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15

Moment Connections

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Why Structural Steel?

Welded and bolted fully-restrained moment connections.

⏱ 110 min★ Advanced✎ 3 objectives
§01Section 01

Engineering story

Engineering story
Chapter 15 · Moment Connections

Welded and bolted fully-restrained moment connections.

A real project narrative for this chapter will be authored as this chapter migrates to the v3.0 structured schema.

§02Section 02

Learning objectives

After this chapter you will be able to
  • Design a bolted end-plate moment connection
  • Check panel-zone strength
  • Detail welded FR connections
§03Section 03

Engineering motivation

§04Section 04

Failure mechanisms

Failure mechanisms & lessons learned

Photographs and lessons-learned case studies for this topic will be added during chapter migration.

AISC Reference Box
  • AISC 360-22 — Specification chapter governing this topic
  • AISC Manual 16th Ed. — Design tables and worked examples

Why This Chapter Matters

Moment connections make a moment frame work. The pre-Northridge W1994 welded flange–bolted web detail failed brittly under seismic loading; modern prequalified detailing (RBS, BFP, WUF-W per AISC 358) is now required.

Learning Objectives

  • Distinguish simple, partially restrained (PR), and fully restrained (FR) connections.
  • Design a bolted flange-plate (BFP) FR moment connection per AISC 358.
  • Check panel-zone shear at the column web (AISC J10.6).
  • Detail continuity plates and doubler plates when required.
  • Understand the Northridge lesson and the role of prequalified connections.

Where This Chapter Is Used

Moment-frame columns in Chapters 17 and 19 and the seismic-force-resisting system of the capstone.

ANSI / AISC 360-22Specification for Structural Steel Buildings16.1-121 to 16.1-150
Chapter
J
AISC 360-22

Chapter J. Design of Connections — Moment Connections

Use this reference to flip directly to the correct page of the AISC 360-22 Specification while solving problems in this course chapter.

§Section titlePage
J1.2Simple, PR, and FR Connections16.1-121
J2Welds (CJP for FR moment connections)16.1-124
J10.6Panel Zone Web Shear16.1-148
J10.7Continuity Plates & Doubler Plates16.1-149

Companion reference: AISC 358 — Prequalified Moment Connections (RBS, BFP, WUF-W) & Manual Part 12

Lecture Notes

Chapter 15 — Moment Connections (AISC 358 / 360-22)

Chapter focus. Moment connections keep beam-to-column rotation small so the frame can resist lateral load. Two families exist: fully restrained (FR) — welded flanges + bolted web, or bolted end plates — and partially restrained (PR). Seismic moment frames must use pre-qualified AISC 358 connections (RBS, BFP, WUF-W, etc.).

1. Fully-Restrained vs Partially-Restrained

A fully-restrained (FR) connection is stiff enough that the rotation between beam and column is negligible: joint rotation θ ≤ 5° of the simple-connection rotation at ultimate load. A partially-restrained (PR) connection develops a predictable but non-rigid M–θ curve and must be modeled with a rotational spring.

2. Pre-Qualified FR Connections (AISC 358)

  • Bolted flange plate (BFP).
  • Bolted unstiffened / stiffened extended end-plate (4E, 4ES, 8ES).
  • Reduced-beam-section (RBS) — dogbone.
  • Welded unreinforced flange (WUF-W) — for OMF/IMF only.

3. Extended End-Plate — Design Basis (AISC 358 §6)

The plate is bolted to the column flange with A325 or A490 pretensioned bolts. Design forces:

Ffu = Mu,cf / (d − tf) → flange force at critical section
Vu,web from beam shear + amplified by hinge moment

Design each of: end-plate flexure (tp), bolt tension, column-flange bending, column-web yielding / crippling / buckling, panel-zone shear.

4. Panel-Zone Shear (§J10.6)

Eq. J10-9 (Pr ≤ 0.4 Pc)Rn = 0.60 Fy dc tw
Eq. J10-11 (inelastic action considered)Rn = 0.60 Fy dc tw [ 1 + (3 bcf tcf²) / (db dc tw) ]

5. Continuity Plates & Doubler Plates

  • Continuity plates required when column flange bending / web yielding governs and demand ≥ capacity.
  • Doubler plate required if panel-zone shear insufficient.
  • Weld continuity plates with CJP welds; doublers may be plug- or fillet-welded per AISC 358.

6. Design Workflow (Extended End-Plate)

  1. Compute Mu,cf at column face and Vu.
  2. Flange force Ffu = Mu/(d − tf).
  3. Pick pre-qualified geometry from AISC 358 Table 6.1; determine tp,req.
  4. Bolt tension: Tu = Ffu / nb,tens.
  5. Check column flange bending, web local yield/crip, panel zone.
  6. Add continuity/doubler plates if required.

Additional Design Aids & Stratified Equations

4-bolt extended end-plate — yield-line pattern beam flange Yp yield-line length used in AISC 358 Eq. 6.9-11
End-plate yield-line pattern (AISC 358 §6) — Y_p from Table 6.2 drives t_p,req.
Flange forceF_fu = M_u,cf / (d − t_f)
End-plate thicknesst_p,req = √( 1.11·γ_r·F_fu·p_f / (φ·F_py·Y_p) )

⚠ Common mistakes

  • Assuming FR without checking rotation stiffness (K > 20·EI/L).
  • Skipping panel-zone shear (a leading cause of Northridge failures).
  • Using snug-tightened bolts in pretensioned connections.
  • Forgetting continuity plates when column flange is too thin for concentrated flange forces.

📖 Using the AISC Manual — Moment Connections

Companion reference: Manual Part 12 (FR moment connections) + Design Guides 4 (extended end-plate) & 16 (flush end-plate). In practice, engineers rarely compute every quantity from first principles — the AISC Steel Construction Manual (16th Ed.) tabulates φRn (or Rn/Ω) for every rolled shape so you can pick a member in seconds. Formulas remain essential for understanding, verifying, and for anything the tables do not cover.

Fully-restrained moment connections have too many geometry variables for a single 2-D table, so the Manual gives step-by-step design procedures keyed to worked examples in Part 12 rather than pre-computed φRn tables. However:

  • Table 12-1 tabulates flange-weld strength per weld size for direct-welded flange connections.
  • Design Guide 4 (4-bolt & 8-bolt extended end-plate) has a fill-in-the-blank spreadsheet template with every limit state.
  • AISC 358 (Prequalified Seismic Connections) tabulates every dimension for RBS, WUF-W, and end-plate moment connections — you copy the geometry, do not design it.
Table vs. formula — which to use? Use tables to select a shape quickly. Use formulas to verify odd geometry, non-standard grades (Fy≠50 ksi), unusual K-factors or Lb, and to answer exam problems that hand you a section not in the current Manual.
Mu, VuFR welded-flange / bolted-web (continuity plates)
Fully-restrained moment connection: CJP flange welds + bolted shear tab + continuity plates.

Formula Sheet

NameEquationAISC Ref
Design strengthφ Rn ≥ RuAISC 360-22 B3.1

Worked Example

Worked Example 15.1 — 4-Bolt Extended End-Plate

Given

  • Beam: W24×76 (d = 23.9 in, tf = 0.68 in), A992.
  • Column: W14×132 (tcf = 1.03 in, bcf = 14.7 in).
  • Mu,cf = 350 k-ft, Vu = 55 k.
  • End-plate A572 Gr. 50, Fy = 50 ksi; 4 A490 bolts (2 above + 2 below tension flange), db = 1 in, Fnt = 113 ksi.
  • Bolt gage pf = 1.75 in.
4-bolt extended end-plate — flange force F_fu couples the moment M_u W14×132 t_p W24×76 beam p_f = 1.75″ F_fu (tension) F_fu (compression) M_u = 350 k·ft d − t_f
Figure 15.1a — Extended end-plate: M_u resolved into flange couple F_fu = M_u/(d−t_f)

Step 1 — Flange Force

Moment coupleFfu = Mu / (d − tf)
Ffu = Mu / (d − tf) = 350·12 / (23.9 − 0.68) = 4200/23.22 = 180.9 k

Step 2 — Bolt Tension Demand

Eq. J3-2Tu = Ffu/nb,tension ;  φRnt = 0.75·Fnt·Ab
Tu = Ffu / 2 = 90.4 k per bolt
φRnt = 0.75·113·(π/4·1²) = 0.75·113·0.785 = 66.6 k  NG

Need 1⅛″ A490 → Ab = 0.994; φRnt = 84.3 k. Still < 90.4. Try 4 bolts each side (8 total) — Tu = 45.2 k/bolt with 1 in bolts:

φRnt per bolt = 66.6 k ≥ 45.2 ✓  (use 8-bolt stiffened configuration, AISC 358 8ES).

Step 3 — End-Plate Thickness (AISC 358 Eq. 6.9-11)

AISC 358 6.9-11tp,req = √( 1.11·γr·Ffu·pf / (φ·Fpy·Yp) )
Simplified yield-line: tp,req = √( 1.11·γr·Ffu·pf / (φ·Fpy·Yp) )
Assume Yp = 90 in (from Table 6.2 for 8ES 24-in-deep beam):
tp,req = √(1.11·1.0·180.9·1.75 / (0.90·50·90)) = √(351.5/4050) = √0.0868 = 0.295 in → use tp = 3/8 in.

Step 4 — Column Panel-Zone Shear

Eq. J10-9Rn = 0.60 Fy dc tw ;  φ = 0.90
dc = 14.66 in, tw = 0.645 in.
Rn = 0.60·50·14.66·0.645 = 283 k; φ = 0.90·283 = 255 k ≥ Ffu = 181 ✓ (no doubler required).

Step 5 — Column Flange Bending

Eq. J10-1φRn = 0.90·6.25·Fy·tcf²
φRn = 0.90·6.25·Fy·tcf² = 0.90·6.25·50·(1.03)² = 298 k ≥ 181 ✓ (no continuity plates).
Use AISC 358 8ES with 8× 1″ A490 pretensioned bolts, 3/8 in A572 Gr. 50 end plate, no doublers, no continuity plates. All limit states check.

FE-Style Worked Examples(6)

Each example mirrors the NCEES FE Civil Reference Handbook style: brief givens, a labeled figure, AISC section reference, step-by-step numeric solution, and a single boxed answer.

Given
Beam Mu=300 k-ft, d≈18 in.
AISC Reference
AISC §J10 / Manual Part 12
Step-by-step solution
  1. Fpr
    Mu/(d − tf) = 300×12/(18 − 0.5) = 206 k (flange tension or compression)
Answer Each flange transmits ~206 k.
Flange force from beam moment
Problem statement image
Mu, VuBolted FR moment connection
DIMDimensions from the problem statement
Mu = 300k
Moment connection — beam to column
  • Flange forces P_f = M / (d − t_f) transferred through welds/bolts
  • Column panel-zone shear = M / d_c (check per §J10)
  • Continuity plates required if column flange too thin

Graded Chapter Quiz(13 FE-style questions · AISC Manual required)

These questions reference AISC Steel Construction Manual (16th ed.) — sections, equations, and tables are cited explicitly. Use a calculator. Each question offers a clue you may reveal before answering. Submissions are recorded to your account once signed in.

C15-01AISC 358-22
1. AISC 358 covers:
FR moment connection · flange plates col. top flange PL bottom flange PL Mu
C15-02AISC 358-22 Ch. 5
2. Reduced beam section (RBS) 'dogbone' moved plastic hinge:
C15-03AISC 358-22 Ch. 7
3. Bolted flange plate (BFP) connection strength governed by:
FR moment connection · flange plates col. top flange PL bottom flange PL Mu
C15-04AISC 358-22 Ch. 6
4. Extended end-plate moment connection: bolt-force distribution:
FR moment connection · flange plates col. top flange PL bottom flange PL Mu
C15-05AISC 360-22 Eq. J10-11
5. Panel zone shear strength (§J10.6):
FR moment connection · flange plates col. top flange PL bottom flange PL Mu
C15-06AISC 360-22 §J10.6
6. Doubler plate needed if panel zone Vu >:
C15-07AISC 360-22 Eq. J10-1
7. Column flange bending due to concentrated tension (beam flange force):
C15-08AISC 360-22 §J10
8. Continuity plates (transverse stiffeners at column) sized for:
C15-09AISC 358-22 §5.8
9. Prequalified RBS parameters a, b, c per AISC 358 §5.8:
FR moment connection · flange plates col. top flange PL bottom flange PL Mu
C15-10AISC 360-22 §I2
10. Composite moment connection design: rebar and stud contribution:
C15-11AISC Manual Part 12
11. Web-plate splice designed for:
C15-12AISC 358 & FEMA 350
12. Directly welded flange moment connection: fracture typically initiates at:
C15-13AISC 358-22 Ch. 6
13. End-plate thickness for 4E extended end plate (Manual Part 12):
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Attach your handwritten or typed step-by-step solution for this chapter's graded quiz. The instructor can download every submission. PDF only, up to 25 MB.

How your upload will be graded

Your file — PDF, Word document, scanned handwriting or a photo — is read page by page like an experienced structural engineering instructor would. The scan is validated first, then your reasoning, structural model, calculations, diagrams, code basis and final answers are graded on process, not just the final number. Design work is additionally reviewed against AISC 360-22 and ACI 318-19. Partial credit applies, and one early mistake carried correctly forward is only penalized once.

Before you attach the file
  • Include every page, in order and right way up — a missing page cannot earn credit.
  • Keep margins in frame: nothing cropped at the edges, especially boxed final answers.
  • Scan or photograph in good, even light — no shadows, glare or blur; 300 dpi or a steady phone scan.
  • Write in dark pen; faint pencil is the most common 'UNREADABLE — INSTRUCTOR REVIEW REQUIRED' flag.
  • Include all diagrams, FBDs, shear/moment diagrams and section sketches — label them.
  • Number each question the same way the assignment does, and note anything you skipped.
  • Show units on every line and box your final answers.
  • Combine everything into ONE file (PDF preferred; Word, JPG or PNG accepted) under 20 MB.
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§11Section 11

Chapter summary

Formula sheet
  • Design strength
    φ Rn ≥ Ru
    AISC 360-22 B3.1
Engineering checklist
  • Module 15: Moment Connections
  • Key limit states and AISC references are listed in the reference box.
  • Use φRn ≥ Ru for every check.
  • Verify section properties with the official AISC Manual.
Professional tips
  • • Mixing ASD and LRFD load combinations in the same problem.
  • • Using nominal strength Rn instead of design strength φRn.
  • • Forgetting to check every limit state listed in the AISC chapter.
§13Section 13

FE exam preparation

FE exam preparation
Concept review
Concept summary coming soon.
Calculator tips

Calculator tips coming soon.

Common exam traps

Traps coming soon.

Time management

Aim for ~3 minutes per FE problem; skip and return to any item that takes longer than 5 minutes.