Remember when your science teacher said: every action has an opposite and equal reaction? Well every building load has an equal “reaction load”. The structural goal of a house is to safely transfer building loads (weights) through the foundation to the supporting soil. A complete analysis of wood’s mechanical properties is complex, but understanding a few basics of lumber strength will allow you to size joists and rafters with the use of span tables. Wood is naturally engineered to serve as a structural material: The stem of a tree is fastened to the earth at its base (foundation), supports the weight of its branches (column) and bends as it is loaded by the wind (cantilever beam). I pointed out that we had a potential sheathing issue, flipped to the lumber sheathing table and I took him up to show that we were using T&G sheathing that was appropriate for the 4' spacing.Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. It looked impressive but was structurally about the same as a stick framed roof, we were simply twice as thick and half as often. The new inspector came on a log and timber job I was doing a few years back, looked up at the roof framed of 4x10 DF and remarked that it had to be one of the strongest looking roofs he had seen. If we are engineering just from tables it would be fine to use the 12" oc 2x4 table for the allowable span. That extra half inch of thickness in the 4x4 accounts for the increase in section properties, nothing mysterious. The section modulus and the moment of inertia for the 4x4 are more than twice that of a 2x4. In that post you roughly equated a 4x4 to a 2x4 and suggested ignoring the the additional thickness. From my read of your original post (#2) it sounds like you are proposing using the 24" oc 2x4 table. In the op the 4x4 rafters are on 2' centers. I pointed out that we had a potential sheathing issue, flipped to the lumber sheathing table and I took him up to show that we were using T&G sheathing that was appropriate for the 4' spacing. They are planning on voting this upcoming January on whether those design values that they put into play in June are valid (they are). Sistering a 2圆 to the existing scrapes by on paper. Allowable span for this situation dropped 8" due to the recent derating. MOE was 1.6 x 106psi so the deflection even using old values is 2.24". Overspanned based on strength alone but not horribly. With the higher bending strength material the section modulus required drops to 7.607"3. Using the old design values I get an adjusted Fb of 2156psi. This was probably pulled before 6/1/12 and who knows what is being enforced, so in trying to make the math fit the existing. Putting 4x4's on 12" ctrs still has 1.28" deflection. certainly passes l/180, 4圆's would have worked. Plugging in Rio's 5.45" depth deflection drops to. I'm curious, Texas I'm assuming #2 SYP, which design values are you using in Strucalc? I tried it with new post 6/1/12 allowable design values and had an adjusted Fb of 1509 psi, which required a rafter section modulus of 10.89"3 remember a 4x4 is 7.146"3.īut the controlling factor is deflection, the new E value is 1.4 million psi giving a deflection of 2.56".
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