Truss Design
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32' Long Trusses with 1/2"x16" OSB Web and 2x4 Chords
- For a 32' long truss with 1/2" web, the limiting factor for span is not the web - it's the chords. In fact, there could be a break in the OSB in the middle, and it would not affect the truss bearing capacity much - but a break near the support points would be catastrophic. [1]
- 32' truss with 24" on center - 1/2" web by 16" tall - has 3200 lb on it at 50 psf, and 1600 shear at ends. Shear capacity of the OSB is 2400 lb, so we are fine on shear. [2]. We are not fine on tension at bottom chord [3]
Solution: use half inch rebar, 20' long, stapled to the bottom of the bottom 2x4 chord of the truss. 100+ staples with the rebar holds a total of 12000 lb of tension - exceeding the tension requirement of a 32' joist bottom chord - for rated design of 50 psf total roof load.- More: shear at 6' from end - at terminus of 20' mid-reinforcing rebar - is 1600 lb for 50 psf total load specification - and tensile requirement for bottom chord is 3600 lb. Quality controlled 2x4 lumber has a tensile strength of 600 psi (12 ksi for ideal)
- Big point: ideal SPF wood is 10 ksi tensile - 30x more than its 300 psi 'design rating' for tensile strength. So as long as we inspect visually for quality, we can expect 1000 psi tensile strength for a visually inspected, defect-free specimen. Thus, we can use a single 20' rebar in the middle of 30' truss - and be fine for the required 3600 lb tension at the 6' point where the 20' stick or rebar ends - if we rely on the bottom chord of 2x4 lumber to hold the remaining tension [4]
