A490 bolt torque and pretension (RCSC 2020)

Minimum bolt pretension (RCSC Table 5.2)

70% of the tensile strength of Group 120 or 150 fasteners x tensile stress area, rounded to the nearest kip. Group 144 uses the Group 150 values.⁠¹

Pre-installation verification pretension (RCSC Table 7.1)

The tension a calibration device must show during daily wrench verification⁠¹.

No torque, a pretension

A490 is the 150 ksi structural bolt, now ASTM F3125 Grade A490 and RCSC Group 150 ⁠¹. RCSC gives it a minimum pretension (Table 5.2 above) and no torque. For the calibrated wrench method, “torque values determined from tables or from equations that claim to relate torque to pretension without verification shall not be used” (Section 8.2.2) ⁠¹. Torque tables were dropped from the Specification in 1954 because the torque-to-pretension ratio can vary by up to ±40% ⁠¹.

The pretension is 70% of the 150 ksi tensile strength times stress area ⁠¹. Because 150 ksi is 25% above A325’s 120 ksi, each A490 value is about 25% higher: 15 vs 12 kips at 1/2 in, 64 vs 51 kips at 1 in, 148 vs 118 kips at 1-1/2 in. ASTM F3148 Grade 144 assemblies use the same values as A490 ⁠¹.

The installation methods for A490

RCSC 2020 permits all five methods for Group 150 ⁠¹. What changes from A325:

• Calibrated wrench. Test at least three assemblies a day for each wrench in a bolt tension measurement device. The highest torque that reaches the Table 7.1 tension (37 kips at 3/4 in) becomes the minimum installation torque. Turn the nut, never the head, with an F436 washer under the nut ⁠¹.
• Turn-of-nut. Same rotations as A325 (Table 8.1). Mean pretension from turn-of-nut is about 1.26 times the A490 minimum, compared with 1.35 for A325 ⁠¹. In RCSC’s tests of 7/8 in bolts with 8 in of thread in the grip, A490 bolts lasted to about 1-1/4 turns past snug, against about 1-3/4 turns for A325 ⁠¹.
• Twist-off (F2280). The A490-strength tension-control assembly, made with a round or heavy hex head, 1/2 to 1-1/4 in ⁠¹ ⁠².
• DTI. Use ASTM F959 Type 490 indicators for Group 150 ⁠¹.
• Combined method. Default initial torque from RCSC Table 7.3 when the supplier gives none, e.g. 210–250 ft-lb (285–339 N·m) at 3/4 in, then 90° or 120° of rotation ⁠¹. See turn-of-nut method.

The commentary notes that nuts for plain A490 bolts are often supplied with lubricant to reduce tightening effort and increase bolt elongation during installation ⁠¹. That lubricant is one reason a torque measured on one lot does not carry over to another.

No galvanizing

RCSC 2020 Section 2.8.1: “Hot-dip or mechanical galvanizing of Group 150 heavy hex bolts or Group 150 spline-end matched bolting assemblies is not permitted” ⁠¹. The commentary explains that zinc applied to A490 by galvanizing, metallizing or mechanical coating is not allowed because its effect on embrittlement and delayed cracking has not been fully investigated ⁠¹.

The concern is hydrogen. Steels of 200 ksi and higher are prone to hydrogen embrittlement. A490 has a 173 ksi maximum tensile strength and 38 HRC maximum hardness to keep a margin, but production close to that limit has to be expected. Hot-dip galvanizing can bring in hydrogen during pickling and seal it in under the zinc ⁠¹.

The permitted alternative is zinc-aluminum coating: ASTM F1136 Grade 3, F2833 Grade 1 or F3019 Grade 4 on the bolt, with nuts overtapped per ASTM F3125 Table A1.2 ⁠¹. Testing to IFI-144 showed a Zn/Al inorganic coating on A490 bolts does not cause internal hydrogen embrittlement ⁠¹.

Rules that apply to A490 only

• No reuse. Group 150 heavy hex bolts shall not be reused. They have enough ductility for one pretensioned installation, not a consistent second one ⁠¹.
• Hardened nuts. Plain Type 1 A490 bolts take ASTM A563 Grade DH or DH3 nuts, and coated ones DH. Type 3 takes DH3. ASTM A194 2H may replace DH ⁠¹. RCSC gives a Grade C nut on an A490 bolt as an example of a mismatch that pre-installation testing should catch ⁠¹.
• Washers on soft steel. When A490 bolts are pretensioned in material with a specified minimum yield under 40 ksi, F436 washers go under both head and nut ⁠¹.
• Joint type. Group 144 and 150 bolts in tension or combined shear and tension must be in pretensioned joints; snug-tightened joints with tensile loads are not permitted for them ⁠¹.

A490 vs Grade 8

A490 and SAE Grade 8 share 150 ksi minimum tensile and 120 ksi proof ⁠³ ⁠⁴. A490 also carries a 173 ksi maximum and a 38 HRC hardness limit as a guard against hydrogen embrittlement ⁠¹. A Grade 8 chart torque from this site targets 75% of proof: about 30,150 lbf on a 3/4-10, while RCSC requires 35,000 lbf, about 16% more. Fastenal states that a Grade 8 cannot replace a specified A490 ⁠⁴. See structural bolt vs Grade 5 and 8.

Sources

1. 1
   Research Council on Structural Connections, RCSC Specification for Structural Joints Using High-Strength Bolts, June 11, 2020Checked 21 Oct 2026
2. 2
   ASTM International, ASTM F3125/F3125M-26 (Active), standard page with abstract and scopeChecked 21 Oct 2026
3. 3
   St. Louis Screw & Bolt (structural bolt manufacturer), St. Louis Screw & Bolt, Heavy Hex Structural Bolts page: Table 4, Tensile Strength Requirements for Bolts Tested Full Size (labeled 'Source: ASTM F3125-15')Checked 21 Oct 2026
4. 4
   Fastenal Company (copy hosted by University of Florida MAE Design Lab), Fastenal Technical Reference Guide, S7028 Rev. 9 (printed 9/13/2005), Fastenal Engineering DepartmentChecked 21 Oct 2026

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