ASTM F3125 Unified Bolt Standard — 2023 Edition
ASTM F3125:2023 supersedes the legacy individual bolt specifications A325, A325M, A490, A490M, F1852, and F2280. Under F3125, all high-strength structural bolts are classified into three groups based on strength, material, and application:
Group A — 120 ksi tensile strength (formerly A325):
- F3125 Grade A325 (heavy-hex structural bolt, 1/2 in. to 1-1/2 in. diameter)
- F3125 Grade F1852 (twist-off-type tension-control bolt, 1/2 in. to 1-1/8 in.)
Group B — 150 ksi tensile strength (formerly A490):
- F3125 Grade A490 (heavy-hex structural bolt, 1/2 in. to 1-1/2 in.)
- F3125 Grade F2280 (twist-off-type tension-control bolt, 1/2 in. to 1-1/8 in.)
Group C — 200 ksi tensile strength (new, F3043):
- F3125 Grade F3043 (heavy-hex, currently limited to 1/2 in. to 1 in.)
- No twist-off variant available for Group C as of the 2023 edition.
The consolidation under F3125 eliminates redundant specifications and standardizes requirements across all high-strength structural bolting products, reducing 6 legacy ASTM standards to a single specification.
Nominal Bolt Strengths — AISC 360-22 Table J3.2
The LRFD design strength of a bolt in shear or tension is phi times the nominal strength (Rn), with phi factors per AISC 360-22 Table J3.2.
Shear Strength (Bearing-Type Connections, Threads Not Excluded)
| Bolt Grade | Nominal Tensile Strength (ksi) | Nominal Shear Strength Fnv (ksi) |
|---|---|---|
| A325 / F1852 (Gr. A) | 120 | 54 (threads in shear plane) |
| A325 / F1852 (Gr. A) | 120 | 68 (threads excluded, X) |
| A490 / F2280 (Gr. B) | 150 | 68 (threads in shear plane) |
| A490 / F2280 (Gr. B) | 150 | 84 (threads excluded, X) |
| F3043 (Gr. C) | 200 | 90 (threads in shear plane) |
| F3043 (Gr. C) | 200 | 112 (threads excluded, X) |
The LRFD resistance factor for bolt shear is phi = 0.75. For a 3/4 in. diameter Group A bolt (A325-N, threads in shear plane, nominal area Ab = 0.442 in^2): phi Rn = 0.75 x 54 x 0.442 = 17.9 kips per bolt.
Tension Strength
| Bolt Grade | Nominal Tension Strength Fnt (ksi) |
|---|---|
| A325 / F1852 (Gr. A) | 90 |
| A490 / F2280 (Gr. B) | 113 |
| F3043 (Gr. C) | 150 |
LRFD resistance factor for bolt tension: phi = 0.75. For a 3/4 in. Group A bolt in direct tension: phi Rn = 0.75 x 90 x 0.442 = 29.8 kips.
Combined Shear and Tension
For bolts subjected to combined shear and tension, AISC 360-22 Equation J3-3a applies for bearing-type connections. The available tensile strength must be calculated per AISC 360-22 Section J3.7 using the reduction formula that accounts for the tension stress interaction index fv/Fnt.
Bolt Diameter Selection — Practical Guidance
Standard bolt diameters for US steel construction range from 1/2 in. to 1-1/2 in. in 1/8 in. increments, with 3/4 in. and 7/8 in. being the most common for beam-to-column connections. The choice of diameter balances shear capacity per bolt against the required number of bolts and the connection geometry.
- 1/2 in. to 5/8 in. — Light framing, purlins, girts, secondary members. Limited to 2-3 bolts in standard shear tabs.
- 3/4 in. — Workhorse size. Beam shear connections, simple shear tabs, single-plate connections. Most economical for connections carrying 20-60 kips.
- 7/8 in. to 1 in. — Heavy beam connections, moment connections, column splices where bolt count must be minimized. Capacity range: 60-120 kips per connection.
- 1-1/8 in. to 1-1/2 in. — Column base plates with large anchor rods, heavy truss connections, bridge bolting. Usually requires wider connection plates and larger edge distances.
Pre-Tensioned vs Snug-Tight Connections
Per RCSC Specification Section 4, bolts in connections subject to direct tension, fatigue loading, or where slip is a serviceability limit state must be fully pre-tensioned. Snug-tight installation is acceptable for bearing-type shear connections in standard building frames.
Pre-tension methods per RCSC Specification:
- Turn-of-nut method — The most common field method. Bolt is brought to snug-tight condition, then rotated a prescribed amount (1/3 to 1 turn depending on bolt length and diameter) to develop the minimum pre-tension.
- Calibrated wrench method — Torque wrench calibrated to deliver required pre-tension. Used where turn-of-nut is impractical (e.g., limited access).
- Twist-off-type tension-control bolts (F1852/F2280) — The splined end shears off at a calibrated torque, providing visual confirmation of pre-tension without torque measurement.
- Direct tension indicator (DTI) washers — Crushable protrusions on a hardened washer flatten at the specified pre-tension, providing a verifiable measure of bolt force independent of torque.
Grade Selection by Application
Standard building frames (bearing-type shear connections): A325 bolts, snug-tight, 3/4 in. or 7/8 in. diameter. The workhorse specification for 90% of US steel building connections per the AISC Manual Table 10-1.
Slip-critical connections (faying surface coatings): A325 bolts, fully pre-tensioned, Class A or B faying surface preparation. Required for connections with slotted holes loaded perpendicular to the slot, and connections where slip would impair serviceability.
Heavy moment frames and column splices: A490 bolts, fully pre-tensioned, 7/8 in. to 1 in. diameter. The Group B 150 ksi bolts provide 25% more shear capacity per bolt, reducing bolt count and connection length compared to Group A.
Seismic applications (AISC 341, SMF/IMF/SCBF): Pre-tensioned high-strength bolts with explicitly required faying surface preparation. Group A or B depending on demand. AISC 341-22 Section D2.2 specifies additional requirements for bolted connections in the seismic load-resisting system.
Bridge bolting (AASHTO LRFD): A325 bolts for primary members. A490 bolts permitted for secondary members and bracing, but not in the main load path of fracture-critical members due to lower ductility at the higher strength. F3043 (Group C) is under evaluation for bridge applications.
Bolt Length Selection
The bolt grip is the total thickness of connected plies plus the nut and washer stack. Nominal bolt length per AISC Manual Table 7-15 accounts for the nut height (one bolt diameter) plus allowance for thread run-out beyond the nut face. A minimum of one full thread must project beyond the nut per RCSC inspection requirements, but excessive projection wastes bolt length and complicates erection.
For a standard 3/4 in. A325 bolt connecting a 3/8 in. single plate to a 1/2 in. beam web (grip = 0.375 + 0.500 = 0.875 in.), the minimum bolt length is approximately 2 in. (0.875 in. grip + 0.75 in. nut + 0.25 in. washer + 0.125 in. projection). The standard length increment is 1/4 in. for bolts up to 6 in. long.
Frequently Asked Questions
What is the difference between A325 and A490 bolts per AISC 360?
A325 bolts (ASTM F3125 Group A) have a nominal tensile strength of 120 ksi, while A490 bolts (Group B) have 150 ksi. In shear with threads excluded, A325 provides Fnv = 68 ksi and A490 provides Fnv = 84 ksi. A490 bolts require specific faying surface preparation for slip-critical connections and are not permitted for galvanized faying surfaces unless tested. A490 bolts are also not permitted in connections subject to painting after assembly unless documented by the EOR.
When should I use fully pre-tensioned bolts vs snug-tight per RCSC?
Pre-tensioned bolts are required when: (1) the connection is subject to direct tension (hanger connections, column splices with uplift), (2) fatigue loading from cranes or machinery applies, (3) the connection is slip-critical for serviceability, or (4) AISC 341 (seismic) or AISC 358 (prequalified moment connections) explicitly requires it. Snug-tight installation is permitted for all other bearing-type connections in statically loaded building frames, which covers the majority of steel building connections in the US.
What is the unified ASTM F3125 standard and which bolt grades does it cover?
ASTM F3125:2023 consolidates the previous separate specifications A325, A325M, A490, A490M, F1852, and F2280 into a single standard. It covers Group A (120 ksi, A325 and F1852), Group B (150 ksi, A490 and F2280), and Group C (200 ksi, F3043). The unified standard simplifies procurement and eliminates inconsistencies between the legacy specifications. All references in AISC 360-22 and the AISC Steel Construction Manual recognise F3125 as the governing bolt specification.
How do I select the right bolt diameter for a beam shear connection?
Start with the required shear per bolt (Vu / number of bolts), then select a bolt diameter that provides adequate shear strength per AISC 360-22 Table J3.2 with phi = 0.75. For standard shear tabs, 3/4 in. A325-N bolts provide 17.9 kips per bolt, so a 4-bolt connection carries approximately 72 kips. If the beam reaction exceeds this, either increase the bolt diameter to 7/8 in. (29.8 kips per bolt with threads excluded) or increase the number of bolt rows. Bolt bearing and tear-out checks on the connected plies must also be satisfied.
Educational reference only. All design values are per AISC 360-22, RCSC Specification 2020, and ASTM F3125:2023. Verify bolt grades against the project specification and the latest adopted building code (IBC 2024 references AISC 360-22). Designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.