Introduction to Steel Structure Bolting
Bolted connections are the most common method of joining structural steel members in buildings, bridges, and industrial structures. High-strength bolting provides reliable, inspectable, and cost-effective connections that can be designed for either slip-critical or bearing-type behavior.
Proper bolting procedures are essential to ensure structural integrity. This guide covers bolt grades, installation methods, tightening procedures, and quality inspection requirements.
High-Strength Bolt Grades
| Grade | Standard | Min. Tensile (MPa) | Material | Application |
|---|---|---|---|---|
| A325 | ASTM A325 | 830-1030 | Medium carbon steel, quenched and tempered | General structural connections |
| A490 | ASTM A490 | 1040-1210 | Alloy steel, quenched and tempered | Heavy structural connections, bridges |
| A325M | ASTM A325M | 830 | Metric equivalent of A325 | Metric structural connections |
| 8.8 | ISO 898-1 | 800 | Medium carbon steel, quenched and tempered | European metric structural connections |
| 10.9 | ISO 898-1 | 1040 | Alloy steel, quenched and tempered | High-strength European connections |
Bolt Types and Configurations
By Head Type
- Hex Head Bolts: Most common for structural steel; used with hex nuts
- Heavy Hex Structural Bolts: Larger bearing surface for structural connections (A325, A490)
- TC (Twist-Off) Bolts: Designed for direct tension indication; splined end twists off at required preload
By Coating
- Plain (Black): No coating; used in interior or painted applications
- Hot-Dip Galvanized: For exterior or corrosive environments; requires over-torqueing to account for zinc friction
- Mechanically Galvanized: Thin zinc coating; for moderate corrosion protection
- Weathering Steel (Type 3): Matches weathering steel members (ASTM A588); forms protective patina
Installation Methods
1. Turn-of-Nut Method
The most common method for structural bolting in North America.
- Snug Tight: Bring all plies into firm contact using a few impacts of an impact wrench, or the full effort of a worker with a standard spud wrench.
- Mark Reference: Place a mark on the nut and a corresponding mark on the bolt or adjacent surface.
- Turn: Rotate the nut the specified additional amount:
- 120 degrees for bolts with grip length up to 4 diameters
- 180 degrees for bolts with grip length over 4 diameters
2. Torque-Control Method
Apply a calibrated torque to achieve the required preload. This method requires:
- Calibrated torque wrench with documented calibration
- Daily verification of torque wrench accuracy
- Prooftesting of 3 bolts per day (or per 100 bolts) to verify calibration
- Typical required torque: T = K * D * P, where K = torque coefficient (typically 0.15-0.25), D = bolt diameter, P = required preload
3. Direct Tension Indicator (DTI) Method
Uses a hardened washer with protrusions that compress under bolt tension.
- DTI washers have built-in indicators that show when proper tension is achieved
- Gap measurement: when gaps are closed to the specified value, the required preload is achieved
- ASTM F959: Standard Specification for Compressible-Washer Type Direct Tension Indicators
4. Twist-Off (TC) Bolt Method
Special bolts with a splined end that twists off when the required tension is reached.
- Uses a special twin-headed wrench
- Visual confirmation: the splined end breaks off cleanly
- ASTM F3125 Grade A490T or A325T
Required Bolt Pretension (Tension)
| Bolt Size | A325 Pretension (kN) | A490 Pretension (kN) |
|---|---|---|
| M16 (5/8 inch) | 75 | 98 |
| M20 (3/4 inch) | 120 | 156 |
| M22 (7/8 inch) | 150 | 195 |
| M24 (1 inch) | 175 | 228 |
| M27 (1-1/8 inch) | 225 | 293 |
| M30 (1-1/4 inch) | 280 | 365 |
Joint Types
Slip-Critical Connections
Designed so that the friction between faying surfaces prevents any movement under service loads. Requirements:
- Specified surface preparation (Class A: unpainted clean mill scale, mu = 0.33)
- Full required pretension in all bolts
- Inspection of all bolts (100%)
Bearing-Type Connections
Allow controlled slip at service loads; bearing of bolt shank against the hole provides ultimate strength. Requirements:
- Minimum snug-tight condition for service loads
- Full pretension required for cyclic loading or where slip is undesirable
- Sample inspection (minimum 10% of bolts per connection, minimum 2 per connection)
Quality Inspection
Inspection Requirements
- Slip-critical: 100% inspection of all bolts
- Snug-tight: Visual inspection that all bolts are snug
- Pretensioned (bearing): Minimum 10% of bolts per connection
Inspection Methods
- Turn-of-Nut: Visual check that the specified rotation has been achieved; re-tighten bolts that have not rotated enough
- DTI: Use feeler gauge to verify gaps are closed per specification
- TC Bolt: Visual verification that splined end has twisted off
- Torque: Re-torque check with calibrated wrench within specified time window
Key Standards
- AISC 360: Specification for Structural Steel Buildings
- RCSC Specification: Structural Joints Using High-Strength Bolts (Research Council on Structural Connections)
- ASTM F3125: Standard Specification for High Strength Structural Bolts (replaces A325 and A490)
- ASTM F959: Direct Tension Indicators
- EN 1090-2: Execution of steel structures – Technical requirements
- ISO 898-1: Mechanical properties of fasteners
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