Bolted Connections
High-strength bolts, ASTM F3125 grades, snug-tight and pretensioned installation, turn-of-nut method, TC bolts, and DTI washers.
- Identify ASTM F3125 Grade A325 and Grade A490 high-strength structural bolts
- Describe snug-tight, pretensioned, and slip-critical installations
- Explain the turn-of-nut method and TC bolt operation
- Describe erection bolting and temporary connection requirements
Lesson 1
ASTM F3125 High-Strength Structural Bolts
The F3125 Standard - What Changed
The standalone standards ASTM A325 and ASTM A490 have been withdrawn. Current Canadian and American structural steel practice uses ASTM F3125, which consolidates all high-strength structural bolt grades into a single standard. When reading drawings or specifications that reference A325 or A490, understand these are now designated:
- ASTM F3125 Grade A325 - standard high-strength structural bolt (replaces standalone A325)
- ASTM F3125 Grade A490 - higher-strength structural bolt (replaces standalone A490)
Twist-off (tension control) bolt grades F1852 (equivalent to A325) and F2280 (equivalent to A490) are also now incorporated within ASTM F3125.
F3125 Grade A325 Properties
ASTM F3125 Grade A325 is the most commonly used structural bolt in steel construction. It is made from medium-carbon or alloy steel and heat treated. A key change in F3125 is the unified tensile strength: all Grade A325 bolts up to 1-1/2 inch diameter are rated at a minimum 120 ksi tensile strength regardless of diameter. Previously, bolts larger than 1 inch had a lower rating (105 ksi) under the old standalone standard.
Grade A325 bolts come in Type 1 (carbon or alloy steel, medium hex head) and Type 3 (weathering steel with atmospheric corrosion resistance).
F3125 Grade A490 Properties
ASTM F3125 Grade A490 bolts are higher-strength, made from alloy steel. Their minimum tensile strength is 150 ksi. A490 bolts have strict coating restrictions due to hydrogen embrittlement risk:
- Hot-dip galvanizing: prohibited (molten zinc immersion drives monatomic hydrogen into the bolt's crystalline lattice, causing delayed brittle fracture)
- Mechanical galvanizing: prohibited for the same reason
- Permitted coatings: zinc-aluminum dispersion coatings meeting ASTM F1136, F2833, or F3393
Hydrogen Embrittlement - A490 Bolts
Never use hot-dip or mechanically galvanized coatings on A490 bolts. During acid pickling and molten zinc immersion, monatomic hydrogen diffuses into the steel's crystalline lattice. Under the high clamping stress of pretensioning, this causes delayed brittle fracture - sudden failure with no visible warning. Zinc-aluminum dispersion coatings (ASTM F1136/F2833/F3393) are approved alternatives.
Bolt Reuse Prohibition
Galvanized Grade A325 bolts and all Grade A490 bolts must never be reused. Pretensioning permanently yields the thread pitch of high-strength bolts. Reusing a pretensioned bolt risks unpredictable tension values and potential brittle failure.
F3125 Grade A325
120 ksi tensile (unified all diameters)
Galvanizing permitted
Galvanized A325: no reuse
TC bolt equivalent: F1852
F3125 Grade A490
150 ksi tensile strength
Hot-dip and mechanical galvanizing prohibited
Zinc-aluminum dispersion coatings permitted
Never reuse after pretensioning
TC bolt equivalent: F2280
Standalone ASTM A325 and A490 are withdrawn - current standard is ASTM F3125 Grade A325 and Grade A490. Grade A325 has a unified tensile strength of 120 ksi for all diameters. Grade A490 must never be hot-dip or mechanically galvanized due to hydrogen embrittlement risk. Neither galvanized A325 nor any A490 bolt may be reused after pretensioning.