| Tensile strength, Fu | 120 ksi / 830 MPa | 150 ksi / 1,040 MPa | 800 MPa (116 ksi) | 1,000 MPa (145 ksi) | | Yield strength, Fy | 92 ksi / 635 MPa | 130 ksi / 900 MPa | 640 MPa (93 ksi) | 900 MPa (130 ksi) | | Fy/Fu ratio | 0.77 | 0.87 | 0.80 | 0.90 | | Elongation (min) | 14% | 14% | 12% | 9% | | Hardness (HRC) | 25–34 | 33–38 | 22–32 | 32–39 | | Material | Medium-carbon steel, quenched & tempered | Alloy steel, quenched & tempered | Medium-carbon steel, quenched & tempered | Alloy steel, quenched & tempered | | Typical bolt sizes | 1/2 in–1-1/2 in (M12–M36) | 1/2 in–1-1/2 in (M12–M36) | M12–M36 | M12–M36 | | Galvanising permitted | Yes (mechanical or hot-dip) | No (hydrogen embrittlement risk) | Yes (hot-dip with precautions) | Limited (hydrogen embrittlement risk) | | Governing standard | ASTM F3125 / AISC 360 J3 | ASTM F3125 / AISC 360 J3 | EN ISO 898-1 / EN 1993-1-8 | EN ISO 898-1 / EN 1993-1-8 |

The key differences: A490 provides 25% higher strength than A325 but cannot be galvanised. Grade 10.9 provides 25% higher strength than Grade 8.8 but with reduced ductility (9% vs 12% elongation). The Fy/Fu ratio increases with bolt strength — from 0.77 for A325 to 0.90 for Grade 10.9 — reflecting the reduced strain-hardening capacity of higher-strength bolt materials.

Bolt Shear Capacity Comparison

AISC 360 Table J3.2 — Nominal Shear Strength Fnv (ksi/MPa):

Design shear strength per bolt (single shear, phi = 0.75): phi rn = phi x Fnv x Ab

EN 1993-1-8 Table 3.4 — Shear Resistance per Bolt:

The European capacity values are higher because EN 1993-1-8 uses alpha_v = 0.6 (threads in) or 0.5 (threads out, but the gross area A is larger) and gamma_M2 = 1.25, while AISC 360 uses phi = 0.75 and Fnv from Table J3.2. The two codes produce different design capacities for the same bolt — always use the capacity per the governing project code.

Bolt Tension Capacity Comparison

AISC 360 Table J3.2 — Nominal Tensile Strength Fnt (ksi/MPa):

Tension capacity phi rn = phi x Fnt x Ab where phi = 0.75 and Ab = nominal body area. The tension capacity is based on the tensile stress area At for threaded bolts per AISC 360 Table J3.2.

EN 1993-1-8 Table 3.4 — Tension Resistance:

Ft,Rd = k2 x f_ub x As / gamma_M2 where k2 = 0.9 and gamma_M2 = 1.25.

Bolt Grade Selection by Application

Selection Workflow — Step by Step

Step 1: Determine the required bolt shear capacity. From the connection analysis, determine the factored shear force per bolt Vu_bolt. For a bolt group with an eccentric load, use the instantaneous centre of rotation method (AISC 360 Part 7) or the elastic method (conservative) to determine the force distribution.

Step 2: Select bolt grade and diameter. Start with A325 (or Grade 8.8 in metric regions). Select the smallest standard diameter that satisfies phi rn >= Vu_bolt. If the required diameter exceeds 1 in (M24), consider: (a) increasing the number of bolts, (b) upgrading to A490/Grade 10.9 to keep the same diameter, or (c) redesigning the connection geometry. A 1-1/2 in (M36) A490 bolt is the largest standard structural bolt — if capacity still insufficient, the connection must be redesigned.

Step 3: Check bearing and tearout at the connected parts. Bolt shear is rarely the governing limit state. Plate bearing (phi rn = phi x 2.4 x d x t x Fu) and tearout (phi rn = phi x 1.2 x Lc x t x Fu) often govern, particularly for thin plates and short edge distances. Always check these limit states — a bolt can have adequate shear capacity but the plate may fail in bearing or tearout at a lower load.

Step 4: Verify bolt spacing and edge distance. AISC 360 Table J3.3 specifies minimum bolt spacing (2-2/3 d, centre-to-centre) and minimum edge distance (from Table J3.4, varying from 3/4 in for 1/2 in bolts to 2 in for 1-1/2 in bolts). The preferred spacing is 3d for wrench clearance — specify this unless geometry forces a tighter layout.

Step 5: Determine if pretension is required. Per RCSC Section 4.1, pretensioned bolts are required for: (a) slip-critical connections, (b) connections subject to direct tension (hanger rods, column splices subject to uplift), (c) connections where bolt loosening under vibration or load reversal is a concern, and (d) all connections in special and intermediate moment frames per AISC 341. If none of these conditions apply, snug-tight bolts are permitted and more economical — pretensioning adds cost (calibrated wrench or DTI washers, inspection verification).

Bolt Grade Cost Comparison

The cost of bolts as a percentage of total connection cost is small (5–10%). The dominant costs are shop fabrication (cutting, drilling, welding) and erection labour (crane time, ironworker hours). Specifying a higher bolt grade to reduce the number of bolts — for example, using A490 in place of A325 so four bolts can do the work of five — can reduce connection size, plate material, and erection time, often offsetting the higher bolt cost many times over.

FAQ

Why can't A490 bolts be galvanised?

A490 bolts (and Grade 10.9 bolts) are manufactured from alloy steel heat-treated to high hardness levels (HRC 33–38). During the hot-dip galvanising process (immersion in molten zinc at approximately 450 deg C), hydrogen from the pickling acid can diffuse into the high-strength steel and cause hydrogen embrittlement — a sudden, brittle fracture under sustained tensile stress. ASTM F3125 explicitly prohibits hot-dip galvanising of A490 bolts. For exterior connections requiring corrosion protection, specify A325 Type 1 bolts with mechanical galvanising (zinc plating per ASTM B695) or weathering steel bolts (ASTM A325 Type 3, for use with unpainted weathering steel structures).

What bolt tightening method should I specify?

Per RCSC Section 8.2, four methods are permitted: (1) turn-of-nut — snug-tight plus an additional 1/3 to 1/2 turn (most common, no special equipment); (2) calibrated wrench — torque tightened with a wrench calibrated daily (requires Skidmore-Wilhelm calibrator); (3) direct tension indicator (DTI) washers — protrusions flatten at specified tension, checked with feeler gauge; and (4) twist-off tension-control (TC) bolts — splined end shears at specified tension. TC bolts are increasingly popular because the sheared spline provides a definitive visual indicator of proper installation. For snug-tight connections (the majority of bearing-type connections), specify snug-tight per RCSC Section 8.1: the full effort of an ironworker with an ordinary spud wrench, bringing the plies into firm contact.

When should I specify slip-critical bolts instead of bearing-type?

Specify slip-critical (SC) bolts when: (1) the connection is subject to stress reversal or fatigue loading (bridges, crane girders, vibrating machinery), (2) oversized or slotted holes are used and slip into bearing would change the structural response, (3) the connection bolts are in combined shear and tension and joint slip at service loads is unacceptable, or (4) the faying surface condition can achieve the required slip coefficient (Class A: 0.30, clean mill scale; Class B: 0.50, blast-cleaned). Slip-critical connections add cost — faying surface preparation, pretension verification, and inspection — and should only be specified when the governing condition requires it.

How do anchor rods differ from structural bolts?

Anchor rods (ASTM F1554) are fundamentally different from structural bolts (ASTM F3125). Anchor rods are cast into concrete foundations and transfer column base reactions through bond, bearing, and tension. They are specified in lower-strength, more ductile grades (F1554 Grade 36, Fy = 36 ksi; Grade 55, Fy = 55 ksi; Grade 105, Fy = 105 ksi) because anchor behaviour benefits from ductility — a ductile anchor rod can yield and redistribute load under seismic uplift, while a brittle high-strength rod would fracture. Anchor rod design follows ACI 318 Chapter 17 (anchoring to concrete) rather than AISC 360 Chapter J (bolted connections). The bolt grade selection logic for structural connections does not apply to anchor rods.

Related pages: Bolt Grades Chart | Bolted Connection Calculator | Bolt Torque Calculator | Bolt Hole Sizes Reference | Bolt Design Code Comparison