Module 5: Introduction to Structural Steel

Module 5: Introduction to Structural Steel

$Learning Objectives$

Explain the elastic, yielding, strain-hardening, and fracture behavior relevant to structural steel design.
Navigate NSCP 2015 Chapter 5 and distinguish member strength, stability, serviceability, and connection provisions.
Recognize common rolled, built-up, and hollow structural shapes and relate section properties to structural behavior.
Use LRFD or ASD consistently from load effects through available strength.
Distinguish axial-compression element classification from flexural compactness terminology.
Evaluate architectural implications of section depth, exposed steel, corrosion protection, fire protection, fabrication, and erection.

$NSCP Code Basis$

Structural steel design in ARCH 4 is governed by NSCP 2015 Chapter 5 — Structural Steel. ASEP states that this chapter adapts the AISC Steel Construction Manual, 14th Edition, whose specification basis is ANSI/AISC 360-10. The introductory provisions and design requirements in Sections 501–503 establish the framework used by the member-specific Sections 504–513.

Later AISC editions can be useful for comparison, but they are not automatically the governing basis for an NSCP 2015 calculation. If a later provision differs, identify the edition explicitly rather than silently importing it into the course workflow.

$Yield strength$

The specified stress $F_y$ associated with the onset of significant inelastic deformation and used by many steel strength limit states.

$Tensile strength$

The specified ultimate tensile stress $F_u$ used by rupture, fastener bearing, connection, and related limit-state calculations.

$Material Behavior$

Structural steel is often idealized as elastic up to yielding, followed by substantial inelastic deformation before fracture. The real stress-strain response depends on grade, manufacturing process, thickness, temperature, strain rate, and other factors.

Ductility is valuable because it allows redistribution and visible deformation before many forms of failure, but ductility does not eliminate brittle fracture, local buckling, connection fracture, fatigue, or fire-temperature concerns.

$Interactive Exploration$

Use Full curve to compare the elastic, idealized yield, hardening, and necking regions, then switch to Elastic detail and move the strain slider through $F_y/E$ . The exact yield breakpoint is retained in the plotted curve, while the finer strain increment lets you inspect the elastic-to-yield transition without treating the schematic post-yield shape as material-test data. Use Reset to return to the reference case.

Controls

345 MPa

450 MPa

0.02000 mm/mm

Elastic modulus E200,000 MPa

Yield strain Fy/E0.00172

Plot rangeFull schematic

Current regionillustrative strain hardening

Illustrative stress352.6 MPa

Only the elastic branch and Fy/Fu anchors are quantitative design inputs. The yield plateau, strain hardening, and necking portions are schematic and must not be used as a material-test prediction. Exact yield, plateau, hardening, and terminal breakpoints are retained in the plotted polyline so the displayed knee is not shifted by sampling.

$Structural Shapes and Section Properties$

Common structural forms include W-shapes, channels, angles, tees, HSS, pipes, plates, and built-up members. Selection is driven by more than cross-sectional area:

$A$ influences axial strength and self-weight.
$I$ controls elastic stiffness.
$S$ and $Z$ influence flexural resistance.
$r=\sqrt{I/A}$ strongly affects column slenderness.
torsional and warping properties influence stability and special connection behavior.

The most efficient shape depends on the dominant force path, unbraced length, connection geometry, fire/protection strategy, and architectural expression.

$Steel Material Properties and Specification-Controlled Strength$

Elastic properties used in ordinary structural-steel analysis are distinct from grade-controlled strength properties. Keep those two categories separate.

Property	Typical structural-analysis value or source	Use
Elastic modulus, $E$	approximately $200{,}000\text{ MPa}$ for ordinary structural steel	Axial/flexural stiffness and Euler buckling
Shear modulus, $G$	approximately $77{,}000\text{ MPa}$	Torsion and shear deformation
Poisson ratio, $\nu$	approximately $0.30$	Elastic material relationships
Thermal coefficient, $\alpha$	approximately $12\times10^{-6}/{}^\circ\text{C}$	Thermal movement
Mass density	approximately $7{,}850\text{ kg/m}^3$	Self-weight
Yield strength, $F_y$	material-specification dependent	Yielding, local slenderness limits, compression/flexure strength
Ultimate tensile strength, $F_u$	material-specification dependent	Rupture, block shear, bolts/bearing and connection checks

Do not infer $F_y$ or $F_u$ from the section designation. The shape table provides geometry; the material specification and grade provide strength.

$Section-Property Reading Guide$

Before a member calculation, identify the properties the governing limit state actually uses:

Symbol	Meaning	Typical role
$A_g$	Gross area	Axial yielding/compression
$I_x,I_y$	Second moments of area	Elastic stiffness
$S_x,S_y$	Elastic section moduli	Elastic bending
$Z_x,Z_y$	Plastic section moduli	Plastic moment benchmark
$r_x,r_y$	Radii of gyration	Column slenderness
$J$	Torsional constant	Torsional/LTB calculations
$C_w$ or related warping property	Warping resistance	Open-section torsional stability
$r_{ts}$ , $h_o$	LTB parameters for applicable I-shapes	$L_r$ and elastic LTB

Use the section database/manual corresponding to the adopted design edition; do not manually approximate tabulated W-shape properties when an official shape table is available.

$Interactive Exploration$

Switch among the W, C, L, and HSS families and compare how open versus closed geometry influences likely beam, column, bracing, torsional, and connection behavior. Dashed centroidal reference axes are shown only for the doubly symmetric W and square HSS sketches; the channel and angle deliberately omit centroid/principal axes because their correct locations require section properties rather than the visual bounding-box center.

W section behavior

Wide-flange sections provide high strong-axis bending efficiency and are widely used for beams and columns; weak-axis stability and connection access still require review.

Final selection depends on area, I, S/Z, radius of gyration, torsional properties, local slenderness, bracing, connection geometry, fire protection, fabrication, and architectural integration.

Dashed axes are shown only for the doubly symmetric W and square HSS sketches. The simplified C and L drawings intentionally omit centroid and principal axes because their actual locations require section properties rather than the visual bounding-box center.

Cross-sections are schematic and not drawn from a specific rolled-section database. Use verified section-property tables for centroid locations, principal axes, section properties, and design; this visual is for recognizing geometry and likely behavior.

$LRFD and ASD$

NSCP steel provisions permit strength to be expressed through either LRFD or ASD formulations where specified. The nominal limit-state strength $R_n$ is the same physical resistance basis, but the resistance factor $\phi$ and safety factor $\Omega$ create different available strengths.

$Steel Member Design Workflow$

The workflow requires the design method, verified material/section properties, governing member action, applicable limit-state family, serviceability, and connection load path to be established before acceptance.

NSCP Structural-Steel Design Workflow

Design-basis, section-classification, member-limit-state, stability, serviceability, and detailing sequence.

Define steel system, load path, loads, and required strengths → Design basis?; Design basis? — LRFD → LRFD: factored demand with φRn available strength; Design basis? — ASD → ASD: required strength from applicable ASD load combinations with Rn/Ω; LRFD: factored demand with φRn available strength → Select trial section and obtain verified material/section properties; ASD: required strength from applicable ASD load combinations with Rn/Ω → Select trial section and obtain verified material/section properties; Select trial section and obtain verified material/section properties → Governing member action?; Governing member action? — Tension → Tension: yielding, rupture, shear lag, block shear, connection region; Governing member action? — Compression → Compression: local slenderness, global buckling, frame stability; Governing member action? — Flexure → Flexure: section classification, local buckling, LTB, shear, interaction; Tension: yielding, rupture, shear lag, block shear, connection region → Check serviceability, bracing, fire, corrosion, and coordination; Compression: local slenderness, global buckling, frame stability → Check serviceability, bracing, fire, corrosion, and coordination; Flexure: section classification, local buckling, LTB, shear, interaction → Check serviceability, bracing, fire, corrosion, and coordination; Check serviceability, bracing, fire, corrosion, and coordination → Check connections and complete force transfer; Check connections and complete force transfer → All applicable limit states and detailing requirements satisfied?; All applicable limit states and detailing requirements satisfied? — Yes → Document governing limit state and utilization; All applicable limit states and detailing requirements satisfied? — No → Revise section, system, bracing, connection, or design basis; Revise section, system, bracing, connection, or design basis → Select trial section and obtain verified material/section properties

Define steel system, load path, loads, and required strengths: terminator
Design basis?: decision
LRFD: factored demand with φRn available strength: process
ASD: required strength from applicable ASD load combinations with Rn/Ω: process
Select trial section and obtain verified material/section properties: process
Governing member action?: decision
Tension: yielding, rupture, shear lag, block shear, connection region: subprocess
Compression: local slenderness, global buckling, frame stability: subprocess
Flexure: section classification, local buckling, LTB, shear, interaction: subprocess
Check serviceability, bracing, fire, corrosion, and coordination: process
Check connections and complete force transfer: process
All applicable limit states and detailing requirements satisfied?: decision
Revise section, system, bracing, connection, or design basis: process
Document governing limit state and utilization: terminator

$Section Classification Is a Design Step, Not a Label$

A section cannot be called "compact," "noncompact," "slender," or "nonslender" in the abstract. The required classification depends on member action, element type, stress distribution, and the governing NSCP limit-state chapter. Reclassify the relevant elements when the governing action changes.

Steel Available Strength

Generic relationship between nominal strength and the LRFD or ASD available strength.

R_{\text{LRFD}}=\phi R_n \qquad R_{\text{ASD}}=\frac{R_n}{\Omega}

Variables

Symbol	Description	Unit
$R_n$	Nominal resistance for the applicable limit state.	-
$\phi$	LRFD resistance factor specified by the governing provision.	-
$\Omega$	ASD safety factor specified by the governing provision.	-

$Do Not Mix Design Philosophies$

A member check must use demand and resistance on the same design basis. Do not compare an LRFD factored load directly with an ASD allowable strength or apply both $\phi$ and $\Omega$ to the same nominal strength.

$Section Classification Depends on the Limit State$

Element slenderness controls local buckling. For axial compression, elements are generally treated as nonslender or slender for the compression-member provisions. For flexure, compact, noncompact, and slender classifications are used where the flexural provisions define them.

Do not describe every steel section under axial compression as compact/noncompact/slender; that terminology belongs to flexural classification.

$Classification Requires an Actual Width-to-Thickness Check$

A shape name such as W, C, L, or HSS does not determine compactness or local slenderness by itself. Compute the relevant plate ratio ( $b/t$ , $h/t_w$ , or $D/t$ ) and compare it with the limit for the specific element and stress state. Module 7 gives axial-compression limits; Module 8 gives the flexural workflow.

$Global Stability$

Steel structures require a stable load path from the member level to the whole building. Bracing, frame stiffness, effective lengths or direct-analysis requirements, imperfections, second-order effects, and connection restraint all influence stability.

A visually slender architectural frame is feasible only if the stabilizing system is explicit rather than assumed.

$Architectural Selection of Steel$

Section choice affects ceiling depth, façade lines, exposed structural rhythm, fireproofing thickness, connection visibility, tolerances, MEP penetrations, corrosion access, and erection sequence. HSS can provide clean exposed geometry but can make connections and internal inspection more difficult; W-shapes are connection-friendly but may require fireproofing and architectural concealment.

$Protection and Fire$

Corrosion protection should match exposure and maintenance access. Coatings and galvanizing are not interchangeable solutions for every detail, especially at welds, bolted faying surfaces, enclosed HSS, and interfaces that trap water.

Steel loses strength and stiffness at elevated temperature. Required fire resistance normally comes from approved protection systems, rated assemblies, or a qualified fire-resistance design—not from room-temperature member strength alone.

Key Takeaways

NSCP 2015 Chapter 5 is the governing basis for structural steel in ARCH 4.
Steel design checks nominal limit states through either a consistent LRFD or ASD framework.
Section properties, local element slenderness, global stability, serviceability, and connection geometry all influence member selection.
Axial compression uses nonslender/slender element terminology; flexure uses compact/noncompact/slender classifications where applicable.
Architectural steel selection must account for depth, fire protection, corrosion, connections, fabrication, and erection in addition to strength.