Truss Design

Day 2

• Tensile strength of SPF - 300 psi, even though 10000 is for perfect wood. National Design Specification states 500 psi for tensile SPF, p11 for 2x4. 2x6 is lower. [1]. But that seems to be for #1 lumber, not #2. 500 seems to be right - considering snow load - [2].

Day 1 - 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. [3]
• 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. [4]. We are not fine on tension at bottom chord [5]
• 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. Yes, nice, but there is no easy way to attach the rebar to the wood. Simpson strips would do, but they are like $2/foot for what we need, so doubling up 2x4 is 1/4 the cost of <10 lb extra weight.
• 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 [6]

Links

• I-Joists