A century of steel — from concept to skyline
You cannot design what you cannot specify.
Fy and Fu — plus the right shape family — control every AISC calculation.
Chapter 3 covers ASTM specifications, the stress–strain curve, and the shape families (W, HP, HSS, angle, channel) that populate the AISC Shapes Database.
What you will be able to do after finishing Chapter 1 — and why each objective matters in practice
Objective 01A992 (Fy=50, Fu=65) is the standard for rolled W-shapes; A500 Gr. C for HSS; A36 for plates and angles.
Objective 02E = 29,000 ksi elastic; yield plateau at Fy; strain-hardens to Fu; ruptures at ~15-25% strain.
Objective 03W (flexure), HP (piles), HSS (columns/braces), L (tension), C/MC (secondary).
Objective 04For each shape: A, d, bf, tf, tw, Ix, Iy, Sx, Sy, Zx, Zy, rx, ry, J, Cw.
Objective 05Charpy V-Notch impact tests screen brittle behaviour at low temperature; AISC-Seismic requires demand-critical welds to satisfy CVN targets.
Objective 06Painted, galvanised, or weathering steel (A588). Loss of section from corrosion reduces Ag and Fy relevance.
What each part of a steel-frame building actually does — and why it exists
Before you design any single member, you have to see the whole system. A steel-frame building is not a collection of independent shapes bolted together — it is a deliberate load path, engineered so that every kilonewton of gravity, wind, or seismic demand has a continuous route from where it starts to the ground where the earth can resist it.
The photograph below shows a typical steel framing detail. Drag each labelled chip onto the structural element it names — the drop is only accepted when it lands inside the correct element's outlined region. A correct answer locks in with a green outline and a short explanation; a wrong answer flashes the region red, tells you what you actually hit, and returns the chip so you can try again. Press Reveal expected placements to see the reference solution (that attempt is then marked as assisted).
Why we design the way we do — six ways steel structures have failed, and what each disaster taught the profession
Every provision in AISC 360 is a scar. Behind each equation, load factor, and detailing rule is a bridge, a walkway, or a tower whose failure cost lives and rewrote the profession. The six case studies below trace the mechanisms that motivate the code you are about to learn.
Read each one as an engineer, not a spectator: identify the load, the limit state, the missing check, and the specific clause that exists today because that check was missed. When you meet those clauses again in Chapters 5–17, they will read as answers, not rules.
Cold-temperature cleavage fracture with little plastic warning.
Low toughness, notch, tri-axial restraint.
Rust reduces effective Ag and creates local pits.
Water + oxygen + no coating.
Design codes intervene at the transition from yield to instability. Everything before yield is elastic and reversible; everything after instability is a race to collapse. LRFD keeps the demand well below the first transition.
The full textbook chapter — figures, equations, and engineering narrative
Two beams have identical geometry but one is A36 and the other is A992. Before running any numbers, where do you expect their behavior to differ — yielding, deflection, ductility, weldability, cost?
Structural design is the planned organization of materials and components to safely resist internal and external forces — dead, live, and environmental loads — without exceeding strength or serviceability limit states. Choosing the right material means balancing strength, stiffness, durability, weight, and economy for the specific load path, exposure, and budget in front of you.
| Property | Structural steel | Reinforced concrete | Timber / mass timber | UHPC | Masonry | FRP composites |
|---|---|---|---|---|---|---|
| Strength | Fy = 50 ksi (A992); equal in tension & compression | f'c = 4–8 ksi compression; tension carried by rebar (fy = 60 ksi) | Fb ≈ 0.9–3 ksi parallel to grain; weak perpendicular to grain | f'c = 18–30 ksi; sustained post-crack tension from steel fibers | f'm = 1.5–3 ksi compression; essentially no reliable tension | Tensile 100–400 ksi (CFRP); linear elastic to rupture — no yielding |
| Stiffness | E = 29,000 ksi | Ec ≈ 57,000√f'c psi; mass helps damping | E ≈ 1,300–2,000 ksi | Ec ≈ 7,000–8,500 ksi | Em ≈ 900 f'm; stiff but brittle | E = 6,000 ksi (GFRP) to 20,000+ ksi (CFRP) |
| Weight | 490 pcf, but very high strength-to-weight | 150 pcf — heavy self-weight drives foundations | 25–40 pcf — lightest structural option | 155 pcf, but sections are far thinner | Heavy — 100–140 pcf walls | ≈ 1/5 of steel — installs without cranes |
| Durability | Corrodes — paint, galvanize, or weathering steel | Excellent fire and water resistance; cover controls corrosion | Decay, moisture, and termites unless treated or kept dry | Near-zero permeability — outstanding freeze-thaw and chloride resistance | Excellent weathering, thermal mass, and fire rating | Immune to corrosion; UV and elevated-temperature limits apply |
| Economy | High material cost, fast erection, low labor hours | Low material cost, high formwork/labor and cure time | Low cost in residential/light commercial; premium for glulam/CLT | Very high per cubic yard; justified by thin sections and long life | Low material cost, labor-intensive, local mason skill dependent | High unit cost, very low installation and downtime cost |
| Best for | Long spans, high tension, moment frames, retrofits, fast schedules | Heavy static loads, compression members, shear walls, slabs, parking, foundations | Light framing, short-to-medium spans, low-carbon mass-timber buildings | Bridge field joints, thin architectural panels, overlays, blast/impact zones | Bearing walls, shear walls in low/mid-rise, fire separations, façades | Strengthening/retrofit wraps, corrosive environments, non-magnetic structures, GFRP rebar |
| Watch out for | Fire protection required; stability (LTB, local buckling) governs often | Cracking/deflection serviceability; heavy seismic mass; long cure schedule | Moisture content, shrinkage, connection crushing, char rate in fire | Specialty mixing/curing; limited local supply; few prescriptive code provisions | Brittle in seismic shear unless reinforced and grouted; out-of-plane loading | Brittle failure — design with large strength-reduction factors; bond/debonding controls |
| Governing code | AISC 360-22 / AISC 341 | ACI 318-19 | NDS / AWC | ACI 239 / AASHTO UHPC guidance | TMS 402/602 | ACI 440.2R / 440.11 |
Trace the force to the foundation first. Tension and long spans favor steel or FRP; sustained compression and bearing favor concrete or masonry; light gravity framing favors timber.
Strength rarely governs slender systems — deflection, drift, and floor vibration do. Compare EI and mass before choosing, not after sizing.
Assess humidity, freeze-thaw cycling, de-icing salts, and industrial chemicals. Corrosive exposure pushes toward concrete cover, galvanizing, UHPC, or FRP reinforcement.
Concrete and masonry are inherently rated; steel needs applied protection; timber relies on sacrificial char depth; FRP resins soften early.
Balance first cost against maintenance, formwork, crane access, schedule, and the skill available in the local labor market.
Compare embodied carbon, recycled content, recyclability, and renewable sourcing. Steel is highly recycled; timber sequesters carbon; cement clinker dominates concrete emissions.
Steel for long spans and tension. Concrete for heavy static compression, fire, and water. Timber for light, low-carbon framing. Masonry for bearing and shear walls with thermal mass. UHPC for thin, aggressive-exposure elements. FRP for corrosion-driven retrofits — never as the sole source of ductility.
Material specification practice
ABET / licensure link. These points map to ABET Student Outcomes 2 and 4 — engineering design within realistic constraints, and recognition of ethical and professional responsibilities. Expect NCEES FE and PE exam questions on the NSPE Code of Ethics, OSHA construction requirements, and the engineer's standard of care.
Grade and shape selection economics
| Line item | Qty | Rate | Cost |
|---|---|---|---|
A36 section, Ag = 9.3 in² (31.6 lb/ft, 30 ft) | 0.47 ton @ A36 | $1,080 | $508 |
A992 section, Ag = 6.7 in² (22.5 lb/ft, 30 ft) | 0.34 ton @ A992 | $1,150 | $391 |
Fabrication saved (lighter piece) | -0.13 ton | $900 | −$117 |
| Estimated total | $782 | ||
Takeaway. The 50 ksi option is lighter and cheaper installed even at a higher $/ton — always price the installed member, not the coupon.
Unit rates are representative US averages for teaching purposes. On a real project, price with current local rates (RSMeans, fabricator quotes, or contractor pricing) and state the estimate date.
Key mechanics visualised — watch the strain profile, stress block, or buckled shape evolve
Elastic → yield plateau → strain hardening → necking. Fy sets design capacity; Fu sets rupture.
Full-page reference diagrams — the visual vocabulary you will use for the rest of the course
Match the family to the loading.
Yield → strain-hardening → rupture.
Little plastic warning.
Coating is a design decision.
Full textbook solutions — problem, theory, step-by-step, verification, interpretation
Lookup Fy, Fu, Sx, Zx from the AISC Manual and compute the shape factor.
Confirm ASTM properties and section properties for a W14x90 (A992), then compute its shape factor Zx/Sx.
The shape factor bounds the plastic reserve above elastic Sx. For rolled W-shapes it is typically 1.10–1.15.
Compute the governing variables — hints unlock as you need them
Identify the specified minimum yield stress F_y and tensile stress F_u for (a) an A500 Grade C round HSS and (b) an A992 W-shape. State the modulus of elasticity E and comment on whether E differs between grades.
Solve the chapter's design task — compute each governing variable
For A992 structural steel (Fy = 50 ksi, Fu = 65 ksi, E = 29,000 ksi), compute the yield strain εy, the strain-hardening strain εsh ≈ 10·εy, and the flange local-buckling slenderness limit λr = 0.56·√(E/Fy) for a rolled I-shape flange.
Pick the shape family + ASTM grade that fits each application
Match the correct steel product/family AND specification to each application: 24-ft office floor beam, 12-ft interior gravity column, bolted single-angle brace diagonal, and a channel stair stringer. Cite AISC Manual Part 1 shape tables and Part 2 material tables.
A mind map of how every concept connects
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.
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.
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.
NCEES-style practice with timer, equation sheet, and mastery tracking
Q1 — An ASTM A36 structural steel plate is tested in tension. As force increases, the steel reaches a critical stress where it begins to permanently stretch and deform without any additional load. Which mechanical property marks this transition from elastic to plastic behavior?