Understanding Structural Bolt Grades

Structural steel connections demand fasteners engineered for predictable, reliable performance under extreme loads. Unlike general-purpose bolts, structural fasteners must meet stringent specifications for material, dimensions, and installation that ensure safe load transfer throughout the life of a structure.

This guide covers high-strength structural bolting—the fasteners, installation methods, and inspection requirements that keep buildings and bridges standing.

Understanding Structural Bolt Grades

Structural steel connections primarily use two ASTM specifications: A325 and A490. These aren't interchangeable with SAE grades—they're specifically designed and tested for structural steel applications.

ASTM A325 Structural Bolts

A325 bolts are medium-carbon or alloy steel, heat-treated to provide reliable high-strength performance:

• Tensile strength: 120,000 PSI minimum (105,000 PSI for diameters over 1")
• Proof load: Approximately 85% of tensile strength
• Common uses: General structural connections, most building frames, standard bridge connections
• Head marking: "A325" with manufacturer identification

Type designations:

• Type 1: Medium-carbon steel (most common)
• Type 3: Weathering steel for unpainted exposed applications

ASTM A490 Structural Bolts

A490 bolts are alloy steel with higher strength than A325:

• Tensile strength: 150,000 PSI minimum (no diameter deduction)
• Proof load: Approximately 85% of tensile strength
• Common uses: Heavy connections, high-load transfer joints, connections requiring fewer bolts
• Head marking: "A490" with manufacturer identification

Important limitations:

• A490 bolts cannot be galvanized (hydrogen embrittlement risk)
• Not used in slip-critical connections with oversized holes
• Requires careful attention to installation torque

Metric Equivalents

For metric specifications:

• ASTM F3125 Grade A325M: Metric equivalent to A325
• ASTM F3125 Grade A490M: Metric equivalent to A490

Connection Types: Snug-Tight vs. Pretensioned

How you install structural bolts depends entirely on the connection type specified by the engineer. Getting this wrong compromises structural integrity.

Snug-Tight Connections

Snug-tight is the minimum acceptable installation: the point where all plies of the joint are in firm contact with no gaps.

When snug-tight is acceptable:

• Bearing-type connections in static loading
• Secondary framing members
• Connections designed for snug-tight in the contract documents
• Bracing and infill framing

What qualifies as snug-tight:

• "Full effort" of a worker with an ordinary spud wrench
• Several impacts of an impact wrench
• No specified torque value—it's a condition, not a number

Snug-tight connections are faster and less costly to install but aren't suitable for slip-critical joints or fatigue-loaded connections.

Pretensioned (Fully Tensioned) Connections

Pretensioned connections require bolts to be tensioned to a specified minimum tension, creating high clamping force that:

• Prevents joint slip under service loads
• Provides fatigue resistance through controlled preload
• Ensures consistent load distribution

When pretensioning is required:

• Slip-critical connections
• Connections subject to fatigue or vibration
• Column splices
• Beam-to-column moment connections
• Connections using oversized or slotted holes
• Wherever specified by the engineer

Minimum pretension values are specified in AISC and RCSC standards—typically 70% of the bolt's tensile strength.

Pretensioning Methods

Several methods achieve required pretension. The appropriate method depends on project specifications, accessibility, and inspection requirements.

Turn-of-Nut Method

The most common field method for pretensioning structural bolts:

Procedure:

1. Install all bolts in the connection snug-tight

2. Match-mark the nut and protruding bolt end with a visible line

3. Rotate the nut (or bolt) the specified additional rotation

4. Visually verify rotation achieved

Required rotation (from snug-tight):

These values apply when both surfaces are perpendicular to the bolt axis. Add rotation for beveled washers or sloped surfaces.

Advantages:

• Simple visual verification
• No special tools required
• Well-understood by ironworkers
• Consistent results with proper training

Calibrated Wrench Method

Uses a torque wrench or power tool calibrated to deliver required bolt tension:

Procedure:

1. Calibrate the wrench daily using a bolt tension calibrator

2. Install bolts snug-tight

3. Apply calibrated torque to each bolt

4. Torque value is NOT specified—it's determined by daily calibration

Important: Published torque charts should never be used directly. Actual required torque varies with lubrication, thread condition, and bolt lot—daily calibration accounts for these variables.

Direct Tension Indicators (DTI)

Direct tension indicators are hardened washers with raised protrusions on one face. These protrusions flatten under load, and gap measurement indicates achieved tension.

Procedure:

1. Install DTI washer under bolt head or nut

2. Tighten bolt while monitoring gap

3. Verify gap closure using feeler gauge

4. Specified gap indicates minimum tension achieved

Gap criteria:

• Initial gap: Approximately 0.015" per protrusion
• Final gap: Per specification, typically ≤0.005" average with no more than 50% of protrusions showing refusal

DTI washers provide direct indication of bolt tension independent of torque, making inspection straightforward.

Twist-Off Type Tension Control Bolts

Also called "TC bolts," these fasteners have a splined end extending beyond the threads. The installation wrench grips both the nut and splined end, torquing until the spline shears off at the calibrated tension.

Advantages:

• Built-in tension indication (sheared end = minimum tension achieved)
• Simple visual inspection
• Consistent factory-calibrated tension
• Fast installation with proper tools

Limitations:

• Higher initial cost than standard bolts
• Requires TC wrench (electric shear wrench)
• Sheared spline creates debris
• Cannot be reused or re-tightened

F436 Hardened Washers

Hardened steel washers meeting ASTM F436 are required under the turned element (nut or bolt head) when installing structural bolts.

Why Hardened Washers Matter

Standard flat washers aren't adequate for structural bolting:

• Prevent galling: Hardened surface reduces friction damage to connected steel
• Consistent friction: Known surface condition enables torque-tension correlation
• Protect base material: Prevents nut rotation from scoring the steel
• Bridge oversized holes: Washers must be hardened when holes exceed standard size

F436 Washer Requirements

• Hardness: 38-45 HRC
• Thickness: Sufficient to prevent dishing under preload
• Marking: Manufacturer identification required

When Washers Are Required

Per RCSC Specification:

• Under the turned element (nut or bolt head being rotated during tightening)
• Under both head and nut when A490 bolts are used
• When holes are oversized or slotted
• When bearing surfaces have slopes exceeding 1:20 (use beveled washers)

Inspection Requirements

Structural bolting requires documented inspection to verify proper installation. Inspection procedures should be established before erection begins.

Pre-Installation Verification

Rotational capacity testing: Before installing each bolt lot, verify the bolts can achieve required pretension:

1. Install bolt in calibration device

2. Tension to required minimum using specified method

3. Verify the bolt can achieve minimum rotation beyond snug-tight without failure

This catches defective bolt lots before they're installed.

Snug-Tight Connection Inspection

• Verify all bolts are present
• Confirm plies in firm contact (no gaps)
• Check correct bolt grade installed
• Visual inspection adequate

Pretensioned Connection Inspection

Turn-of-nut method:

• Verify match-marks present on all bolts
• Confirm required rotation achieved
• Sample verification using systematic procedure

DTI inspection:

• Verify gap closure using feeler gauges
• Check gap at multiple protrusions per washer
• Document acceptance/rejection

TC bolt inspection:

• Verify splined ends properly sheared
• Check for any loose bolts (indication of missed installation)
• Visual inspection of shear surface

Documentation

Maintain inspection records including:

• Connection identification
• Bolt lot numbers
• Calibration records (if calibrated wrench method)
• Pre-installation verification results
• Inspection results and dates
• Non-conformance reports and resolutions

Common Installation Issues

Under-Tensioned Bolts

Causes:

• Insufficient rotation in turn-of-nut method
• Calibration drift in wrench method
• Lubrication issues affecting torque-tension
• Worker fatigue or rushing

Prevention:

• Regular training and certification
• Daily calibration verification
• Supervision and inspection
• Proper bolt preparation (lubrication, cleaning)

Over-Tensioned Bolts

Causes:

• Excessive rotation
• Mis-calibrated equipment
• Mixing bolt lots with different properties

Consequences:

• Bolt failure during installation or service
• Thread stripping
• Fatigue life reduction

Joint Relaxation

After initial tensioning, bolts may lose some tension as paint compresses, surfaces settle, or material creeps. For critical connections:

• Re-snug all bolts after initial installation
• Re-pretension in proper sequence
• Consider final pass after time delay

Structural Fastener Summary

Final Recommendations

1. Verify connection type before selecting installation method—snug-tight and pretensioned are not interchangeable

2. Use specified materials only—A325 and A490 are not interchangeable with SAE grades

3. Install F436 washers under the turned element, always

4. Calibrate daily when using calibrated wrench method

5. Document everything—inspection records protect all parties

6. Never reuse TC bolts or visibly yielded fasteners

7. Train your crew—proper structural bolting requires understanding, not just following steps

Structural bolting done right creates connections that perform reliably for the life of the structure. Done wrong, it creates hidden deficiencies that may not reveal themselves until catastrophic failure. Follow specifications, inspect thoroughly, and maintain documentation.