Trusses

question on trusses. contractor is building a stand-alone garage

24'x30'. 2nd floor is for storage. he's using 2x8 lumber for the floor joists, mated in the middle with a gusset plate.

is this sufficient for bearing weight on the 2nd floor for storage? some reason I thought wood-composite i-beams were better. he said the gusset-type should be fine.

yes? no? opinions?

Reply to
themamayada
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They would be ok, but I would go with the TGI beams.

Reply to
Lamey

I think you have a reason to be concerned. Search the internet using google. I think you will find many stories of gusset plate failures when used in floors. Ok for roofs but I would never have a floor built that way. One story I read a while back was a condo that they had to rip out ceilings and sister every truss with a solid beam. It is going back a few years but if I remember correctly the original manufacturer who had invented the system had misrepresented strength and engineers had been relying on those numbers when designing floors.

Reply to
Art

is the 2x8 part of an attic truss? if so, then they would be engineered and will be fine. I joists might support more load, but I assume the project has some budget---I joists would require second floor framing or perhaps a hand framed roof, all more expensive.

Reply to
marson

A couple of things don't seem quite right here: there are commercial trusses stacked up in the yards at all the box stores and lumber yards and I have (so far) never seen any up to 30' or so with a bottom chord bigger than a 2 x 4. Seems to me that a 2 x 8 is a huge overkill. You're not going to be storing a dozen or so Chevy 350 crate engines up there, are you? Truss chord mambers are often (maybe usually) mated with 'nailing plates' at junctions with other truss bracing elements. If your contractor is building his own trusses, better back off and order the proper engineered and factory built types. If he doesn't have a properly architect-designed system it may not pass your local building inspector. HTH

Joe

Reply to
Joe

As long as there is a bearing wall or heavy beam under the joists where they meet in the middle, it will be fine. Otherwise, forget it.

Let me guess: either you live in some backwoods community with no building codes, or you are trying to bootleg a building in without a permit. What you are describing would never pass a structural plan check or inspection.

Do a quick structural calc. Suppose you want to load just the center

12' of the upstairs with something fairly heavy, say 100 psf. If the joists are 16" on center, that means each joist has to support 1560 lbs, plus the weight of the lumber and decking. Divide by 2 (two joists) times 12' of lever arm, divided by 2/3 of a foot (8 inch joist) gives 14,040 lbs. of tension on the bottom edge of that gusset plate. Now how do you think it's going to work?
Reply to
Larry Caldwell

Joists or trusses? Your confusion alone is worrisome.

You want engineered floor trusses for that span and load. Not joists assembled by the contractor.

Reply to
Richard J Kinch

Standard roof trusses are designed to carry weight on the *roof*, not on the bottom truss members.

Unless specifically engineered for it, the bottom truss members will only be designed to handle the weight of the ceiling and insulation, with enough extra to let people walk around up in the attic safely.

Chris

Reply to
Chris Friesen

rip it out and do it the right way. floors can NOT have splices or nailers in them.

24 ft span...........2x8......................under sized for storage. TJI's would work best......................1 piece.............no joints.

or add a strong back to the top of the spliced joists.

Reply to
Cindy Boucher

I'm a bit confused. You say question about trusses but I don't think you are describing trusses? To me, you are describing a 24' long 2x8 spliced in the center of the span, right? Also, are you building a 2 story garage so there is a floor at the 2nd story or is it just a normal garage where the ceiling chords are going to be used for storage above?

Assuming a 2 story garage with a long 2x8 with a splice in the middle of the span and NO support in the middle (like a column or beam) you do NOT want this. Not only will it not support a FLOOR load but it will also DEFLECT too much. And if these members are only ceiling chords as in a normal garage, I'd still want no splices and at least

2x10 members.

Please do yourself a favor and hire an engineer or architect for adequate member sizes. It sounds to me like the contractor is not good or is trying to keep the cost very low by building a subquality garage. Over the years you will be sorry about this garage.

Reply to
under construction

There are a lot of people posting on this thread that have never seen a set of attic trusses. If your contractor is using attic roof trusses, they commonly come with spliced 2x8's for bottom chords. These will be engineered and gauranteed for a certain load. They do come with some caveats--usually the specs call for no holes and of course no alterations. I have installed many sets of these. Again, they do have splice plates. I have used them in houses as well as garage storage areas. Trusses usually come with a packet that will spell out the assumed loads--your contractor should have that. It does not mean that your contractor is not good. Floor joist can have splices. You do not have to rip it out and do it the right way. What you are doing would indeed pass inspection.

Reply to
marson

Here is a drawing of an attic truss. Note the splice plate in the bottom chord.

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Reply to
marson

First of all, your diagram (other post) is NOT a "floor joist" but rather a chord member for a truss. That said, floor joists should not have splices especially in the middle (in commercial structures they use drop in beams). The reason your chord member (in your diagram) has a splice in the middle is that it is a tension member. If you are going to help at least give correct advice.

Reply to
under construction

Here is a link for the engineering of an attic truss. Note the splice in the bottom chord. Note the bottom chord loading of 40 psf.

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Why would you use an attic truss if you couldn't use the space created for a room?

The OP used the term "floor joist", but I suspect that he is looking at the bottom chord of an attic truss. He needs to clarify this. Since most attic trusses will have a spliced bottom chord, I suspect that is what he has, and his contractor hasn't given him a very good answer.

Reply to
marson

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

Reply to
HerHusband

Just to clarify, I agree that bottom chords can have bending between panel points if engineered accordingly but generally they are tension members. Notice tho even as in floor joists, the splice is not in the middle to avoid maximum stresses.

I agree w/ your 2nd paragraph.

Reply to
under construction

Actually, take a look at truss A288 at the same site. The splice is dead-center.

Chris

Reply to
Chris Friesen

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

Reply to
Chris Friesen

And why would that be any different than any other joist or rafter?

Reply to
marson

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