14.4 Structural Steel, Bolting, Welding, Joists, Light-Gauge Framing & Subpart R

Structural Steel Framing (AISC Standards)

Structural steel erection is governed by specifications published by the American Institute of Steel Construction (AISC) and the AISC Steel Construction Manual.

Common Structural Steel Shapes & Designations

                       PRIMARY STRUCTURAL STEEL SHAPES
      W-Shape                S-Shape               C-Channel             HSS Tube
    (Wide-Flange)          (American Std)         (Channel)            (Hollow Struct)
    ┌───────────┐          ┌───────────┐          ┌───────────┐         ┌───────────┐
    └───┐   ┌───┘          └───┐   ┌───┘          └───┐                 │   ┌───┐   │
        │   │                  │   │                  │   │                 │   │   │   │
        │   │                  │   │                  │   │                 │   │   │   │
    ┌───┘   └───┐          ┌───┘   └───┐          ┌───┘                 │   └───┘   │
    └───────────┘          └───────────┘          └───────────┘         └───────────┘
    Parallel Flanges       Tapered Flanges        C-Profile             Square/Round

• Wide-Flange Shapes (W-Shapes): The primary beam and column section in modern steel framing. Designated as $W \text{Depth} \times \text{Weight}$ (e.g., $W14 \times 90$ indicates a wide-flange member with a nominal depth of 14 inches and a linear weight of $90\ \text{pounds per linear foot}$). Flanges have parallel inner and outer surfaces.
• American Standard Beams (S-Shapes): Historical I-beams featuring narrow flanges with a $16\text{-}2/3\%$ inner flange slope.
• Channels (C-Shapes): C-shaped cross section (e.g., $C10 \times 30$). Commonly used for stair stringers, lintels, and bracing.
• Hollow Structural Sections (HSS): Cold-formed structural tubing in square, rectangular, and round profiles (e.g., $\text{HSS } 8 \times 8 \times 1/2$ indicates an 8-inch square tube with a nominal wall thickness of $1/2\ \text{inch}$). Superior resistance to torsional twist and high bi-axial buckling strength in compression columns.
• Angles (L-Shapes): Equal or unequal legs (e.g., $L4 \times 4 \times 3/8$). Used for roof trusses, diagonal bracing, ledger angles, and shelf angles.

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Structural Steel Metallurgy & Material Grades

• ASTM A992 ($F_y = 50\ \text{ksi}$): The definitive standard structural steel for all W-shapes. Minimum yield strength $F_y = 50\ \text{ksi}$ ($50,000\ \text{psi}$); minimum tensile strength $F_u = 65\ \text{ksi}$. Formulated with tightly controlled carbon equivalents and maximum yield-to-tensile ratio ($F_y / F_u \le 0.85$) ensuring ductile plastic deformation and reliable weldability during seismic events.
• ASTM A36 ($F_y = 36\ \text{ksi}$): Mild carbon structural steel used for plates, base plates, angles, channels, and anchor rods. Yield strength $F_y = 36\ \text{ksi}$; tensile strength $F_u = 58$ to $80\ \text{ksi}$.
• ASTM A500 (Grade B / Grade C): Standard steel for cold-formed Hollow Structural Sections (HSS). Grade B yields $F_y = 46\ \text{ksi}$ for shaped tubing; Grade C yields $F_y = 50\ \text{ksi}$.
• ASTM A572 (Grade 50): High-strength low-alloy columbium-vanadium steel used for heavy plates, built-up plate girders, and structural shapes.

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High-Strength Structural Bolting

Structural steel bolted connections are governed by the Research Council on Structural Connections (RCSC) Specification for Structural Joints Using High-Strength Bolts and ASTM F3125.

Bolting Grades under ASTM F3125

ASTM F3125 consolidates legacy bolting specifications into one unified standard:

• Grade A325: High-strength medium carbon steel bolts. Minimum tensile strength $120\ \text{ksi}$ for all diameters under F3125. Head markings display "A325".
• Grade A490: Heat-treated alloy steel bolts. Minimum tensile strength $150\ \text{ksi}$. Head markings display "A490". Grade A490 bolts cannot be hot-dip galvanized because acid pickling causes hydrogen embrittlement, risking catastrophic brittle fracture under tension.

                       STRUCTURAL BOLT JOINT TYPES

  1. SNUG-TIGHT:             All plies brought into firm contact;
                             no minimum clamping tension verified.
                             (Permitted for bearing-type connections)

  2. PRETENSIONED:           Bolts tightened to ≥ 70% of minimum tensile
                             strength; prevents bolt backing-out.
                             (Required for cyclic fatigue, impact loads)

  3. SLIP-CRITICAL:          Pretension creates intense clamping force;
                             load transferred purely through surface FRICTION.
                             (Required for oversized holes, fatigue, etc.)

Joint Connection Classifications

1. Snug-Tight Joints: All connection plies are brought into firm contact using an ordinary spud wrench or several impacts of an impact wrench. Bolt tension is unmeasured. Permitted only in bearing connections where joint slip under load will not impair structural stability.
2. Pretensioned Joints: Bolts must be tightened to achieve a verified minimum clamping tension equal to at least 70% of the bolt's minimum tensile strength (e.g., $39\ \text{kips}$ for a $7/8\text{-inch}$ A325 bolt). Required where joints undergo stress reversal, severe vibration, or dynamic cyclic fatigue.
3. Slip-Critical Joints: High clamping force produces friction between the contacting steel surfaces (faying surfaces). The connection is designed so it does not slip. The RCSC Specification requires slip-critical joints for:
   • Joints with fatigue loading and reversal of load direction;
   • Joints with oversized holes, or with slotted holes where the load is not perpendicular to the slot;
   • Joints where slip at the faying surfaces would harm the structure's performance.
   • Seismic frames: AISC 341 requires bolts in seismic-force-resisting systems to be pretensioned, with faying surfaces prepared to Class A or better.
   • Faying Surface Classes: Class A ($\mu = 0.30$) is unpainted clean mill-scale steel, or blast-cleaned steel with a qualified Class A coating. Class B ($\mu = 0.50$) is unpainted blast-cleaned steel, or blast-cleaned steel with a qualified Class B coating. Ordinary paint is not allowed on slip-critical faying surfaces unless the coating has been qualified by testing.

The Four Certified Bolt Tensioning Methods

To achieve required clamping pretension in pretensioned and slip-critical joints, ironworkers must use one of four RCSC-approved installation methods:

1. Turn-of-Nut Method: Bolts are brought to snug-tight, match-marked with paint across the bolt tip, nut, and steel plate, and rotated an additional specified fraction of a turn (e.g., $1/3$, $1/2$, or $2/3$ turn, depending on bolt diameter and grip length). Match marks allow visual verification of completed rotation.
2. Calibrated Wrench Method: Torque wrenches are calibrated daily on site using a hydraulic tension calibrator (Skidmore-Wilhelm device) for each bolt diameter, length, and lot. Wrenches must be set to achieve tension 5% above the required minimum.
3. Twist-Off Tension-Control (TC) Bolts (ASTM F3125 Grade F1852 / F2280): TC bolts feature a splined tip extending beyond the threaded shank. An electric shear wrench engages the nut with an outer socket while holding the splined tip with an inner socket in counter-rotation. When the engineered torsional resistance corresponding to required tension is reached, the splined tip shears off, providing immediate visual proof of full pretension.
4. Direct Tension Indicator (DTI) Washers (ASTM F959): Hardened washers featuring raised metal protrusions. As the bolt is tensioned, the protrusions flatten. An inspector checks the remaining gaps with a tapered feeler gauge (commonly $0.005\ \text{inch}$); when the gauge is refused in the number of spaces required by ASTM F959 and the RCSC Specification, the required pretension has been reached.

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Structural Welding & Non-Destructive Testing (AWS D1.1)

Structural welding on buildings is governed by the American Welding Society (AWS) D1.1 (Structural Welding Code – Steel). All structural welds must be executed by welders possessing current AWS qualifications matching the specific process, position, and joint geometry.

Structural Welding Processes

• SMAW (Shielded Metal Arc Welding / "Stick"): An electric arc struck between a flux-coated consumable electrode and the base metal. Flux decomposes to form a shielding gas envelope and molten slag blanket protecting the weld puddle. Highly versatile for outdoor jobsite repair and tie-ins, but deposition rates are slow and electrodes require hot-box storage.
• GMAW (Gas Metal Arc Welding / "MIG"): Continuous solid wire electrode shielded by an externally supplied gas mixture (typically 75% Argon / 25% $\text{CO}_2$). Highly productive in shop fabrication, but susceptible to shielding gas disruption by wind on outdoor jobsites.
• FCAW (Flux-Cored Arc Welding): Continuous tubular wire containing internal fluxing agents, used with external gas (FCAW-G) or self-shielding (FCAW-S). The dominant field welding process in structural steel erection: high deposition rates and deep penetration. Self-shielded wires (for example, E71T-8) tolerate wind far better than gas-shielded processes. AWS D1.1 does not allow gas-shielded welding in a draft or wind unless the weld is sheltered, and limits wind at the weld to 5 mph.

Weld Joint Types

• Fillet Welds: Triangular cross section joining overlapping or perpendicular plates (lap, tee, and corner joints). Sized by nominal leg length (e.g., $5/16\ \text{inch}$ fillet weld). Economical because base metal edges require no beveling or torch-cutting preparation.
• Complete Joint Penetration (CJP) Groove Welds: The weld metal extends completely through the full joint thickness, fusing base metal plies into a solid monolithic element. Designed to develop the full strength of the joined members. Common in welded beam-flange-to-column moment connections and in many column splices.
• Partial Joint Penetration (PJP) Groove Welds: Weld metal penetrates only a specified portion of the joint thickness. Used where joint stresses are lower and full-depth fusion is structurally unnecessary.

                      NON-DESTRUCTIVE TESTING (NDT) METHODS

  NDT METHOD   PRINCIPLE               DETECTABLE DEFECTS       COMMONLY USED ON
  ─────────────────────────────────────────────────────────────────────────────
  VT           Visual examination via  Surface cracks, undercut, 100% of all structural
  (Visual)     gauges and flashlights. porosity, overlap, sizing welds (baseline)
  ─────────────────────────────────────────────────────────────────────────────
  MT           Magnetic flux leakage   Surface and shallow sub-  Heavy fillet welds,
  (Magnetic)   via iron powder.        surface cracks/laminat.   column base plates
  ─────────────────────────────────────────────────────────────────────────────
  PT           Capillary action of     Surface-breaking fissures Stainless steel, non-
  (Penetrant)  colored/fluorescent dye.in non-porous metals.     ferrous alloys
  ─────────────────────────────────────────────────────────────────────────────
  UT           High-frequency sound    Deep internal cracks,     CJP groove welds;
  (Ultrasonic) wave reflection.        lack of fusion, slag.     AISC 341 seismic QA
  ─────────────────────────────────────────────────────────────────────────────
  RT           X-ray or gamma-ray      Volumetric internal voids,Bridge girders, pressure
  (Radiograph) photographic film.      inclusions, cracks.       vessels (permanent rec)
  ─────────────────────────────────────────────────────────────────────────────

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Open-Web Steel Joists, Joist Girders & Steel Deck

The B trade exam lists Steel Joist Institute (SJI) Technical Digest No. 9, Handling and Erection of Steel Joists and Joist Girders, as a reference.

• Joist series: K-series joists are standard open-web joists for shorter spans. LH (longspan) and DLH (deep longspan) joists carry longer spans and heavier loads. Joist girders are primary members that support joists at panel points. A designation such as 24K8 means a 24-inch-deep K-series joist with section number 8; a higher section number means a stronger joist at the same depth.
• Bridging: Horizontal or diagonal bridging braces the top and bottom chords laterally. Until bridging is installed and anchored, a joist is unstable and can roll over under a person's weight. The bridging rows needed come from the SJI tables and OSHA 1926.757.
• Handling: Lift and land joists so they are not bent, and never use bridging, chords, or web members as hoisting points unless the manufacturer allows it. Damaged joists are not erected until the manufacturer or engineer has approved a repair.
• Steel deck: Roof deck (commonly 1-1/2 inches deep), composite floor deck that acts with the concrete slab, and form deck are fastened to the framing with arc-spot (puddle) welds, screws, or powder-actuated fasteners in the pattern on the drawings. Side laps are stitched with screws, welds, or button punches. The fastening pattern creates the roof diaphragm that carries wind and seismic loads to the braced frames or shear walls, so missing fasteners are a structural defect, not a cosmetic one.

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Cold-Formed (Light-Gauge) Steel Framing

Cold-formed steel (CFS) studs, joists, and track are common in Nevada commercial interiors, exterior curtain-wall backup, and some mid-rise framing.

• Member designations (SSMA/AISI): 600S162-54 means a 6.00-inch web depth (stated in hundredths of an inch), S for a stud or joist section with lips, a 1.62-inch flange, and a minimum base-metal thickness of 54 mils (0.054 inch). Other letters: T for track, U for channel, F for furring.
• Thickness in mils: 33 mils is about 20 gauge, 43 mils about 18 gauge, 54 mils about 16 gauge, 68 mils about 14 gauge, and 97 mils about 12 gauge. Thinner non-structural interior studs are lighter still. The exam may give either term, so learn both.
• Structural vs. non-structural: Structural members carry axial loads, lateral loads, or both, and follow the AISI S240 framing standard. Non-structural interior partitions follow AISI S220. Do not substitute lighter studs without the engineer's approval.
• Fastening: Self-drilling tapping screws are the standard connection. They must extend through the joined steel with at least three exposed threads. Use welds and powder-actuated fasteners only where the design shows them.
• Corrosion protection: Members are galvanized. Structural framing typically requires a minimum G60 coating, and cut or welded areas must be touched up with a zinc-rich coating.
• Openings and bracing: Factory punch-outs in the web carry utilities. Field cuts larger than allowed need an engineer's detail. Stud walls need bridging or strap bracing at the spacing on the drawings to prevent the studs from twisting under load.

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Steel Erection Safety (OSHA 29 CFR 1926 Subpart R)

Structural steel erection is among the most hazardous construction operations. OSHA enforces strict safety mandates under 29 CFR 1926 Subpart R:

• Column anchorage (§ 1926.755): All columns must be anchored by at least four anchor rods (anchor bolts). Each column and its anchor-rod assembly must resist an eccentric gravity load of 300 pounds placed 18 inches from the column's outer face, in each direction at the top of the shaft. A competent person decides whether guying or bracing is needed. Anchor rods may not be repaired, replaced, or field-modified without the approval of the project structural engineer of record. Before the column is erected, the controlling contractor must notify the steel erector in writing of any such change.
• Releasing beams (§ 1926.756(a)): Do not release a solid-web member from the hoisting line until each connection has at least two bolts of the size and strength shown on the erection drawings, drawn up wrench-tight. Diagonal bracing needs at least one bolt per connection.
• Perimeter columns (§ 1926.756(e)): Perimeter columns extend at least 48 inches above the finished floor, with holes or devices at 42 to 45 inches and at the midpoint, so perimeter safety cables can be installed before the next tier is erected.
• Plumbing-up (§ 1926.754(d)): When a competent person decides it is needed, install plumbing-up equipment as erection proceeds. It must be in place before the structure is loaded with bundles of joists, decking, or bridging, and it comes off only with a competent person's approval.
• Multi-story limits (§ 1926.754(b)): Permanent floors are installed as erection proceeds, with no more than eight stories between the erection floor and the uppermost permanent floor unless the design maintains structural integrity. There may be no more than four floors or 48 feet, whichever is less, of unfinished bolting or welding above the foundation or the uppermost permanently secured floor. A fully planked or decked floor, or nets, must be kept within two stories or 30 feet, whichever is less, below any erection work.
• Multiple-lift rigging ("Christmas treeing," § 1926.753(e)):
  1. Use a multiple-lift rigging assembly, hoist no more than five members per lift, lift only beams and similar members, and use only trained employees.
  2. Each assembly is certified with a 5-to-1 safety factor for all components.
  3. Members hang at their center of gravity, reasonably level, rigged from the top down, and at least 7 feet apart.
  4. Members are set from the bottom up, and controlled load lowering is used whenever the load is over the connectors.
• Fall protection (§ 1926.760(a)): Each employee on a walking or working surface with an unprotected side or edge more than 15 feet above a lower level must be protected by guardrails, safety nets, personal fall arrest, positioning, or fall restraint systems.
• Connectors (§ 1926.760(b)): Connectors must be protected from falls of more than two stories or 30 feet, whichever is less, and must have completed connector training. Between 15 and 30 feet, each connector must be provided with a personal fall arrest, positioning, or fall restraint system and must wear the equipment needed to tie off.
• Controlled decking zone (§ 1926.760(c)): A CDZ may be used only over 15 and up to 30 feet where metal decking is first being installed at the leading edge. Only trained leading-edge workers may enter. The zone may be no more than 90 feet wide and 90 feet deep from the leading edge, unsecured decking may not exceed 3,000 square feet, and each deck panel needs at least two safety attachments.
• Deck openings (§ 1926.754(e)): Deck over roof and floor holes, or protect workers at openings that cannot be decked. Covers must support twice the weight of the employees, equipment, and materials that may be on them, be secured, and be painted in a high-visibility color or marked "HOLE" or "COVER." Install shear connectors only after the deck is in place, using it as a working platform.
• Joist erection (§ 1926.757):
  • Where columns are not framed in at least two directions with solid-web members, a joist is field-bolted at the column. Each column gets a vertical stabilizer plate at least 6 by 6 inches, extending at least 3 inches below the bottom chord, with a 13/16-inch hole for guying or plumbing cables.
  • Joists at or near columns that span 60 feet or less are designed so one employee can release the hoisting cable without erection bridging. Joists at or near columns that span more than 60 feet are set in tandem with all bridging installed, unless a qualified person designs an equivalent method.
  • Bridging depends on the spans in OSHA Tables A and B. At or above the listed span, a row of bolted diagonal erection bridging goes near midspan before the cable is released, and only one employee may be on the joist until all other bridging is installed. Over 60 through 100 feet, two rows go near the third points, and no more than two employees may be on the joist. Over 100 through 144 feet, all bridging is installed before the cable is released.
  • K-series ends are attached with two 1/8-inch fillet welds 1 inch long or two 1/2-inch bolts. LH and DLH ends and joist girders use two 1/4-inch fillet welds 2 inches long or two 3/4-inch bolts.
  • No construction loads go on joists until all bridging is installed and anchored and all bearing ends are attached. Two limited exceptions apply. A bridging bundle of up to 1,000 pounds may rest on at least three joists secured at one end, within 1 foot of the secured end. A decking bundle of up to 4,000 pounds may rest on at least three joists attached at both ends, with one row of bridging installed, when a qualified person has documented the capacity in the site-specific erection plan.
  • No modification affecting joist strength is allowed without the structural engineer of record's approval. Joists may not be used as fall-arrest anchorage unless a qualified person has approved it in writing.

Exam trap: The rule is "four floors or 48 feet" of unfinished bolting or welding. Thirty feet is a different rule: the maximum distance down to a decked floor or net, and the connector fall-protection trigger.