Built-up and spaced columns
Wood columns made of several pieces, by NDS 2024 15.3 and 15.2, with ASCE 7-22 combinations. LRFD or ASD. A built-up column is two to five laminations nailed or bolted together. A spaced column is two or three members held apart by blocks and joined at the ends by split rings or shear plates in end blocks.
Inputs of the column #
- Column: nailed built-up (15.3.3), bolted built-up (15.3.4) or spaced (15.2).
- Laminations or members: sawn lumber of one species, grade and size (Supplement Tables 4A, 4B, 4D), and their number: 2 to 5 laminations, or 2 or 3 spaced members. If the pieces differ, enter the weakest (15.3.1, 15.2.3.4).
- Unbraced lengths l1 and l2:
- Built-up column: d1 is the face width of a lamination and d2 the thickness of the assembly (n t).
- Spaced column: d1 is a member's thickness (perpendicular to its wide faces) and d2 its width.
- Ke (Appendix G) gives le = Ke l. For a spaced column, le is not taken less than l (15.2.3.1).
- During construction: raises the le/d limit of a built-up column from 50 to 75 (15.3.2.3).
- Nailing or bolting: the nail (common wire nails) or the bolt diameter, the rows, the spacing in a row and between rows, the end distance and the edge distance. They are checked against the rules of 15.3.3.1 or 15.3.4.1.
- Spaced column end blocks:
- Connector position: the centroid of the end-block connectors from the column end. Within l1/20 is condition a (Kx = 2.5); within l1/10 is condition b (Kx = 3.0).
- l3: from the spacer block to the end connectors.
- Connector capacity: the end-block connectors' capacity at each contacting surface, from your timber connector calculation.
- Kx in FcE: see Limitations.
- Loads and service: axial dead, live, roof live and snow loads; wet service; temperature. The load duration factors are in the code options.
Auto-design (editor toolbar) varies the lamination or member size and the number of laminations or members. Objective: the least wood. Species, grade, lengths, fasteners, end blocks and loads stay yours.
Checks performed #
| Check | Basis |
|---|---|
| Built-up column stability | CP by Eq. 15.3-1 on le1/d1 (Kf = 1.0) and on le2/d2 (Kf = 0.6 nailed, 0.75 bolted); the smaller CP, but not below that of a lamination as a solid column (15.3.2.2) |
| Built-up slenderness | le/d ≤ 50, or 75 during construction (15.3.2.3) |
| Nailing (15.3.3.1) | 15D ≤ end distance ≤ 18D; 20D ≤ spacing in a row ≤ 6 tmin; 10D ≤ spacing between rows ≤ 20D; 5D ≤ edge distance ≤ 20D; two or more rows where d > 3 tmin; nails through all laminations and 3/4 of the outermost |
| Bolting (15.3.4.1) | 7D ≤ end distance ≤ 8.4D (softwoods); 4D ≤ spacing in a row ≤ 6 tmin; 1.5D ≤ spacing between rows ≤ 10D; 1.5D ≤ edge distance ≤ 10D; two or more rows where d > 3 tmin; washers and tight nuts (note) |
| Spaced column stability | CP by Eq. 15.2-1 on le1/d1 with Kx, not more than the member as a solid column on l2/d2 (15.2.3.5) |
| Spaced column limits | l1/d1 ≤ 80, l2/d2 ≤ 50, l3/d1 ≤ 40 (15.2.3.2); end-block connectors within l1/10 of the end (15.2.2.1) |
| End spacer block constant | Ks by species group (Table 13A from G): connector capacity at each contacting surface ≥ member area × Ks (15.2.2.5) |
| Compression | fc = P / A against Fc' = Fc* CP for every combination |
The drawings are the column elevation (length, axial load, mark) and the column section (the laminations and fastener rows, or the spaced members).
Limitations #
- Kx in FcE: the NDS 2024 printing of 15.2.3.3 writes FcE = 0.822 Emin' / (le/d)² and still defines Kx (2.5 for condition a, 3.0 for condition b) without using it. NDS 2018 writes FcE = 0.822 Kx Emin' / (le/d)². The calculation applies Kx by default (the 2018 form). You can clear the box to follow the 2024 text as printed, which is conservative. This is to be confirmed against the code text and AWC errata.
- Axial load only: bending of built-up and spaced columns (3.9, 15.2.3.7) is not covered.
- Sawn lumber only: one species, grade and size for all pieces.
- End blocks: the split rings or shear plates in the end blocks are designed in the timber connector calculation; here their capacity is an input. Spacer block details (15.2.2.2, 15.2.2.4) are notes.
- Built-up fastener layout: the fastener rules are checked as entered. The calculation does not lay out the nails or bolts.
Verification examples #
Verification status: 14 values compared in 6 examples (2 published, 4 hand calculations from the cited equations): 14 within tolerance, 0 recorded deviations. Thin: only 2 published examples. Code basis: not yet recorded for this module.
The code's own Figures 15C and 15D (typical nailing and bolting schedules) pass every fastener rule. The column capacities are hand calculations of Eqs. 15.3-1 and 15.2-1 with the Supplement values (Douglas Fir-Larch No. 2):
- three 2x6 nailed and bolted, in ASD and LRFD;
- two 3x6 spaced, with and without Kx, with the spacer block constant Ks.
No worked example of a built-up or spaced column is on disk. AWC's Structural Wood Design Examples are listed as needed.