If, as you indicated, you need more stiffness, not more strength, then if
you were to rigidly attach (with closely spaced fasteners and glue) plywood
to the bottom chords (i.e., install a plywood ceiling skin) you would
effectively have made a sandwich panel (see, for example
http://www.oneoceankayaks.com/Sandcore.htm) of the entire
floor/truss/ceiling structure and that puppy would NOT deflect --
noticeably. It might also require less plywood than would sheathing the
sides of the trusses. However, your electrician/plumber/HVAC technician
might hate you for destroying access -- drop ceilings in basements are
wonderful things. As a practical matter, I think longshot's suggestion of
X-bracing would buy you much more than has been suggested by others
(consider steel stringer bridge deck design) and, as you yourself noted,
would have negligible effect on utilities access (thinking steel X-bracing).

Disclaimer: Either way, I would strongly suggest that you discuss the
matter with the truss manufacturer and the local building code inspector to
tap into practical experience and avoid unpleasant surprises relating to
issues that you, I and others may not have foreseen (code, fire, moisture,
relative wood shrinkage, critters, etc). If you have doubts concerning the
real qualifications of these two resources then a site visit by a bona fide,
residential, structural engineer might be a good investment.

David Merrill


"Dan Oelke" wrote in message
...

In my current abode they used floor trusses for both the first and
second floor. From the basement I have easy access (for now) to the
first floor trusses. They are great in that they allow for such a long
span and a very open floor plan. However - there is a fair amount of
bounce in the floors, especially when two kids get a little excited
running around the house. This is especially true in the kitchen area
which has cabinets all around, heavy appliances, island, etc. so I
suspect that this added "normal" load makes them a little more flexible.

Does anyone have any suggestions for how to stiffen or strengthen these
trusses? There is currently one strongback going across the trusses
about mid-span between the supported ends. This isn't something that I
am afraid has been under-engineered to start with. From my reading of
some span tables, etc the spans are well within what they can be for
that size of truss. I'm just looking to reduce the rattling of
glasses/light fixtures, etc when the kids go jumping.

Some ideas I have had a

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.

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.

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?

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.

Any other brainstorms?

Thanks - Dan