Timber Connection Calculator

This calculator estimates the load capacity of a bolted wood connection using a simplified version of the NDS dowel-bearing yield model. It starts with the wood's bearing strength (Fe), computed differently depending on load direction: parallel-to-grain bearing scales linearly with specific gravity as 11,200 × G, while perpendicular-to-grain bearing follows the steeper NDS relationship 6,100 × G^1.45 / √D, reflecting how much weaker wood is when a bolt bears across the grain rather than along it. From there, a simplified reference design value per bolt is derived from bolt diameter, main member thickness, and Fe, divided by a fixed reduction constant standing in for the full multi-mode yield-limit equations NDS actually uses.

Two adjustment factors then scale that per-bolt value down to a real connection capacity: a group action factor (Cg) that steps down from 1.0 to 0.85 as bolt count climbs past 2, 4, and 8 — reflecting how load doesn't distribute evenly across many fasteners in a row — and a geometry factor that penalizes bolt spacing tighter than the 4-diameter minimum NDS requires for full capacity. Multiplying adjusted per-bolt capacity by bolt count gives total connection capacity, while a separate adequacy flag checks only that minimum spacing. Because the yield equation here is a simplified stand-in rather than NDS's full Mode Im/Is/II/IIIs/IIIm/IV set, treat results as a preliminary screen and verify final connections against the complete NDS bolted-connection tables.