Moment and eccentric fastener groups
A group of bolts or lag screws joining a side member (wood or steel) to a main member under moment and eccentric shear: knee braces, portal frame knees, brackets. It uses the elastic method with each fastener checked at its own angle to the grain of each member, by NDS 2024 Chapters 11 and 12. LRFD or ASD.
Inputs of the moment group #
- Fasteners: bolts (single or double shear) or lag screws (single shear) with their diameter and, for lag screws, their length.
- Pattern: fasteners along the main member's grain and rows across it, with their spacings.
- Members:
- Shear planes and side members: single or double shear; wood side members, or steel plates (Fe = 1.5 Fu, Appendix I).
- Species and thicknesses: G gives the dowel bearing strengths.
- Angle β: the side member's grain to the main member's grain (45° for a knee brace to a post).
- Loads:
- P by type (dead, live, snow, wind) at its direction α from the main member's grain, acting at the eccentricity ex, ey from the group centroid.
- Moments by type at the centroid.
- Combined by ASCE 7-22.
- End and edge distances: the main member's end distance (0 for a continuous post) and edge distance, and the side member's end distance along its grain and its edge distance.
- Service conditions: moisture (Table 11.3.3) and temperature. Load duration is in the code options.
Auto-design (editor toolbar) varies the fasteners along the grain, the rows and the diameter. Objective: the fewest fasteners, then the smaller diameter.
Checks performed #
| Check | Basis |
|---|---|
| Fastener forces | Elastic method: P/n plus M r / J perpendicular to each radius, J = Σ(x² + y²) |
| Each fastener's Z | Yield limit equations (Table 12.3.1A) with Feθ by Hankinson (Eq. 12.3-11) at the fastener's own angle to each member's grain, Rd with Kθ at the larger angle (Table 12.3.1B) |
| Governing fastener | The largest load over adjusted capacity among all fasteners, not only the most loaded one |
| Adjustment | CD ≤ 1.6 or KF φz λ (λ ≤ 1.0), CM, Ct, CΔ by Tables 12.5.1A and 12.5.1B in each member for the governing fastener's direction, the least applied to all |
| Group action | Cg = 1.0 (see Limitations) |
| Edge distance and row spacing | Table 12.5.1C (4D at a loaded edge across the grain), Table 12.5.1D |
| Lag screw penetration | At least 4D without the tip (12.1.4.6) |
The drawings are the joint (members, pattern, governing fastener, the load at its eccentricity) and a section through the joint.
Limitations #
- Group action: Cg = 1.0 is our reading. Eq. 11.3-1 is derived for rows of fasteners sharing an axial load through stretching members, while the forces of an elastic moment group follow its rotation. For a group loaded mainly in direct shear along a row, use the wood fastener connection calculation with Cg.
- Direction for CΔ: parallel below 5°, perpendicular above 85°, the lesser of both between; our reading.
- Side member spacing: the side member's grain is not aligned with the pattern, so its spacing is the least center-to-center distance of the pattern; our reading.
- Not designed:
- the members' net section and moment capacity at the joint;
- local stresses (Appendix E);
- the steel plates and the fasteners' steel strength (AISC 360);
- withdrawal (lag screws loaded laterally only);
- the plastic (instantaneous center) distribution.
Verification examples #
Verification status: 10 values compared in 7 examples (5 published, 2 hand calculations from the cited equations): 10 within tolerance, 0 recorded deviations. Code basis: not yet recorded for this module.
Published values: NDS Table 12A reference values (a single bolt along and across the grain of each member). The elastic method on a single fastener reduces to the table's connection.
Hand calculations: a 2 × 2 group under a pure moment (every bolt at 45° to the grain) and the default knee brace group.
No worked example of an eccentric wood fastener group is on disk. Breyer's Design of Wood Structures and AWC's Structural Wood Design Examples are listed as needed.