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

kevin wrote:

Can anyone give me a hint as to how much load a very simply built howe
truss can take?

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.

using a simple beam instead, even though it costs more... $60+ versus
some stock lumber I already have and can reuse later.

Small price compared to the consequences of failure.

Mike

kevin wrote:
....

... gain access for some work to be done in the basement. ...
....
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, ...

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?

On Thu, 14 Sep 2006 11:33:13 -0400, Michael Daly
wrote:

kevin wrote:

Can anyone give me a hint as to how much load a very simply built howe
truss can take?

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.

using a simple beam instead, even though it costs more... $60+ versus
some stock lumber I already have and can reuse later.

Small price compared to the consequences of failure.

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.

Who is going to do the work and how???

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.

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?

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

Goedjn wrote:
On Thu, 14 Sep 2006 11:33:13 -0400, Michael Daly
wrote:

kevin wrote:

Can anyone give me a hint as to how much load a very simply built howe
truss can take?

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.

using a simple beam instead, even though it costs more... $60+ versus
some stock lumber I already have and can reuse later.

Small price compared to the consequences of failure.

COnsidering you can re-use the 2x12's later, too.

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...

Is there a window in the wall-section you're supporting,
Yes, but it has been boarded up for a long time (siding right over it).

is the inner wall open to the studs, and is this
Yes.

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

Screwing 2x4s to the inside
of the wall in a giant 'A' would go a long way towards
stiffening the system.

Thats basically exactly what I was thinking of doing. The bottom beam
would just be used to strap the floor joists below.

-Kevin

COnsidering you can re-use the 2x12's later, too.

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...


Ok, skip the 2x12s and use more 2x8s.

Goedjn wrote:
COnsidering you can re-use the 2x12's later, too.

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...


Ok, skip the 2x12s and use more 2x8s.

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

kevin wrote:
Who is going to do the work and how???

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.

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.