Roof Truss Wind Uplift — Bottom Chord and Web Bracing Worked Example
A light roof in a windy place: net uplift reverses every member force in the truss. The bottom chord, sized as a tie, becomes a column, and so do the long webs. The forces are small, but the NDS slenderness limit doesn't care how small they are. This example shows the checks that catch it and the bracing that fixes it.
The truss
| Truss | Same 28 ft, 6/12 Fink as the gravity example, 24 in on center, DF-L No. 2, 2x6 chords and 2x4 webs |
| Dead load | TCDL 7 psf (light metal roof), BCDL 5 psf |
| Snow | 20 psf |
| Wind | Net uplift W = 30 psf on the top chord (given; from ASCE 7 wind pressures for the site) |
| Combinations (ASD) | D, D + S, D + 0.6W, 0.6D + 0.6W, each with its own CD (0.9, 1.15, 1.6, 1.6) |
Defaults, with TCDL 7, BCDL 5 and wind uplift 30. Then try the bracing options.
Open the truss calculator →0.6D + 0.6W
0.6W = 0.6 × 30 psf × 2 ft × 28 ft = 1,008 lb up
Net = 604.8 lb up → R = −302.4 lb at each bearing (uplift)
That 302 lb is the uplift your truss-to-wall connector must resist. Connector catalogs list allowable uplift loads with CD = 1.6 already included, so compare ASD to ASD.
Gravity tension becomes uplift compression
| Member | D + S (lb) | 0.6D + 0.6W (lb) |
|---|---|---|
| Bottom chord, heel panels 1–2, 3–4 | +1,321 | −468 |
| Bottom chord, center 2–3 | +880 | −312 |
| Top chord, heel panels | −1,477 | +523 |
| Long webs 2–6, 3–6 | +453 | −107 |
| Short webs 2–5, 3–7 | −341 | +174 |
468 lb in a 9.33 ft 2x6: depends entirely on the ceiling
Fc* = 1350 × 1.6 × 1.1 = 2,376 psi; FcE = 1,149.7 psi; CP = 0.4222; Fc′ = 1,003 psi
fc = 468 / 8.25 = 56.7 psi, plus ceiling bending (0.6 × 5 psf × 2 ft = 6 plf) fb = 103.7 psi → 0.054 ✓
No ceiling, braced only at panel points: le/d = 9.333 × 12 / 1.5 = 74.7 > 50 → not permitted (NDS 3.7.1.4)
107 lb that you cannot ignore
With a continuous lateral restraint (CLR) at mid-length: le/d = 33.65 → OK; the web then governs in D + S tension at 0.087
This is why truss drawings call for CLRs on long webs even when the gravity forces are tension. In the calculator, set "Web out-of-plane bracing" to "Lateral restraint at mid-length". The maximum demand/capacity then drops from "∞" to 0.405, which is the top chord under D + S.
Gravity sizes the members; wind sizes the bracing and the anchors
With the ceiling attached and the webs braced, the worst ratio is the top chord under D + S (0.405). Uplift didn't change a single lumber size. It added two lines to the drawings, the bottom chord bracing and the web CLR, and set the connector capacity at 302 lb. Those lines are what fail in hurricanes.
Tools used in this example
Truss calculator with wind uplift and the bracing options, the gravity worked example for the same truss, and the roof truss how-to guide for choosing the inputs.
Frequently asked questions
Why does wind uplift put the bottom chord of a truss in compression?
When the net load on the roof acts upward, it reverses the direction of every member force. The bottom chord, a tension tie under gravity, carries compression, and because it is a long member it can buckle unless the ceiling or permanent bracing restrains it.
What is the maximum slenderness ratio for a wood compression member?
NDS Section 3.7.1.4 limits the slenderness ratio le/d of a solid wood column to 50 in service (75 during construction). A 2x member laid in a truss has d = 1.5 in about its weak axis, so it needs lateral bracing at no more than 75 in (6.25 ft) when it carries any compression.
Which load combination gives the maximum uplift?
In ASD, 0.6D + 0.6W (ASCE 7 §2.4.1): only 60% of the dead load is counted as resisting uplift. In LRFD, the equivalent is 0.9D + 1.0W.