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LecturesModule 19

Steel Framing Systems

Braced frames, moment frames, dual systems, gravity systems.

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

Lecture Notes

This module introduces steel framing systems. Lecture content here covers the governing physics, LRFD philosophy, and how the relevant AISC 360-22 chapter organizes the limit states.

Instructors can replace this text in Admin Mode. Each section is structured around: (1) behavior, (2) failure modes, (3) AISC limit-state equations, (4) design workflow, (5) detailing requirements.

A short comparison to ASD is included only where the resistance factor / safety factor relationship clarifies the LRFD design check.

Braced (CBF)Moment (SMF)Dual system
Lateral systems: concentrically braced frames, moment frames, and dual systems (AISC 341).

Formula Sheet

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

Worked Example

Steel Framing Systems

Given
Replace with project-specific given data (loads, geometry, material).
Load combination
Controlling LRFD load combination from ASCE 7.
Required strength
Compute required strength Ru from the controlling combination.
Limit states
  • Limit state 1
  • Limit state 2
AISC reference
AISC 360-22 — applicable chapter
Solution steps
  1. 1. Required strength
    Compute Ru.
  2. 2. Trial section
    Pick a trial from AISC shape tables Instructor should verify with official AISC Manual.
  3. 3. Check each limit state
    Apply φ Rn ≥ Ru for every governing limit state.
  4. 4. Iterate
    Resize until the most economical section satisfies all checks.
Final design decision
Select the lightest section that satisfies all LRFD limit states.
Common mistakes in this example
  • Skipping a limit state
  • Using the wrong φ factor
  • Forgetting serviceability checks

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
  1. Trib width
    10 ft
  2. w (service)
    100 psf × 10 ft = 1.0 k/ft
Answer Infill beam carries 1.0 k/ft service load.
w (kip/ft)LSimply supported beam, UDL w

Practice Problems

  1. [E] List the four common steel framing systems.
  2. [E] Define a diaphragm.
  3. [E] State economical column-spacing range for offices (~25–35 ft).
  4. [E] List four typical lateral-resisting systems (BRBF, SCBF, OMF, SMF).
  5. [E] State the AISC seismic specification (AISC 341).
  6. [M] Tributary gravity on an interior base column of a 4-story bldg, 25 x 30 ft bay, 90 DL, 50 LL.
  7. [M] Choose between W18x35 and W21x44 for a 30 ft floor beam, 100 psf floor + 30 psf partition, 10 ft o.c.
  8. [M] Diaphragm shear at 100 x 200 ft floor under 30 psf wind.
  9. [M] Lateral system: 6-story office, SDC C. Braced frame vs SMF — recommend.
  10. [M] Drift check for a 4-story braced frame, H = 48 ft, ΔW = 0.8 in. Pass H/400?
  11. [H] Preliminary design of a 30 x 30 ft bay: composite W21x50 beams at 10 ft o.c. + W24x62 girders. Verify gravity + serviceability.
  12. [H] Full lateral design of a 5-story braced frame, V = 240 k, design HSS diagonals per story.
  13. [H] Dual system SMF + BRBF: allocate seismic base shear per AISC 341 §G3.
  14. [H] Floor diaphragm: 120 x 240 ft composite metal deck, 80 k wind. Shear flow + chord force.
  15. [H] SMF strong-column / weak-beam ratio ≥ 1.0 — verify for chosen sections.

Quiz

1. Which AISC 360-22 chapter primarily governs steel framing systems?
2. In LRFD, the basic design inequality is:

Common Student Mistakes

  • 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.

"Professor Explains" Script

Today we're talking about steel framing systems. Think of this topic as one step in the LRFD workflow: identify the demand, identify the limit states from the relevant AISC chapter, then check that φ·Rn is at least equal to Ru. We'll walk through the failure modes, the equations, and a worked example. Pay close attention to where the resistance factor changes — that's where students lose points on exams.