Block Shear — Definition, Failure Mode & AISC 360 Design
Block shear is a limit state in bolted steel connections where a block (or "chunk") of material tears out of the connected member. The failure involves a combination of tension fracture on one plane (the "tension face" — perpendicular to the applied load) and shear yielding or fracture on parallel planes (the "shear faces" — parallel to the applied load). This combined tension-shear failure is distinct from net section tension rupture (straight-across fracture) and bolt bearing failure.
Block shear is the governing limit state for many short bolted connections, particularly at beam end connections, gusset plate attachments, and tension member splices where the bolt group is compact.
Failure Mechanism
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.
In a typical bolted connection loaded in tension, the potential block shear failure surface consists of:
- Tension face: The plane perpendicular to the load at the end of the bolt group, where the material fractures in tension (net area Ant subject to ultimate tensile stress Fu)
- Shear faces: One or two planes parallel to the load along the bolt lines, where material yields or fractures in shear (gross area Agv or net area Anv subject to shear stress)
The block tears out when the combined tension + shear resistance is exceeded. The controlling mechanism depends on whether shear yielding (ductile) or shear fracture (brittle) dominates.
AISC 360-22 Section J4.3 — Block Shear Strength
The nominal block shear strength Rn is:
Rn = 0.60 * Fu * Anv + Ubs * Fu * Ant <= 0.60 * Fy * Agv + Ubs * Fu * Ant
Where:
- Fu = specified minimum tensile strength of steel (ksi)
- Fy = specified minimum yield stress (ksi)
- Anv = net area subject to shear (in^2)
- Ant = net area subject to tension (in^2)
- Agv = gross area subject to shear (in^2)
- Ubs = non-uniform stress factor (1.0 for uniform tension, 0.5 for non-uniform)
Design strength (LRFD): phi * Rn, where phi = 0.75.
The equation has two terms:
- 0.60 _ Fu _ Anv: Shear fracture capacity on the net shear area (never yield-dominated)
- Ubs _ Fu _ Ant: Tension fracture on the net tension area
The upper bound 0.60 _ Fy _ Agv + Ubs _ Fu _ Ant limits capacity to gross shear yielding + tension fracture (prevents the formula from predicting higher capacity than gross-section yielding allows).
Ubs — Non-Uniform Stress Factor
| Ubs = 1.0 | Ubs = 0.5 |
|---|---|
| Uniform tension stress distribution | Non-uniform tension stress |
| Multiple bolt rows distributing tension | Single bolt row in tension |
| Standard connections with symmetric bolt patterns | Coped beams (reduced ability to distribute stress) |
| Gusset plates with well-distributed forces | Gusset plates with Whitmore section eccentricity |
| Angles loaded in tension with staggered bolts | Single-row connections at angle legs |
Practical impact: Using Ubs = 0.5 instead of 1.0 reduces the tension-component contribution by 50%. For connections where the tension component is large relative to the shear component, this can significantly reduce block shear capacity.
Net and Gross Areas
Gross Shear Area (Agv)
Agv = (number of shear planes) * (edge distance + bolt spacing * (n-1)) * t
Where t is the member thickness and n is the number of bolts along the shear line.
Net Shear Area (Anv)
Anv = Agv - (number of bolt holes in shear plane) * (dh + 1/16") * t
Where dh is the nominal bolt hole diameter. The additional 1/16" accounts for hole damage per AISC 360.
Net Tension Area (Ant)
Ant = (gage distance between shear lines - bolt holes) * t
Worked Example — Block Shear of Tension Member Splice
Problem: A 3/8" thick A36 plate (Fu = 58 ksi, Fy = 36 ksi) is spliced with two rows of three 3/4" diameter A325 bolts. Bolt spacing is 3" center-to-center, end distance = 1.5", edge distance = 1.25", gage = 3.5". Determine block shear capacity.
Step 1: Compute areas
Plate thickness t = 0.375 in
Hole diameter dh = 3/4" + 1/16" = 13/16" = 0.8125 in
Bolt rows = 2, bolts per shear plane = 3
Gross shear area (both planes):
Agv = 2 * [(1.5 + 3.0 + 3.0) * 0.375] = 2 * [7.5 * 0.375] = 2 * 2.813 = 5.625 in^2
Net shear area (both planes, 3 holes per plane):
Anv = Agv - 2 * [3 * 0.8125 * 0.375] = 5.625 - 2 * [0.914] = 5.625 - 1.828 = 3.797 in^2
Net tension area:
Ant = [3.5 - 2 * 0.8125] * 0.375 = [3.5 - 1.625] * 0.375 = 1.875 * 0.375 = 0.703 in^2
Step 2: Compute block shear strength
Ubs = 1.0 (multiple bolt rows providing uniform stress distribution)
Rn_shear_frac = 0.60 * 58 * 3.797 + 1.0 * 58 * 0.703
= 132.1 + 40.8 = 172.9 kips
Upper bound:
Rn_upper = 0.60 * 36 * 5.625 + 1.0 * 58 * 0.703
= 121.5 + 40.8 = 162.3 kips
Rn = min(172.9, 162.3) = 162.3 kips
Step 3: Design strength
phi * Rn = 0.75 * 162.3 = 121.7 kips (LRFD)
Code Comparison — Block Shear
| Code | Section | Formula | phi / gamma_M |
|---|---|---|---|
| AISC 360 | J4.3 | 0.6FuAnv + UbsFuAnt <= 0.6FyAgv + UbsFuAnt | phi = 0.75 |
| AS 4100 | 9.1.10 | Vf + Tf, similar mechanism, different notation | phi = 0.75 |
| EN 1993-1-8 | 3.10.2 | Veff,Rd based on net shear area + tension | gamma_M2 = 1.25 |
| CSA S16 | 13.11 | Similar to AISC, uses phi = 0.75 | phi = 0.75 |
All codes use the same fundamental approach: shear resistance on parallel planes + tension resistance on perpendicular plane, with the controlling mechanism being the smaller of net-section shear fracture or gross-section shear yielding.
Preventing Block Shear — Design Strategies
| Strategy | Description |
|---|---|
| Increase edge distance | Larger Le provides more shear area Agv |
| Increase bolt spacing | Larger pitch increases Agv without affecting hole count |
| Add bolt rows | More rows distribute tension force, increasing Ant and allowing Ubs = 1.0 |
| Increase gage | Wider gage increases Ant |
| Increase plate thickness | Directly scales all areas proportionally |
| Use higher-strength steel | Fu and Fy directly scale capacity |
| Stagger bolts | Creates zigzag failure path with greater effective net area |
Frequently Asked Questions
What is block shear failure? Block shear is a tearing-out failure in bolted connections where a block of material separates from the connected member. It combines tension fracture on one plane with shear yielding or fracture on parallel planes. The failure surface looks like a U-shaped or rectangular block pulling out from the plate edge.
When does block shear govern over net section tension? Block shear governs when the bolt group is compact (short and wide) rather than elongated. A connection with few bolts but a wide gage is more likely governed by block shear. Net section tension governs for long connections with many bolts in a line. Always check both limit states.
What is the Ubs factor? Ubs accounts for non-uniform tensile stress distribution on the tension face. Ubs = 1.0 when the tension stress is uniform across the net tension area (multiple bolt rows), and Ubs = 0.5 when stress is concentrated (single bolt row, coped beams, or gusset plates). AISC 360 Commentary provides detailed guidance.
Does block shear apply to welded connections? Block shear is primarily a bolted connection limit state. However, similar combined tension-shear block failure can occur in welded connections at gusset plates and coped beams. The weld line replaces the bolt line as the shear failure plane. AISC 360 J4.3 may be applied by analogy using appropriate shear areas.
Related Terms and Pages
- Shear Lag — Definition & Design Effect
- Prying Action — Definition & Bolt Force
- Web Crippling — Definition & Design Check
- Compact Section — Definition & Limits
- Bolted Connection Calculator — Free Online Tool
- Bolt Capacity Table — AISC Reference
- Bolt Bearing & Tearout — Full Guide
- Connection Design Workflow
Educational reference only. Block shear must be checked per the governing design code (AISC 360 J4.3, AS 4100 9.1.10, EN 1993-1-8 3.10.2) by a licensed Professional Engineer for all construction applications.
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.