Engineering story
Chapter 19 · Steel Framing Systems
Braced frames, moment frames, dual systems, gravity systems.
A real project narrative for this chapter will be authored as this chapter migrates to the v3.0 structured schema.
§01Section 01
Why Structural Steel?
Braced frames, moment frames, dual systems, gravity systems.
⏱ 75 min★ Core✎ 3 objectives
§02Section 02
Learning objectives
After this chapter you will be able to
- Compare braced vs moment frames
- Lay out a gravity system
- Select a lateral system
§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
The choice of lateral system — braced frame, moment frame, dual, or shear-wall-with-steel-gravity — sets the cost, the erection sequence, and the seismic performance of the whole building.
Learning Objectives
- Compare concentrically braced frames (CBF), eccentrically braced frames (EBF), and buckling-restrained braced frames (BRBF).
- Compare special (SMF), intermediate (IMF), and ordinary (OMF) moment frames.
- Understand response-modification factor R and how it drives design forces.
- Layout a floor framing plan with beams, girders, columns, and lateral system.
- Estimate steel tonnage per square foot for common systems.
Where This Chapter Is Used
This chapter directly precedes the capstone (Chapter 20), where students choose and design a full lateral system.
Lecture Notes
Chapter 19 — Steel Framing Systems
Chapter focus. Individual members do not stand alone — they combine into a system: a gravity system that carries vertical load down to the foundation, and a lateral system (braced frames, moment frames, or shear walls) that resists wind and earthquake. This chapter shows the common framing schemes and how a designer chooses among them.
1. Gravity Systems
- Filler beam + girder — typical office. Beams @ 8–10 ft o.c. onto composite girders.
- Joist + girder — long-span (≥ 40 ft) or light live-load; open-web steel joists onto W-shape girders.
- Staggered-truss — hotel / residential; truss depth = story height.
2. Lateral Systems
3. Dual Systems
Moment frame + braced frame combined. Moment frame must resist ≥ 25% of the design story shear independently; higher R and greater redundancy result.
4. Capacity Design (AISC 341)
In seismic frames the fuse element (brace in SCBF, link in EBF, RBS in SMF) is designed for the code seismic force. Every other element is designed for the amplified capacity of the fuse — Ry·Fy·Afuse — so the fuse yields first and ductility is guaranteed.
5. Selecting a System
- Determine seismic design category (SDC) and wind design pressure.
- Check height, plan/vertical irregularity limits (ASCE 7 Table 12.2-1).
- Balance architectural openness (favors MF) vs stiffness (favors CBF/BRBF).
- Estimate story drift → pick a system that meets Δa without ballooning member sizes.
Additional Design Aids & Stratified Equations
Four common steel lateral systems — SCBF, chevron, EBF (link), SMF.
Seismic base shearV = C_s · W, C_s = min( S_DS/(R/I_e), S_D1 / (T·R/I_e) )
SCBF capacity designP_expected = R_y · F_y · A_g; R_y = 1.1 for A992
⚠ Common mistakes
- Using OCBF or OMF in SDC D without checking height limit → prohibited.
- Applying non-seismic AISC 360 provisions to SCBF/SMF (must include AISC 341).
- Ignoring capacity design — sizing gussets and columns for code brace force instead of Ry·Fy·Abr.
- Dual-system design without verifying 25% moment-frame shear rule.
Formula Sheet
| Name | Equation | AISC Ref |
|---|---|---|
| Design strength | φ Rn ≥ Ru | AISC 360-22 B3.1 |
Worked Example
Worked Example 19.1 — Lateral System Selection for Cardinal Square
Given
- 6-story office building, plan 120 ft × 90 ft, hsx = 13 ft, roof 78 ft.
- Site: Cincinnati, OH. SDS = 0.18, SD1 = 0.08 → SDC B.
- Basic wind V = 115 mph (ASCE 7); Exposure B.
- Base shear (approx): Vwind = 380 k, Vseismic = 210 k.
Figure 19.1a — Plan showing chevron braced-bay locations (2 per direction)
Step 1 — SDC Constraints
ASCE 7 Table 12.2-1Vdesign = V/R (R = response modification factor); Governing lateral = max(Vwind, Vseismic/R)
SDC B → most systems permitted with no height limit. AISC 341 seismic detailing not required for R ≤ 3 systems (R = 3 "steel systems not specifically detailed for seismic").
Step 2 — Candidate Systems
Step 3 — Drift Check (Wind, H/400)
ServiceabilityΔstory ≤ H/400 (wind, ASCE 7 CC.1.2); Δx ≤ Cd·Δxe/Ie ≤ 0.020 hsx (seismic)
Allowable Δ = 13·12/400 = 0.39 in/story.
R = 3 chevron CBF at each end + interior gravity columns → estimated Δstory = 0.25 in ✓.
OMF alternative Δstory ≈ 0.55 in → NG without column upsizing.
R = 3 chevron CBF at each end + interior gravity columns → estimated Δstory = 0.25 in ✓.
OMF alternative Δstory ≈ 0.55 in → NG without column upsizing.
Step 4 — Decision
Adopt a chevron braced frame (R = 3) along each principal axis. Simpler than OCBF, meets wind drift, and passes seismic by inspection since SDC B. Deliver as 4 braced bays total (2 per direction) with HSS diagonals.
Reflection: Had the site been in high-seismic California (SDC D+), an SCBF or BRBF with capacity design under AISC 341 would be required and OMF would be prohibited above 35 ft.
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
Bay 30×30 ft, infill beams at 10 ft o.c.; service floor load 100 psf.
AISC Reference
ASCE 7 §4.7
Step-by-step solution
- Trib width10 ft
- w (service)100 psf × 10 ft = 1.0 k/ft
Answer Infill beam carries 1.0 k/ft service load.
Problem statement image
DIMDimensions from the problem statement
No numeric parameters detected in the given statement — refer to the figure above for geometry.
Beam schematic — uniform load w over span L
- Left support A (roller/pin) at x = 0
- Right support B at x = L
- Uniform load w along the full span
- R_A = R_B = wL/2 · M_max = wL²/8 at midspan
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.
C19-01AISC 341-22
1. Braced frame types include:
C19-02AISC 341-22 Ch. E
2. Moment frame types (seismic):
C19-03AISC 341-22 §E3
3. SMF requires beam-column connection qualified for interstory drift of:
C19-04ASCE 7-22 §12.10
4. Composite slab diaphragm chord force resisted by:
C19-05AISC 341-22 §E3.4a
5. Capacity design (seismic) requires:
C19-06ASCE 7-22 §12.8
6. Base shear V for seismic (equivalent lateral force):
C19-07ASCE 7-22 Table 12.2-1
7. R-factor for SMF:
C19-08ASCE 7-22 §12.10
8. Diaphragm design forces Fpx (ASCE 7 §12.10):
C19-09AISC 341-22 §F3
9. EBF link is designed to:
C19-10AISC 341-22 §F4
10. BRBF brace strain range (design):
C19-11AISC 341-22 §E3.6g
11. Column splices in SMF located:
C19-12ASCE 7-22 Comm. 12.10
12. Diaphragm chord: analog to a beam:
C19-13ASCE 7-22 Table 12.2-1
13. When both braced + moment frames used (dual system) R factor:
Upload your worked solution (PDF)
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.
Sign in to upload your worked solution.
§11Section 11
Chapter summary
Formula sheet
- Design strengthφ Rn ≥ RuAISC 360-22 B3.1
Engineering checklist
- Module 19: Steel Framing Systems
- 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
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.