Behaviour and detailing of bolted and nailed joints

Mechanical fasteners (nails, screws, bolts) transfer load between timber members mainly by bearing of the fastener on the wood together with bending of the fastener.

Behaviour of bolted joints

• A bolt in a loaded joint bears against the wood. Early load–slip behaviour is nearly linear, then wood crushes under the bolt and the bolt bends, giving a ductile yielding response.
• A bolt placed in a hole 1–1.5 mm larger than its diameter first slips before taking load, so tight holes and washers (about $3d$ in size) under the head and nut are needed.
• Capacity depends on the bolt diameter $d$, member thickness $t$ (ratio $t/d$), the angle between load and grain (Hankinson's formula) and whether the joint is in single or double shear.

Behaviour of nailed joints

• Nails carry lateral load by bearing and bending; under axial load they resist withdrawal by friction.
• Joints with many small nails are ductile. Nails should not be used to carry permanent withdrawal loads.
• Hard timber is pre-drilled to avoid splitting.

Failure modes

1. Wood crushing under the fastener (bearing failure).
2. Bending / yielding of the fastener.
3. Splitting along the grain when end distance or spacing is too small.
4. Shear-out of a block (plug) at the loaded end.
5. Tension failure of the net section through the holes.
6. Withdrawal of nails or screws.

Detailing rules (typical code values; check the relevant code)

Other rules: stagger the fasteners in adjacent rows, avoid placing all bolts on one grain line, provide at least two bolts per connection, and use nail penetration in the second member of not less than $10d$ (clinched where the nail passes through).