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itsnotrequired 04-30-2012 09:52 PM

stiffening wood i-joist floor
wanted to get some opinions on this. home has wood i-joist floor joists with some fairly long spans and it gets a little bouncy. how's this for an idea to stiffen them up:
  1. install 2x4 blocking perpendicular to the joists, on the bottom chord, at approximately the mid-span of the joists. 2x4 would be flush with the bottom of the chord and laid flat, such that the 3.5" side faces down.
  2. drill a small hole through each joist above the 2x4 blocking and run an aircraft cable or similar strong wire through the holes.
  3. anchor one end of the cable at the end of the floor with eye bolts, washers, additional vertical blocking, etc. similar on the other end.
  4. tighten bolts at one end of the floor to pull the cable taught. this puts pressure on the floor joist chords and helps stiffen up the floor.
  5. finish basement ceiling with drywall.
thoughts? will the cable and blocking have any noticeable effect? or am i wasting my time/money? don't get me wrong, it isn't like i'm bouncing off the walls right now but 'firmer' floors would be nice.

joecaption 04-30-2012 09:58 PM

The manufactures suggest adding 3/4 plywood or osb in the web with screws and constrution adhesive.

itsnotrequired 04-30-2012 09:59 PM


Originally Posted by joecaption (Post 911496)
The manufactures suggest adding 3/4 plywood or osb in the web with screws and constrution adhesive.

yes, i've seen that technique but seems labor intensive. just curious if my proposal has any merit or is doa...

another one i've seen is simply sheeting the whole ceiling in osb or plywood, creating one giant, monolithic structure but man, that sounds expensive.

Fix'n it 04-30-2012 10:17 PM

basement or crawl space ?

itsnotrequired 04-30-2012 10:24 PM


Originally Posted by Fix'n it (Post 911523)
basement or crawl space ?

basement. i am finishing it off right now and figure now is the time to stiffen it up if i want to.

Fix'n it 04-30-2012 10:29 PM

ok, then my idea would not work. well, it would work real good. but you would not want more beams and posts in the middle of the room.

i would go with joe's idea.

itsnotrequired 04-30-2012 10:31 PM


Originally Posted by Fix'n it (Post 911536)
i would go with joe's idea.

gets in the way of all the utilities in the joist cavities.

Daniel Holzman 05-01-2012 07:28 AM

Blocking is useful in reducing lateral deflection, and it is necessary to prevent lateral buckling in certain installations. It does not increase the stiffness of the floor. I was not totally clear about exactly what you had in mind with the airplane cable, but if you are planning to install it perpendicular to the I joists at the center, which I think is what you were talking about, it isn't going to do anything useful. You would need to tension the cable parallel to the I joist to get any effect, and you would need to rigidly attach the cable to the I joist, kind of a post tensioned I joist.

Oddly enough, this type of thing is actually done, but not with I joists. I have seen it done in wooden mill buildings with large format lumber, where a cable is stressed and attached to the bottom of the beams, held in place by a groove. This increases the moment of inertia of the beam, and is useful in seismic retrofit operations, or if you need to increase the floor loading. Not gonna work on I joists.

The idea of adding to the web is good, but it may not be adequate. You get more effect if you can add to the flange, but this is difficult with an I joist, since the flange is narrow.

bill01 05-01-2012 07:41 AM

I doubt you could get enough pressure on the steel cable to make a difference. You could try a cable near the floor for a test and stand on it. I suspect it will still deflect at least a inch. I doubt your floor deflects even that much...if it does you will crack the new ceiling you are installing.

I would start with a 8 foot strip of plywood nailed to each side of your ijoists as close to the center of the span as possible. The more ijoists you can do and the more length you can do the stiffer it will get. If you go to one of the ijoist sites they show details of how to nail these in the section that talk about cantilevering. If you could do the total length you would end up with a plywood box beam which are very strong

itsnotrequired 05-01-2012 07:41 AM

yes, the cable would be perpendicular to the joists. i have read about cables in parallel with joists like you describe but was intrigued by this perpendicular approach.

my thinking was that the joists are held rigidly in place by the floor decking above above but can move freely on the bottom of the joists. i was thinking if that free movement was reduced, it would help in stiffening up the floor as a whole. or would the ceiling drywall below do more than the cable and blocking ever would? it seems that every unfinished-to-finished basement project i have seen has resulted in the first floor above feeling 'stiffer'. or is it all psychological?

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