Dan Oelke wrote:

J wrote:
Engineering is difficult to do over usenet, but here are some general
comments.

Agreeded - but this is for additions to something that is sufficiently
strong to start with and I'm just looking for ideas. I do tend to
calculate what is needed, double it for my plans, and then double it
again while building such that SWMBO constantly asks why the lumber bill
is so big. grin

The question that comes to my mind is why exactly the flex is occuring--is
the truss as a whole flexing as a rigid body or is there movement within
the truss? Is it actually the truss itself or is it some other portion of
the structure on which the truss is supported? It is really inadequate
stiffness or is the kid hitting a resonance in the structure?

It's important to know what specifically you're fixing before you try to fix
it.

1) Add additional strongbacks. How big is useful? Would a 2x6 do much
or do I need a 2x10 or 2x12 before it does any good? Going around
heating ducts and drain lines could prove somewhat a challenge to get
bigger sized material in place.


All this does is spread the load over adjoining trusses. What you have
their already is probably doing most of the work. Not the biggest bang
for the buck.



2) Put in some kinds of cross-bracing between the trusses - like you
normally see for normal joists. Allows for easier maneuvering around
utilities. This of course begs the question - wood or metal.


Again, just spreads the load a bit, but doesn't add much in the way of
strength.

Yeah - I know that both of these spread the load - but with trusses at
2' on center I was thinking that being able to spread some of the point
load (kid jumping) across 3 or more trusses such that it helps with the
temporary deflection. BUT this is an area where I am just guessing and
don't have much real world experience. I haven't pulled a string, but I
believe that the trusses have stayed pretty straight - they just bounce
a little more than we like.


3) Add plywood gussets on the truss sides where I can. Because of
heating ducts, plumbing, etc, there are some places that I couldn't, but
most places I could. Does it help/hurt to only do this on one side? Do
the extra fasteners into the 2x4's hurt their strength?


Slapping 1/2 plywood on both sides would stiffen things up immensely. One
side would probably be OK. The amount of eccentricity created is fairly
minimal. I'd probably use glue and nail every 6 inches or so. The
strength of the plywood should more than make up for any issues created
by nailing

I wasn't initially thinking of gluing the plywood - but that does make a
lot of sense - Duh! Reduces the number of needed fasteners, and makes
it stronger overall. "just a few nails to hold it while the glue dries"
(Saint Norm).

4) Do some/any of the above after jacking up the center of the truss by
a small amount (1/4" ??) so that the re-enforcing is taking the load
right away.


If your floor is deflected without load and you want to fix it, then do
this. Otherwise there is not much need.


Any other brainstorms?


You could double up the bottom chord. This will help somewhat if it is
well attached.

Hmmmm - not a bad idea. I am planning on putting in a ceiling "sometime
soon" and this would be easy and doable without affecting ceiling height
too much. Going with that idea - what about a 2x running alongside the
truss at the top or bottom, glued & nailed. Kind of like putting
plywood on the side, but with 2x material instead. This would leave
more open-space in the middle of the truss (one of the GREAT things
about trusses) but hopefully stiffen things up some. Even better since
I can probably buy some rough sawn air-dried 2x hemlock pretty
reasonable compared to plywood.

Dan

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)