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This look at fastener load sharing was published during a free Kindle week for four engineering books in November 2026.
Line up six fasteners to join two plates, and it's natural to assume each carries a sixth of the load. It doesn't. In a simple lap joint the outer fasteners usually do much more than those in the middle, and that fact shapes how aircraft joints are designed.
Think of both plates as stiff springs, and each fastener as a smaller spring connecting them. Load starts in one plate and must transfer to the other. At the first fastener, the first plate is stretched the most relative to the second, so that fastener is pushed hardest. In the middle of the joint, the plates stretch at much the same rate, so little needs to transfer.
That's why the end fasteners attract the most load, and why adding more and more fasteners gives diminishing returns.
How a joint is checked
Analysts estimate each fastener's share using its flexibility along with the plates' stiffness. Then they check the different ways a joint can fail.
- Fastener shear, where the fastener itself is cut through.
- Bearing, where the fastener deforms the hole.
- Net section tension, where a plate tears across a row of holes.
- Edge tear-out, if a fastener sits too close to the edge.
At high enough loads, metal helps by yielding. Local yielding allows load to spread more evenly among fasteners, which is why ultimate strength checks can often assume more even sharing than fatigue checks.
Fatigue is usually the bigger concern
Joints are a classic source of fatigue cracks. Each hole concentrates stress, and the highly loaded end fastener combines hole stress with load transfer. Designers reduce this with tapered plates that lower stiffness at the ends, careful fastener spacing and methods that improve the fit.
Countersunk fasteners, used to keep outer skins smooth, need extra care because the countersink removes material. On the aerodynamic surfaces of an aircraft, that compromise is usually worth making.
For more on how holes carry load, see our earlier explainer on how lugs fail.
The four books
Practical Airframe Stress Analysis, Volume 1: Metallic Structure
GD&T Professional Exam Guide
Practical Airframe Stress Analysis, Volume 2: Composite Structure
Python for Aerospace Stress Engineers (Volume 3)
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Related gear
- Practical Airframe Stress Analysis, Volume 1: Metallic Structure (Sandeep Bandyopadhyay) on Amazon — Related gear
- Aero Structures and Composite Materials with Artificial Intelligence (book) — Sandeep's book
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