9.2 Steel Framing Systems: Beams, Girders & Open-Web Joists

9.2 Steel Framing Systems: Beams, Girders & Open-Web Joists

Structural steel is one of the most versatile framing materials in modern architecture. Characterized by high strength-to-weight ratios, uniform isotropic properties, ductile failure behavior, and rapid shop fabrication, steel is the predominant framing choice for mid- to high-rise commercial offices, long-span sports arenas, and institutional facilities.

For the ARE candidate, understanding structural steel involves decoding member designations, mastering the gravity load hierarchy, optimizing composite action between steel beams and concrete floor slabs, selecting open-web joist series, specifying code-compliant passive fireproofing, and evaluating column buckling mechanics.

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Structural Steel Metallurgy & Material Properties

Structural steel is an alloy composed primarily of iron combined with small percentages of carbon (typically 0.15% to 0.30%) and trace alloying elements including manganese, phosphorus, sulfur, and silicon. Controlling carbon content is critical: higher carbon increases tensile strength but reduces ductility (the ability of a material to deform plastically before fracture) and compromises field weldability.

Material Benchmarks

• Modulus of Elasticity ($E$): Approximately $29,000\text{ ksi}$ ($29,000,000\text{ psi}$) for all carbon and low-alloy structural steels. This constant governs elastic deflection and column buckling stiffness.

• Density / Specific Weight: Approximately $490\text{ lbs/cu ft}$ ($0.284\text{ lbs/cu in}$).

• Thermal Expansion Coefficient: Approximately $6.5 \times 10^{-6}\text{ in/in/}^\circ\text{F}$, which closely matches reinforced concrete, preventing destructive differential thermal stresses in composite systems.

Common Structural Steel Specifications

ASTM A500 yield strengths depend on grade and section shape. Do not use one value for every round and rectangular HSS. Use the specified product standard, grade and published properties in the structural design.

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Decoding Wide-Flange Member Designations

Wide-flange shapes (designated by the prefix $W$) feature parallel inner and outer flange surfaces. The American Institute of Steel Construction (AISC) establishes standardized nomenclature for all structural profiles:

$\mathbf{W24 \times 68}$

• $W$: Designates the cross-sectional shape profile as a Wide-Flange section.

• $24$: Represents the nominal cross-sectional depth in inches along the vertical web axis (actual depth for a $W24 \times 68$ is $23.73\text{ inches}$). Deep sections ($W24, W30, W36$) are optimized for high bending moment resistance in beams and girders.

• $\times$: Read as 'by'.

• $68$: Represents the weight of the member in pounds per linear foot ($68\text{ lbs/ft}$). A $30\text{-foot}$ piece of $W24 \times 68$ weighs $30 \times 68 = 2,040\text{ lbs}$.

Other AISC Member Designations

• $S12 \times 35$: Standard I-beam (American Standard beam with sloped 16.67% inner flange faces).

• $C10 \times 20$: Structural Channel ($10\text{ inches}$ deep, $20\text{ lbs/ft}$).

• $L6 \times 4 \times 1/2$: Structural Angle ($6''$ vertical leg, $4''$ horizontal leg, $1/2''$ thickness).

• $HSS8 \times 8 \times 1/2$: Square Hollow Structural Section ($8'' \times 8''$ outer profile, $1/2''$ wall thickness).

• $WT12 \times 34$: Structural Tee split from a $W24 \times 68$ wide-flange ($12\text{ in}$ nominal depth, $34\text{ lbs/ft}$).

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Gravity Framing Hierarchy & Load Paths

In standard structural steel building construction, gravity loads travel through a strictly ordered structural hierarchy:

1. Concrete Floor Slab: Typically 3.25 to 4.5 inches of lightweight or normal-weight concrete.

2. Corrugated Metal Decking: 2-inch or 3-inch fluted galvanized steel spanning 6 to 10 feet between infill beams.

3. Infill Beams: Secondary members spaced at 6 to 10 feet on center, spanning 25 to 40 feet to carry floor decking.

4. Primary Girders: Transfer beams spanning column-to-column, supporting intermediate beam end reactions.

5. Columns: Vertical compressive members transmitting cumulative loads downward story-by-story to foundations.

6. Base Plates & Foundations: Spreading column point loads into soil or bedrock via concrete footings.

Simple vs. Moment Connections

A simple connection is idealized as having negligible moment resistance for analysis. AISC's PR designation means partially restrained, not a synonym for an idealized pinned/simple connection. Real connections have stiffness; the engineer chooses a consistent frame and connection model.

• Moment Connections (Rigid Framing / AISC Type FR): Connect both the web and the top/bottom flanges rigidly to the supporting column using full-penetration groove welds or heavy bolted flange plates. Moment connections resist both shear and bending moments, preventing rotational slip and enabling the structural frame to resist lateral wind and seismic forces without diagonal bracing.

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Composite Steel Construction

In noncomposite design the slab and beam are not credited with composite flexural action, though each still has its own load-transfer role. Composite design deliberately transfers interface shear through designed connectors to mobilize their combined stiffness and strength. Compare shoring, deck, connections, serviceability and cost; composite action is not a universal fixed percentage gain.

The Mechanics of Composite Action

Composite construction unites the concrete floor slab and the underlying structural steel beam into a single structural unit behaving like an integrated T-beam:

1. Headed Shear Studs: Steel studs ($3/4\text{ inch}$ diameter, $4\text{ to }6\text{ inches}$ long) are fusion-welded through the flutes of the corrugated metal deck directly to the top flange of the steel beam using an electric stud welding gun.

2. Resisting Horizontal Shear: When downward gravity loads cause the assembly to bend, the concrete slab tries to slip horizontally relative to the steel beam flange. The welded shear studs mechanically lock the two components together, transferring longitudinal shear across the interface.

3. Optimized Stress Distribution: Under positive bending, the concrete topping slab functions as the compressive flange of the T-beam, while the bottom flange and web of the steel wide-flange section resist the tensile forces.

Technical and Economic Advantages

• 30% to 50% Increase in Flexural Strength: Enables significantly higher load capacities across identical spans.

• Reduced Steel Beam Depth: Beam depths can be reduced by several inches (e.g., downsizing from a $W21$ to a $W16$ or $W18$), directly reducing overall floor-to-floor heights.

• Weight Savings: Reduces overall steel framing tonnage by 15% to 25%, leading to smaller columns and lighter foundations.

• Enhanced Floor Stiffness & Vibration Damping: The concrete compression flange substantially increases the cross-sectional moment of inertia ($I_x$), damping footfall-induced vibrations critical in laboratories, operating rooms, and high-end offices.

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Open-Web Steel Joists & Joist Girders

Open-web steel joists are lightweight shop-welded steel trusses with parallel chords and triangulated web systems, engineered and standardized by the Steel Joist Institute (SJI).

Standard SJI Joist Classifications

Decoding SJI Joist Designations

• Example: $24K8$

  • $24$: Nominal joist depth in inches ($24\text{ inches}$). Depth-to-span rule of thumb for roofs is typically $L/24$.
  • $K$: K-Series standard open-web steel joist.
  • $8$: Chord size designation number (larger integers indicate heavier chord angles and greater load capacity).

• Example Joist Girder: $48G8N10K$

  • $48G$: $48\text{ inches}$ nominal depth.
  • $8N$: $8$ joist spaces (panel points).
  • $10K$: Designed to carry a $10\text{ kip}$ concentrated point load at each joist panel seat.

Technical Trade-Offs of Open-Web Joists

• Advantages: Unmatched strength-to-weight ratio; rapid erection; open triangulated web voids allow horizontal passage of major HVAC ductwork, electrical conduit, and plumbing piping without cutting or coring.

• Limitations: Inherent fire vulnerability due to slender thin-walled chord angles; poor acoustic mass; susceptibility to wind uplift requiring continuous horizontal or diagonal bridging lines.

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Cellular & Castellated Beams

Castellated and cellular beams are engineered structural members manufactured by flame-cutting the web of a standard wide-flange rolled section in a continuous zig-zag or semi-circular pattern. The two split halves are separated, offset, and re-welded together along the high points:

• Increases structural beam depth by approximately 50% and moment of inertia ($I_x$) by up to 200% with zero added steel weight.

• The resulting regular hexagonal or circular web openings allow mechanical ducts, fire sprinkler mains, and conduits to pass directly through the structural depth.

• Ideal for long clear spans ($40\text{ to }90\text{ ft}$) in parking decks, office floor framing, and roofs requiring integrated MEP distribution.

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Fireproofing Methods for Structural Steel

Steel stiffness and strength decrease at elevated temperature. Fire resistance depends on the full protected or unprotected assembly, section properties, loading and tested or approved design method. A single pair of temperature/strength percentages is not a substitute for a required rating.

1. Spray-Applied Fire-Resistive Materials (SFRM): Cementitious wet slurries or sprayed mineral fiber coatings applied directly to steel surfaces. Economical and fast; standard for concealed plenums above suspended acoustic ceilings. Vulnerable to mechanical abrasion and air-erosion dusting.

2. Intumescent Fireproofing Coatings: Thin-film organic paints applied by roller or airless spray. Under fire temperatures ($300\text{–}400^\circ\text{F}$), the chemical coating undergoes an endothermic reaction, swelling up to 50 times its original thickness into an insulating multicellular carbonaceous char. Ideal for Architecturally Exposed Structural Steel (AESS) where structural profiles remain visible to occupants.

3. Gypsum Board Encasement: Multi-layer assemblies of $5/8\text{-inch}$ Type X or Type C gypsum board framed around steel columns and beams. Contains chemically bound crystalline water ($H_2O$) that vaporizes slowly (calcination), keeping steel temperatures below critical thresholds.

4. Concrete Encasement: Pouring monolithic normal-weight concrete around the steel section. Highly durable and impact-resistant; standard in industrial loading docks and heavy manufacturing facilities.

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Steel Column Buckling & Euler's Formula

Unlike short stocky compression blocks that fail by direct material yielding ($P_y = F_y \cdot A$), long slender steel columns fail by sudden elastic lateral buckling. In 1757, Leonhard Euler formulated the critical buckling load ($P_{cr}$):

$P_{cr} = \frac{\pi^2 E I}{(KL)^2}$

• $P_{cr}$: Critical axial compressive buckling load (lbs or kips).

• $E$: Modulus of elasticity of steel ($29,000\text{ ksi}$).

• $I$: Moment of inertia about the governing weak buckling axis ($I_y$, in $\text{in}^4$).

• $L$: Actual unbraced column length between lateral restraints (inches).

• $K$: Effective Length Factor, reflecting end rotational and translational restraint:

For an ideal elastic Euler column with unchanged end conditions and section properties, halving unbraced length multiplies critical load by four: that is 400% of the original value, or a 300% increase. Real design must also check yielding, inelastic buckling, local buckling and connection/bracing capacity.

Structural spans, member-depth ratios and preliminary capacities in this section are design-screening assumptions, not completed structural designs. Check loads, serviceability, fire rating, vibration, connections, bracing, durability and constructability with the engineer. IBC 2021 references ASCE 7-16; an exam-provided reference or a locally adopted edition may differ. Never transfer a numerical limit between editions or structural systems without checking its scope.