--------------------- | -------------- | ------------- | ----------------- | | As-received (dry, steel) | 0.18 - 0.22 | 0.20 | RCSC | | Zinc-coated (hot-dip) | 0.20 - 0.25 | 0.22 | RCSC | | Lubricated (oil or wax) | 0.12 - 0.18 | 0.15 | RCSC | | Galvanized | 0.22 - 0.28 | 0.25 | RCSC | | Cadmium-plated | 0.12 - 0.16 | 0.14 | Historical | | PTFE-coated | 0.08 - 0.12 | 0.10 | Manufacturer data | | Weathered (rusty) | 0.25 - 0.35 | 0.30 | Field estimate |

Critical note: The K-factor for the same bolt can vary by 20-30% between lots and even between bolts in the same lot. This is why calibrated wrench installation requires verification with a Skidmore-Wilhelm device and why torque-based preload is inherently less reliable than direct tension indicators or turn-of-nut methods.

Sensitivity of preload to K-factor

Because F = T / (K × D), the preload is inversely proportional to K. A 10% error in K produces a 10% error in the estimated preload:

If K is assumed = 0.20 but actual K = 0.22:
  Actual preload = 0.20/0.22 × assumed preload = 0.909 × assumed
  → 9% less clamp force than expected

If K is assumed = 0.20 but actual K = 0.18:
  Actual preload = 0.20/0.18 × assumed preload = 1.111 × assumed
  → 11% more clamp force (risk of over-torquing)

This ±10% scatter is inherent to torque-based installation and is the primary reason AISC requires pre-installation verification for calibrated wrench methods (RCSC Section 8.2.2).

AISC Minimum Bolt Pretension — Reference Table

AISC 360-22 Table J3.1 specifies the minimum bolt pretension for A325 and A490 bolts. These values represent approximately 70% of the specified minimum tensile strength of the bolt.

Note: Values are from AISC 360-22 Table J3.1 (Metric equivalents in the AISC Manual). The pretension value is the same regardless of whether the bolt is used in a slip-critical or pretensioned bearing connection. Snug-tightened connections do not have a specified minimum pretension.

Required torque from pretension

Using T = K × D × F, the torque required to achieve the AISC minimum pretension:

Example: 7/8” A325 bolt, K = 0.20 (as-received)
  T = 0.20 × 0.875 in × 39,000 lbf
  T = 6,825 lb-in
  T = 568.75 lb-ft

Table of required torques (K = 0.20, as-received condition):

These values are for initial estimation only. Field installation must use a calibrated method verified with a Skidmore-Wilhelm device.

Worked Example — Torque Required for Slip-Critical Connection

Problem: A slip-critical connection uses 1-inch A325 bolts. The joint requires a minimum pretension of 51 kips per AISC Table J3.1. The bolts are as-received (no lubrication, no coating). Determine the installation torque using the calibrated wrench method.

Step 1 — Identify parameters

Bolt: ASTM F3125 Grade A325, 1 inch diameter
Required pretension: F = 51 kips = 51,000 lbf (AISC Table J3.1)
Nut factor: K = 0.20 (as-received, dry steel per RCSC)
Bolt diameter: D = 1.0 in

Step 2 — Calculate required torque

T = K × D × F
T = 0.20 × 1.0 × 51,000
T = 10,200 lb-in
T = 850 lb-ft

Step 3 — Verify with Skidmore-Wilhelm

Per RCSC Section 8.2.2, the calibrated wrench method requires testing at least three bolt assemblies from the installation lot in a Skidmore-Wilhelm device. The wrench is set to the torque calculated above (850 lb-ft) and the resulting tension is measured.

Acceptable range: 51 kips (minimum) to bolt proof load. For a 1-inch A325 bolt, the proof load is approximately 0.70 × Fu × As = 0.70 × 120 × 0.606 = 50.9 kips. This is very close to the required pretension, confirming that the 70% Fu assumption in AISC is consistent.

Step 4 — Document the verification

Record in the installation report: bolt lot number, diameter, grade, K-factor used, calculated torque, Skidmore-Wilhelm readings (minimum 3), average tension achieved, wrench calibration date, and inspector name.

Installation Methods Comparison

AISC 360-22 and the RCSC Specification recognize four pre-installation verification methods and four installation methods for pretensioned and slip-critical joints:

The calibrated wrench method is the most common torque-based approach but has the highest scatter because it depends on the K-factor, which varies with surface condition, lubrication, temperature, and reuse.

Bolt torque and preload — summary of key relationships

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Worked Example

Problem: Determine the required torque for a 3/4-inch diameter A325 structural bolt in a slip-critical connection with standard holes.

Given:

• Bolt: 3/4 in. dia. A325 (ASTM F3125 Grade A325)
• Connection type: Slip-critical, Class A faying surface (blast-cleaned, no coating)
• Hole type: Standard (dh = 13/16 in.)
• One 3/4 in. A325 bolt in direct tension (no reduced area per RCSC)

Solution:

Step 1 -- Minimum bolt pretension per AISC 360-22 Table J3.1M:

Tb = 28 kips (for 3/4 in. dia. A325 bolt)

Step 2 -- Slip resistance per AISC 360-22 Section J3.8:

Rn = mu * Du * hf * Tb * ns
   = 0.30 * 1.13 * 1.0 * 28 * 1
   = 9.5 kips per bolt

where mu = 0.30 (Class A surface), Du = 1.13 (multiplier reflecting mean slip coefficient to bolt pretension ratio), hf = 1.0 (no fillers), ns = 1 (single shear plane).

Step 3 -- Torque from modified Short Formula:

T = K * D * P
  = 0.20 * 0.75 * 28
  = 4.2 kip-in = 350 ft-lb

The 0.20 nut factor (K) assumes clean, lightly lubricated threads. Actual K values range from 0.11 to 0.25 depending on lubrication condition and thread condition. The installer must verify pretension by calibrated wrench, turn-of-nut method, or direct tension indicator per RCSC Specification.

Result: Apply 350 ft-lb torque to achieve approximately 28 kips pretension. Confirm with calibrated wrench verification.

Frequently Asked Questions

What is the K-factor (nut factor) in the bolt torque-tension equation? The K-factor, also called the nut factor or torque coefficient, is a dimensionless constant in the equation T = K × D × F where T is the applied torque, D is the bolt nominal diameter, and F is the desired preload (clamp force). K accounts for friction at the bolt thread flanks, friction under the nut face, and thread geometry. For standard as-received steel bolts without lubrication, K is typically 0.20; for lubricated or zinc-coated bolts K may range from 0.12 to 0.18. Using the wrong K value introduces the largest single source of error in torque-based preload estimation.

What pretension does AISC require for A325 and A490 bolts? AISC 360 Table J3.1 specifies minimum bolt pretension for fully tensioned joints: for A325 (ASTM F3125 Grade A325) bolts the required pretension ranges from 12 kips for 5/8" diameter up to 51 kips for 1-1/8" diameter; for A490 bolts the values are approximately 25% higher, ranging from 15 kips to 64 kips over the same diameter range. These pretension values represent approximately 70% of the bolt’s minimum tensile strength and are the target clamp force when calculating required installation torque.

What is the difference between snug-tight and fully pretensioned bolt installation? Snug-tight means the bolt is tightened until the full plies are in firm contact with the joint — typically defined as the effort of an ironworker using a spud wrench or a few impacts of an impact wrench. Snug-tight connections rely only on shear capacity of the bolt shank (bearing-type) and are not suitable where slip under service loads is a concern or where the joint is subject to dynamic or fatigue loading. Fully pretensioned connections require a calibrated method (turn-of-nut, twist-off bolt, direct tension indicator, or calibrated wrench) to reach the AISC minimum pretension values.

How do you verify bolt pretension using a calibrated torque wrench? A calibrated torque wrench method requires pre-job testing using a Skidmore-Wilhelm bolt tension calibrator or equivalent device. At least three representative bolt assemblies from the lot must be tested to establish the torque that achieves the required pretension for that specific bolt diameter, grade, and surface condition. The wrench is then set to that verified torque and used in the field. This process accounts for batch-to-batch variation in K-factor and must be repeated if the bolt lot, lubricant, or nut brand changes.

Why is over-torquing a bolt harmful? Over-torquing a high-strength bolt can stretch the shank beyond its proof load into the inelastic range, permanently reducing the clamp force after the torque is released and risking thread stripping or bolt fracture during installation. For A325 bolts the transition from elastic to plastic behavior begins at roughly 85–90% of ultimate tensile strength; applying excessive torque pushes the bolt past this threshold. Over-torquing also damages the thread flanks of the nut, which further reduces the effective K-factor and makes subsequent torque readings unreliable.

How does temperature affect bolt torque readings? Cold temperatures increase the viscosity of any lubricant or coating on the bolt threads, which raises the effective friction coefficient and therefore increases the K-factor. A bolt installed at 0°C may require 10–20% more torque than the same bolt installed at 20°C to achieve the same preload. Conversely, hot weather can reduce viscosity and lower K, leading to under-torquing if the target torque was calibrated at a different temperature. Field calibration tests should be performed at temperatures representative of actual installation conditions.

Related pages

• Bolted connections calculator
• Splice connection calculator
• Gusset plate calculator
• Steel grades reference
• Unit converter
• Anchor bolts reference
• Tools directory
• Reference tables directory
• Guides and checklists
• How to verify calculator results
• Disclaimer (educational use only)
• standard bolt hole sizes reference
• bolt bearing and tearout capacity
• Bolt Grades

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