11.1 Structural Steel, Heavy Timber & Framing Systems

Structural Steel Shapes, Standards & Metallurgy

Commercial structural steel framing is engineered to support substantial gravity, wind, and seismic loads with high strength-to-weight efficiency. In commercial construction, steel shapes are standardized by the American Institute of Steel Construction (AISC) and manufactured to rigid metallurgical standards established by ASTM International.

Structural Steel Shape Designations

Contractors and estimators must interpret standard structural drawing callouts accurately. The standard nomenclature identifies the cross-sectional shape profile followed by primary nominal dimensions:

• Wide-Flange Shapes (W-Shapes): The primary framing member for columns, girders, and beams. Designated as W Depth x Weight (for example, W12x26 indicates a wide-flange beam with a nominal depth of 12 inches and a nominal weight of 26 pounds per linear foot). Wide-flange beams feature parallel inner and outer flange surfaces with uniform thickness, optimizing resistance to bending moments.
• American Standard Beams (S-Shapes): Formerly known as I-beams, designated as S Depth x Weight (e.g., S12x35). Unlike W-shapes, S-shape flanges have a sloped inner surface with a 16.67% (2:12) slope on the interior flange faces.
• American Standard Channels (C-Shapes & MC-Shapes): Designated as C Depth x Weight (e.g., C10x20). Used for stair stringers, wall purlins, girts, and perimeter framing. MC-shapes represent miscellaneous channels with non-standard flange proportions.
• Structural Angles (L-Shapes): Designated by leg lengths and thickness, formatted as L Leg1 x Leg2 x Thickness in inches (e.g., L4x4x1/2 indicates an equal-leg angle with 4-inch legs and 1/2-inch wall thickness; L6x4x3/8 indicates an unequal-leg angle). Used extensively for lintels, bracing, truss members, and shelf angles supporting brick masonry veneers.
• Structural Tees (WT, ST, MT): Produced by splitting standard W, S, or MC shapes longitudinally down the center of the web. Designated as WT Depth x Weight (e.g., WT6x13 is split from a W12x26 beam).
• Hollow Structural Sections (HSS): High-efficiency tubular members manufactured in round, square, and rectangular geometries. Designated by nominal outside dimensions and wall thickness (e.g., HSS8x6x3/8 indicates a rectangular section with an 8-inch depth, 6-inch width, and 3/8-inch nominal wall thickness; round tubing is designated by outside diameter and thickness, such as HSS6.625x0.375). HSS members provide superior torsional resistance and bi-axial buckling resistance, making them ideal for commercial columns and exposed architectural bracing.

Steel Grades and Metallurgy Matrix

Structural steel properties depend on carbon content and alloying elements such as manganese, silicon, vanadium, and columbium. Modern building codes require specific ASTM grades for structural framing:

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Steel Erection & OSHA Subpart R (29 CFR 1926.750)

Commercial steel erection involves significant structural stability and fall hazards. General contractors must ensure strict compliance with OSHA Safety and Health Standards for Steel Erection (29 CFR Part 1926, Subpart R).

Site-Specific Steel Erection Plan & Foundation Readiness

Prior to commencing erection, the steel erector must establish a Site-Specific Steel Erection Plan coordinated with the general contractor. Under 29 CFR 1926.752, the general contractor must provide written notification to the steel erector certifying that:

1. Foundation concrete has achieved either 75% of the intended minimum compressive design strength (f'c) or sufficient strength to support loads imposed during erection, based on verified cylinder break reports.
2. Any repairs, replacements, or modifications to anchor bolts were executed with the written approval of the Structural Engineer of Record (SER).
3. Staging and laydown areas provide firm, properly graded, drained, and compacted ground conditions adequate for heavy crane operations and delivery transport.

Column Anchor Bolts and Base Plate Stability

Column erection is the critical foundation of structural stability. OSHA mandates explicit physical controls:

• Minimum Four Anchor Bolts: Under 29 CFR 1926.755(a)(1), all columns must be anchored by a minimum of 4 anchor bolts (anchor rods). Two-bolt column bases are strictly prohibited in structural steel erection because they create a dangerous pin-connection hinge line prone to column toppling during hoisting line release.
• Absolute Ban on Unauthorized Field Modifications: Under 29 CFR 1926.755(b)(1), anchor bolts cannot be repaired, replaced, or field-modified (such as torch-cutting base plate holes, field-bending bent rods, or welding extensions) without the prior written approval of the project Structural Engineer of Record (SER). An unauthorized torch-cut on an anchor bolt violates federal law and compromises structural liability.
• Plumbing-Up and Guying: Before releasing temporary supports or loading structural members, columns and frames must be plumbed using turnbuckles, wire ropes, and guy cables. Plumbing cables must remain in place until sufficient permanent framing, decking, and diaphragm connections are secured.

Hoisting, Rigging & Multi-Lift Rigging ("Christmas Treeing")

Cranes must be operated strictly within rated load charts. For multi-lift rigging—commonly known in the trade as Christmas treeing, where multiple structural steel members are hoisted simultaneously on a single crane line—OSHA enforces rigid safety limits under 29 CFR 1926.753(e):

• A maximum of five (5) members may be hoisted per lift, and only beams and similar structural members may be lifted this way.
• Every employee engaged in multiple-lift rigging must be trained under 1926.761, and the rigging assembly must be manufactured by a qualified rigger.
• Members are rigged from the top down, attached at their center of gravity, kept reasonably level, and hung at least 7 feet apart.
• The total load may not exceed the rated capacity of the crane or the rigging assembly.
• Members are set from the bottom up.

+------------------------------------------------------------------------------------------------+
|                       OSHA SUBPART R MULTI-LIFT RIGGING ("CHRISTMAS TREEING")                  |
+------------------------------------------------------------------------------------------------+
|                                                                                                |
|                                    Crane Hoisting Line                                         |
|                                            |                                                   |
|                                   [Master Rigging Ring]                                        |
|                                            |                                                   |
|                             +--------------+--------------+                                    |
|                             |                             |                                    |
|                     Member 1 (Top)                Rigging Sling                                |
|                     =============================================                              |
|                                            |                                                   |
|                                            |  <--- Minimum 7 Feet Vertical Separation          |
|                                            |                                                   |
|                     Member 2 (Middle)      |                                                   |
|                     =============================================                              |
|                                            |                                                   |
|                                            |  <--- Minimum 7 Feet Vertical Separation          |
|                                            |                                                   |
|                     Member 3 (Bottom)      |                                                   |
|                     =============================================                              |
|                                                                                                |
|  RULES: Maximum 5 members | Minimum 7-ft separation | Rig top-down, set bottom-up | Trained crew|
+------------------------------------------------------------------------------------------------+

Fall Protection Thresholds in Steel Erection (29 CFR 1926.760)

OSHA Subpart R enforces tiered fall protection rules that differ from general construction:

• General Steel Workers (15-Foot Rule): All ironworkers, detailers, and bolters exposed to fall hazards greater than 15 feet above a lower level must be protected by personal fall arrest systems (PFAS), safety net systems, or guardrail systems.
• Structural Connectors (Initial Connection): Connectors actively connecting structural steel members must be provided with fall protection equipment and wear harnesses when working between 15 feet and 30 feet (or two stories) above a lower level. Full 100% tie-off is legally mandated once connectors work above 30 feet or two stories, whichever is less.
• Controlled Decking Zones (CDZ): A leading-edge metal decking crew may establish a CDZ up to 30 feet or two stories without conventional fall arrest, provided the zone does not exceed 90 feet by 90 feet, access is strictly limited to authorized deckers, and perimeter control lines are secured.

High-Strength Bolted Connections

Structural steel connections transfer axial, shear, and moment forces between structural members. Bolted connections are governed by the Research Council on Structural Connections (RCSC) Specification for Structural Joints Using High-Strength Bolts.

Bolt Specifications: ASTM A325 vs. ASTM A490

Structural steel connections rely almost universally on heavy hex structural bolts standardized under the unified ASTM F3125 specification:

• ASTM F3125 Grade A325: Medium carbon steel bolts with a minimum tensile strength of 120 ksi in all F3125 diameters (the old A325 standard dropped to 105 ksi above 1 inch). A325 bolts may be hot-dip galvanized (ASTM F2329) or mechanically galvanized for corrosion resistance.
• ASTM F3125 Grade A490: Quenched and tempered alloy steel bolts with a minimum tensile strength of 150 ksi to 173 ksi. Critical Safety Warning: Grade A490 bolts must never be hot-dip galvanized using acid pickling. The acid-cleaning process introduces atomic hydrogen into the high-strength alloy steel matrix, causing catastrophic hydrogen embrittlement and sudden, brittle fracture under sustained service tension. Where corrosion protection is required, A490 bolts use only the coatings the standard permits, such as zinc/aluminum flake coatings (ASTM F1136 Grade 3 or F2833), as approved by the engineer.

Joint Classifications: Snug-Tight, Pretensioned & Slip-Critical

The engineer of record designates the required joint type on the structural contract drawings:

1. Snug-Tight Joints: The plies of the joint are brought into firm, solid contact. Attained by a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench. Snug-tight bolts carry shear through direct bearing of the bolt shank against the side of the hole. Permitted only in applications where neither pretensioning nor slip resistance is structurally specified.
2. Pretensioned Joints: Bolts are tightened to a specified minimum clamping pretension (equal to 70% of the minimum tensile strength of the bolt). Required where joints are subjected to significant cyclic load reversals, dynamic live loads, or tension fatigue.
3. Slip-Critical Joints: Clamping tension is applied to press the connecting steel plies together with sufficient force that the joint resists shear through frictional resistance between the contact surfaces (faying surfaces) rather than bolt shank bearing. Slip-critical joints are mandatory for connections with oversized or slotted holes, structures subject to fatigue or seismic load reversals, and bridge framing.
   • Faying Surface Preparation: To maintain design slip coefficients, faying surfaces in slip-critical joints must be blast-cleaned unpainted steel (Class A surface, slip coefficient 0.30) or coated with an engineered high-friction zinc primer (Class B surface, slip coefficient 0.50). Faying surfaces must never receive standard gloss oil paints or grease.

Bolt Tightening and Verification Methods

Where pretensioned or slip-critical joints are specified, the contractor must verify that every bolt achieves the mandatory minimum tension using one of four approved methods:

Welded Connections & AWS D1.1 Structural Welding Code

Welded connections fuse steel components into continuous, monolithic structural frames. In commercial building construction, all structural welding is governed by the American Welding Society (AWS) D1.1 / D1.1M Structural Welding Code - Steel.

Primary Structural Welding Processes

1. Shielded Metal Arc Welding (SMAW): Commonly known as "stick" welding. Uses a consumable electrode coated in chemical flux that decomposes into shielding gas and protective slag. Highly portable, versatile, and suitable for adverse outdoor jobsite conditions.
2. Flux-Cored Arc Welding (FCAW): Uses a continuously fed tubular wire containing internal flux. Often operated self-shielded (FCAW-S) without external gas cylinders, making it the preferred high-deposition process for commercial structural steel field erection.
3. Gas Metal Arc Welding (GMAW / MIG): Uses a solid wire electrode shielded by an external compressed gas mixture (e.g., argon and carbon dioxide). High productivity in shop fabrication, but sensitive to jobsite wind drafts that blow away shielding gas.
4. Submerged Arc Welding (SAW): High-speed automated shop process where the arc is completely buried beneath a granular blanket of fusible flux, used for fabricating heavy plate girders and built-up columns.

Structural Weld Types and Configurations

• Fillet Welds: Triangular cross-section deposited at the intersection of two overlapping or perpendicular steel surfaces (e.g., tee joints, lap joints, corner joints). Sized by the nominal leg length (w). The effective throat dimension of an equal-leg fillet weld equals 0.707 x w. Fillet welds are designed primarily to resist shear stresses.
• Groove Welds: Deposited in a prepared channel or groove between two abutting members (butt joints):
  • Complete Joint Penetration (CJP) Groove Welds: The weld metal extends completely through the entire thickness of the base metal, fusing the full cross section. When executed properly with backing bars or back-gouging, a CJP weld develops the full design strength of the connected base metal.
  • Partial Joint Penetration (PJP) Groove Welds: The weld metal extends only to a specified depth into the joint, leaving an unfused root area. Used where full joint strength is not required.
• Plug and Slot Welds: Circular or elongated holes in one plate filled with weld metal to join it to an underlying plate in lap joints.

Quality Assurance & Nondestructive Examination (NDE/NDT)

Structural steel welding must be executed by certified welders qualified under specific Welding Procedure Specifications (WPS) and inspected by an independent Certified Welding Inspector (CWI):

• Visual Testing (VT): Performed on 100% of all structural welds before, during, and after welding. Inspectors check for fit-up, joint gap, preheat temperature, undercut, overlap, surface porosity, and crack formation.
• Magnetic Particle Testing (MT): Ferromagnetic particles applied to magnetized welds detect surface and shallow subsurface cracks, especially in fillet welds and beam-to-column flange connections.
• Dye Penetrant Testing (PT): Liquid penetrant and developer reveal surface-breaking cracks and discontinuities in non-porous metals.
• Ultrasonic Testing (UT): High-frequency sound waves transmitted through the weld reflect back from internal discontinuities (such as slag inclusions, incomplete fusion, and internal cracks). UT is the primary method for non-destructively verifying Complete Joint Penetration (CJP) groove welds.
• Radiographic Testing (RT): Industrial X-ray or gamma-ray imaging produces photographic film of internal weld integrity, providing a permanent archival record of internal sound metal.

Open-Web Steel Joists (SJI) & Metal Decking

Open-web steel joists are lightweight, shop-fabricated steel trusses designed according to the specifications of the Steel Joist Institute (SJI) to support commercial floor and roof assemblies.

SJI Joist Classifications

Joist Bridging & Hoisting Cable Release Mandate

Open-web joists have high vertical load-carrying capacity but extremely low lateral torsional stiffness until braced. Unbraced joists will buckle laterally under minimal dead load or the weight of a worker:

• Bridging Types: Joists require either horizontal bridging (continuous steel angles welded or bolted to top and bottom chords) or diagonal bridging (cross-bracing angles forming an "X" between adjacent joists).
• OSHA & SJI Hoisting Cable Release Rule (29 CFR 1926.757): The crane hoisting cables shall not be released from any steel joist until the designated bolted diagonal bridging is fully installed, bolted, and anchored, or until joist ends are secured to supporting framing per SJI tables. Releasing hoisting cables prematurely on an unbridged joist is a major cause of progressive framing collapse.
• Bearing Support Dimensions: K-series joists must bear at least 2.5 inches on structural steel or 4 inches on masonry or concrete walls. LH- and DLH-series joists require at least 4 inches of bearing on steel and 6 inches on masonry or concrete.

Metal Floor and Roof Decking

Metal decking forms the structural diaphragm that braces steel joists and distributes lateral wind and seismic loads:

• Composite Floor Decking: Manufactured with rolled mechanical indentations (embossments) along the ribs. When concrete is placed over the deck, the embossments interlock with the cured concrete, transforming the assembly into a composite floor slab where the steel deck acts as positive tension reinforcement.
• Non-Composite Roof Decking (B-Deck): Standard 1.5-inch deep ribbed profile (Wide Rib Type B) supporting rigid roof insulation boards and commercial membrane roofing.
• Fastening Methods: Decking is secured to structural steel joists and beams using arc spot puddle welds (minimum 5/8-inch diameter welds through the deck into the support flange), powder-actuated drive pins, or pneumatic fasteners. Sidelaps between adjacent deck sheets are fastened using mechanical crimping (button punching) or self-drilling screws spaced 12 to 36 inches on center.

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Heavy Timber Construction (IBC Type IV) & Mass Timber

Heavy Timber construction—designated as Type IV-HT under Chapter 6 of the International Building Code (IBC)—utilizes large-dimension solid-sawn timber or engineered mass timber to create structural frames with inherent fire endurance.

Minimum Nominal Member Dimensions for Type IV-HT

Under IBC Section 2304.11, wood members in Type IV-HT construction must satisfy strict minimum nominal dimensions. These minimum cross sections prevent rapid structural failure during a fire:

• Columns Supporting Floor Loads: Minimum 8 inches by 8 inches nominal (wood columns supporting roof and ceiling loads only may be 6 inches by 8 inches nominal).
• Floor Beams and Girders: Minimum 6 inches wide by 10 inches deep nominal.
• Roof Beams and Girders: Minimum 4 inches wide by 6 inches deep nominal (or 3 inches nominal width if engineered glulam).
• Wood Floor Decking: Minimum 3 inches nominal tongue-and-groove or splined solid wood planks (or laminated wood planks), covered by a 1-inch nominal finish wood floor, 15/32-inch wood structural panel, or 1.5-inch non-combustible gypsum concrete underlayment.
• Wood Roof Decking: Minimum 2 inches nominal tongue-and-groove planks or 1-1/8-inch tongue-and-groove wood structural panels.

Natural Fire Resistance and Charring Mechanics

Heavy timber members perform remarkably well in building fires due to the physics of wood combustion:

• When exposed to fire temperatures exceeding 550°F, wood pyrolyzes, creating a surface layer of carbonized charcoal.
• Wood char has low thermal conductivity (approximately 1/6 that of raw wood). This dense char layer insulates the interior unburned timber core, slowing the penetration of heat.
• Heavy timber chars at a predictable rate of approximately 1.5 inches per hour (or 1/40 inch per minute). Because the internal timber core retains its ambient temperature and full structural load-bearing capacity, heavy timber structures resist collapse during fires far longer than unprotected steel beams (which lose 50% of their structural yield strength at 1,000°F).
• No Concealed Spaces: Traditional Type IV-HT construction prohibits hollow walls, soffits, or dropped ceiling cavities where hidden fire can spread undetected, unless all concealed cavities are fully protected by non-combustible materials or sprinklered.

Engineered and Mass Timber Systems

Modern commercial framing incorporates high-performance engineered wood systems:

• Glued-Laminated Timber (Glulam): Individual dimensional lumber laminations (typically 1.5 inches thick) bonded with moisture-resistant structural adhesives, with grain running parallel longitudinally. Glulam allows long clear spans and curved architectural arches.
• Cross-Laminated Timber (CLT): Formed by stacking solid-sawn wood boards in orthogonal (alternating 90-degree) layers glued under hydraulic pressure. CLT panels create rigid two-way structural floor slabs, roof decks, and shear walls, enabling multi-story mass timber buildings up to 18 stories under IBC Types IV-A, IV-B, and IV-C.
• Laminated Veneer Lumber (LVL): Wood veneers bonded under heat and pressure with grain running parallel, providing uniform, high-strength structural headers and rim boards.

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Light-Gauge Cold-Formed Steel Framing (CFS)

Cold-formed steel (CFS) framing utilizes thin-gauge galvanized sheet steel roll-formed at room temperature into structural C-studs, tracks, and channels. CFS is non-combustible, dimensionally stable, and resistant to termites and rot.

CFS Member Profiles & AISI Nomenclature

Under the American Iron and Steel Institute (AISI) standardized labeling system, CFS members are designated by a four-part code: Depth (in 1/100 inches) + Profile Letter + Flange Width (in 1/100 inches) - Mil Thickness:

• Profile Letters: S = Stud (C-shape with stiffening lip); T = Track (unlipped channel); U = U-channel (cold-rolled channel); F = Furring channel (hat channel).
• Example: 600S162-43 designates a 6.00-inch deep C-stud with a 1.625-inch flange width and a metal thickness of 43 mils (0.043 inches, corresponding to 18 gauge).

Gauge Number vs. Mil Thickness

In steel framing, mil thickness (1 mil = 0.001 inch) is the precise legal measure. Contractors must understand that a lower gauge number denotes thicker, stronger steel:

Deflection Tracks and Structural Isolation

In multi-story commercial buildings, floor slabs deflect downward under live loads. Interior non-load-bearing partitions must be structurally isolated from this vertical movement:

• If an interior drywall partition is rigidly fastened to the underside of a structural slab, slab deflection will impose crushing axial loads onto the light-gauge studs, causing partition buckling, drywall blowout, and door jamming.
• Deflection Track Assemblies: Contractors install slotted deflection tracks (tracks with vertical slots along the legs) or nested double tracks at the top of the wall. Studs are cut 1/2 to 3/4 inch short of the track web, and screws are driven into the center of the vertical slots. This allows the structural floor slab to deflect freely without transferring vertical axial loads into the non-load-bearing wall.