How do I stiffening floor trusses?


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 are:
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
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I would try the metal cross braces that should lock everything up nice & tight & be a fairly easy fix. at least on the basement ceiling level.
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Ask a competent structural engineer. Any comments made here would made without sufficient structural facts.
OTHO, Having had to stiffen a floor in one of our dealerships, I boxed the floor trusses with 3/8" plywood, glued and nailed. Made it very stout, however I had no obstructions to deal with.
My engineer winced, shrugged his shoulders and walked off shaking his head.
Dave

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Teamcasa wrote:

As an engineer (but not structural) I know that anything discussed here is not what I am going to base my decision on - BUT it can be a great way to get ideas and help sort out some of the options. On purpose I didn't give enough details for anyone to even start making calculations so that this would be kept more as a brainstorming situation.
Of course - this isn't to add "needed" strength - just to reduce the flex that is normal in any floor. (save maybe a poured on grade concrete slab). Because this isn't necessary to keep the building standing I feel a lot more comfortable with hill-billy style engineering <grin>.
Basically boxing the trusses was one of my ideas - with a few appropriate cutouts where needed.
Thanks for the input!
Dan
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Engineering is difficult to do over usenet, but here are some general comments.

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.

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

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

You could double up the bottom chord. This will help somewhat if it is well attached.
-j
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J wrote:

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>

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.

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).

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
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Dan Oelke wrote:

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.

--
--John
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snip

Properly installed X bracing between joists does not add strength but it does add stiffness, just what the OP wants.
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For my shed, the building inspector suggested doubling up every third joist. Not only does it strengthen the floor, but it makes the bounciness non-uniform. Unfortunately, you'd have to install these when you build as they'd need to go the full length of the truss to get the most benefit.
You could add some small steel I-beams at 1/4 and 3/4 span.
You could spray in polyurethane (structural) foam. This won't add load strength, but may dampen vibrations.
But to know for sure, you really need a structural engineer to look at it and figure out what's going to work.
As a test, you could try just bracing some of the trusses at the 1/4 or 3/4 point with a 2x4 to the floor and see if it makes a difference. Use one 2x4 to brace across a number of trusses, and a second to go from that brace to the floor, like a "T".
Note that truss joists are *supposed* to have cross-bracing between them to keep them vertical. They have much less bendy-strength than solid wood joists. IMHO the easiest way to add them is to cut a bunch of plywood strips as wide as your trusses are tall, custom cut off squares to fit between the trusses, and use pocket screws to attach them at the four corners to the two trusses.
I don't know if that will help with bounciness, though.
FYI our house uses TJ-25 floor joists (manufactured I-joists) for 18 foot clear spans and we don't have much bounce in our floors.
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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

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On Wed, 16 Mar 2005 11:53:33 -0600, the inscrutable Dan Oelke

I watched an old TOH classic where their plumber used strongbacks on his own house. 'Twas steel plate bolted through the joists. They jacked it up to remove the sag, drilled, bolted, and let 'er down.
That or a post or two would go a long way toward reducing dish rattling until:
A) you get dem wildarsed kids under control OR B) they grow up. ;)
- Yea, though I walk through the valley of Minwax, I shall stain no Cherry. http://diversify.com
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Hey - I remember seeing that now that you mention it. (very few of those that I haven't seen at some point). There were a couple of structures where they replaced or added a big beam, but I remember them adding steel alongside existing joints in once case just to stiffen the floor. Steel was used because you would flex as you put it up into place and then turned on edge, bolted, etc it would stiffen the joist. Used 1/8" plate (probably 10ga) if I remember correctly. There was also an episode where they ripped up the floor (to preserve the ceiling below) and sistered in glu-lam joists to repair plumber work.

Posts are out - space below there is supposed to house my pool table someday soon.
Getting the kids under control.... yeah sure..... besides what happens if the "kid" is me sometimes?
Some days it seems like the kids will never be growing up. Besides about then there are grandkids (just ask my parents)
<grin>
Dan
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I would do the above but as has been mentioned you might want to check with a truss company. I've seen this done on new homes but the plywood was applied prior to the installation of the trusses and completely covered one side. This was not required but was requested by the home owner. The duct work and mechanics then had to cut the plywood for access. The truss company allowed this as long as the trusses were covered from end to end. My guess is that this would transfer the load across the entire length of the truss and to the bearing points. There were still places where there was only 2'' of plywood on the top and bottom of the truss where ductwork passed through. There were also butted joints every 8'. If I found that I could do this (from the truss guys) I would probably use a construction adhesive just to help with any squeaks that I might create with the added fasteners.
Mike O.
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You have access to the ground floor at least from the basement: Strapping [furring strips] every 12". Ties them all together.
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In my experience of actually having to stiffen some undesized floor trusses (12" deep trusses, spaning ~20')
Adding material (2x6) to the bottom chord was the easiest effective means to stiffen. Glue & pin nail for best stiffness. slight jacking would also be helpful. You could just put a temporary post at midspan..
Cross bracing is not very effective for impact load sharing unless it its VERY well fit & also glued.
With a 2' truss spacing perhaps the plywood floor deck is part of the problem? Like a plywood drum.
cheers Bob
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I strengthened a floor by adding a 2 x 4 to the bottom of some floor joists in a manner similar to the bottom flange of an "I" beam. I glued it with construction adhesive and screwed it about every 6-8 inches. I ran the stiffener from support beam to support beam. Wish I had done the entire house rather than just the kitchen.
There was a study done several years ago concerning the use of solid vs "X" brace blocking, and several alternatives. The researchers determined that the best way to stiffen a floor was to use solid blocking between the joists, nailed tight, and then to install a very strong continuous strap pulled tightly across the bottom of the joists directly under the blocking and nailed in place. The effect is to distribute the load between joists. The problem found with normal X brace blocking or even solid blocking is that the tension in the bottom chord of the blocking causes it to pull away from the joist so that the blocking depends upon the compressive strength of the blocking installation only. The metal strap pulls everything together and transfers the tension across the joists. While I know that you indicated that your abode uses trusses, I'm sure that you could make the idea work.
I think this research came from the Canadian Building Code council (or a similar Canadian organization), but couldn't seem to Google up anything. I have probably filed the article away, and will see if I can resurrect it from the archives later today.
Jim Ray, President McFeely's Square Drive Screws www.mcfeelys.com

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Jim's comment about the value of a continuous tension tie across the bottom of the blocking is right on the money.
The CBA(?) has a number of research projects about the stiffness / "feel" of wooden floor systems & has developed criteria for avoiding springiness.
The tension tie w/ well fit blocking could help develop "two way slab" behavior in the wooden floor system
cheers Bob
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found it!
CBD-173
Canadian Building Digest
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BobK207 wrote:

Thanks Bob - that is some interesting reading..... now I just need to digest it.
Dan
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Dan-
if you have any questions, feel free to contact me
I use gmail
rkazanjy
cheers Bob
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