------------------ | ------------------- | ---------------------------------------------- | | Snug-tightened (bearing) | Yes (minimal) | Bolt bears against connected material | | Pretensioned (bearing) | Yes (specified) | Bearing, but bolt preloaded for serviceability | | Slip-critical | Yes (specified) | Friction between faying surfaces |

Snug-tightened: The plies are in firm contact. Achieved by a few impacts of an impact wrench or full effort of a worker with a spud wrench.

Pretensioned: The bolt is tightened to a specified minimum tension. Required for connections subject to fatigue, significant load reversal, or AISC Section J3.1 conditions.

Slip-critical: Faying surfaces are prepared to achieve a specified slip coefficient. The bolt pretension creates the clamping force that generates friction.

Minimum Required Pretension

Per AISC Table J3.1 and RCSC Specification Table 5.2:

Bolt Diameter (in) A325 (kips) A490 (kips) A325 Area (in²) A490 Area (in²)
1/2 12 15 0.196 0.196
5/8 19 24 0.307 0.307
3/4 28 35 0.442 0.442
7/8 39 49 0.601 0.601
1 51 64 0.785 0.785
1-1/8 56 80 0.994 0.994
1-1/4 71 102 1.227 1.227
1-3/8 85 121 1.485 1.485
1-1/2 103 148 1.767 1.767

Note: Pretension values are approximately 70% of the bolt's minimum tensile strength.

When Pretensioning Is Required

Per AISC Specification Section J3.1, pretensioning (beyond snug-tight) is required for:

  1. Joints subject to fatigue load with reversal of the loading direction
  2. Joints where bolts are subject to tension
  3. Joints in AISC 341 (seismic) lateral force-resisting systems requiring SC (slip-critical) or PT (pretensioned) connections
  4. Joints with A490 bolts subject to tension or combined shear and tension
  5. Joints with oversized or slotted holes in the outer ply where load is applied parallel to the slot

When none of these conditions apply, snug-tight installation is acceptable.

Installation Methods

Per RCSC Specification Section 8:

Turn-of-Nut Method

The most common field installation method. After snug-tightening, the nut is rotated an additional amount:

Bolt Length (L) Rotation from Snug-Tight
L ≤ 4d 1/3 turn (120°)
4d < L ≤ 8d 1/2 turn (180°)
8d < L ≤ 12d 2/3 turn (240°)

where d = bolt diameter. For both faces normal to bolt axis.

Condition Additional Rotation
Both faces normal to bolt As shown above
One face normal, other inclined ≤ 1:20 Add 1/6 turn
Both faces inclined ≤ 1:20 Add 1/3 turn

Calibrated Wrench Method

The bolt is tightened with a calibrated torque wrench until the specified pretension is achieved. Requires daily calibration of at least three bolts per diameter and grade using a tension-indicating device.

Approximate torque: T = K × D × F

where T = torque (ft-lb), K = nut factor (typically 0.20 for unlubricated, 0.15 for lubricated), D = bolt diameter (in), F = bolt pretension (lb).

Bolt Size Approx. Torque for A325 (ft-lb) Approx. Torque for A490 (ft-lb)
3/4 280-350 350-440
7/8 440-560 560-700
1 660-850 850-1060

Torque values assume K ≈ 0.20. Actual torque varies significantly with lubrication, surface condition, and thread condition.

Twist-Off Type Tension-Control Bolts (F3148)

These bolts have a splined end that twists off when the specified pretension is achieved. Installation requires a special wrench that engages both the nut and the spline.

Advantages:

Limitations:

Direct Tension Indicators (DTI)

Compressible washer with protrusions that flatten under load. Gap measurement with a feeler gauge indicates whether the bolt has been tensioned to the specified minimum.

DTI Type Inspection Method
Standard DTI Feeler gauge gap at 4 locations
Squirter DTI Silicone compound squirts out when tension achieved
Electronic DTI Load cell embedded in washer

Slip-Critical Connection Design

The slip resistance of a slip-critical connection is:

φRn = φ × μ × D × Fpt × nb × ns

where:

Slip Coefficients

Surface Class Condition Slip Coefficient (μ)
A Clean mill scale, blast cleaned Class A 0.30
B Blast cleaned with Class B coating (inorganic zinc) 0.50
C Hot-dip galvanized, roughened 0.30
Uncoated, blast cleaned Blast cleaned to bare steel 0.50

Slip Resistance per Bolt (Single Shear Plane)

Bolt Size Class A (kips) Class B (kips)
3/4 A325 8.4 14.0
3/4 A490 10.5 17.5
7/8 A325 11.7 19.5
7/8 A490 14.7 24.5
1 A325 15.3 25.5
1 A490 19.2 32.0

Values shown are φRn per bolt, single slip plane (φ = 0.85).

Frequently Asked Questions

What is the difference between snug-tight and pretensioned? Snug-tight means all plies are in firm contact (a few impacts of an impact wrench or full worker effort). Pretensioned means the bolt has been tightened to a specified minimum tension per AISC Table J3.1 (e.g., 28 kips for a 3/4 in A325 bolt).

When is slip-critical required? Slip-critical is required when joint slip would compromise the structure's serviceability or when specified by the engineer for fatigue, aesthetics, or other reasons. It is not required for most building connections.

Can I use A325 bolts in slip-critical connections? Yes. Both A325 (F3125 Gr A325) and A490 (F3125 Gr A490) bolts can be used in slip-critical connections. A490 provides higher slip resistance due to higher pretension.

What is the turn-of-nut method? After snug-tightening, the nut is rotated an additional 1/3 to 2/3 turn depending on bolt length. This elongates the bolt to produce the required pretension. It is the most common field installation method.

How do I inspect pretensioned bolts? Turn-of-nut: Match-mark the nut and bolt before and after rotation. Calibrated wrench: Verify wrench calibration. Twist-off: Verify spline has sheared. DTI: Verify gap closure with feeler gauge.

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Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.

Bolt Design Methods

Design for Combined Shear and Tension

Most bolted connections in structural steel design are subject to combined shear and tension. The interaction is checked using:

For AISC 360 LRFD:

For EN 1993-1-8:

The interaction formula for combined loading:

Slip-Critical Connections

For slip-critical connections, the slip resistance depends on the bolt pretension force and the slip coefficient of the faying surface:

Where μ is the slip factor (0.30-0.50 depending on surface treatment) and n is the number of slip planes.

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Frequently Asked Questions

What is the recommended design procedure for this structural element?

The standard design procedure follows: (1) establish design criteria including applicable code, material grade, and loading; (2) determine loads and applicable load combinations; (3) analyze the structure for internal forces; (4) check member strength for all applicable limit states; (5) verify serviceability requirements; and (6) detail connections. Computer analysis is recommended for complex structures, but hand calculations should be used for verification of critical elements.

How do different design codes compare for this calculation?

AISC 360 (US), EN 1993 (Eurocode), AS 4100 (Australia), and CSA S16 (Canada) follow similar limit states design philosophy but differ in specific resistance factors, slenderness limits, and partial safety factors. Generally, EN 1993 uses partial factors on both load and resistance sides (γM0 = 1.0, γM1 = 1.0, γM2 = 1.25), while AISC 360 uses a single resistance factor (φ). Engineers should verify which code is adopted in their jurisdiction.

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