Steel Fasteners — Bolt Grades, Design Strengths, and Installation
Structural steel fasteners include high-strength bolts (A325/F3125 Grade A325, A490/F3125 Grade A490), common bolts (A307), and specialty fasteners (tension-control bolts, lockbolts, self-drilling screws). AISC 360-22 Chapter J and the RCSC Specification for Structural Joints govern bolt design and installation. The correct selection of fastener grade, installation method, and connection type directly affects connection capacity and structural performance.
Bolt grades and design strengths (AISC 360-22 Table J3.2)
| Grade | ASTM | Fnv (ksi) N/X | Fnt (ksi) | Fu (ksi) | Common Sizes |
|---|---|---|---|---|---|
| Group A (A325) | F3125 Gr. A325 | 54 / 68 | 90 | 120 | 3/4", 7/8", 1" |
| Group B (A490) | F3125 Gr. A490 | 68 / 84 | 113 | 150 | 3/4", 7/8", 1" |
| A307 Grade A | A307 | 27 | 45 | 60 | 1/2" - 1-1/2" |
N vs X: N = threads included in the shear plane (default assumption, lower capacity). X = threads excluded from the shear plane (higher capacity, but requires careful detailing to ensure threads are not in the shear plane).
Nominal bolt area
Ab = pi/4 * db^2
phiRn_shear = 0.75 * Fnv * Ab [per bolt, single shear]
| Diameter | Ab (in^2) | phiRn A325-N (kips) | phiRn A325-X (kips) | phiRn A490-N (kips) |
|---|---|---|---|---|
| 3/4" | 0.4418 | 17.9 | 22.5 | 22.5 |
| 7/8" | 0.6013 | 24.3 | 30.7 | 30.7 |
| 1" | 0.7854 | 31.8 | 40.1 | 40.1 |
| 1-1/8" | 0.9940 | 40.3 | 50.7 | 50.7 |
Installation methods
Snug-tight (AISC 360 Section J3.1)
Bolts tightened to the "snug-tight" condition -- full effort of a single worker with an ordinary spud wrench, or a few impacts of an impact wrench. This is the minimum installation requirement and is permitted for:
- Bearing-type connections in static loading
- Connections not subject to fatigue, vibration, or load reversal
- Non-slip-critical applications
Pretensioned (Section J3.1, Table J3.1)
Bolts tightened to a minimum tension equal to 70% of the bolt tensile strength. Required for:
- Slip-critical connections
- Connections subject to fatigue
- Column splices with tensile loads
- Connections to bracing members
| Diameter | Min. Pretension A325 (kips) | Min. Pretension A490 (kips) |
|---|---|---|
| 3/4" | 28 | 35 |
| 7/8" | 39 | 49 |
| 1" | 51 | 64 |
| 1-1/8" | 64 | 80 |
Pretensioning methods (RCSC Specification)
- Turn-of-nut: Snug-tight, then rotate the specified amount (1/3 to 1 full turn depending on bolt length and slope).
- Calibrated wrench: Torque wrench calibrated daily against a bolt tension indicator.
- Twist-off (tension-control) bolts: Splined tip shears off at the required tension. Most common method in modern practice.
- Direct tension indicator (DTI): Hardened washer with protrusions that flatten under the required tension.
Slip-critical connections (Section J3.8)
Slip-critical connections resist loads by friction between faying surfaces, not bolt bearing:
phiRn = mu * Du * hf * Tb * ns [per bolt]
phi = 1.00 (serviceability) or 0.85 (strength)
Where mu = slip coefficient (Class A = 0.30, Class B = 0.50), Du = 1.13, hf = filler factor, Tb = bolt pretension, ns = number of slip planes.
Class A surfaces: Clean mill scale, or blast-cleaned with Class A coating. Class B surfaces: Blast-cleaned bare steel, or blast-cleaned with Class B coating. Class B provides 67% higher slip resistance than Class A.
Combined shear and tension (Section J3.7)
When bolts carry both shear and tension simultaneously:
F'nt = 1.3*Fnt - (Fnt/(phi*Fnv))*frv <= Fnt
Where frv = required shear stress per bolt. This equation linearly reduces the available tensile strength as shear demand increases.
Practical tip: bolt selection for economy
For most building connections, 3/4" A325-N in snug-tight bearing-type connections provides the best economy. Moving to A490 is justified when: (1) connection geometry is limited and more capacity per bolt is needed, (2) reducing the bolt count improves erection speed enough to offset the material premium, or (3) the specification requires slip-critical connections and A490 pretension provides more friction force.
Avoid mixing A325 and A490 on the same project -- it creates installation confusion and the risk of undertensioning A490 bolts (or over-tightening and breaking A325 bolts by using the wrong calibration).
Common mistakes
- Assuming threads excluded (X) without verifying. The default is threads included (N). Threads-excluded capacity requires controlling thread length and stick-through, which is often impractical.
- Using A490 bolts in galvanized connections. A490 bolts must not be galvanized due to hydrogen embrittlement risk. Use A325 (galvanizable) or F3125 Grade A354 Grade BD (galvanizable alternative).
- Not checking bearing and tearout. Bolt shear capacity is an upper bound. If the connected plate is thin, bearing or tearout at bolt holes may govern at a lower load.
- Specifying slip-critical when not required. Slip-critical connections cost more due to surface preparation and pretensioning requirements. Use bearing-type connections for static loading when code permits.
- Forgetting the 0.75 resistance factor. Bolt design uses phi = 0.75, not 0.90. This is frequently confused with member design resistance factors.
Run this calculation
Related references
- Bolt Grades Reference
- Bolt Capacity Table
- Bolt Spacing Requirements
- Bolt Bearing and Tearout
- How to Verify Calculations
Disclaimer
This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against AISC 360-22 Chapter J and the RCSC Specification. The site operator disclaims liability for any loss arising from the use of this information.