A Bolt Through a Bamboo Culm Does Not Use the Culm's Strength: Splitting and Bearing Bounds at Pinned Joints, and a Lashing Rule That Does
Year: 2026
Type: article
Abstract: Bolting is the connection method a builder trained on timber or steel reaches for first, and in bamboo it is applied with an expectation this paper tests and disproves by arithmetic: that a bolted joint transfers something like the culm's own strength. For a reference culm of 100 mm diameter with a 10 mm wall — crushing strength 155.5 kN, safe axial load at 2.0 m unsupported length 38.6 kN — a single 12 mm through-bolt bears only on the two thin walls it passes through, a bearing area of 240 mm². At a stated bearing strength of 35 MPa its ultimate capacity is 8.4 kN, which is 5.4% of the culm's crushing strength; with a joint safety factor of 3.0 its safe transfer is 2.8 kN, or 7.3% of the culm's safe axial load. The competing failure, shear-out splitting of the material ahead of the bolt, gives an ultimate bound of 20.7 kN at a generous 80 mm end distance and takes over as the governing mode only when the end distance falls below 36 mm — close to the culm end, which is where workshop bolts are often placed. Enlarging the bolt helps slowly: a 20 mm bolt raises the safe transfer only to 4.7 kN while removing more of the wall. Two alternatives are computed on the same basis. A lever-tightened lashing of 12 turns of 10 mm cord transfers a safe 3.75 kN — modestly more than the hollow bolt, with no hole drilled and with the capacity recoverable by re-tensioning — and a double lashing transfers a safe 6.75 kN. A bolted joint with the culm filled at the joint by a fitted plug, so the bolt bears across the full 100 mm section instead of two 10 mm walls, transfers a safe 12.0 kN, which is 4.3 times the hollow bolt's value and 31.1% of the culm's safe axial load — the best single joint computed, and still nowhere near full-strength. The selection rule originated here assigns each joint type a load ceiling and forbids designing any single joint as if it were the culm. This study is simulated, not measured, and no joint described here has been load-tested.
Author: Aldred, William Alexander
Language: English
Subfield: Plant Science
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W7225739617
FWCI:
Cites:
Cited by:
Related to: