Wood fastener connection

Bolts, lag screws, wood screws, nails and dowels in wood to NDS 2024 Chapters 11 and 12: the yield limit equations, withdrawal, head pull-through, combined lateral and withdrawal loads, the adjustment factors and the spacing, end and edge distance requirements, for a group of fasteners in rows.

Source note. Every provision, equation and tabulated value of this module was checked against ANSI/AWC NDS-2024 (with Commentary); the steps cite the section, table or equation and the printed page. The results are compared with 116 values of the published NDS 2024 design value tables (below).

Inputs of the wood connection #

• Fastener: the type; bolt or dowel diameter (3/8 to 1 in), lag screw diameter (1/4 to 1 in), wood screw gauge (No. 6 to 24) or nail (common, box, sinker, Table L4); the length of lag and wood screws and the thread length (0 = the standard minimum: lag screws the lesser of 6 in and L/2 + 1/2 in, Table L2; cut thread wood screws 2L/3, rolled thread the greater of 4D and 2L/3, Table L3); the lag screw body (reduced body, the basis of Tables 12J and 12K, or full body) and the wood screw thread (rolled, the basis of Tables 12L and 12M, or cut). The diameter in the yield equations follows 12.3.7: D for nails, bolts and dowels; the root diameter Dr for reduced body and rolled thread screws; for full-body screws D only where the thread bearing length in the main member is at most 1/4 of lm (12.3.7.2), otherwise Dr. Fyb is the NDS default for the fastener (Table I1) or entered.
• Members: single shear (two members) or double shear (three members, bolts and dowels); wood or steel side members; species of the main and side members (G, NDS Table 12.3.3A); thicknesses; the load angle to grain in each member (Fe∥ = 11,200 G, Fe⊥ = 6,100 G^1.45 / √D, Hankinson between; Fe = 16,600 G^1.84 below 1/4 in); steel Fu (Fe = 1.5 Fu hot rolled, 1.375 Fu cold formed, Appendix I.2); the member widths and E for the group action factor. Dowel bearing lengths: lm = p − E/2 where the penetration p includes the tapered tip E (12.3.5.3; E from Table L2 for lag screws, 2D for wood screws and nails); toe-nails lm = L cos 30° − L/3 and ls = the lesser of ts and L/3 (12.3.9.2, Commentary C12.5.3).
• Fastener pattern: rows, fasteners per row, spacing in a row and between rows, end distance (member in tension or compression), the loaded and the opposite edge distance.
• Load: the total connection load P (factored for LRFD, the ASD combination for ASD), its angle α to the wood surface (0 lateral, 90 withdrawal) and the load duration (CD for ASD, λ for LRFD).
• Service conditions and factors: moisture at fabrication and in service (Table 11.3.3: dry / dry, fabricated wet / in service dry with the exception of footnote 2, fabricated dry / in service wet, wet / wet), temperature (Table 11.3.4), end grain (Ceg; with D ≥ 1/4 in the main member takes Fe⊥, 12.3.3.4) and toe-nails (Ctn; CM does not apply to toe-nails in withdrawal, 12.5.3.1). NDS 2024 has no diaphragm factor Cdi (NDS 2018 12.5.3 had Cdi = 1.1; it is not in NDS 2024 Table 11.3.1).

Auto-design (editor toolbar) varies the fastener size of the chosen type (bolt, lag, screw or nail) and the number per row and of rows. Objectives: least fastener steel; fewest fasteners, then least steel; lowest cost index. Every other input is kept; see Auto-design.

Checks performed #

For a member loaded at an angle to grain (0° < θ < 90°) the parallel and the perpendicular geometry requirements are both applied (the more restrictive governs).

Drawings #

The drawing sheet (Drawing sheet / DXF buttons, and the report appendix) shows the fastener layout on the main member near its end, with the loaded edge at the top: the rows and fasteners at the designed end distance, spacing, edge distances and row spacing (equal spacings as one dimension, 3 @ 4" = 1'-0"), the fastener callout and the load P at its angle to grain; and the section across the members through a fastener column: the side member(s) or steel side plate and the main member with their thicknesses, the fasteners at the rows (a lag screw, wood screw or nail at its length with its penetration into the main member). Washers, the steel plate design and its holes, net section and row / group tear-out are not part of the calculation; the sheet notes say so.

Limitations #

• One fastener type, one or two shear planes; lag screws, wood screws and nails in single shear only.
• Not checked: the steel side plate (bearing, tear-out, net section) and the steel strength of the fastener (AISC 360), local stresses in the wood members (NDS 11.1.2, Appendix E: net section, row and group tear-out), eccentric connections, split rings, shear plates, timber rivets, structural screws with evaluation reports.
• Smooth shank carbon steel nails only (no stainless, ring shank, post-frame or metal hardware nails), no wood structural panel or CLT members (Table 12.3.3B, CLT narrow edge), no connections with four or more members (12.3.8) and no load at an angle to the fastener axis other than 90° (12.3.9.1).
• Rows are taken as identical (n = rows × per row) with the same Cg.
• The same end and edge distances apply to the main and the wood side member.
• Pull-through uses the side member thickness as the net thickness tns (enter a reduced ts for countersunk heads).
• Perpendicular to grain the spacing for CΔ = 1.0 is taken as 4D (Table 12.5.1B gives "required spacing for attached members").

Verification examples #

The reference values are compared with the published NDS 2024 design value tables (inputs per the table footnotes: one fastener, all factors 1.0, penetration 8D for lag screws and 10D for wood screws and nails, reduced body lag screws and rolled thread wood screws with D = Dr, A36 plates of 1/4 in and A653 Grade 33 below). The Z tables are rounded to 10 lb; every value agrees within 5 lb or 2 % (backend/tests/test_verify_wood_connection.py).

Published worked examples, mode by mode, from free documents (research/sources/references and design_examples): AWC Technical Report 12 (General Dowel Equations, 2015) and the AWC Structural Wood Design Examples 2015/2018 E2.2 and E2.5 as quoted on the free Trimble Tekla Tedds 2026 verification pages. Both take Fe from NDS Table 12.3.3 (rounded to 50 psi); the module computes Fe from G (Table 12.3.3 footnote), so the modes differ by up to 0.6 %.

The equations of the yield modes, the adjustment factors and the combined loading are also verified against hand calculations derived step by step (every mode of each case is compared):

Corrections made when the module was checked against NDS 2024: Redwood G = 0.37 (was 0.44); CM = 0.7 for fasteners under 1/4 in fabricated wet and in service dry (was 1.0); nail withdrawal CM 1.0 when fabricated and in service wet and no CM on toe-nails in withdrawal; the diaphragm factor Cdi removed; lag and wood screws use Dr per 12.3.7 (the default lag screw used the shank D, which overstated Z by up to about 50 %); lm = p − E/2 (was p − E for lag screws and p for nails and wood screws); the lag screw minimum penetration excludes the tip; the lag screw thread length is capped at 6 in; toe-nail bearing lengths; drift pins at 75 %; nail combined loading by Eq. 12.4-2; head pull-through and the Table 12.5.1E checks added.