Trusses

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marson wrote:

Note that the 40psf is a live load and so is expected to be temporary. The specified dead load for the bottom chord in the room area is only 5psf.
Chris
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And why would that be any different than any other joist or rafter?
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On Fri, 18 May 2007 10:28:56 -0600, Chris Friesen

Agreed temporary in so far as it can be removed but it doesn't have to be. Regardless, it still has to be designed for as a bending stress in the chord member.
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Yep, "bottom chord of an attic truss" is what I am referring to. Your link is precisely what the contractor is using. After reading these posts, I obtained the diagrams from the truss company, and it appears they can bear 60 psf (40 live load, the rest I'm assuming dead load).
Thanks.

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snipped-for-privacy@gmail.com (marson) says...

You need to learn to read a spec sheet. The bottom chord is designed for a 10 psf live load (note 9).
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wrote:

That's for the bottom chord as a whole. For the room area (10-12) live load is 40 psf. Wouldn't meet code in most places if it wasn't. Again, attic trusses have been in use for many years. I gotta wonder how much experience you have if you've never seen them.
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If the 2x8 is the bottom chord of an engineered "attic truss", it's probably sized for the intended loads. The other chords of the truss help carry the load, the same way a 2x4 is typically used to build a standard truss. The 2x8 would essentially be sized to carry the load between the truss chords (maybe 10 feet for a typical attic truss?). Assuming the truss is being built by a truss company, they should be able to supply you (or your contractor) with a copy of the engineering diagrams and the loads it is designed to carry. In fact, that's one of the required items for building permits around here, so it may already be part of the plan package.
On the other hand, if you're talking individual 2x8's for spanning 24' you're going to have big problems. Even when sized for a ceiling joist, a 2x8 at 16" OC is only good for about 17 feet, and that's for a very light load. You'll never get by spanning 24 feet with a 2x8.
Just for reference, I spanned 24' in my garage using 2x12's at 16" OC. Even that is way undersized for a floor joist. It works OK for my light attic storage (empty boxes, Christmas decorations, suitcases, etc.), but it is far too bouncy to be considered a usable floor joist.
14" I-joists would probably work well if you have the vertical space to install them.
"IF" you have a beam running down the middle of the garage, your span would only be 12', in which case 2x8 joists would probably be perfect for floor joists. You could even splice shorter joists over the beam if needed.
However, if you DO NOT have a beam, any splice in the joist is going to be an immediate failure point (unless it's part of an engineered truss).
Anthony
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Yep - bottom chord of an engineered attic truss from a truss company. After reading your post, got the truss company to send over the engineering diagrams - turns out it can support 60 psf (40 ll + remaining dead load I'm assuming). Wasn't required for our permit, but I'm sure the building inspector will be happy to see it in writing.
Thanks.

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