Wood Column Design Step by Step — Worked Example, Column Stability Factor CP (NDS 2024)

What you will see

• Starting from an empty module, every input entered on camera
• Step 1: the column's unbraced length, 8 ft
• The end-condition buckling mode picked from NDS Table G1 (Appendix G), with a diagram of how the column buckles in each mode. Fixed at both ends but free to sway gives Ke = 1.2
• Each finished step collapsing to a blue summary line
• Step 2: dead and live axial loads entered separately by ASCE 7-22 load type, with member self-weight added into D automatically, and CD taken from the governing ASD combination (NDS Table 2.3.2)
• Step 3: Douglas Fir-Larch No. 2 selected from the real NDS Supplement Table 4A rows, Fb = 900 and Ft = 575 psi on the row as it is clicked
• The section chosen from NDS Supplement Table 1B, so a 4x6 is the dressed 3.5 in x 5.5 in it really is
• Step 4: the NDS adjustment factors, wet or dry service (CM), temperature (Ct) and incising (Ci)
• Step 5: the 4x6 comes back overstressed and fails, because weak-axis slenderness, not axial stress, limits the capacity
• Going back for a deeper section and re-running until every demand-capacity ratio is under 1.0
• A load-combination row opened to show fc = P/A, Fc* = Fc x CM x Ct x CF x Ci x CD, CP per NDS 3.7.1.5 Eq. 3.7-1, F'c = Fc* x CP, and the ratio

The column in this example

8 ft unbraced height, fixed-fixed with sway, Ke = 1.2 from NDS Table G1, so Le = 1.2 x 96 in = 115.2 in. Axial dead load 4000 lb and live load 5000 lb, plus the member's own self-weight. Douglas Fir-Larch No. 2 from NDS Supplement Table 4A: Fc = 1350 psi, Fb = 900 psi, Ft = 575 psi, E = 1,600,000 psi, Emin = 580,000 psi. The first trial, a 4x6 (dressed 3.5 in x 5.5 in, A = 19.25 sq in), fails. Both trial sections are only 3.5 in wide, so buckling is governed by the weak axis at Le/d = 115.2/3.5, about 32.9, unchanged by going deeper. The fix is a 4x8 (dressed 3.5 in x 7.25 in, A = 25.38 sq in), which wins on area, and it passes. Behind CP: NDS 3.7.1.5 Eq. 3.7-1 with FcE = 0.822 Emin' / (Le/d) squared and c = 0.8 for sawn lumber, holding Le/d to the 50 limit of NDS 3.7.1.4.

Codes referenced: NDS 2024 Section 3.7, 3.7.1.4, 3.7.1.5 Eq. 3.7-1, Table 2.3.2, Table 2.3.3, Appendix G Table G1; NDS Supplement Tables 1B and 4A; ASCE 7-22 Chapter 2.