Structural bolting under the RCSC Specification
High-strength structural bolts (ASTM F3125 Grade A325 and A490) are installed to a minimum pretension, not to a torque from a chart. The 2020 RCSC Specification sets that pretension at 70% of the bolt's tensile strength and lists five installation methods, each checked on the job before use.
Contents6 sections
Key takeaways
- The Engineer of Record picks the joint type: snug-tightened, pretensioned or slip-critical 1.
- Pretensioned and slip-critical joints need a minimum pretension, e.g. 28 kips for a 3/4 in A325 and 35 kips for a 3/4 in A490 1.
- RCSC 2020 lists five pretensioning methods: turn-of-nut, calibrated wrench, twist-off bolt, direct tension indicator and the new combined method 1.
- For calibrated-wrench installation, torque from tables or equations without verification shall not be used (Section 8.2.2) 1.
Three joint types
The 2020 RCSC Specification sorts bolted joints into three types, and the Engineer of Record must name the type in the contract documents 1.
- Snug-tightened. The plies are in firm contact and each bolt is as tight as a few impacts of an impact wrench or the full effort of an ironworker on an ordinary spud wrench makes it. No pretension is specified 1.
- Pretensioned. Each bolt gets at least the minimum pretension in RCSC Table 5.2. Required for load reversal, fatigue, and A490 bolts in tension 1.
- Slip-critical. Pretensioned as well, and the faying surfaces are prepared so friction resists slip. Required for oversized holes, most slotted holes and reversed fatigue loads 1.
See snug-tight vs pretensioned for the full list.
Five ways to reach pretension
RCSC 2020 gives five methods “without preference” 1:
Turn-of-nut
Snug the joint, then turn the nut 1/3 to 1 turn depending on bolt length and face slope. See turn-of-nut method.
Calibrated wrench
The installation torque is measured on the job in a bolt tension device, daily and for each wrench.
Twist-off tension-control bolt
ASTM F3125 Grade F1852 or F2280 assemblies; the wrench shears off the spline.
Direct tension indicator (DTI)
ASTM F959 washers whose protrusions close to a feeler-gage gap.
Combined method
New in 2020: an initial torque followed by a set rotation.
Each method is first checked on three sample assemblies in a bolt tension measurement device, which must show 1.05 times the Table 5.2 pretension (Table 7.1) 1.
Why there is no torque chart
RCSC printed a torque-to-pretension table in its first edition (1951). Research then showed the torque-to-pretension ratio can vary by as much as ±40% unless it is found separately for each lot, diameter and bolt condition, so the 1954 edition withdrew it 1. Lubrication, thread finish, rust, washer friction and air supply all shift the result. That is why the A325 and A490 pages here list pretension in kips, not ft-lb.
In this section
- A325A325 bolts have no RCSC torque value. Minimum pretension is 28 kips for 3/4 in and 51 kips for 1 in; the torque is found daily on the job for each wrench.
- 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.
- 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.
- 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.
- Joint typesRCSC 2020 joint types: when a structural bolted joint may be snug-tightened and when it must be pretensioned or slip-critical, with install and inspection rules.
- 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.
Sources
- 1
Spotted a value that doesn’t match its source? Report an error
Related
- Grade 8Grade 8 bolt torque from 1/4 in to 1 in, dry, zinc-plated and lubricated, with SAE J429 proof, yield and tensile strength and the six-line head marking.
- Socket size chartSocket sizes for inch hex cap screws, heavy hex structural bolts and metric bolts, including M10, M12, M14 and M22, where ISO and DIN heads differ.
- Bolt torque calculatorBolt torque calculator for SAE and metric bolts by size, thread, grade and lubrication, using T = K × D × F with the proof loads from SAE J429 and ISO 898-1.