Foundry roof — wind uplift

Closes the last open item from Chapters 1–2 — does wind try to lift the foundry roof off, using Charleston, WV's real design wind speed?

Concepts pulled in

Velocity pressure (q = 0.00256 × Kz × Kzt × Kd × V²)
Net pressure coefficients for an open/canopy roof

The problem: closes the last open item from Chapters 1–2 — does wind try to lift the foundry roof off? Same 10×10 ft shed roof. Real data: Charleston, WV's own city building documents give the design wind speed directly — Vult = 107 mph (ultimate), Vasd = 82.9 mph (allowable-stress basis, matching the lumber stress values used in Ch. 1–2).

Concepts pulled in: velocity pressure (q = 0.00256 × Kz × Kzt × Kd × V²) and net pressure coefficients for an open/canopy roof — higher than a sealed building, since there's no wall holding pressure inside. Assumed: Exposure B, Kz = 0.57, Kzt = 1.0, Kd = 0.85, gust factor G = 0.85, representative net uplift coefficient Cp ≈ 1.2 for an open shed roof.

Calculation

qz = 0.00256 × 0.57 × 1.0 × 0.85 × 82.9² ≈ 8.5 psf
Uplift pressure: p = qz × G × Cp ≈ 8.5 × 0.85 × 1.2 ≈ 8.7 psf
Total uplift on the 100 sq ft roof: 8.7 × 100 ≈ 870 lb
Compare to roof dead weight: 7 psf × 100 sq ft = 700 lb

Answer

Wind uplift (870 lb) exceeds the roof's own dead weight (700 lb) — in a real gust this roof wants to lift off, not just sit there. The allowable-stress load combination for uplift (0.6 × dead load + 0.6 × wind) lets only 60% of the dead weight resist, so the net to design for is about 870 − 420 = 450 lb across the whole roof, not the 170 lb you get by counting all of the dead weight. Beam-to-post and post-to-footing connections need actual uplift-rated hardware (hurricane ties, post base anchors, through-bolts), not gravity/friction alone.

Honest flag: open/canopy structures carry their own ASCE 7 pressure-coefficient provisions, different from a sealed building. This chapter uses a representative coefficient to get the concept and order of magnitude right; a permit-level design should pull the actual open-structure coefficients before finalizing hardware specs.

This closes the open items from Chapters 1–2 for the foundry roof. The NDS column check on the post (Ch. 2's open item) is now done in Breyer Chapter 4, and Breyer Chapter 6 works out what a through-bolt or steel plate holds against this uplift.

Try your own numbers

Starts on this chapter's foundry roof. Change anything to test your own open roof. This finds the order of magnitude of the lift with representative coefficients, and the real hardware needs the actual open-structure values from the code. The ground elevation factor Ke from ASCE 7-16 (never above 1.0) is left out, which is conservative.

Design wind speed (mph)82.9 = 107 × √0.6, the ASCE 7 conversion from the ultimate speed to the allowable-stress level. The 107 is the chapter's reading of Charleston's city building documents and was not independently verified: look your site up in the ASCE 7 Hazard Tool. Keep the basis consistent with your other numbersExposure coefficient, Kz0.57 = Exposure B at 15 ft or less, as in the chapter. ASCE 7-16 lowered Exposure B Kz for low roofs; older practice used 0.70, which would raise the uplift about 23%Topographic factor, Kzt1.0 = flat ground; a hilltop is higherDirectionality factor, KdGust factor, GNet uplift coefficient, Cp1.2 is a representative open-roof value, not a code lookupRoof length (ft)Roof width (ft)Roof dead weight (psf)roofing + framing

Wants to lift off869 lb of uplift

Velocity pressure: qz = 0.00256 × 0.57 × 1 × 0.85 × 82.9² = 8.5 psf
Uplift pressure: p = qz × G × Cp = 8.5 × 0.85 × 1.2 = 8.7 psf
Total uplift on 100 sq ft: 8.7 × 100 ≈ 869 lb
Roof dead weight: 7 psf × 100 sq ft = 700 lb
Uplift beats the dead weight by about 169 lb
The allowable-stress rule for uplift (0.6 × dead load + 0.6 × wind, ASCE 7-16 2.4.1) lets only 60% of the dead weight resist: 0.6 × 700 = 420 lb, so the net uplift to design the connections for is about 449 lb across the whole roof

In a real gust this roof wants to lift off, not just sit there, so the beam-to-post and post-to-footing connections need uplift-rated hardware (hurricane ties, post base anchors, through-bolts), not gravity or friction alone.

Open and canopy structures have their own pressure-coefficient provisions, different from a sealed building. Pull the actual values for a permit-level design before choosing hardware.

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