Stregthen trusses

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RicodJour wrote:

Have you ever met an EE who didn't _think_ he understood every other engineering discipline?

Sure they will, if he sisters enough of them (like built up to about 4" thick) alongside all of the existing 2x4 members in the present trusses.
That certainly wouldn't be my first choice about how to do it...
--

FF


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snipped-for-privacy@spamcop.net wrote:

Notice how the OP didn't mention the connections, gussets nor anything else? If he bolted 1/2" by 3 1/2" steel plate to the existing truss members it wouldn't triple the strength of the trusses. The weak point, the limiting factor, is the existing gang nail plates.
R
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RicodJour wrote:

Yes but I assumed he wasn't so ignorant as to assume that a series of unconnected segments would add strength to the trusses.
He may be thinking about adding them to make a T- section, rather than sistering them. That might do what he wants, provided he joins them all properly where all the pieces intersect but it would be a major pain compares to just sistering them. Sistering them with 2x6 (properly connected with nailers might triple the strength too, keeping in mind that by nailing the 2x6 to the existing 2x4 (again with proper connections at the joints) they become one truss, not two trusses sharing the load unequally due to unequal stiffness.
--

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snipped-for-privacy@spamcop.net wrote:

I am with you on giving people the benefit of the doubt...on matters that won't cause big problems. On those matters I prefer to err on the side of caution.

The operative phrase being "proper connections at the joints". Those are the areas where someone will run into trouble.

around the ridge and plates?
In any event, the OP will do what he wants based on what information he has and what he believes to be the important factors. He will then make up his mind that he either did or did not do the right thing. If the thing fails, well, that's clear, if the thing doesn't fail, he'll believe that he did the right thing - whether he did or not. That's simply human nature.
R
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kevin wrote:

IF you understand the principles around which beams are designed it is trivial to make a homemade beam that is better than a pre-manufactured I-beam. Instead of an OSB web, join the flanges with plywood sides instead of a 2x3 flange, use a 2x4, instead of #2 SPF, use #1 hem-fir. Viola, a better beam. It will cost you more in both materials and labor, but it will be better.
To be cost-competative the manufacturers have to make their product as cheaply as possible while still (barely) meeting code. They are so good at this, that the material cost of homemade beams will almost certainly exceed the cost of pre- manufactured beams of the same stiffness.
OTOH if you do not understand the relevant principles it is easy to design beams that are heavier and not as stiff as the pre-manufactured ones.
For the existing roof, it would be trivial to double the strength and stiffness by sistering a new truss next to each old one, exactly duplicating the dimensions and materials of the old. This is where there may be an advantage to the do-it-yourself approach as it will not be necessary to take the whole roof down and rebuild it from scratch.
If that is not good enough, then probably the thing to do is to replace the old trusses with new, designed to carry the new load. Here the problem will not be with the roof, which OP is going to redo anyhow, but the interior ceiling which is likely to be slung from joists that are integral to the trusses.
Either way, the local building codes will probably require a PE to sign off on the design. Pre-fab trusses will usually be pre-approved having already been designed by an engineer, as noted above.
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