building howe truss

Hi,
(No, this isn't a troll...)
Can anyone give me a hint as to how much load a very simply built howe truss can take?
The relevant dimessions are something like this (standard pine house-framing lumber): Bottom and top chords: double 2x6s Vertical and diagonal webbing members: single 2x4s Height: anything from 0 to 6ft is doable Span: 10ft plus 1ft bearing surface on each end If it were 6' high, the webbing would be three vertical members (each end, and center), and two diagonal members at 45deg.
Could this hold about 2 tons dead load, evenly distributed across the span? Or, could it hold as much as, say, just a simple pair of 2x10s.
Why you ask? Long story, but if you are curious, I'll tell you...
I am doing some work on a 15'x20' garage. It has a walk-in basement made of (crumbling) block walls, and an upper story stick framed, with a simple truss roof spanning the 15 ft between the side walls. The upper floor is thick planks over 2x8 (actual size) rough cut lumber. The side block walls each have 2 smallish windows (3'x2') with a poured concrete header. I want to temporarily enlarge one of the window openings to gain access for some work to be done in the basement. The opening will be 10' wide and about 3' high, going right up to the floor joists.
For a few weeks, while the work is going on, I need to support the floor joists over this 10' span, and the wall above, and the roof trusses and roof. There is a center beam for the floor, but even neglecting that I have at most (10 linear feet of) approx 8' wide floor, 8' of roof and trusses, and 7' high wood-framed wall. Live load will be small, as the building will be basically empty and unoccupied.
Looks like doubled 12ft 2x12s would hold this fine, though I need to double check at the lumber store. BUT, it would be much better if I can put the beam ABOVE the floor so I don't loose 10 inches or more from my cut opening. I could just rest the beams on the floor above, just inside the wall, then use joist hangers right through the floor to suspend the floor joists below the beam. And I could fasten it to the wall studs and bottom plate too, for good measure. And, of course, put a column on either side of the opening below, to hold the beam. But it occured to me that I have a the full height of the upstairs in which to build this beam, so why not make a simple howe truss, if it would be stronger and less expensive. Or I have plenty of plywood, I could use a doubled 2x6 on the bottom, and a doubled 2x6 on the top, and solid pieces of plywood (say, two pieces 8' long and 2' high) as the webbing sandwiched between them.
Don't bother telling me to get an engineer -- this is a temporary support for an empty structure, no one will be in or under the building, and if the howe truss idea is crazy I will just go back to using a simple beam instead, even though it costs more... $60+ versus some stock lumber I already have and can reuse later.
-Kevin
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kevin wrote:

The load capacity will be as much a function of the joint assembly as anything else. You'd be better off with a simple beam made of multiple 2x12s so as to remove this complication from the task.

Small price compared to the consequences of failure.
Mike
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wrote:

COnsidering you can re-use the 2x12's later, too.
Is there a window in the wall-section you're supporting, is the inner wall open to the studs, and is this a side-wall or a gable-end? Screwing 2x4s to the inside of the wall in a giant 'A' would go a long way towards stiffening the system.
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Goedjn wrote:

I can't imagine what for, really -- they are not great pieces of lumber usually, and are too big for most things I need -- usually 2x4, 2x6, and maybe occationally a 2x8 are useful to me for most projects. And at $30 compared to $4 for a 2x6...

Yes, but it has been boarded up for a long time (siding right over it).

Yes.
Sorry -- yes, a side wall. The roof is minimal -- just some 2x4s rafters and 1x6 ties.

Thats basically exactly what I was thinking of doing. The bottom beam would just be used to strap the floor joists below.
-Kevin
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Ok, skip the 2x12s and use more 2x8s.
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Goedjn wrote:

Hmm. Maybe four or six 2x8s. I'll have to go check the tables. But oh, I could put two 2x8s on top of two more 2x8s. The top and bottom pieces would shear, of course, so this is no better than four side by side. and is not the same as two 2x16s by any means (because of the shearing). If only there were an easy way to keep them from shearing. Some kind of, stuff, to keep the top and bottom chords from shearing...
Thanks for the help in any case! -Kevin
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kevin wrote: ...

...
Who is going to do the work and how???
While one could certainly figure out a way, not going to try w/o seeing directly to comment on this. Sounds to me like would be more effort for apparent little gain -- is that extra foot of clearance possibly gained of any real necessity?
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I will be. But that doesn't tell you much, since you don't know my work, past experience, training etc. I do happen to be an engineer by training, sort of, which still tells you nothing, because there are a lot of mechanically disinclined engineers out there.

Yes, the clearance would be very helpful. We are going to be pouring and finishing a large concrete slab (one that extends right through this opening, actually), and I imagine it will be a pain enough trying to finish the part of the slab that goes through this opening. Don't ask why we are doing this... I'll post some pictures in a week or two of either a completely failed project, or a crazy but successful project to rehabilitate an old garage.
-Kevin
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kevin wrote:

You missed my point completely, methinks, by eliding the quoted material---the point is you claimed it's not a critical support (and therefore isn't worthy of a professional opinion) because the building is unoccupied and nobody is either in or under it but by the same token are planning work in the basement. I was asking (the rhetorical question of) "if nobody is there to potentially get hurt when your makeshift beam/truss fails, how is the work going to get done?" to try to make the point it isn't such a throwaway question as you seem to want to make it.
I'm sticking with if you don't have specific experience appropriate to the task, best not try something particularly exotic and as someone else noted, the difference in some minimal material cost now could easily save much later.
Perhaps an alternative is a combination of both--use 3 or 4 or so 2x8 and add a small truss-like structure above to add some additional stiffness. But, I'd surely want to look at the overall structural stability and load transferral some before committing to knocking out the supporting wall.
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