(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.