Fire tower legs: posts and timbers under the NDS

Hibbeler Chapter 7 found that a 16 ft 4×4 leg for the test fire tower carries its load easily but is too slender: its le/d is 54.9 against the NDS cap of 50. Two fixes were offered: a 6×6 leg, or bracing the 4×4 at mid-height (Chapter 12). Both used simple Euler buckling. This chapter checks both with the NDS column method from Breyer Chapter 4, and it needs the stress values for posts and timbers, which are not the same as for 4× lumber.

What changes at 5×5 and larger

• The NDS calls square or nearly square lumber 5×5 (nominal) and larger "posts and timbers" (4.1.3.4), graded primarily for use as posts or columns, and gives it its own table. For Southern Pine the Southern Forest Products Association's Table 3 lists six grades.
• The stress values are lower than for 4× dimension lumber, not higher. No. 2 timber has Fc = 525 psi and Emin = 440,000 psi, against 1,450 psi and 510,000 psi for a No. 2 4×4. A 6×6 wins on size (30.25 in² against 12.25 in²), not on stress. The source tables give the numbers but not the reason, and I did not find one.
• Other grades: No. 1 has Fc 825 and Emin 550,000; Select Structural has Fc 950 and Emin 550,000; the Dense grades add a little. The calculator has all six.
• No wet service factor: the timber values are for dry or wet service as they stand. And no incising factor, which the NDS (4.3.8) gives for dimension lumber only.

The tower leg: 16 ft, 112 lb, ordinary live load, pinned

• 4×4 unbraced: le/d = 192 in ÷ 3.5 in = 54.9, over 50. Not allowed. (Plain Euler buckling for it is 4,687 lb, matching Chapter 7, but the NDS caps slenderness regardless of load.)
• 4×4 braced at mid-height, so the unbraced length is 8 ft: le/d = 27.4, FcE = 557 psi, CP = 0.35, Fc′ = 504 psi, so about 6,170 lb per leg. That is about 55 times the 112 lb load.
• 6×6 No. 2 timber at 16 ft: le/d = 34.9, FcE = 297 psi, Fc* = 525 psi, CP = 0.48, Fc′ = 251 psi, so about 7,590 lb per leg, about 68 times the load.
• Better grades help the 6×6: No. 2 Dense about 8,290 lb, No. 1 about 9,915 lb, Select Structural about 10,120 lb. An 8×8 No. 2 at 16 ft carries about 20,900 lb.
• The tallest allowed unbraced leg (pinned both ends): 14.6 ft for a 4×4, 22.9 ft for a 6×6, 31.3 ft for an 8×8.

Answer

Both fixes pass with a huge margin on the straight-down load: the braced 4×4 and the unbraced 6×6 each carry dozens of times the 112 lb per leg. Slenderness, not strength, was the problem, and both fixes solve it. The 6×6 is simpler to build: no bracing geometry, no extra connections. The braced 4×4 uses far less lumber but needs the bracing members and their lag-screw connections (Breyer Chapter 2). With a margin this size, the choice is about wind, cost and build effort, not column strength.

Where to be cautious

• A tower leg is part of a frame, not a lone post. If the frame can sway, the legs are not pinned at the braces; use a larger Ke (Table G1 runs to 2.1 and 2.4 for ends free to move). The calculator has the fixed-free case: a 6×6 held only at its base is allowed up to 10.9 ft.
• Straight-down capacity is not the tower's limit. Wind on a 16 ft tower puts bending on the legs and an overturning moment on the whole frame, with uplift at the windward legs and the anchorage. None of that is in this chapter.
• Bracing only counts if it really holds the leg sideways: a brace that is loose or flexible is not a support point.
• The 112 lb per leg is Chapter 7's estimate for a one-person platform. A platform with more people or equipment, or snow, changes it, and so does the load duration factor.
• Large timbers check and shrink as they season; the NDS values assume the grade stamp and drying are what they claim.
• Everything here is a worked example. A tower people climb is for a structural engineer to design and stamp.

Open items for a later pass

The wind load on the tower and the resulting forces in the legs and braces (Chapter 12's open item), frame sway and the real effective length of a braced leg, uplift and overturning at the anchors, the connections, and rectangular timbers such as 6×8 (the calculator covers square posts only). The buckling length coefficients are the NDS recommended values as listed in a university course table, since Table G1 itself could not be read.

Sources

• American Wood Council, NDS 2018 with Commentary (free at awc.org): 4.1.3.4 (posts and timbers), 4.3.8 (incising, dimension lumber only), 3.7 (column stability factor and the le/d limit of 50), Appendix G.
• Southern Forest Products Association, Southern Pine Use Guide, 2013: Table 3 (timbers 5×5 and larger, dry or wet service) and Table 1 (4× dimension lumber).
• Written from these public documents, not from the Breyer textbook itself. If the book disagrees, the book and the current NDS win.