Lag Screw Connection Design

NDS® 2018 Ch. 12 — ASD yield-limit method — single shear, wood side member or steel side plate

Lag Screw Connection — Design Calculation

NDS® 2018 for Wood Construction, Chapter 12 (Dowel-Type Fasteners) · Allowable Stress Design
Project
—
Connection
—
Engineer
—
Date
—

1 · Design parameters input echo

2 · Fastener geometry & penetration NDS 12.1.4.6, App. L

3 · Installation — lead holes NDS 12.1.4.1–12.1.4.5

4 · Reference withdrawal design value NDS 12.2.1, Eq. W = 1800 G3/2 D3/4

5 · Reference lateral design value NDS 12.3.1, Table 12.3.1A (single shear: Im, Is, II, IIIm, IIIs, IV)

QuantityExpressionValue
Yield modeEquationZ (lb)

6 · Spacing, end & edge distance — CΔ NDS 12.5.1, Table 12.5.1A–F

CheckProvidedMin (reduced)Full valueRatioStatus

7 · Adjusted design values & demand check NDS Table 11.3.1, 12.4.1

8 · Notes & assumptions

  1. Lateral capacity uses the 2015/2018 NDS yield-limit (ASD) method, evaluating all six single-shear yield modes — Im, Is, II, IIIm, IIIs, and IV — per Table 12.3.1A (Eq 12.3-1 through 12.3-6); the minimum governs. Reduction terms Rd are from Table 12.3.1B (4Kθ for Im/Is, 3.6Kθ for II, 3.2Kθ for III/IV), with Kθ = 1 + 0.25(θmax/90).
  2. Dowel bearing strengths follow Table 12.3.3: Fe∥ = 11,200·G and Fe⊥ = 6,100·G1.45/√D, combined at an angle to grain by the Hankinson formula (Eq 12.3-11). Dowel diameter is Dr (root) per 12.3.7.1, with full-body D permitted per 12.3.7.2 when the threaded bearing length in the main member is ≤ ¼ of the bearing length there. For lag screws with nominal D ≥ 1/4″ but Dr < 1/4″ (1/4″ and 5/16″ sizes), Rd = KD·Kθ per Table 12.3.1B Note 1.
  3. Bearing length: per 12.3.5.3, the main-member dowel bearing length lm = (embedded length) − E/2, crediting half of any embedded tapered tip. Minimum penetration p (excluding the tip) ≥ 4D per 12.1.4.6. Withdrawal uses thread penetration pt (threaded length in the main member, excluding the tapered tip) per 12.2.1.2.
  4. Withdrawal: W = 1800·G3/2·D3/4 lb/in. of thread penetration (Eq 12.2-1), times pt. End-grain withdrawal applies Ceg = 0.75 (12.2.1.3). Per 12.2.1.4, the lag screw tensile strength at the net (root) section, Aroot = πDr²/4, must not be exceeded — verify against the fastener steel; not checked here.
  5. Standard lag screw dimensions — root diameter Dr, tapered tip E, and minimum thread length T = min(6″, L/2 + 0.5″) — are taken from NDS Appendix L, Table L2; verify against the manufacturer's data. Fyb per the NDS lag screw lateral tables: 70 ksi (1/4″), 60 ksi (5/16″), 45 ksi (≥ 3/8″).
  6. Group action factor Cg (NDS 11.3.6) depends on member EA and fastener spacing and must be computed separately and entered. Cg and CΔ apply to the lateral value Z only.
  7. Fastener metal capacities (head pull-through, washer bearing) and, for steel side plates, the plate itself (bearing, tearout, net section, bending) per AISC are not checked here. For a steel side member, Fes = 1.5·Fu (87,000 psi for A36) per the Table 12.3.3 basis.
  8. Lag screws shall be installed in properly sized lead holes (12.1.4.2) and turned in with a wrench, not driven with a hammer; soap or other lubricant may be used (NDS 12.1.4.4–12.1.4.5).
  9. This tool assists a qualified design professional and does not replace engineering judgment or code review.