Ogopogo modular joints — fastener-limited vs. member-limited connections

The problem: the Ogopogo (formerly Pathfinder) modular building system's standard "moon eye"/"melon head" joints — when is a lag-bolted connection enough, and when should it switch to integral joinery (mortise-and-tenon, French cleat)? Standard fasteners: 3/8" and 1/2" lag bolts for most connections; integral joints for high-load joints by design instinct already.

Concepts pulled in — new, connection design, not member sizing: fastener-limited vs. member-limited joints. A bolted connection's capacity is capped by the fastener's own shear strength — a comparatively sudden, brittle failure mode. An integral joint transfers load through the wood's own bearing/shear capacity, which scales with the wood section (can be sized arbitrarily large) and tends to fail more gradually.

Published NDS single-shear values for one lag screw (2018 NDS Table 12J, Southern Pine, a 1-1/2" side member, 8 diameters of penetration into the main member, before adjustment factors): 3/8" lag ≈ 210 lb loaded along the grain and 140 lb across it; 1/2" lag ≈ 410 lb along the grain and 230 lb across it. Breyer Chapter 2 works this joint in full with a calculator.

The threshold logic (the actual standing rule for the system)

• Up to ~1,000–1,200 lb per joint: three half-inch lag bolts carry about 1,200 lb loaded along the grain in Southern Pine; across the grain the same three carry only about 680 lb, and it takes about five to reach 1,000 lb
• Beyond that, adding more bolts starts clustering fasteners into a shrinking wood area — splitting risk goes up faster than capacity does

Answer / rule of thumb (confirmed against Zach's own practiced instinct)

Count the bolts a joint's load requires in standard sizes. Past 4 bolts/screws between two boards, switch to a more secure fastener or an integral joint rather than adding more hardware — this matches the fastener-cluster math almost exactly, and the reason is the same either way: past that count, a bolt pattern needs more edge/spacing distance than the board has left, so you run out of usable width before you run out of needed capacity.

Field evidence behind the rule: 5 lag bolts in an 8 ft 2×6 will split the board — the count where the spacing math actually breaks down in practice, one past the 4-bolt working limit. (Breyer Chapter 2 checks this against the NDS: neither the published minimum distances nor the NDS wood tear-out check forbids a fifth half-inch lag in an 8 ft 2×6, so treat four as a conservative working limit that comes from experience with splitting, not from a published cutoff.)

Open items for a later pass

The NDS lag-screw values and spacing minimums are now worked in Breyer Chapter 2. Still open: the real moon-eye and melon-head joint geometry, which sets the load direction relative to the grain, and whether the fasteners can meet the minimum end and edge distances in the actual parts.