Combined loadings — when a post carries weight and wind at the same time

Chapters 2 and 3 checked the foundry roof post two different ways: compression/buckling under the roof's dead weight, and the connections under wind uplift pulling the roof up. What neither chapter checked is wind pushing sideways on the post while it's still carrying the roof's weight straight down — both loads happen at once, not one after the other.

Why this matters: a post that passes a pure-compression check and would separately pass a pure-bending check can still fail when both act together, because the combined stress isn't just the bigger of the two — it's closer to the sum. The combined-stress check is: σ_total = P/A + Mc/I, where P/A is the axial (compression) stress from Chapter 2, and Mc/I is the bending stress from the lateral wind force pushing on the post's exposed face.

Worked case

Same 4×4 treated post from Chapter 2, carrying the 675 lb roof load in compression. Wind pushing on the post's own face (not the roof — just the post itself, since the roof uplift case was already covered separately) adds a bending moment at the base. Even with a generous wind estimate, the bending stress this adds is small next to the post's bending capacity, and the combined total stays well under the allowable — this post was never close to its limit in either direction alone, so combining them doesn't change the verdict.

Answer

The teaching point is the method, not a surprise failure: check both loads together, don't assume passing each one separately is enough, especially as a build gets taller or the post gets slenderer.