Bearing-Type Connection — Bolts in Shear and Bearing
A bearing-type connection is the standard bolted joint in structural steel — the bolts carry load by shear through their shanks and by bearing against the hole walls of the connected plates. Slip between the faying surfaces is permitted under service loads. The connection is designed for ultimate strength: the bolt must not shear, the plate must not tear out, and the bolt must not crush the plate in bearing.
Bearing-type strength per bolt = min( φ × Fnv × Ab, φ × Rn_bearing, φ × Rn_tearout )
Where:
Fnv = nominal shear strength (54 ksi A325-N, 68 ksi A325-X, 68 ksi A490-N, 84 ksi A490-X)
Ab = nominal bolt cross-sectional area (gross or tensile stress area)
Rn_bearing = varies by edge distance and bolt spacing
PRELIMINARY — NOT FOR CONSTRUCTION. All content is for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.
Bolt Shear Strength — The Threads Question
The most consequential design decision in bearing-type connections is whether threads fall within the shear plane. This seemingly small geometric detail changes the bolt's shear strength by approximately 25%.
Threads Excluded (Condition X)
When the bolt shank (the unthreaded portion) crosses the shear plane, the entire gross area resists shear. This requires the bolt grip length to position the threaded portion entirely outside the shear plane — the bolt must be long enough that threads start only after the last connected ply. The nominal shear stress for condition X:
A325-X: Fnv = 68 ksi (φ = 0.75 per AISC 360 J3.6)
A490-X: Fnv = 84 ksi
For a 3/4-inch A325-X bolt in single shear: φRn = 0.75 × 68 ksi × 0.442 in² = 22.5 kips.
Threads Included (Condition N)
When threads intersect the shear plane, the reduced area at the thread roots governs. The effective area is the tensile stress area (approximately 75% of gross area for UNC threads). The shear stress is accordingly lower because shear fracture must pass through the thread roots where the cross-section is smallest:
A325-N: Fnv = 54 ksi (54/68 = 0.794 — approximately 80% of condition X)
A490-N: Fnv = 68 ksi
For the same 3/4-inch A325-N bolt: φRn = 0.75 × 54 ksi × 0.442 in² = 17.9 kips — a 20% reduction from the condition X value. For a connection with 6 bolts, switching from X to N costs one additional bolt.
Practical Implications
Most shop connections are detailed as condition X because the detailer can select bolt lengths that exclude threads. Field connections are frequently condition N because the exact grip length depends on accumulated plate tolerances, and erectors carry a limited range of bolt lengths. Conservative design assumes threads included (N) for all field connections unless the engineer is confident that condition X can be achieved and verified during erection.
Bolt Bearing on Steel — The Pin-Crushing Limit
Bearing failure is not a bolt failure — the bolt is harder than the plate. The plate material crushes locally at the hole edge under the bolt's bearing stress. AISC 360 J3.10 provides the nominal bearing strength:
When deformation at bolt hole IS a design consideration (standard holes):
Rn = 1.2 × Lc × t × Fu ≤ 2.4 × d × t × Fu
When deformation at bolt hole is NOT a design consideration (or for long-slotted holes):
Rn = 1.5 × Lc × t × Fu ≤ 3.0 × d × t × Fu
Where Lc is the clear distance between hole edges in the direction of force — from the bolt hole edge to the adjacent bolt hole edge, or to the material edge. The first term (1.2 × Lc × t × Fu) represents tearout — the plate shearing between holes. The second term (2.4 × d × t × Fu) represents pure bearing crushing. The minimum of the two governs.
Edge distance matters. For a 3/4-inch bolt in 3/8-inch A36 plate (Fu = 58 ksi), with Lc = 1.25 inches (standard edge distance 1 inch minus half the hole diameter of 13/16 inch), the bearing strength is 1.2 × 1.25 × 0.375 × 58 = 32.6 kips. The bolt shear strength (A325-X, 22.5 kips) governs — bearing rarely controls for typical bolt spacings unless the plate material is thin.
Close spacing reduces strength. If bolts are spaced at 2.25 inches center-to-center (3d), Lc = 2.25 − 0.8125 = 1.4375 inches. Bearing strength = 1.2 × 1.4375 × 0.375 × 58 = 37.5 kips. If spacing is reduced to 2.0 inches (2.67d), Lc drops to 1.1875 inches, and bearing drops to 31.0 kips — a 17% reduction. Tight bolt patterns trade material economy (smaller gusset plates) against bearing capacity.
Tearout — The Plate Shears Between Holes
Tearout is a distinct failure mode from bolt bearing. Instead of the bolt crushing the plate locally, the plate material shears along two parallel planes running from the bolt hole to the plate edge (or to the adjacent bolt hole). The total shear area is 2 × Lc × t — twice the clear distance times the plate thickness.
Tearout governs when edge distance or bolt spacing is small. For standard connections with bolt spacing ≥ 2.67d and edge distance ≥ 1.5d, bearing crushing (2.4d × t × Fu) governs and tearout does not control. For connections with tight spacing — common in gusset plates for trusses where plate size is minimized — tearout may govern, and the engineer must check both the 1.2Lc and 2.4d terms.
Comparison: Bearing-Type vs Slip-Critical
| Aspect | Bearing-Type | Slip-Critical |
|---|---|---|
| Slip at service loads | Permitted | Not permitted |
| Surface preparation | None required (mill scale OK) | Class A/B/C per RCSC |
| Bolt pretension | Not required (snug-tight sufficient) | Required (70% Fu minimum) |
| Bolt quantity | Fewer bolts (governed by strength) | More bolts (governed by friction) |
| Oversized holes | Not permitted for bearing-only unless otherwise qualified | Permitted with slip-critical design |
| Cost | Lower | Higher (more bolts, pretension labor, surface prep) |
| Inspection | Routine | Pretension verification required |
For 90% of building connections — beam-to-column shear tabs, beam-to-girder clip angles, simple truss joints — bearing-type connections are the correct and economical choice. Slip-critical is reserved for the special conditions enumerated in AISC J3.8 and RCSC Section 4.3.
Frequently Asked Questions
Why not make all connections slip-critical for extra safety?
Because slip-critical connections cost more — more bolts (often 50-100% more), more labor (pretensioning), more inspection (tension verification), and more material (larger plates to accommodate the additional bolts). This cost buys no additional ultimate strength — a slip-critical connection and a bearing-type connection with the same bolts have identical ultimate capacities. The only benefit is preventing slip at service loads. Over-specifying slip-critical where not required adds cost without improving safety.
Does a snug-tight bolt carry any shear?
Yes, but not reliably. A snug-tight bolt develops some friction (approximately 5-10% of pretensioned capacity), but the primary load path at ultimate is bearing — the bolt shank bears against the hole wall. Snug-tightness is sufficient for bearing-type connections because bearing does not require clamping force. The snug-tight condition is defined as "the tightness attained by a few impacts of an impact wrench or the full effort of a worker using an ordinary spud wrench" — just enough to bring plies into firm contact.
Can I use A490 bolts in bearing-type connections?
Yes. A490 bolts can be used in both bearing-type and slip-critical connections. For bearing-type, A490-N (threads included) provides Fnv = 68 ksi and A490-X (threads excluded) provides Fnv = 84 ksi — approximately 25% stronger than the corresponding A325 values. However, A490 bolts are more expensive and less ductile (lower elongation at fracture) than A325. Most building connections default to A325 because the additional strength of A490 rarely reduces bolt count enough to offset the cost premium.
International Code References
- AISC 360-22: Section J3.6 — Bolt shear strength. Section J3.10 — Bearing strength at bolt holes. Table J3.2 — Nominal shear stress values.
- RCSC Specification (2020): Section 4.1 — Snug-tight joints (bearing-type). Section 4.2 — Pretensioned joints.
- AS 4100: Section 9.3.2 — Bearing-type bolted connections. Table 9.3.2 — Nominal shear capacity Vfn. Section 9.3.2.4 — Ply bearing capacity.
- EN 1993-1-8: Table 3.4 — Shear resistance per bolt (αv × fub × A / γM2). Bearing resistance per bolt (k1 × αb × fu × d × t / γM2).
Educational reference only. Bearing-type connection design must be performed per AISC 360 Section J3 by a licensed Professional Engineer. Bolt hole type, edge distance, and spacing must comply with AISC 360 Section J3.3-J3.5 for code-compliant design.
Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.