Canadian Steel Bolting — ASTM A325M, A490M Fasteners & CSA S16-19
Complete reference for Canadian structural bolting covering ASTM A325M and A490M bolts, washers, nuts, installation methods (snug-tight, pretensioned, slip-critical), inspection requirements, and quality control per CSA S16-19 Clauses 22-23 and ASTM F3125M.
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Canadian Bolt Types and Standards
Canadian structural bolting uses metric-dimensioned bolts conforming to ASTM F3125M (which consolidated A325M and A490M into a single standard in 2016). The two primary bolt grades are:
| Bolt Grade | Tensile Strength Fu (MPa) | Yield Strength Fy (MPa) | Hardness (HRC) | ASTM Standard |
|---|---|---|---|---|
| A325M | 830 | 660 | 24-35 | ASTM F3125M Grade A325M |
| A490M | 1035 | 895 | 33-41 | ASTM F3125M Grade A490M |
A325M Bolts (Metric High-Strength)
A325M is the standard structural bolt in Canada, equivalent to ASTM A325 in imperial dimensions but with metric thread profiles. A325M bolts are used for the vast majority of structural steel connections.
Manufacturing requirements:
- Medium-carbon steel (typically 0.30-0.50% carbon)
- Heat-treated (quenched and tempered)
- Minimum tensile strength: 830 MPa
- Minimum yield strength: 660 MPa
- Available diameters: M16, M20, M22, M24, M27, M30, M36
A490M Bolts (Metric High-Strength / Higher Strength)
A490M bolts provide approximately 25% higher tensile strength than A325M and are used in heavy connections where A325M bolts would require too many fasteners or excessively large connection plates.
Manufacturing requirements:
- Alloy steel (typically 0.35-0.50% carbon with Cr, Ni, Mo additions)
- Heat-treated (quenched and tempered)
- Minimum tensile strength: 1035 MPa
- Minimum yield strength: 895 MPa
- Available diameters: M16 through M36
A325M vs A490M — Key Differences
| Property | A325M | A490M |
|---|---|---|
| Fu (MPa) | 830 | 1035 |
| Fy (MPa) | 660 | 895 |
| Relative strength | 1.00 (baseline) | 1.25 |
| Maximum thickness (mm) | No limit | 100 mm (galvanised) |
| Galvanising | Permitted (embrittlement risk) | Not permitted (hydrogen embrittlement) |
| Cost index | 1.00 (baseline) | 1.5-2.0x |
| Typical application | All standard connections | Heavy columns, large tension splices |
Important limitation: A490M bolts must NOT be hot-dip galvanised due to the risk of hydrogen embrittlement. If galvanised fasteners are required, A325M bolts must be specified.
Bolt Dimensions and Thread Configuration
Canadian structural bolts use ISO metric coarse thread series (M profile):
| Size | Thread Pitch (mm) | Body Diameter d (mm) | Body Area Ab (mm^2) | Tensile Stress Area (mm^2) | Head Width (mm) | Nut Width (mm) |
|---|---|---|---|---|---|---|
| M16 | 2.0 | 16 | 201 | 157 | 24 | 24 |
| M20 | 2.5 | 20 | 314 | 245 | 30 | 30 |
| M22 | 2.5 | 22 | 380 | 303 | 32 | 32 |
| M24 | 3.0 | 24 | 452 | 353 | 36 | 36 |
| M27 | 3.0 | 27 | 573 | 459 | 41 | 41 |
| M30 | 3.5 | 30 | 707 | 561 | 46 | 46 |
| M36 | 4.0 | 36 | 1018 | 817 | 55 | 55 |
The body area Ab = pi _ d^2 / 4 is used for bolt shear and tension capacity calculations per CSA S16-19 Clause 13.12. The tensile stress area (through threads) is approximately 0.78 _ Ab for standard metric thread pitch.
Bolt Length Identification
Bolt length is measured from the underside of the head to the extreme end of the bolt. Standard lengths:
| Bolt Diameter | Standard Lengths (mm) |
|---|---|
| M16 | 35, 40, 45, 50, 55, 60, 65, 70, 80, 90, 100 |
| M20 | 40, 45, 50, 55, 60, 65, 70, 75, 80, 90, 100, 110, 120 |
| M22 | 45, 50, 55, 60, 65, 70, 75, 80, 90, 100, 110, 120, 130 |
| M24 | 50, 55, 60, 65, 70, 75, 80, 90, 100, 110, 120, 130, 140 |
| M27 | 55, 60, 65, 70, 75, 80, 90, 100, 110, 120, 130, 140, 150 |
| M30 | 60, 65, 70, 75, 80, 90, 100, 110, 120, 130, 140, 150, 160 |
The bolt length must be selected such that after installation, the threaded portion extends through the nut by at least one full thread, and the bolt point (unthreaded portion) protrudes beyond the nut by a minimum of 2 mm. The grip length (total thickness of connected plies, excluding washers) determines the required bolt length.
Nut Specifications
Structural nuts per ASTM A563M or ASTM A194M:
| Bolt Grade | Nut Grade | Nut Specification | Proof Load (MPa) | Identification |
|---|---|---|---|---|
| A325M | Grade 10S | ASTM A563M | 1040 | Three circumferential marks or "10" |
| A490M | Grade 10S | ASTM A563M | 1040 | Three circumferential marks or "10" |
Nuts must be compatible with the bolt thread series (6H thread class per ISO metric standards). Heavy hex nuts (ASTM A563M Grade 10S) are the standard for structural connections. The heavy hex configuration provides greater bearing area for pretensioned installation.
Washer Specifications
Washers per ASTM F436M:
| Bolt Size | Washer ID (mm) | Washer OD (mm) | Min Thickness (mm) | Hardness |
|---|---|---|---|---|
| M16 | 17 | 34 | 3.0 | 38-45 HRC |
| M20 | 21 | 39 | 3.5 | 38-45 HRC |
| M22 | 23 | 42 | 3.5 | 38-45 HRC |
| M24 | 25 | 46 | 4.0 | 38-45 HRC |
| M27 | 28 | 50 | 4.0 | 38-45 HRC |
| M30 | 31 | 56 | 4.5 | 38-45 HRC |
| M36 | 37 | 66 | 5.0 | 38-45 HRC |
Washer Requirements by Connection Type
| Connection Type | Under Head | Under Nut | Notes |
|---|---|---|---|
| Snug-tight (bearing) | Not required | Not required | Optional for surface tolerance |
| Pretensioned (non-slip-critical) | Required (hardened) | Required | One hardened washer each side |
| Slip-critical (A325M) | Required | Required | Hardened washers, quenched |
| Slip-critical (A490M) | Required (both sides) | Required | Two washers each side |
For slip-critical connections with A490M bolts, two hardened washers are required under the turned element (nut or head) to prevent galling during installation. This is because A490M's higher hardness makes it more susceptible to thread galling during the high-torque installation required for pretensioning.
Installation Categories
CSA S16-19 Clause 22.6 defines three installation categories:
1. Snug-Tight (Bearing-Type Connections)
- Method: A few impacts of an impact wrench or full effort of a person using a spud wrench
- Tension achieved: Approximately 10% of specified pretension
- Application: Static loading, non-seismic, bearing-type connections
- Hole types permitted: Standard holes only (or short slotted in one ply)
Snug-tight connections are the most common in Canadian building construction for non-seismic applications. The full factored shear resistance of the bolt is developed through bolt bearing on the connected plies.
2. Pretensioned (Non-Slip-Critical)
- Method: Full pretensioning per Clause 23.2 (any approved method)
- Tension achieved: 100% of specified Ti
- Application: All connections in seismic frames, column splices, crane runway beams, connections subject to significant vibration
- Hole types permitted: Standard, oversized (in some cases), short slotted
Pretensioned bolts develop the same bearing-type shear capacity as snug-tight bolts but the clamping force provides additional stiffness and prevents loosening under cyclic loading.
3. Slip-Critical
- Method: Full pretensioning + faying surface preparation
- Tension achieved: 100% of specified Ti
- Application: Connections where slip cannot be tolerated (load reversal, fatigue, oversized holes)
- Hole types permitted: Standard, oversized, short slotted, long slotted
- Faying surface: Class A (mu=0.30), Class B (mu=0.50), or tested coefficient
Slip-critical connections are designed to resist load through friction between the connected plies, not through bolt bearing. The design resistance is governed by slip resistance, not bolt shear capacity.
Bolt Installation Methods
CSA S16-19 Clause 23.2 permits the following methods for achieving required pretension:
Turn-of-Nut Method
| Bolt Length (L) | Additional Turn Past Snug-Tight |
|---|---|
| L <= 4d | 1/3 turn (120 degrees) |
| 4d < L <= 8d | 1/2 turn (180 degrees) |
| L > 8d | 2/3 turn (240 degrees) |
| All lengths, connection surface to bolt axis slope > 1:20 | 3/4 turn (270 degrees) |
The turn-of-nut method is the most reliable and most widely used in Canada. Calibration testing per CSA W59 Annex A is required at the start of each project to verify the method achieves the specified pretension.
Tension Control (TC) Bolt Method
TC bolts (ASTM F1852M) have a splined end that shears off when the bolt reaches specified pretension. They are installed with a specialised TC wrench that simultaneously rotates the nut while holding the splined end.
Advantages:
- Fast installation (no torque calibration required)
- Simple visual inspection (splined end broken = bolt properly tensioned)
- Consistent pretension (manufacturer-calibrated spline break point)
Disadvantages:
- Higher bolt cost than A325M
- Splined end must be ordered separately (TC bolt assembly)
- Not available for all diameters (typically M16-M24)
Direct Tension Indicator (DTI) Method
DTI washers have formed protrusions on one face that compress as bolt tension increases:
| Bolt Grade | DTI Specification | Gap Measurement | Max Gap |
|---|---|---|---|
| A325M | ASTM F959M Type 325 | Feeler gauge between washer and bolt head/nut | 0.25 mm |
| A490M | ASTM F959M Type 490 | Feeler gauge between washer and bolt head/nut | 0.25 mm |
Installation:
- Place DTI under the bolt head (preferred) or under the nut
- Tighten using any convenient method
- Check gap at the DTI protrusions using a 0.25 mm feeler gauge
- Required pretension is achieved when the 0.25 mm feeler gauge cannot be inserted at 2 of 3 locations around the DTI
Calibrated Torque Method
Rarely used in Canadian practice due to variability in torque-tension relationships.
Bolt Tension Testing (Skidmore-Wilhelm)
Per CSA S16-19 Clause 23.5.1, the pretension must be verified using a bolt tension calibrator (Skidmore-Wilhelm device):
- Frequency: 3 bolts per diameter per project for TC bolts; 3 bolts per lot for A325M bolts
- Acceptance criteria: Average tension >= specified Ti; no individual bolt less than 0.95 Ti
- Test bolts: Same length, grade, and lot as production bolts
The Skidmore-Wilhelm test is the only reliable field method to verify actual bolt tension. Torque-wrench correlation testing (measuring torque required to achieve specified tension) is required for the calibrated torque method.
Inspection Requirements
Per CSA S16-19 Clause 23.5 and CSA W59 Annex B:
Daily Pre-Installation Inspection
- Verify bolts, nuts, and washers are correct grade and clean
- Check bolt lot markings (manufacturer identification, grade marking, ASTM designation)
- Verify DTI washers (if used) are within shelf life
- Check TC bolt spline break torque calibration
During Installation Inspection
| Item | Acceptance Criteria | Method |
|---|---|---|
| Snug-tight condition | Plies in firm contact, no gap | Visual, 0.25 mm feeler gauge |
| Turn-of-nut rotation | Marking required for rotation measurement | Chalk or paint mark |
| TC bolt spline | Splined end broken off | Visual |
| DTI gap | 0.25 mm feeler gauge not insertable at 2 of 3 locations | Feeler gauge |
Post-Installation Verification
- Snug-tight: 100% visual inspection of connection fit
- Pretensioned: 100% visual + daily Skidmore-Wilhelm verification of installation procedure
- Slip-critical: 100% visual + procedure verification + faying surface inspection
Rejection and Replacement
- Bolts that fail to achieve required pretension must be removed and replaced
- Bolts that have been tensioned to 100% Ti must not be reused (plastic deformation occurs)
- Damaged threads: bolts with damaged threads must be replaced
- Galled threads: if galling occurs during installation, the bolt-nut assembly must be replaced
Corrosion Protection
| Protection Method | Suitable For | Notes |
|---|---|---|
| Hot-dip galvanising | A325M only (not A490M) | ASTM F3125M permits galvanised A325M |
| Mechanically deposited galvanising | A325M, A490M (with caution) | Lower embrittlement risk |
| Painted assemblies | Both A325M and A490M | Cover bolt heads after installation |
| Zinc-rich primer | Both A325M and A490M | Touch up after installation |
| Weathering steel (350AT) | A325M (weathering grade) | A325M bolts for 350AT structures |
For A490M bolts in corrosive environments, painting after installation is the preferred protection method. Hot-dip galvanising of A490M is prohibited by ASTM F3125M due to hydrogen embrittlement risk during the galvanising process.
Frequently Asked Questions
What is the difference between A325M and A490M bolts in Canadian construction? A325M bolts (Fu = 830 MPa) are the standard structural bolt used for most connections. A490M bolts (Fu = 1035 MPa) provide 25% higher strength and are used where higher capacity is needed without increasing bolt size. A490M bolts are more expensive and cannot be hot-dip galvanised due to hydrogen embrittlement risk. A490M installation requires two hardened washers (under both head and nut) to prevent galling during pretensioning.
What are the three tightening categories for bolts per CSA S16-19? Snug-tight (approximately 10% of full pretension) is used for bearing-type connections in non-seismic applications. Pretensioned (100% of specified pretension Ti) is required for connections in seismic frames, column splices, and crane runway beams. Slip-critical (100% pretension + faying surface preparation) is required for connections subject to load reversal, fatigue, or oversized/slotted holes where slip cannot be tolerated.
What installation methods are permitted for achieving bolt pretension per CSA S16? Four methods are permitted: (1) Turn-of-nut — additional 1/3 to 2/3 turn past snug-tight; (2) Tension control (TC) bolts — splined end shears off at specified pretension; (3) Direct tension indicators (DTI) — washer protrusions compress, measurable with 0.25 mm feeler gauge; (4) Calibrated torque — requires Skidmore-Wilhelm tension calibration for each bolt lot. The turn-of-nut method is the most widely used in Canadian practice.
Are A325M and A490M bolts interchangeable? No. A325M and A490M bolts have different tensile strengths (830 MPa vs 1035 MPa), different installation requirements (A490M requires two washers), and different corrosion protection limitations (A490M cannot be galvanised). They must not be mixed in the same connection. On drawings and in specifications, the bolt grade must be clearly stated for each connection type. Substituting A490M for A325M requires verification of ductility demands and connection rotation capacity.
What washers are required for slip-critical connections? For A325M bolts in slip-critical connections: one hardened washer under the head and one under the nut (per ASTM F436M). For A490M bolts: two hardened washers under the turned element (typically the nut) plus one under the head — a total of three washers per bolt. The additional washer for A490M prevents thread galling during high-torque pretensioning. All washers must be hardened to 38-45 HRC per ASTM F436M.
Related Pages
- CSA Bolt Capacity — A325M & A490M Tables
- CSA Bolt Hole Sizes — Standard & Oversized Holes
- CSA Bolt Pretension — S16 Required Clamping
- Canadian Steel Properties — G40.21 Fy & Fu by Grade
- CSA S16 Code Overview
- Bolted Connection Calculator
- Bolt Torque Calculator
- Anchor Bolt Design Guide
This page is for educational reference. All bolting data per CSA S16-19, ASTM F3125M, and CSA W59. Verify fastener specifications against the current code edition and project requirements. Bolts must be installed per approved procedures and verified by qualified inspectors. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.