8.2 Bolted Connections, Welding & Nondestructive Testing

8.2 Bolted Connections, Welding & Nondestructive Testing

High-Strength Bolting: ASTM F3125, Joint Types & Installation

Structural steel connections transfer axial, shear, and moment forces between intersecting members. Under the Research Council on Structural Connections (RCSC) Specification and ASTM F3125, structural bolts are manufactured to high-tensile standards.

Bolt Grades Under ASTM F3125

• Grade A325: High-strength structural steel bolts manufactured from medium carbon, heat-treated alloy steel, providing a minimum tensile strength of 120 ksi for diameters up to 1-1/2". Used for standard shear and tension connections.
• Grade A490: Quenched and tempered alloy steel bolts with a minimum tensile strength of 150 ksi. Provides higher load capacity per fastener, allowing fewer bolts in compact moment connections, but cannot be hot-dip galvanized due to the risk of hydrogen embrittlement.
• Grade F1852 & Grade F2280: The tension-control (TC) twist-off bolt equivalents of Grade A325 and Grade A490, respectively.

Bolted Joint Classifications

1. Snug-Tight Connections: Defined as the tightness achieved by the full effort of an ironworker with an ordinary spud wrench or a few impacts of an impact wrench. Plies of steel must be brought into solid contact. Permitted for members carrying static shear or where bolts are not subject to significant cyclic fatigue or tension.
2. Pretensioned Connections: Bolts are installed to a specified minimum clamping force (equal to 70% of the bolt's minimum tensile strength). Required in connections subject to load reversal, fatigue, or where bolts resist cyclic axial tension.
3. Slip-Critical Connections: Bolts are pretensioned to clamp connection plies tightly together, relying on surface friction across the faying surfaces (contact planes) to resist shear rather than bolt bearing against the hole edge. Required in structures subject to dynamic fatigue, seismic force-resisting systems, crane runways, and connections utilizing oversized or slotted bolt holes. Faying surfaces must meet Class A (unpainted clean mill scale or blast-cleaned, $\mu = 0.30$) or Class B (unpainted blast-cleaned with high slip coefficient, $\mu = 0.50$).

Four Standard Bolt Pretensioning Verification Methods

  ┌─────────────────────────────────────────────────────────────────────────┐
  │                HIGH-STRENGTH BOLT TIGHTENING METHODS                    │
  └─────────────────────────────────────────────────────────────────────────┘
     │
     ├─► 1. TURN-OF-NUT METHOD
     │      Bring assembly to snug-tight; matchmark nut, bolt, and steel plies;
     │      apply specified rotation (e.g., 1/3, 1/2, or 2/3 turn based on L/D ratio).
     │
     ├─► 2. CALIBRATED WRENCH METHOD
     │      Impact or hydraulic wrench calibrated daily using a Skidmore-Wilhelm
     │      tension calibrator for each bolt diameter, grade, and lot.
     │
     ├─► 3. TWIST-OFF TENSION CONTROL (TC) BOLTS (ASTM F1852 / F2280)
     │      Special electric shear wrench holds bolt spline while turning nut;
     │      splined end shears off when exact engineered pretension is achieved.
     │
     └─► 4. DIRECT TENSION INDICATORS (DTI / SQUIRT WASHERS)
            Hardened washers with raised protrusions; protrusions compress or expel
            orange elastomeric silicone paste when target clamp tension is reached.

Turn-of-Nut Rotation Schedule

Under RCSC Table 8.2, after achieving snug-tight condition, the required nut rotation depends on bolt length relative to bolt diameter ($D$):

• Bolt Length $\le 4D$: Requires $1/3$ turn ($120^\circ$) past snug-tight.
• $4D < \text{Bolt Length} \le 8D$: Requires $1/2$ turn ($180^\circ$) past snug-tight.
• $8D < \text{Bolt Length} \le 12D$: Requires $2/3$ turn ($240^\circ$) past snug-tight.

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Structural Welding Processes & AWS D1.1 Code

Welding permanently fuses structural steel elements through localized thermal melting and alloy filler metal deposition. Structural welding in the United States must strictly comply with AWS D1.1 (Structural Welding Code – Steel).

Common Structural Welding Processes

• Shielded Metal Arc Welding (SMAW / "Stick"): An electric arc is sustained between a flux-covered consumable metal electrode and the weld pool. The flux coating decomposes to generate a protective shielding gas and slag layer. Highly portable, wind-resistant, and ideal for field erection, field repairs, and tight access points.
• Flux-Cored Arc Welding (FCAW): Uses a continuous hollow wire electrode containing an internal core of flux. May be self-shielded (FCAW-S, excellent for outdoor field framing) or gas-shielded (FCAW-G, utilizing external CO2 or Argon/CO2 mixes for high-deposition shop fabrication).
• Gas Metal Arc Welding (GMAW / "MIG"): Utilizes a solid wire electrode with an externally supplied shielding gas (typically 75% Argon / 25% CO2). Highly productive in shop environments but susceptible to wind drafts in field erection that blow away shielding gas, causing severe porosity.
• Submerged Arc Welding (SAW): An automated process where the arc is completely buried beneath a blanket of granular fusible flux. Produces deep penetration, high-speed, defect-free welds for heavy built-up plate girders and column fabrications in the shop.

AWS Standard Welding Symbols Anatomy

AWS welding symbols on structural shop drawings convey complete joint fabrication instructions in a compact graphic format:

                          [Finish / Contour Symbol]
                         [Groove Angle / Root Opening]
                         [Weld Size]  (Weld Type)  [Length - Pitch]
                                      │     │
              Field Weld Flag ──┐     │     │     ┌── Weld-All-Around Circle
                                │     │     │     │
                                ▼     ▼     ▼     ▼
        ───────────────────────────┬───────────────────┬─────────────► Arrow
          (Reference Line)         │                   │               (Points to Joint)
                                   └───────────────────┘
                                 [Other Side: Above Line]
                                 [Arrow Side: Below Line]
        Tail ◄────────────────
        (Welding Process / WPS Reference)

1. Reference Line (Horizontal): The anchor of the symbol. Instructions placed below the reference line apply to the arrow side of the joint; instructions placed above the reference line apply to the other side of the joint.
2. Arrow: Points directly to the joint where the weld is to be executed.
3. Weld Type Symbols:
   • Triangle ($\Delta$): Fillet weld.
   • Two parallel vertical lines ($||$): Square groove weld.
   • V-shape ($V$): Single-V groove weld.
   • Single vertical line with angle line ($|/$): Bevel groove weld.
4. Dimensions:
   • Weld Size (Leg Size): Placed to the left of the weld type symbol (e.g., 5/16 for a 5/16" fillet weld).
   • Weld Length: Placed to the right of the weld type symbol (e.g., 2 for a 2" long weld).
   • Pitch (Center-to-Center Spacing): Placed to the right of length, separated by a hyphen (e.g., 2-6 denotes 2" long intermittent welds spaced 6" center-to-center).
5. Supplementary Symbols:
   • Field Weld Flag: A solid black flag placed at the junction of the arrow and reference line indicating that the weld is to be executed in the field during erection, rather than in the fabrication shop.
   • Weld-All-Around Circle: A circle placed at the arrow/reference line intersection indicating the weld must be continuous around the entire joint perimeter.
   • Tail: Located at the end opposite the arrow; contains references to specific Welding Procedure Specifications (WPS), electrode types (e.g., E7018), or NDT requirements.

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Non-Destructive Testing (NDT) & Quality Control

Under IBC Chapter 17 (Special Inspections) and AWS D1.1, critical structural steel welds must undergo independent Non-Destructive Testing (NDT) to identify discontinuities such as cracks, lack of fusion, incomplete penetration, porosity, and slag inclusions without damaging the structural component.

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OSHA 29 CFR 1926 Subpart R: Structural Steel Erection Safety

Steel erection is one of the most hazardous operations in commercial building construction. General contractors and steel erection subcontractors must strictly comply with federal OSHA Subpart R (29 CFR 1926.750 – 1926.761).

1. Column Base Plates & Anchor Bolt Anchorage (1926.755)

• Minimum 4 Anchor Bolts: Every column base plate assembly must be anchored by a minimum of four (4) anchor bolts to provide stability against accidental displacement and eccentric loads.
• Overturning Stability: All columns must be engineered to withstand a minimum eccentric gravity load of 300 pounds placed at 18 inches from the column face in each direction at the top of the column shaft.
• Anchor Bolt Modification: Anchor bolts cannot be repaired, cut, heated, or modified in the field without the express prior written approval of the Structural Engineer of Record (SER).

2. Plumbing Up & Temporary Stability (1926.754)

• Turnbuckles, guy cables, and temporary diagonal braces must be installed to bring the structural steel frame plumb and true.
• Temporary plumbing-up guys must remain in place until sufficient permanent framing, moment connections, cross-bracing, and metal decking are installed to ensure overall structural stability.

3. Fall Protection Criteria in Steel Erection (1926.760)

OSHA Subpart R enforces specialized fall protection thresholds that differ from the standard 6-foot construction trigger:

                      OSHA SUBPART R FALL PROTECTION THRESHOLDS
                                          │
         ┌────────────────────────────────┼────────────────────────────────┐
         ▼                                ▼                                ▼
  GENERAL STEEL WORKERS              CONNECTORS               CONTROLLED DECKING ZONES (CDZ)
  • Mandatory Fall Arrest /       • Initial placement of      • Installing metal decking
    Restraint at 15 FEET            beams and girders           leading edge
  • Applies to bolting, welding,  • Harness required; must    • Authorized up to 30 FEET
    rigging, and detailing          tie off above 30 FEET       or 2 STORIES without tie-off
    above a lower level             (or 2 stories)              under strict written CDZ plan

• Standard Fall Protection (15-Foot Rule): All ironworkers and trades engaged in steel erection activities on a walking/working surface with an unprotected edge or opening more than 15 feet above a lower level must be protected by guardrails, safety nets, or Personal Fall Arrest Systems (PFAS).
• Connectors (30-Foot / 2-Story Rule): Connectors—workers performing the initial connecting of structural members at the hoisting point—must be provided with personal fall protection equipment and wear harnesses at all times. Connectors must tie off when working above two stories or 30 feet above a lower level (between 15 and 30 feet, they must have access to tie-off anchors but are legally permitted to unhook to maneuver for structural stability).
• Controlled Decking Zones (CDZ): A designated area up to 30 feet or 2 stories where metal deckers may lay initial roof/floor decking leading edges without conventional fall arrest systems, provided the CDZ is demarcated by warning lines, limited to authorized trained personnel, and capped at a maximum working area of 90 feet by 90 feet.