Bolt Torque Worked Example — A325 Bolt per AISC and EN 1993-1-8

Problem: Determine the required tightening torque for a 7/8-inch-diameter A325 bolt in a slip-critical connection. The bolt is to be installed using the turn-of-nut method. Compute the target preload per AISC 360-22 J3.1, select the appropriate nut factor (torque coefficient), and determine the torque required for installation. Also provide the equivalent EN 1993-1-8 preload for comparison.

Step 1: Bolt Material and Geometry

• Bolt: ASTM A325, 7/8 in diameter
• Nominal diameter (d): 0.875 in
• Tensile stress area (A_b): 0.462 in² (from AISC Manual Table 7-1)
• Minimum tensile strength (F_u): 120 ksi (A325, per ASTM)
• Yield strength (F_y): 92 ksi (typical for A325)
• Connection type: Slip-critical, standard holes, Class A faying surface

Step 2: Required Preload per AISC 360-22 J3.1

AISC 360 Equation J3-1 defines the required bolt pretension for slip-critical connections:

P_req = 0.70 × F_u × A_b (for A325 bolts)

P_req = 0.70 × 120 ksi × 0.462 in² P_req = 38.8 kips

For comparison, the alternative provision J3-1b uses:

P_req = 0.85 × F_y × A_b (for A325 when F_y is well-defined) P_req = 0.85 × 92 × 0.462 = 36.1 kips

The larger value (38.8 kips) governs for design.

Result: Target preload P = 38.8 kips

Step 3: Torque Calculation — T = K × D × P

The standard torque-preload relationship is:

T = K × D × P

Where:

• T = target torque (kip·in or ft·lb)
• K = nut factor (torque coefficient, dimensionless)
• D = nominal bolt diameter (in)
• P = bolt preload (kips)

K Factor Selection

For this example, assume as-milled clean steel surfaces (Class A): K = 0.22

Compute Torque

T = 0.22 × 0.875 in × 38.8 kips T = 7.47 kip·in

Convert to more practical units:

T = 7.47 kip·in × (1000 lb/kip) = 7,470 lb·in T = 7,470 / 12 = 623 ft·lb

Result: Target installation torque = 623 ft·lb (approximately 620 ft·lb for field use)

Step 4: Verification by Turn-of-Nut Method

Per AISC 360 J3.2, the turn-of-nut method provides an alternative to torque-based installation:

For a 7/8 in bolt, 4D = 3.5 in. If the grip length is 4.5 in (common for a plate-to-plate connection), then L > 4D, requiring 1/2 turn from the snug-tight condition.

The snug-tight condition is defined as the tightness achieved by a few impacts of an impact wrench or the full effort of a worker using a spud wrench — approximately 10-20% of the target preload.

Step 5: EN 1993-1-8 Equivalent Preload

For European design, EN 1993-1-8 Section 3.6 defines the preload for preloaded bolts (category E, F, or G connections) as:

F_p,C = 0.70 × f_ub × A_s

For a comparable M22 (22 mm diameter, closest metric equivalent to 7/8 in):

• f_ub = 800 MPa (grade 8.8, roughly equivalent to A325)
• A_s = 303 mm² (tensile stress area for M22)

F_p,C = 0.70 × 800 × 303 / 1000 = 169.7 kN

For conversion: 38.8 kips ≈ 172.6 kN. The AISC and EN 1993-1-8 preload values align within approximately 2%, confirming consistency between the two standards.

Step 6: Practical Considerations

Torque Wrench Calibration

Torque wrenches should be calibrated within ±5% accuracy. For our target of 623 ft·lb:

• Acceptable range: 592 - 654 ft·lb
• If using a hydraulic torque wrench, adjust for hose length and pump pressure

Lubrication Effects

The K factor is highly sensitive to lubrication:

• Dry (as-received): K ≈ 0.30 — increases torque by 36% for same preload
• Light oil: K ≈ 0.22 (used in this example)
• Moly paste: K ≈ 0.12 — reduces torque by 45%

Important: Always verify K factor by calibration test on actual joint components. Laboratory tests show K can vary by ±30% even within the same production lot.

Solvent Effects on Lubricated Bolts

If bolts are washed with solvent (galvanized bolts often are), lubricant is removed and K increases to 0.30-0.35. Re-lubrication is essential for consistent preload.

Summary of Results

Try the Calculator

Use the Bolt Torque Calculator to compute target torques for your own bolt sizes, grades, and faying surface conditions. Supports AISC, AS 4100, EN 1993, and CSA S16 standards.

Frequently Asked Questions

What is the difference between K factor and nut factor? The terms are used interchangeably in structural engineering. The nut factor K (or torque coefficient) relates applied torque to achieved preload: T = K×D×P. It accounts for thread friction, underhead friction, and lead angle effects. K is dimensionless and typically ranges from 0.10 to 0.35.

Should I use torque control or turn-of-nut for slip-critical connections? AISC 360 J3.2 permits both methods. Turn-of-nut is generally preferred for slip-critical connections because it is less sensitive to friction variations — preload scatter is ±15% versus ±30% for torque control. Torque control is simpler for field inspection but requires calibrated wrenches and verified K factors.

Does the bolt torque calculator handle metric bolts for EN 1993? Yes. The calculator supports both imperial (A325/A490) and metric (grade 8.8/10.9) bolts. Select the appropriate standard to switch between AISC and EN 1993-1-8 preload provisions, and the calculator adjusts the K factor range and torque formula accordingly.

See Also

• Bolted Connections Calculator
• Welded Connections Calculator
• Bolt Torque Calculator
• Bolt Hole Sizes Reference
• Connection Design Workflow