Bearing vs Slip-Critical Bolts — AISC 360 & AS 4100 Selection Guide

Not all bolted connections are equal. A connection that works perfectly as a bearing-type joint can fail prematurely if slip-critical performance is required — and a connection over-designed as slip-critical when bearing would suffice wastes bolt pretension, surface preparation costs, and inspection effort. The decision between bearing-type and slip-critical bolted connections is among the most consequential choices in steel connection design, yet it is frequently misunderstood.

In this guide: We explain what slip-critical means, when it is required under AISC 360-22 Section J3.8 and AS 4100 Section 9.3.4, how faying surface classes and pretension methods affect capacity, and we walk through a fully worked example comparing bearing and slip-critical design on the same M20 connection. You can verify every number with our free bolted connections calculator.

PRELIMINARY — NOT FOR CONSTRUCTION. All results discussed are for educational and reference use only. Must be independently verified by a licensed Professional Engineer or Structural Engineer before use in any project.

What you will learn

• The fundamental difference between bearing-type and slip-critical bolted connections
• When slip-critical connections are required per AISC 360 J3.8 and AS 4100
• Faying surface classes (AISC Class A/B/C, AS 4100 categories) and their slip coefficients
• Four pretension methods: turn-of-nut, calibrated wrench, DTI, and tension-control bolts
• The slip resistance formula and the factors that influence it
• A worked example comparing bearing and slip-critical design on the same M20 connection
• Practical application selection criteria for different connection types

Copyright and standards notice

This site does not reproduce copyrighted code clauses or proprietary tables verbatim. Discussion of AISC 360 and AS 4100 here is high-level and intended to help you understand verification workflows. Always consult the official published standard (AISC 360-22, AS 4100:2020) for authoritative requirements.

Step 1 — Understand bearing-type bolted connections

Bearing-type connections are the default for most structural steel connections. In a bearing connection, the load transfers between connected plies through direct bearing of the bolt shank against the edges of the bolt holes. Under service loads, a small amount of slip occurs as the bolts engage the plates — the holes are typically 1.6 mm to 3.2 mm larger than the bolt diameter, and this gap must close before the bolt bears.

The key characteristics of bearing-type connections are:

1. Load transfer by bearing. Shear force passes from one ply through the bolt shank to the adjacent ply via bearing stress at the hole wall.
2. Slip at service loads is accepted. The small movement as bolt-to-hole clearance closes is typically harmless for statically loaded structures.
3. Bolt shear governs at ultimate. At the ultimate limit state, the bolt material shears through its cross-section, or the plate experiences bearing failure, tear-out, or block shear.
4. Pretension not required for strength. Bearing connections can use snug-tight bolts. The bolt tension does not contribute to the calculated shear strength (except for the minor frictional effect ignored in design).

The design strength of a bearing-type connection per AISC 360 is the lesser of bolt shear, bolt bearing on the connected plies, and block shear. Snug-tight installation is sufficient. No faying surface preparation is needed. No pretension verification is required.

Step 2 — Understand slip-critical bolted connections

Slip-critical connections do not rely on bolt bearing to transfer load. Instead, they rely on friction between the connected plies — the bolt pretension clamps the plies together, and the resulting friction at the faying surfaces resists slip. The bolt shank should never bear against the hole edge at the service load level.

The defining characteristics of slip-critical connections are:

1. Load transfer by friction. The shear force is resisted entirely by the friction developed at the contact surface between connected plies. The bolt acts as a clamp; the bolt shank does not bear on the hole edge.
2. No slip at service loads. The connection remains rigid with no relative movement between plies. This is critical for fatigue-sensitive structures and connections where misalignment would cause problems.
3. Surface preparation is essential. The faying surfaces must be prepared to a specific roughness and cleanliness to achieve the design slip coefficient. Paint, oil, rust, and mill scale all affect the slip coefficient.
4. Bolt pretension is mandatory. Every bolt must be tensioned to at least the minimum specified pretension in the governing code. Installation must be verified by one of the approved methods.
5. Lower design capacity than bearing. For the same bolt diameter, slip-critical connections typically have 40–60% lower design shear capacity than bearing connections because friction resistance is less than bolt shear strength.

Once slip occurs (if the friction is overcome), the bolt bears against the hole edge, and the connection reverts to bearing behaviour. Slip-critical design typically checks two conditions: slip resistance at the serviceability limit state and ultimate bearing strength as a backup. Both must be satisfied.

Step 3 — When slip-critical connections are required

Neither AISC 360 nor AS 4100 mandates slip-critical connections for every application. The selection depends on the connection's function, hole type, and loading characteristics.

Per AISC 360-22 Section J3.8

Slip-critical connections are required when:

1. Oversized or slotted holes are used. Per AISC 360 J3.2, connections with oversized holes or short-slotted holes loaded parallel to the slot require slip-critical bolts. Long-slotted holes loaded perpendicular to the slot do not necessarily require slip-critical bolts.
2. Fatigue loading with stress reversal. Cyclic loading that includes full stress reversal (tension to compression and back) demands slip-critical connections to prevent progressive loosening. AISC 360 Commentary J3.8 references AISC Specification Appendix 3 for fatigue criteria.
3. Wind or seismic column splices in moment frames. Slip in a column splice can amplify drift, so slip-critical bolts are specified when drift sensitivity is high.
4. A490 bolts over 1 in. diameter in tension. AISC 360 J3.8 also requires pretension for A490 bolts larger than 1 in. diameter when loaded in tension, due to concerns about brittle fracture.

Slip-critical connections are also recommended (not required) for connections subject to vibration, impact loading, or where slip would cause misalignment of connected machinery or equipment.

Per AS 4100:2020 Section 9.3.4

AS 4100 uses a different classification system. Bolts are designated by installation category:

• 8.8/S: Snug-tight, bearing-type connection. No pretension required. Standard for most connections.
• 8.8/TB: Tensioned, bearing-type connection. Bolts are pretensioned but the connection is still designed as bearing at ultimate. The pretension is primarily for fatigue control and vibration resistance.
• 8.8/TF: Fully tensioned, friction-type (slip-resistant) connection. The design capacity is based on slip resistance at the serviceability limit state. This is the AS 4100 equivalent of AISC slip-critical design.

The TF (friction-type) category is required per AS 4100 when: slip at service loads would be detrimental to the structure, the connection is subject to fatigue, or oversize/slotted holes are used with the slot perpendicular to the load direction.

Step 4 — Faying surface classes and preparation

The faying surface is the contact surface between connected plies. Its condition — roughness, cleanliness, and coating — determines the slip coefficient μ (AISC) or k_s (AS 4100), which directly multiplies the available slip resistance.

AISC 360 Table J3-1 — Faying Surface Classes

The difference between Class A (μ = 0.30) and Class B (μ = 0.50) is a 67% increase in slip resistance. Specifying Class B faying surfaces can reduce the number of bolts required by nearly half, but the cost of blast-cleaning and the schedule impact must be weighed against the bolt reduction.

Class C galvanized surfaces require roughening because the as-galvanized surface is too smooth. Without roughening, galvanized surfaces can have slip coefficients below μ = 0.20. Wire brushing after galvanizing restores adequate friction.

AS 4100 Faying Surface Categories

AS 4100 uses friction coefficient k_s in the slip resistance formula, with values comparable to AISC:

Step 5 — Bolt pretension methods

Slip-critical connections require every bolt to achieve a minimum pretension verified by an approved method. Simply tightening "until it feels tight" is not acceptable. AISC 360 Section J3.8 references RCSC Specification Section 9 for pretension requirements, four of which are permitted:

1. Turn-of-Nut Method

After all bolts in the connection are brought to the snug-tight condition (full effort of a worker using an ordinary spud wrench, or a few impacts of an impact wrench), each bolt is turned a specified additional fraction of a turn. For bolts up to 200 mm length, the additional turn is typically one-third turn (120°) for bolts where the outer face of the nut is perpendicular to the bolt axis; longer bolts require two-thirds turn (240°). This method requires no torque measuring equipment and relies on the linear relationship between nut rotation and bolt elongation in the elastic range.

2. Calibrated Wrench Method

A torque wrench calibrated on a daily basis to deliver the specified pretension. A sample of at least three bolts of each diameter, length, and grade in a Skidmore-Wilhelm calibrator or equivalent device determines the torque-pretension relationship. The wrench is set to the average torque required. This method is widely used but is sensitive to lubrication condition — a dry bolt can require twice the torque of an oiled bolt to achieve the same pretension.

3. Direct Tension Indicator (DTI) Method

ASTM F959 DTI washers have small compressible protrusions (bumps) on one face. As the bolt is tightened, the protrusions crush. When the gap between the DTI washer and the bolt head/nut reduces to a specified feeler gauge thickness (typically 0.4 mm), the minimum pretension has been achieved. DTI washers provide a visible, inspectable indication after installation — an inspector can check the gap with a feeler gauge without specialised equipment.

4. Twist-Off Type Tension-Control Bolts

ASTM F1852 (A325 equivalent) and ASTM F2280 (A490 equivalent) bolts feature a splined end that shears off when the specified pretension is reached. The installer uses a shear wrench that engages both the nut and the splined tip, turning them in opposite directions. When the pretension exceeds the torsional capacity of the reduced section at the spline, the tip shears off cleanly. This method provides the most reliable pretension indication and eliminates the need for torque calibration or feeler gauge checks.

Step 6 — Slip resistance formula

Per AISC 360-22 Section J3.8

The nominal slip resistance per bolt per shear plane is:

$$R_{n} = \mu \times D_u \times h_f \times T_b \times n_s$$

Where:

• μ = slip coefficient from Table J3-1 (Class A = 0.30, Class B = 0.50)
• D_u = 1.13, a multiplier that accounts for the ratio of mean installed pretension to minimum specified pretension
• h_f = filler factor (1.0 for one filler, 0.85 for two or more fillers between plies)
• T_b = minimum bolt pretension from AISC 360 Table J3-1 (in kips or kN)
• n_s = number of slip planes (1 for single shear, 2 for double shear)

The LRFD design slip resistance applies φ = 1.0 for standard holes, φ = 0.85 for oversized and short-slotted holes, and φ = 0.70 for long-slotted holes perpendicular to the load.

Per AS 4100 Section 9.3.4

The slip resistance for friction-type connections under AS 4100 follows:

$$V_{sf} = \phi \times \mu \times n_{ei} \times N_{tf} \times k_s$$

Where:

• φ = 0.70 (capacity factor for slip resistance)
• μ = 0.35 (assumed slip factor unless otherwise determined by testing)
• n_{ei} = number of effective interfaces (shear planes)
• N_{tf} = minimum bolt tension from AS 4100 Table 9.3.1
• k_s = friction coefficient for the specific faying surface category

Step 7 — Worked example: bearing vs slip-critical on the same connection

Problem: A lap splice connection transfers a factored shear force of V_u = 240 kN (LRFD). The connection uses four M20 bolts in single shear, arranged in a 2×2 grid. Plates are Grade 300 steel, 10 mm thick, with standard holes (22 mm for M20). Compare the design as bearing-type versus slip-critical.

Given data

Design as bearing-type connection

Bolt shear capacity (AISC 360 LRFD):

Bolt nominal area A_b = 314 mm². Threads in shear plane (N): F_nv = 0.563 × F_u.

For Group A bolt (equivalent F_u ≈ 896 MPa for A325): F_nv = 0.563 × 896 = 504 MPa

Single bolt shear capacity: φR_n = 0.75 × 504 × 314 / 1000 = 118.7 kN per bolt.

Total bolt shear capacity: 4 × 118.7 = 474.7 kN.

Utilisation: 240 / 474.7 = 0.51 — OK.

Bolt bearing on plate:

For standard holes, edge distance 35 mm, bolt spacing 70 mm:

Clear distance for edge bolt: l_c = 35 − 22/2 = 24 mm. Bearing per edge bolt: φR_n = 0.75 × 1.2 × 24 × 10 × 440 / 1000 = 95.0 kN.

Clear distance for interior bolt: l_c = 70 − 22 = 48 mm. Bearing per interior bolt: φR_n = 0.75 × 1.2 × 48 × 10 × 440 / 1000 = 190.1 kN.

Total bearing: 2 × 95.0 + 2 × 190.1 = 570.2 kN > 240 kN — OK (utilisation 0.42).

Block shear check (simplified): Block shear capacity > applied shear — assumed satisfied.

Bearing design outcome: The connection passes all checks comfortably. The governing limit state is bolt shear at utilisation 0.51. Four M20 A325 bolts are adequate for bearing-type design.

Design as slip-critical connection

Slip resistance per bolt (Class A, μ = 0.30):

Per AISC 360 J3.8 LRFD for standard holes (φ = 1.0):

$$R_{n} = \mu \times D_u \times h_f \times T_b \times n_s$$

$$R_{n} = 0.30 \times 1.13 \times 1.0 \times 142 \times 1 = 48.1 \text{ kN per bolt}$$

Total slip resistance: 4 × 48.1 = 192.4 kN.

Utilisation (slip): 240 / 192.4 = 1.25 — FAILS.

The slip-critical connection with four M20 bolts and Class A faying surfaces does not provide adequate slip resistance for the 240 kN factored shear.

What would make it work?

Option 1 — Upgrade to Class B faying surfaces (μ = 0.50):

Per bolt: R_n = 0.50 × 1.13 × 1.0 × 142 × 1 = 80.2 kN.

Total: 4 × 80.2 = 320.8 kN. Utilisation: 240 / 320.8 = 0.75 — OK.

Option 2 — Increase to six M20 bolts with Class A surfaces:

Total: 6 × 48.1 = 288.6 kN. Utilisation: 240 / 288.6 = 0.83 — OK.

Option 3 — Upgrade to M24 A325 bolts with Class A surfaces:

T_b for M24 A325 = 201 kN. Per bolt: 0.30 × 1.13 × 1.0 × 201 × 1 = 68.1 kN.

Total (4 bolts): 4 × 68.1 = 272.5 kN. Utilisation: 240 / 272.5 = 0.88 — OK.

Slip-critical design outcome: The connection that passed bearing design easily fails slip-critical design with the same four M20 bolts on Class A surfaces. To meet slip-critical requirements, you must either increase the bolt count, upgrade to a higher slip coefficient surface, or use larger bolts. The slip-critical requirement directly governs the connection size.

Side-by-side comparison

Key insight: A connection that works for bearing may require 50% more bolts or a Class B surface upgrade to satisfy slip-critical requirements. Never assume that a bearing design can be converted to slip-critical simply by adding pretension — the numbers must be checked.

Step 8 — Bearing vs slip-critical selection guide

Common mistakes in bearing vs slip-critical selection

1. Designing as bearing, specifying as slip-critical on drawings. A connection may pass all bearing checks but fail slip resistance by 30% or more. The bolt count, surface prep, and pretension must match the design assumption. Drawing notes that add "pretension all bolts" without a slip-critical check are unsafe.

2. Assuming slip-critical always needs more bolts. In some arrangements — for example, Class B surfaces with short, high-shear connections — the slip-critical bolt count can be similar to bearing if bearing is governed by plate tear-out rather than bolt shear. Always run both checks.

3. Using bearing friction factor for slip resistance. Bearing-type connections develop some incidental friction from the snug-tight clamping force, but this friction is unreliable and is explicitly excluded from bearing capacity calculations. Never credit this incidental friction in bearing design.

4. Forgetting the filler factor h_f. When multiple fillers are used between connected plies (packing plates to adjust for beam depth differences or connection fit-up), the h_f factor reduces to 0.85 for two or more fillers. This 15% reduction catches designers who copy single-filler values to multi-filler details.

5. Substituting Class A for Class B without checking. A common value-engineering move: change the faying surface specification from Class B (blast-cleaned) to Class A (mill scale) to save surface preparation cost. But if the slip-critical bolt quantity was based on μ = 0.50, reducing to μ = 0.30 cuts slip resistance by 40%. The connection will fail unless bolt count increases.

6. Neglecting the post-slip bearing check. Slip-critical connections must also satisfy the bearing limit states at ultimate load. After the friction is overcome — which occurs somewhere between service load and ultimate load — the connection behaves as a bearing-type joint. The bolt shear, bearing, and block shear checks are required for slip-critical connections as well. A slip-critical connection that passes friction but fails bearing at ultimate is not safe.

Frequently Asked Questions

What is the difference between bearing-type and slip-critical bolted connections?

Bearing-type connections transfer shear through direct bolt bearing against the connected plates and allow limited slip between plies at service loads. Slip-critical connections are pretensioned to a specified minimum bolt tension so that the friction between faying surfaces resists shear without any slip. Per AISC 360 J3.8, slip-critical connections are required for connections with oversized holes, connections subject to fatigue or significant load reversal, and connections where slip would impair structural performance. Slip-critical connections have lower design shear capacity than bearing connections because friction resistance is typically less than bolt shear strength.

When are slip-critical connections required per AISC 360?

Per AISC 360-22 Section J3.8, slip-critical connections are required when: (1) the connection uses oversized holes or slotted holes with the long dimension perpendicular to the load direction, (2) the connection is subject to fatigue loading with reversal of stress direction, (3) the connection is in a column splice in a wind or seismic frame where slip would alter the drift characteristics, or (4) slip would cause serviceability problems such as misalignment of connected equipment. The Commentary to J3.8 also notes that slip-critical connections should be considered for connections with A490 bolts of diameter greater than 1 in. loaded in tension.

What is the difference between AISC Class A, B, and C faying surfaces?

Class A (μ = 0.30) is unpainted clean mill scale, the default for structural steel as-delivered from the mill. Class B (μ = 0.50) requires abrasive blast cleaning to SSPC-SP 10 near-white metal standard, providing 67% more slip resistance than Class A. Class C (μ = 0.35) is hot-dip galvanized and roughened — the roughening by wire brushing after galvanizing is essential because smooth galvanized surfaces have slip coefficients below 0.20. Class B provides the highest slip resistance but at additional fabrication cost; Class A is the economic default; Class C is used when corrosion protection demands galvanizing.

How do you verify bolt pretension in the field?

Four methods are accepted by AISC 360 / RCSC: (1) turn-of-nut — a marked nut rotation beyond snug-tight, verified visually, (2) calibrated wrench — torque wrench calibrated daily, (3) DTI washers — feeler gauge check of the gap after tightening, and (4) tension-control bolts — visual confirmation that the splined tip has sheared off. DTI washers and tension-control bolts are preferred where inspection access is limited or where torque wrench calibration is impractical, as they provide permanent, visible evidence of achieved pretension.

Can you mix bearing and slip-critical bolts in the same connection?

No. A connection is either bearing-type or slip-critical for a given load case. All bolts in the connection must be installed to the same category. Mixing snug-tight and pretensioned bolts in one connection creates different slip characteristics at adjacent bolt locations and leads to unpredictable load sharing between bolts.

How does AS 4100 handle slip-critical design differently from AISC 360?

AS 4100 uses a three-tier bolt category system (8.8/S, 8.8/TB, 8.8/TF) that separates the installation method from the design model. 8.8/S (snug-tight, bearing) and 8.8/TB (tensioned, bearing) both design for bearing at ultimate — 8.8/TB adds pretension for fatigue and vibration without changing the shear capacity. 8.8/TF (fully tensioned, friction-type) uses slip resistance as the design basis, similar to AISC slip-critical. AISC does not have a separate 8.8/TB intermediate category — in AISC, if a bolt is pretensioned, the slip-critical provisions apply. AS 4100 also uses a lower capacity factor for slip resistance (φ = 0.70) compared to bolt shear (φ = 0.80), further widening the gap between bearing and friction design capacities.

Is this calculator a replacement for professional engineering judgment?

No — this is an educational reference only. All bolted connection designs must be independently verified by a licensed Professional Engineer before use in any project. Results are PRELIMINARY — NOT FOR CONSTRUCTION.

Key Takeaways

• Bearing connections allow slip and transfer load through bolt bearing. They are the economic default for most statically loaded connections.
• Slip-critical connections prevent slip through friction generated by bolt pretension. The faying surface condition directly determines capacity.
• A connection sized for bearing will typically fail slip-critical checks. For Class A surfaces with standard holes, slip resistance is 40-60% of the bolt shear strength.
• Class B (blast-cleaned) surfaces provide 67% more slip resistance than Class A. The cost of blast-cleaning must be weighed against the bolt count reduction.
• All four pretension methods are equal in the design calculations. The choice depends on site conditions, inspection access, and crew experience.
• Slip-critical design includes mandatory post-slip bearing checks. Never run only the friction check — the backup bearing capacity at ultimate must also satisfy the limit states.

Run This Calculation

Bolted Connections Calculator — Complete bolted connection design including bearing and slip-critical checks per AISC 360, AS 4100, EN 1993, and CSA S16. Eccentric bolt group analysis, block shear, bearing, tear-out, and slip resistance all checked simultaneously. No signup required.

Bolt Torque Calculator — Calculate required installation torque for pretensioned bolts per AISC 360 J3.8 and AS 4100 Clause 15.2.5.2. Includes k-factor compensation for lubrication condition and bolt diameter.

Further Reading

• How to Calculate Bolt Capacity — AISC 360, EN 1993, AS 4100 Worked Example
• Structural Steel Connection Design Guide — Types, Checks & Worked Examples
• AS 4100 Bolt Group Design — M20 8.8 Worked Example
• AS 4100 Fillet Weld Design — SP/GP Capacity Per mm Run
• EN 1993-1-8 Steel Connection Design — Bolt & Weld Checks
• Gusset Plate Design Example — Whitmore Section, Block Shear, Buckling
• Base Plate Design Example — AISC 360, AS 4100, EN 1993 & CSA S16
• Steel Beam Splice Design — Bolted & Welded Splices per AISC 360
• Fillet Weld Size Selection Guide — Minimum Size, Strength, and Design
• Bolt Capacity Table — Shear and Tension for Common Bolt Grades
• Bolt Hole Sizes Reference — Standard, Oversize, and Slotted
• Steel Fy & Fu Reference — Yield and Tensile Strength by Grade

Disclaimer (educational use only)

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