------------------------- | ------------ | -------- | ----------------- | ------------------------------------- | | Turn-of-nut | 8.2 | +/- 15% | Moderate | Most building connections | | Calibrated wrench | 8.3 | +/- 10% | High | Projects with systematic bolt tension | | Twist-off tension-control bolts | 8.4 | +/- 10% | Low (visual) | High-production, limited access | | Direct tension indicator (DTI) | 8.5 | +/- 10% | Moderate | Critical connections, verification |

Turn-of-Nut Method (RCSC Section 8.2)

The turn-of-nut method is the most commonly specified method for building construction in the US. It relies on the relationship between bolt elongation and nut rotation: one full turn of the nut advances it by one thread pitch, producing a predictable amount of bolt stretch, which corresponds to a predictable pretension force.

Procedure:

1. Snug-tight condition: Tighten all bolts in the connection to the snug-tight condition — defined as the tightness achieved by the full effort of a worker using an ordinary spud wrench, or a few impacts of an impact wrench. This brings the plies into firm contact ("metal-to-metal").

2. Match-mark: After snug-tightening, match-mark the nut and bolt tip (or the nut and plate) with a paint marker or center punch. This provides a visual reference for the rotation.

3. Apply rotation: Turn the nut the specified amount from the snug-tight position. The rotation depends on the bolt diameter and length.

Required Nut Rotation from Snug-Tight Condition (RCSC Table 8.1):

Bolt length is measured from the underside of the head to the end of the bolt. For bolts with washers, the rotation is applied from the snug-tight condition with washers in place.

Key requirement: The turn is applied to the NUT while the bolt head is held stationary. If the bolt head turns during the operation, the rotation measurement is invalid because the bolt stem is rotating in the hole rather than stretching. In tight access conditions where the nut cannot be turned, the bolt head may be turned provided the nut is held stationary — but this requires verification that the bolt head bears on a surface with sufficient friction to prevent the bolt from spinning.

Calibrated Wrench Method (RCSC Section 8.3)

A calibrated wrench (torque wrench or pneumatic impact wrench with torque-control) is used to apply a predetermined torque correlated with the required pretension. The wrench must be calibrated daily using a Skidmore-Wilhelm tension calibrator or equivalent device.

Calibration procedure:

1. At the start of each day, insert three representative bolts (same diameter, length, grade, and lot as those to be installed) into the Skidmore-Wilhelm calibrator.

2. Tighten each bolt with the wrench until the calibrator gauge reads the minimum required pretension T_b from RCSC Table 8.2.

3. Record the average torque reading T from the three tests.

4. The installation torque is the average T. The wrench must be set to achieve this torque.

5. Recalibrate if the bolt lot changes, the air pressure to the wrench changes, or after 4 hours of continuous use.

Calibrated wrench method advantages: Provides verifiable, documented bolt tension. Preferred for large projects with many bolts where the Skidmore-Wilhelm calibration can be integrated into the QA/QC program.

Calibrated wrench method limitations: Torque is an indirect measure of tension. Variables such as thread condition (clean vs. rusty), lubrication (lubricated vs. dry), and washer hardness affect the torque-tension relationship by up to 30%. The daily calibration on representative bolts from the same lot mitigates this variability.

Twist-Off Tension-Control (TC) Bolts (RCSC Section 8.4)

TC bolts (also called twist-off bolts or AISC "F3125 Grade F") have a splined end that shears off at a calibrated torque corresponding to the required pretension. The installation tool has an inner socket that engages the splined end and an outer socket that holds the nut. The tool turns the inner socket relative to the outer socket, twisting the splined end until it fractures — at which point the bolt has achieved the specified pretension.

Advantages:

• Visual inspection: if the splined end is sheared off, the bolt is pretensioned. No further verification needed.
• Single-side access: the tool only needs access to one side of the connection (the nut side).
• Consistent tension: factory-calibrated shear torque eliminates operator variability.

Limitations:

• Higher bolt cost (approximately 1.5-2x standard A325 or A490 bolts).
• Requires the special installation tool (electric or pneumatic shear wrench).
• Not suitable when bolt length is insufficient for the tool's outer socket to engage the nut (minimum stick-out approximately 1-1/2 in. beyond the nut).

TC bolts are the dominant pretensioning method for high-production steel connections — particularly in seismic moment frames where 100% of bolts require pretension verification.

Direct Tension Indicator (DTI) Method (RCSC Section 8.5)

DTI washers (ASTM F959) are hardened steel washers with raised protrusions (dimples or bumps) on one face. When placed under the bolt head or nut with the protrusions bearing against the harder surface (typically the bolt head), the gap between the washer body and the bolt head decreases as the bolt is tensioned.

Procedure:

1. Place the DTI washer under the bolt head with the protrusions facing the bolt head bearing surface.

2. Tighten the bolt until the feeler gage of the specified thickness cannot be inserted into at least half of the gaps between the protrusions.

3. The feeler gage acceptance criteria:

   • Feeler gage Type A (0.015 in.): Must NOT enter at any gap — indicates minimum pretension achieved. This is the standard acceptance check.
   • Feeler gage Type B (0.005 in.): Must enter at least one gap — indicates the bolt is not over-tensioned beyond the maximum allowed pretension.

DTI washer advantages:

• Direct measurement of tension, not torque.
• Visual and feeler gage verification — no Skidmore-Wilhelm calibrator needed for inspection.
• Works in any orientation (horizontal bolts, overhead).
• Permanent record of pretension (the crushed protrusions remain visible after installation).

DTI washer limitations:

• Additional cost (approximately $0.50-1.00 per bolt for the DTI washer).
• Requires bolt length increase to accommodate the washer thickness (approximately 1/8 in. extra per DTI washer).
• Not all protrusion configurations work with all bolt grades — specify DTI washers matched to the bolt grade per manufacturer.

Minimum Bolt Pretension (RCSC Table 8.2)

The minimum pretension T_b is the tension force that must exist in the bolt shank after installation. AISC 360-22 uses T_b in the slip resistance equation (J3-4) and for tension fatigue checks.

For A325 bolts: T_b = 0.70 x F_u x A_b, where F_u = 120 ksi (1/2 in. to 1 in.) or 105 ksi (1-1/8 in. to 1-1/2 in.). For A490 bolts: T_b = 0.70 x F_u x A_b, where F_u = 150 ksi (all diameters of F3125 Grade B).

Most common US structural bolt (3/4 in. A325): T_b = 28 kip. This is the pretension specified for standard shear connections in slip-critical joints, moment frame beam-to-column connections, and braced frame gusset plate connections.

The ratio of pretension to nominal tensile strength is exactly 0.70 — meaning the bolt is intentionally tensioned to 70% of its ultimate tensile capacity during installation. This leaves a 30% margin for the additional tension from applied loads before the bolt reaches its ultimate capacity.

Verifying Pretension in the Field — Inspection

Per RCSC Section 10, pretensioned bolts must be inspected. The inspection method matches the installation method.

Turn-of-Nut Inspection

The inspector observes the installation process, verifying:

1. Plies are in firm contact (snug-tight) before rotation begins.
2. Match marks are applied.
3. Nut rotation matches the RCSC Table 8.1 requirement.
4. The nut is turned, not the bolt head (unless the reverse-rotation method is used and documented).

Inspection is typically "observation-based" — the inspector watches the ironworker perform the turn and verifies the rotation against the match marks. For critical connections (moment frame beam-to-column joints), 100% inspection is typical. For secondary connections, sampling inspection (10-25%) is common.

DTI Washer Inspection

The inspector uses a feeler gage:

1. Insert Type A feeler gage (0.015 in.) into each protrusion gap. Gage must NOT enter at any gap if the bolt has achieved minimum pretension.
2. For verification of non-over-tensioned condition (optional, typically for A490 bolts where hydrogen embrittlement is a concern): Insert Type B feeler gage (0.005 in.). Gage must enter at least one gap.

DTI inspection does not require observation of the installation process — it can be performed after all bolts are installed. This is the primary advantage of DTI washers for QA/QC.

TC Bolt Inspection

Visual inspection: the splined end is sheared off. No measurement tools required. This is the simplest inspection of the four methods and is why TC bolts are popular for large projects.

Snug-Tight vs. Pretensioned vs. Slip-Critical — Clarifying the Terminology

These three terms are frequently confused. They represent different design conditions for the same bolt:

All slip-critical connections require pretension. Not all pretensioned connections are slip-critical — a connection with bolts in direct tension (hanger rods) requires pretension to prevent loosening but the shear is transferred by bearing, not friction.

Common misconception: Specifying "pretensioned bolts" does NOT automatically make the connection slip-critical. The designer must explicitly designate the connection as slip-critical per AISC 360 Section J3.8, and the faying surface preparation must comply with RCSC Section 7.

Worked Example — Turn-of-Nut Pretension Verification

Given: 3/4 in. A325 bolts, 3-1/2 in. bolt length (from underside of head to tip), connecting two 3/4 in. plates (total grip = 1-1/2 in.). Slip-critical connection, Class A faying surface.

Step 1 — Check if turn-of-nut is applicable: Bolt is 3/4 in. A325. Turn-of-nut applies. d = 0.75 in.

Step 2 — Bolt length classification: Bolt length = 3.50 in. 4d = 4 x 0.75 = 3.00 in. 8d = 6.00 in. 3.50 in. is greater than 4d (3.00 in.) and less than 8d (6.00 in.). Bolt length is in the 4d < L <= 8d range.

Step 3 — Required rotation: From RCSC Table 8.1: 1/2 turn (180 degrees) from snug-tight.

Step 4 — Procedure:

1. Install bolt with ASTM F436 hardened washer under the turned element (nut).
2. Snug-tighten using a spud wrench to bring plies into firm contact.
3. Match-mark nut and bolt tip with paint.
4. Turn nut 1/2 turn (180 degrees) while holding bolt head stationary.
5. Verify the match mark on the nut is opposite (180 degrees from) the mark on the bolt tip.

Step 5 — Slip resistance check (after pretension): T_b = 28 kip (for 3/4 in. A325). Slip resistance per bolt (single shear plane): R_n = mu x D_u x h_f x T_b x n_s

For Class A surface (unpainted, clean mill scale): mu = 0.30. D_u = 1.13 (for 3/4 in. bolt). h_f = 1.0 (no fillers). n_s = 1 (single shear plane).

R_n = 0.30 x 1.13 x 1.0 x 28 x 1 = 9.49 kip per bolt phi R_n = 1.00 x 9.49 = 9.49 kip (LRFD, standard holes)

For a connection with 6 bolts: total phi R_n = 6 x 9.49 = 56.9 kip.

Step 6 — Bearing check (can the bolt also bear?): In slip-critical connections, the bolts may bear AFTER slip has occurred at the required strength level. The bearing check uses the standard bearing formulas (AISC 360 J3.10) with the bolt diameter, plate thickness, and Fu. For this example with two 3/4 in. plates (A36, Fu = 58 ksi), clear distance Lc = 2.0d typical:

phi R_n_bearing = 0.75 x 2.4 x d x t x Fu = 0.75 x 2.4 x 0.75 x 0.75 x 58 = 58.7 kip per bolt

Bearing capacity exceeds slip resistance. Slip controls at service loads, bearing governs at ultimate.

Lubrication Requirements for Pretensioned Bolts

Per RCSC Section 2.3, A325 and A490 bolts must be installed with the as-manufactured lubricant in place. If bolts have been in storage for more than 6 months, or if they are visibly dry or rusted, they must be re-lubricated:

• A325 bolts: Apply a thin coat of stick wax or propriety bolt lubricant to the threads and the nut bearing surface. Do NOT lubricate the bolt head bearing surface (the bolt head must be held stationary during tightening).
• A490 bolts: Same lubrication requirement, but use ONLY lubricants certified for A490 bolts. Some lubricants contain sulfur compounds that can cause hydrogen embrittlement in the higher-strength A490 alloy.

Warning: Over-lubrication is as dangerous as under-lubrication. If excess lubricant drips onto the faying surfaces, the slip coefficient mu is reduced — potentially below the assumed design value. Protect faying surfaces during bolt lubrication.

Common Pretension Installation Errors

Regional Standards Comparison

The international approach is largely harmonized — the 0.70 x F_u x A_b minimum pretension is universal across all major steel design codes, as is the turn-of-nut method. The primary differences are in documentation and inspection requirements, which are governed by each country's quality assurance standards.

Frequently Asked Questions

Do all bolts in a steel structure need to be pretensioned? No. Most bearing-type connections in simple shear (beam web connections, simple shear tabs, angle seats) only require snug-tight installation per AISC 360 Section J3.1. Pretension is required only when (a) the connection is designated slip-critical, (b) the bolts are in direct tension with fatigue loading, (c) the connection uses oversized or slotted holes parallel to the load direction, (d) the connection is in a Seismic Design Category D, E, or F designated moment frame per AISC 341, or (e) the engineer of record specifies pretension for a specific reason (vibration, connection stiffness). In a typical office building, only 10-20% of bolts require pretension — the rest are snug-tight.

What is the difference between snug-tight and pretensioned? Snug-tight is the condition achieved by the full effort of a worker using an ordinary spud wrench, or a few impacts of an impact wrench. It brings the plies into firm contact but does not produce a known, consistent tension. Pretension applies a specific, minimum tension (T_b per RCSC Table 8.2) using a controlled method. A snug-tight bolt has an unknown tension — typically 20-40% of T_b. A pretensioned bolt has a verified tension of at least T_b. For bearing-type connections, snug-tight is sufficient because the bolt only needs to hold the plies together while the shank bears against the hole edges.

Can I use a torque wrench set to a specific value instead of the turn-of-nut method? Only if the torque wrench is calibrated daily on a Skidmore-Wilhelm tension calibrator using bolts from the same lot as those being installed (this is the "calibrated wrench" method per RCSC Section 8.3). A standard torque wrench set to a calculated torque value (T = K x D x P) is NOT sufficient because the nut factor K varies by 30% depending on thread condition and lubrication. The daily calibration on representative bolts removes this uncertainty. For small projects with fewer than 50 bolts, the turn-of-nut method is typically more practical than setting up a calibrator.

Why are A490 bolts not reusable? A490 bolts (F3125 Grade B) are heat-treated to achieve a tensile strength of 150 ksi minimum. The 0.70 x F_u pretension brings the bolt to 70% of ultimate strength — very close to its yield point. During the first installation, the bolt undergoes plastic elongation. Reinstalling a bolt that has already yielded will not achieve consistent pretension — the torque-tension relationship has changed because the bolt is now work-hardened. AISC 360 Section J3.1 explicitly prohibits the reuse of A490 bolts. A325 bolts may be reused once at the discretion of the engineer of record, provided the bolt shows no visible signs of distress (necking, thread damage, corrosion pitting).

Try it now: Check your bolt pretension with our free Bolted Connection calculator →

Related pages

• AISC Bolt Hole Sizes — Table J3.3
• Bolt Capacity Table — A325 & A490 Shear and Tension
• Bolt Spacing & Edge Distance — AISC J3
• Bolted Connection Example — Worked Problem
• Steel Fy & Fu Reference — Yield and Tensile Strength
• Slip-Critical Connection Design
• Bolted Connections Calculator
• Disclaimer (educational use only)

Educational use only. Verify against RCSC Specification 2024, AISC 360-22 Sections J3, and the governing project specification.

Disclaimer (educational use only)

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Design Resources

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• Bolted Connection Calculator — AISC 360
• Bolt Torque Calculator — Tension to Torque Conversion
• Slip-Critical Connection Calculator
• Bolt Group Calculator — Elastic Method