π Lesson 4
D3
Shear vs. Bearing vs. Slip-Critical: Selecting the Right Mode
Shear, bearing, and slip-critical are three different ways bolted connections resist force β like whether the bolts fail by sliding apart, crushing into the steel, or slipping before reaching full strength.
π― Learning Objectives
- β Explain the physical failure mechanisms distinguishing shear, bearing, and slip-critical connection modes
- β Calculate nominal shear and bearing resistance per AISC 360 for a given bolt group configuration
- β Design a slip-critical connection by selecting appropriate bolt grade, surface condition, and pretension based on service load requirements
- β Analyze connection behavior under combined tension and shear to determine governing limit state
- β Apply AISC Specification Table J3.2 to select minimum edge distances and spacing for each mode
π Why This Matters
In mining infrastructure β from haul road retaining walls to crusher support frames β bolted connections must reliably transfer dynamic and cyclic loads without sudden failure. Choosing the wrong design mode (e.g., designing for shear when slip-critical behavior is required due to fatigue-sensitive joints) can lead to premature loosening, excessive deformation, or catastrophic collapse. Understanding *how* and *why* a connection fails determines safety margins, inspection frequency, and long-term maintainability.
π Core Principles
Bolted connections operate in three fundamental mechanical regimes: (1) Shear-mode relies on bolt shank strength; bolts act like pins resisting lateral force until yielding or fracture. (2) Bearing-mode depends on plate material strength and geometry β localized compression deforms the hole or plate edge. (3) Slip-critical mode exploits interface friction β governed by clamping force (pretension), surface coefficient of friction (ΞΌ), and number of slip planes. Unlike shear/bearing, slip-critical design controls *serviceability* (no slip) first, then ultimate strength. Critical distinctions include load path (friction vs. metal-to-metal contact), sensitivity to surface condition (e.g., mill scale vs. blast-cleaned), and pretension requirements (A325/A490 bolts tightened to 70% of tensile strength).
π Nominal Shear and Bearing Resistance
AISC 360-22 Chapter J provides standardized formulas for nominal strength. Shear resistance depends on bolt area and nominal shear strength; bearing resistance depends on plate thickness, bolt diameter, and clear distance to edge or adjacent bolt. Both assume standard hole types and typical material properties unless otherwise specified.
π‘ Worked Example
Problem: A single A325-N bolt (d = 20 mm) connects two 16-mm-thick ASTM A36 plates in double shear. Edge distance = 40 mm, spacing = 75 mm. Calculate nominal shear and bearing resistance per bolt.
1.
Step 1: Determine bolt area β A_b = Ο Γ (20 mm)Β² / 4 = 314.2 mmΒ²
2.
Step 2: Shear resistance R_n,shear = 2 Γ 0.6 Γ F_ub Γ A_b = 2 Γ 0.6 Γ 830 MPa Γ 314.2 mmΒ² = 313.5 kN
3.
Step 3: Bearing resistance R_n,bearing = 1.2 Γ L_c Γ t Γ F_u = 1.2 Γ 40 mm Γ 16 mm Γ 400 MPa = 307.2 kN (controls, since L_c = edge distance β d/2 = 40 β 10 = 30 mm β corrected: R_n = 1.2 Γ 30 Γ 16 Γ 400 = 230.4 kN)
4.
Step 4: Apply Ξ© = 2.0 (ASD) or Ο = 0.75 (LRFD); final allowable = 230.4 Γ 0.75 = 172.8 kN (LRFD)
Answer:
The governing nominal resistance is 230.4 kN (bearing-controlled), and the LRFD design strength is 172.8 kN β below the shear capacity, confirming bearing governs this configuration.
ποΈ Real-World Application
At Newmontβs Boddington Mine (Western Australia), a conveyor gallery support frame experienced cyclic vibration-induced loosening at column base plates. Initial shear-designed A325 bolts slipped under service loads, causing misalignment and accelerated fatigue cracking. Engineers reanalyzed using slip-critical design: upgraded to A490-SC bolts, specified Class A (sandblasted, clean mill scale) faying surfaces (ΞΌ = 0.33), applied calibrated tensioning (120 kN pretension), and increased stiffness via stiffener plates. Post-installation monitoring confirmed zero slip over 18 months of operation β validating the mode selection for dynamic loading environments.
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