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NAIBC / IRC codes
References & Sources
Standards Cited
- NDS — National Design Specification for Wood Construction
Standard for structural wood design in North America - PS 20 — American Softwood Lumber Standard (ALSC)
Defines nominal and actual dimensions for dimension lumber
Formulas Used
- Member count (area-based):
count = ceil(perpendicularSpan / spacing) + 1
Source: NDS — Perpendicular span = area / member length; +1 for the end member - Total linear length:
totalLength = memberCount × memberLength
Source: Basic geometry - Volume:
volume = totalLength × crossSectionWidth × crossSectionDepth
Source: Basic geometry - Board feet:
BF = actualWidth_in × actualDepth_in × length_ft / 12
Source: PS 20 / ALSC — Board foot is the standard NA unit for lumber volume (1 BF = 144 cubic inches)
Key Assumptions
- Lumber sizes use actual (dressed) dimensions: 2×4 = 1.5" × 3.5" (38 × 89 mm) — PS 20
- Standard lumber lengths: 8 ft, 10 ft, 12 ft, 14 ft, 16 ft — Industry standard