A325 bolt torque and pretension (RCSC 2020)
The RCSC Specification gives no torque for A325 bolts. It gives a minimum pretension (28 kips, 125 kN, for a 3/4 in bolt), and for the calibrated wrench method the torque that produces it is measured on site, daily and for each wrench.
Contents9 sections
Key takeaways
- No torque value: RCSC 2020 says torque values from tables or equations without verification shall not be used (Section 8.2.2) 1.
- Minimum pretension is 70% of the 120 ksi tensile strength times stress area: 12 kips at 1/2 in up to 118 kips at 1-1/2 in 1.
- Calibrated wrench torque = the highest torque from three test assemblies reaching the Table 7.1 tension, set daily for each wrench 1.
- Turn-of-nut, twist-off (F1852), DTI and the combined method avoid relying on a torque reading alone 1.
Quick facts
| Standard | ASTM F3125 Grade A325 (RCSC Group 120) |
|---|---|
| Min. tensile strength | 120 ksi, all sizes 1/2 to 1-1/2 in |
| 3/4 in pretension | 28 kips (125 kN); verify at 29 kips |
| Matching twist-off bolt | ASTM F3125 Grade F1852 |
Minimum bolt pretension (RCSC Table 5.2)
| Diameter | A325 / F1852, kips | A490 / F2280, kips |
|---|---|---|
| 1/2 in | 12 | 15 |
| 5/8 in | 19 | 24 |
| 3/4 in | 28 | 35 |
| 7/8 in | 39 | 49 |
| 1 in | 51 | 64 |
| 1-1/8 in | 64 | 80 |
| 1-1/4 in | 81 | 102 |
| 1-3/8 in | 97 | 121 |
| 1-1/2 in | 118 | 148 |
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)
| Diameter | Group 120, kips | Group 144/150, kips |
|---|---|---|
| 1/2 in | 13 | 16 |
| 5/8 in | 20 | 25 |
| 3/4 in | 29 | 37 |
| 7/8 in | 41 | 51 |
| 1 in | 54 | 67 |
| 1-1/8 in | 67 | 84 |
| 1-1/4 in | 85 | 107 |
| 1-3/8 in | 102 | 127 |
| 1-1/2 in | 124 | 155 |
The tension a calibration device must show during daily wrench verification1.
Why there is no A325 torque value
Search for “A325 bolt torque” and you will find charts in ft-lb. The RCSC Specification, which governs A325 installation in buildings, does not publish one and forbids using one. Section 8.2.2 says: “Torque values determined from tables or from equations that claim to relate torque to pretension without verification shall not be used” 1.
The reason is history. The first RCSC edition (1951) had a torque-to-pretension table. Research soon showed that the ratio between torque and pretension can vary by as much as ±40% unless it is found for each bolt lot, diameter and condition, so the table was withdrawn in 1954 1. The variables RCSC lists include thread finish and tolerance, lubrication, dirt or rust, friction under the turned nut, air supply to impact wrenches and the repeatability of the wrench itself 1.
What A325 bolts do have is a required pretension, the tension in the bolt after installation. The table above (RCSC Table 5.2) lists it by diameter. It equals 70% of the 120 ksi minimum tensile strength times the tensile stress area, rounded to the nearest kip 1. RCSC 2020 also revised the values for Group 120 bolts over 1 in 1, because ASTM F3125 now requires 120 ksi for A325 at every size up to 1-1/2 in 2. The old A325 dropped to 105 ksi above 1 in 3.
Calibrated wrench: how the torque is found
The calibrated wrench method does use torque, but only a torque measured on the bolts being installed (Sections 7 and 8.2.2) 1:
Test at least three assemblies
Use each combination of diameter, length, grade and lot at the job site, with an ASTM F436 washer under the nut 1.
Snug, then tighten in a bolt tension measurement device
Use the same wrench, components and method as the work. Find the torque at which each bolt reaches the Table 7.1 pretension, which is 1.05 times Table 5.2 (29 kips for 3/4 in) 1.
Use the highest of the three
The highest torque measured is the minimum installation torque 1.
Turn the nut only
Turning the bolt head is prohibited with this method, because friction between the shank and the steel would make the calibration meaningless 1.
Repeat daily
Testing is done daily before installation for each installation wrench. Per the commentary, recalibrate also when any component lot changes, after relubrication, when thread or washer surface condition changes noticeably, and when wrench lubrication, hose or air supply change 1.
Pneumatic impact wrenches must have enough air to pretension each bolt in about 10 seconds up to 1-1/4 in diameter, about 15 seconds above that 1. A higher torque than the minimum installation torque is not cause for rejection 1.
The other four methods
- Turn-of-nut. Snug the joint, match-mark, then turn the nut 1/3 to 1 turn by bolt length and face slope (Table 8.1, tolerance +60°/−0°). Mean pretension with this method is about 1.35 times the A325 minimum 1. See turn-of-nut method.
- Twist-off (F1852). The A325-strength tension-control bolt. After snugging, check the spline is still on; the installation wrench then shears it off. Only the manufacturer may relubricate these assemblies 1.
- Direct tension indicator. ASTM F959 Type 325 DTIs for Group 120. The bolt is done when a feeler gage is refused in more than half of the spaces between the protrusions 1.
- Combined method. An initial torque (supplier value, or RCSC Table 7.3 if none is given) then 90° or 120° of rotation 1.
A325 vs a Grade 5 torque chart
A325 and SAE Grade 5 share 120 ksi tensile, 85 ksi proof and 92 ksi yield up to 1 in 2 3, so a Grade 5 chart looks like a shortcut. It aims at a different tension. Torque charts on this site, like Fastenal’s, target 75% of proof load 3; for a 3/4-10 bolt that is about 21,360 lbf. RCSC requires 28,000 lbf for a 3/4 in A325, about 31% more, and the same gap of 31–33% holds from 1/2 in to 1 in. A Grade 5 chart value for a 3/4-10, 267 ft-lb dry, is set for less tension than RCSC requires. Fastenal also states that a Grade 5 bolt must not be used where an A325 is specified 3. See structural bolt vs Grade 5 and 8.
Coatings and reuse
A325 bolts may be hot-dip galvanized (ASTM F2329) or mechanically galvanized (ASTM B695 Class 55), with all threaded parts coated by the same process 1. Galvanized bolts and lubricated, overtapped nuts must come from one supplier as a tested assembly. Galvanizing raises thread friction and makes torque-induced pretension less consistent 1. Plain A325 bolts may be reused in pretensioned joints only with Engineer of Record approval; galvanized ones may not be reused 1. The commentary’s rule of thumb: a plain A325 is fit for reuse if the nut runs the full thread by hand 1.
Sources
- 1
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- 3
Spotted a value that doesn’t match its source? Report an error
Related
- A490A490 bolts have no RCSC torque value. Minimum pretension is 35 kips for 3/4 in and 64 kips for 1 in; RCSC bans galvanizing and reuse of A490 heavy hex bolts.
- Turn-of-nutTurn-of-nut per RCSC 2020 Table 8.1: 1/3 to 1 turn past snug by bolt length, tolerance +60°/−0°. Match-marking, verification and the new combined method.
- ASTM F3125ASTM F3125 replaced A325, A325M, A490, A490M, F1852 and F2280. Grades, 120 and 150 ksi strengths, sizes, types and the head markings RCSC requires.
- vs Grade 5 / 8A325 matches Grade 5 strength only up to 1 in, and A490 matches Grade 8 minimums. Heavy hex heads, shorter threads and RCSC rules keep them out of each other's joints.
- Grade 5Grade 5 bolt torque from 1/4 in to 1 in, dry, zinc-plated and lubricated, with SAE J429 strength, the three-line head mark and the step down to 74 ksi above 1 in.