 · ΣM_pf / d_b ≥ 0.6 F_yc · d_c · t_w → if not, add doubler

Check 4 — Strong-column-weak-beam (AISC 341 E3.4a):

ΣM_pc*/ ΣM_pb* > 1.0 with M_pcexpected column flexural strength reduced for axial load, M_pb expected beam flexural strength projected to column centreline

All checks pass with W14×370 column + 25 mm continuity plates + 12 mm doubler. Connection prequalified per AISC 358 § 6 — no project-specific testing required.

Example C — Welded flange-plate moment (WFP)

Demand: W21×62 beam (A992, Z_x = 2.08 × 10⁶ mm³) to W14×120 column. Probable max moment:

M_pr = 1.15 · 1.1 · 345 · 2.08 × 10⁶ = 908 kN·m (670 kip·ft) Beam d = 533 mm, t_f = 15.6 mm; flange-force lever arm ≈ d − t_f = 517 mm Flange force T = M_pr / (d − t_f) = 908 × 10⁶ / 517 = 1756 kN (395 kip)

Flange plate selection: 200 mm × 25 mm (8” × 1”) A572 Gr 50 plate, top and bottom:

φP_y = 0.90 · 345 · 200 · 25 = 1553 kN — undersized Increase to 230 mm × 25 mm: φP_y = 0.90 · 345 · 230 · 25 = 1785 kN ✓

Plate-to-beam-flange weld — fillet weld each side of plate, full length L_w:

Demand 1756 kN, weld size 14 mm (9/16”), E70 fillet: φr = 0.75 · 0.60 · 483 · 0.707 · 14 · 2 sides = 4302 N/mm L_w,req = 1756 × 10³ / 4302 = 408 mm — use 450 mm plate length, 14 mm fillet, both sides

Plate-to-column-flange weld — CJP groove on each flange plate. Develops full plate strength; matching filler E70 → no separate check. AISC 341 § A3.4b designates this a demand-critical weld (DCW):

• Filler metal certified to AWS D1.8 Annex A: CVN ≥ 27 J at −18 °C (20 ft·lb at 0 °F) by Charpy V-notch at the weld root location
• Preheat 65–95 °C (150–200 °F) for plate ≥ 25 mm and F_y = 345 MPa per AWS D1.1 Table 5.8
• Backing bar removed and root back-gouged, or left in place with reinforcing fillet (AISC 358 § 3.5)
• Weld tabs (run-off tabs) removed and end ground flush

Web shear plate (single plate): bolted to beam web with 4× 22 mm A325-X bolts in slotted holes, fillet-welded to column flange. Carries V_u = 489 kN only (flanges carry M).

Continuity + doubler at column joint per AISC 341 § E3.6e/f as in Example B.

Connection class: FR, not AISC 358 prequalified, requires project-specific qualification testing per AISC 341 § K2 (cyclic loading protocol) — OR — substitute one of the eight prequalified configurations (BFP § 7 is the closest analogue).

5. Seismic detailing (AISC 341 + 358)

Seismic moment-frame and braced-frame connections are designed to a different philosophy than gravity connections: capacity design. The connection is sized to develop the expected member strength (R_y · F_y, where R_y = 1.1 for A992) — not the analysis demand — so that inelastic action is forced into the member at a known location (the plastic hinge), and the connection itself stays elastic.

Demand-critical welds (DCW) — AISC 341 § A3.4b lists the welds (e.g. CJP beam-flange to column-flange in SMF, gusset-to-column in SCBF, BRB-core-to-end-plate) whose fracture would lead to system-level collapse. DCW require:

• Higher CVN toughness on filler: 27 J at −18 °C (Charpy V-notch) per AWS D1.8
• Lower hydrogen filler (H8 or H4 designation)
• Stricter preheat / interpass
• More aggressive NDE (100 % UT or PAUT typically)

Protected zone — AISC 341 § D1.3 defines a length on the beam (for RBS, the reduced section; for BUEP, between column face and end-plate weld; for WUF-W, a length 1·d from column face) within which no welds, no fasteners, no shear-stud holes, no plate attachments, no decking attachments may be installed. Any attachment in the protected zone reduces low-cycle fatigue life of the plastic hinge.

Continuity plates — AISC 341 § E3.6f. Transverse stiffeners in the column web aligned with the beam flanges, transferring the flange tension/compression couple across the column web. Required unless the column web/flange satisfies all of: J10.2 web yielding, J10.3 web crippling, J10.5 web compression buckling, J10.6 web sidesway buckling, J10.10 flange flexural yielding (the t_cf ≥ 0.4√(b_b · t_b · F_yb · R_yb / (F_yc · R_yc)) rule).

Doubler plate — AISC 341 § E3.6e. Required when panel-zone shear demand R_v = 0.8 · ΣM_pf,exp / d_b exceeds the column-web panel-zone strength φR_n = 0.60 · F_yc · d_c · t_w · (1 + 3 b_cf · t_cf² / (d_b · d_c · t_w)).

Strong-column-weak-beam (SCWB) — AISC 341 § E3.4a. ΣM_pc*/ ΣM_pb* ≥ 1.0 at every SMF beam-column joint (M_pcreduced for axial, M_pb projected to column centreline). Forces the plastic mechanism into beams, not columns. Exceptions: top-storey, non-SMF storey, low-axial columns (P_u/P_y ≤ 0.3).

System detailing summaries:

System	AISC 341 chapter	Plastic hinge location	Required moment connection
SMF Special Moment Frame	E3	In beam, away from column face	AISC 358 prequalified
IMF Intermediate Moment Frame	E2	In beam	AISC 358 prequalified for IMF list
OMF Ordinary Moment Frame	E1	At column face (R ≤ 3.5)	Conventional FR moment, less restrictive
SCBF Special Concentrically Braced Frame	F2	In brace + gusset	Gusset 2t-clearance line, expected brace yield
OCBF Ordinary CBF	F1	In brace	Less restrictive
EBF Eccentrically Braced Frame	F3	In link (shear yielding)	Link-to-column, link-to-brace, brace-to-collector
BRBF Buckling-Restrained Braced Frame	F4	In BRB core (axial yielding)	BRB end connections per project tests (AISC 341 K3)
SPSW Special Plate Shear Wall	F5	In web plate (tension field)	Boundary-element connections

6. Edge cases and engineering judgement

• Block shear often controls in thin plates — shear tabs ≤ 10 mm thick, gusset plates ≤ 13 mm thick, and angle legs ≤ 13 mm regularly fail block-shear check before any other limit state. The combined-tension-and-shear failure surface peels a “block” out of the plate around the bolt group, and the limit-state equation (AISC J4-5) combines 0.6 F_u A_nv on the shear plane with U_bs F_u A_nt on the tension plane.

• Prying action on T-stub and end-plate tension bolts — When a flange transmits tension through bolts to a plate that bends under that tension, the plate’s deflection generates a contact reaction Q at the plate tip that adds to bolt tension: B = T + Q. AISC Manual Chapter 9 + Eqn 9-21 through 9-30 gives the closed-form: t_min,thick = √(4.44 · T_d · b′ / (φ · F_y · p)) and Q = 0 if t ≥ t_min,thick (the “thick-plate limit”). For thin plates, Q can equal T — doubling the apparent bolt demand.

• Galvanising — A325 OK, A490 NOT — Hot-dip galvanised A490 bolts have failed catastrophically by hydrogen-induced cracking (HIC). RCSC 2020 explicitly prohibits HDG of A490 bolts (and F1852/F2280 by extension). HDG is permitted on A325/F3125 Gr A325 with mechanical galvanising preferred per ASTM F2329. For corrosive environments with high-strength bolts, use weathering steel (A325 type 3) or zinc-aluminium (ASTM F3393) or stainless A325M.

• High-strength bolt installation — four valid methods:

  • Turn-of-nut (T-N): from snug, rotate nut 1/3 turn (L ≤ 4d), 1/2 turn (4d < L ≤ 8d), 2/3 turn (L > 8d). Cheap, robust, the legacy standard.
  • Calibrated wrench (CW): preset torque calibrated daily on a Skidmore-Wilhelm bolt-tension calibrator. Sensitive to lubrication state — RCSC 2020 § 9.2.2 requires daily recalibration.
  • Twist-off-type tension control (TC) bolts (F1852, F2280): nut-side spline breaks off at the certified pretension. Installed with an inner-and-outer-socket TC gun.
  • Direct tension indicator (DTI) washers (ASTM F959, “Squirter”): protrusions on washer compress under pretension; visual gap (or expelled silicone) measured with feeler gauge to confirm tension achieved.

• Eccentric load on a bolt group — combine direct shear V (uniform on each bolt) with rotational shear from the moment V · e (proportional to radius from group centroid). Closed-form via vector sum or, more accurately, instantaneous-centre-of-rotation (ICR) method (AISC Manual Tables 7-7 through 7-13 give the C coefficients directly).

• Coped beams — When the top flange of a beam is cope-cut to fit under a girder flange, the resulting tee section is checked for: (a) flexural rupture of the cope (AISC Manual Eqn 9-1, 9-2), (b) lateral-torsional buckling of the coped section, (c) local buckling of the cope flange. AISC Manual Eqn 9-7 gives the flexure-shear interaction at the cope.

• Welder + WPS qualification — AWS D1.1 § 6 (procedure) and § 6.16 (welder) require every WPS to be qualified by a procedure-qualification record (PQR), and every welder to be qualified to the WPS on a coupon. Position-specific (1G/2G/3G/4G), process-specific (SMAW/FCAW/GMAW), thickness-specific. AISC 341 § J5 demands enhanced toughness testing (Charpy at low T) for seismic DCW filler metals.

• Field vs shop welds — Shop welds: AWS-certified fab shop (AISC certification + AWS QC1 inspector), controlled environment, position-of-choice, generally cheaper. Field welds: site weather, position-as-built, overhead/vertical common, expensive (~3–10× shop weld cost per inch). Industry rule: detail to shop-weld + field-bolt wherever possible.

• Galvanic corrosion at mixed-material connections — Stainless bolt on weathering steel, aluminium gusset on steel column, copper grounding lug on galvanised plate. Use isolation washers (G-10, Teflon), dielectric paint, or specify the same EMF class throughout.

• Tolerance stack-up — AISC Code of Standard Practice (303-22) allows ±3 mm (±1/8”) on shop fabrication of W-shape lengths, ±5 mm (±3/16”) on field-erected column locations, and standard 1.5 mm (1/16”) oversize on bolt holes. Slotted holes (short-slot 1.5 d, long-slot 2.5 d) accommodate field misfit. Connection designer must check that slip-critical surface area is preserved when slots are used.

• Hyatt Regency 1981 — review every shop-drawing change. Original design: continuous threaded rod through second-floor walkway suspended from fourth-floor walkway, common hanger from roof. Fabricator’s shop-drawing change: each walkway hung from its own rod section, with the second-floor washer bearing on the box-beam of the fourth-floor walkway — doubling the load on the fourth-floor connection. Failed at design dance-traffic load. Engineer of Record had stamp on shop drawings. Lesson: every shop drawing change must be reviewed against the design intent by the EOR; “review for general conformance” is not enough.

• Northridge 1994 — untested moment connections fail brittle. Pre-1994 WUF-B (Welded Unreinforced Flange–Bolted web) used CJP groove welds at beam flanges with E70T-4 filler (no required toughness) and left backing bars in place (creates a built-in notch at root). Hundreds of frames cracked at the bottom-flange root in moderate ground motion, before any meaningful inelastic action. Triggered the SAC Steel Project (1994–2000), FEMA 350/355, and AISC 358 (prequalified connections tested under cyclic protocol per AISC 341 K2). Modern rule: a moment connection in an SMF must be on the AISC 358 prequalified list, or it must be tested per K2 for the specific geometry.

7. Tools and software

AISC publications (canonical, all in-print):

• AISC Steel Construction Manual, 16th edition (2023) — the practitioner’s lookup. Parts 7–15 are the connection tables.
• AISC Design Examples v15.1 (2023) — free PDF, worked LRFD + ASD examples of every Manual procedure.
• AISC 360-22 Specification — the code itself. Free PDF download for AISC members.
• AISC 341-22, AISC 358-22 — seismic provisions and prequalified moment connections.
• AISC Design Guides 1–43 (see Reference table below).

Connection-specific design software:

• IDEA StatiCa Connection — component-based finite-element method (CBFEM) on the connection; checks every weld, bolt, plate yielding pattern, and stiffener with 3D solid modelling. Industry standard for non-standard connections.
• RAM Connection (Bentley) — integrated with RAM Structural System; AISC 360 + AISC 341 + AISC 358 prequalified moment connection design automated.
• Hilti PROFIS Engineering / PROFIS Anchor — anchor-rod and post-installed anchor design per ACI 318 Ch.17 + AISC DG-1.
• SDS/2 (Allplan) — connection design + detailing + CNC fabrication automation. The dominant fabrication-shop package in North America.
• Risa-Connection — light analytical package, common in small consultancies.
• Limcon (Bentley) — legacy AISC + AS 4100 connection design.
• Descon — legacy AISC connection design (now discontinued).

Detailing / BIM:

• Tekla Structures (Trimble) — BIM-driven structural-steel modelling + auto-detailing + connection libraries (PowerConnect, Tekla Connection Design). Dominant tool for fabricators globally.
• Advance Steel (Autodesk) — Revit-integrated steel detailing.
• ProSteel (Bentley) — MicroStation-based steel detailing.

Specialty fabrication:

• DSTV-NC (DIN-based standard for CNC fabrication) is the universal interchange format.
• CNC equipment: Peddinghaus (drill-saw-coper line, beam-thermic line), Voortman (plate processor, beam line, robotic welding cells), Ficep (plate + beam).
• Robotic weld cells: ABB IRB-2600, Kuka KR-16, Yaskawa MA-2010 with off-line programming from Tekla.

AISC Design Guide series (selection):

DG #	Title	Use
DG-1	Base Plate and Anchor Rod Design (2nd ed.)	Column base plates, anchor rod groups, shear lugs
DG-4	Extended End-Plate Moment Connections	Detailed companion to AISC 358 § 6
DG-13	Stiffening of Wide-Flange Columns at Moment Connections	Continuity + doubler plate design
DG-16	Flush and Extended Multiple-Row Moment End-Plate Connections	Murray & Sumner; specialised end-plate geometry
DG-17	High-Strength Bolts: A Primer for Structural Engineers	RCSC pretensioning practice
DG-21	Welded Connections — A Primer for Engineers	AWS D1.1 procedural design
DG-24	Hollow Structural Section Connections	AISC 360 Ch.K worked examples
DG-29	Vertical Bracing Connections — Analysis and Design	UFM gusset-plate design
DG-30	Sound Isolation and Noise Control in Steel Buildings	(Adjacent)
DG-32	Design of Modular Steel-Braced Frames for Schools	(Adjacent)
DG-39	End-Plate Moment Connections	Re-issue of DG-4 + DG-16 content under new numbering

8. QC and inspection

AISC 360 Chapter N and AISC 341 Chapter J define the quality control (QC = fabricator + erector internal) and quality assurance (QA = owner’s special inspector) tasks. Three inspection tiers: observed (O) for routine bolts and welds, performed (P) for critical bolts (slip-critical, oversized hole) and most welds, and demand-critical (DCW) for seismic.

Welding inspection — AWS D1.1 § 6 + § 8. Visual (VT) on 100 % of welds by an AWS Certified Welding Inspector (CWI). Nondestructive examination as required:

• VT (visual): always
• MT (magnetic particle): surface and near-surface flaws on ferritic steels
• PT (penetrant): surface flaws on any material, especially non-ferritic
• UT / PAUT (ultrasonic / phased-array ultrasonic): volumetric, the standard for CJP groove welds in moment connections
• RT (radiographic): volumetric, alternate to UT, slower and requires evacuation of the inspection zone

Bolt inspection — RCSC 2020 § 9 + AISC 360 Ch.N Table N5.6-1/2/3:

• Snug-tight: visual confirm at full ironworker effort
• Pretensioned (T-N): visual confirm matchmarks rotated correct angle from snug
• Pretensioned (TC bolts): visual confirm spline broken off
• Pretensioned (DTI): feeler-gauge gap measurement, or silicone-expelled witness
• Slip-critical: faying-surface inspection before assembly, then pretension method as above

Pre-construction meeting (PCM) — AISC 341 § J6 + AWS D1.1 § 5.1. Mandatory walkthrough among EOR, fabricator’s QC, owner’s QA inspector, erector, and welding superintendent before any seismic-critical welding starts. Reviews WPS, filler-metal certifications, NDE plan, preheat plan, weld sequence, and protected-zone restrictions.

9. Cross-references

• steel-design — member design that produces the connection demands
• fasteners-bolts — preload mechanics, bolt-tension theory, Junker, VDI 2230
• joining-welding — AWS D1.1 process selection, WPS, welder qualification, HAZ metallurgy
• mechanics-of-materials — combined stress, shear-flow, bending of plates
• fracture-mechanics — root-notch sensitivity in welded connections, K_IC of CJP groove welds
• fatigue-analysis — fatigue category at bolted vs welded connections (AASHTO Table 6.6.1.2.3-1, AISC 360 Appendix 3)
• structural-analysis — modelling assumption (pinned/fixed/PR) at every joint
• structural-dynamics — connection ductility demand under cyclic loading (AISC 341 K2 protocol)
• reinforced-concrete — anchor rod embedment per ACI 318 Ch.17
• fem-fea — CBFEM in IDEA StatiCa, contact + plasticity in 3D solid models
• planned masonry-timber — adjacent material systems with their own connection codes

10. Citations

Standards and codes:

• AISC Steel Construction Manual, 16th ed. (2023) — the canonical practitioner’s reference
• AISC 360-22 Specification for Structural Steel Buildings (2022), Chapter J: Design of Connections
• AISC 341-22 Seismic Provisions for Structural Steel Buildings (2022)
• AISC 358-22 Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications (2022)
• RCSC Specification for Structural Joints Using High-Strength Bolts (2020), Research Council on Structural Connections
• AWS D1.1/D1.1M:2024 Structural Welding Code — Steel, American Welding Society
• AWS D1.8/D1.8M:2021 Structural Welding Code — Seismic Supplement
• AISC 303-22 Code of Standard Practice for Steel Buildings and Bridges (2022)
• AISC Design Examples v15.1 (free PDF, 2023)
• AISC Design Guides 1, 4, 13, 16, 17, 21, 24, 29, 39 (varies)
• ASTM F3125 / F3125M-22 (consolidated A325, A490, F1852, F2280 bolt spec)
• ASTM F1554-23 (anchor rods, Gr 36/55/105)
• ASTM F959 / F959M-22 (direct tension indicators)
• ASTM F2329 (mechanical galvanising for bolts)

Textbooks:

• McCormac & Csernak, Structural Steel Design, 6th ed. (Pearson, 2018)
• Salmon, Johnson, & Malhas, Steel Structures: Design and Behavior, 5th ed. (Pearson, 2009)
• Geschwindner, Murray, & Disque, Unified Design of Steel Structures, 3rd ed. (Wiley, 2017)
• Kulak, Fisher, & Struik, Guide to Design Criteria for Bolted and Riveted Joints, 2nd ed. (RCSC reprint, 2001) — the historical reference for bolted-joint behaviour
• Murray & Sumner, AISC Design Guide 4 + Design Guide 16, Extended End-Plate Moment Connections (2003 + 2003)

Vendor / software documentation:

• IDEA StatiCa Connection Theoretical Background (Brno, 2023) — CBFEM derivation
• Tekla Structures Connection Design Guide (Trimble, 2024)
• SDS/2 Connection Library Reference (Allplan, 2023)