Elimniating / Reducing “bounce” I Joist
Jackaroo
| Posted in Construction Techniques on
Building a home and have read article on reducing the “bounce” in I-Joist over some longer spans. I am sure I am making more of it (since I readthe article), but I can feel a little “bounce” in the sub floor. I read the article about how to do it, my questions is how much is ordinary( And will any of this dissapate when the hardwood floor or carpet and pad are put down)? The article from Oct/Nov I believe mentioned reinforcing the i joist with support blocks on one or several strongbacks. Is this an effective technique and should I approach the gc about it offering this as a solution? I do not want or have any adequate location for any posts. Thanks for the input.
Replies
Reading the thread on Floor Vibration will help you understand what's happening.
What depth and spans of I-joists are in your house? and what stage of construction are you in?
This post made me think of an issue that I have that may or may not be related. Excuse me in advance if i'm hijacking it.
I have an early 70's house with 2x8 floor joists running above the basement, holding up the first floor. The span runs just short of 12, which i understand is close to the limit. The joists are 16" o/c and sit on a base plate on one side and 2 2x10's (sitting on columns) on the other.
Now my situation: our floor bounces like crazy. a wall unit running parallel to the joists has odds and ends that I'm sure would eventually bounce off. Even my 2 year old shakes it without trying (God forbid if I jump on the floor).
Now my question: is there any way to retro fit the joists to add more strength (if that's needed)? I've been thinking about trying to sister the 2x8's over the wall unit to help carry the load, but I'm not sure if this will work. Is this a common problem? Should I run for the hills?
Thanks for the help, and I hope this is related.
Nap
This type of thing has been discussed endlessly. Try the advanced search with the word "bounce" and see what you come up with.
Inflation is when the buck doesn't stop anywhere.
Boss
Thanks for the quick post. I'll check into it.
Nap
Do you have blocking between those joists? If not, it's the first thing to try. The subfloor holds the tops of the joists from bending sideways, but the bottoms are free to move, and that's a major factor in bounce. Blocking crammed in there good and tight stabilizes the bottom edges, and also spreads some of the load to adjacent joists.
If blocking doesn't help enough, step two is to increase the tension carrying capacity of the bottoms of the joists. Glue and nail 2x4's on the flat to the bottoms of the joists, creating a sort of upside-down "T". Your floor is strong enough, you just want to make it stiffer.
-- J.S.
If you're in the planning stages, the simplest approach is to increase the depth of the joists another couple of inches above the minimum. If the plans are already pretty hard, you can increase the number of joists. If the joists are already in you can spec additional bridging between joists.
Keep in mind that if you only have subfloor down, that will flex some and make the bounce seem worse. Once the rest of the flooring is down it gets stiffer (though how much depends on what the rest of the flooring is). At this stage you may still be able to spec a thicker underlayment or some such to increase stiffness.
These are two different things, though: "Bounce" is where the entire floor (including joists) goes up and down, and things on a table three feet away will rattle. "Flex" is where the flooring bends between the joists, with the joists staying relatively stationary. You'll feel it underfoot, but it won't make the china rattle.
One solution on floors that are already down and in place is to have the insulation guy spray foam under the affected floor ( Or floor area) . (save your self some work and do it after all the wiring etc is in place and checked)
You'd be amazed at how stiff that will make a floor! If you are in the planning stages try to change wood used.; Short runs in my timberframe house I use pine (eastern white) longer runs I use Tamarack and for the greatest span of all I switch to ash. While some white oak (for example swampy white) has nearly the same strength oak has tannic acids that rust nails and cause decay in the oak..
Ash is only slightly more expensive than pine and as long as you nail it while it's still relatively green easy enough to nail.; once it dries to 7% moisture you won't be able to drive nails with a sledge hammer.
you may be able to slide a flooring truss/joist between existing joists and stiffen the floor up too (again depending on how far along you are in the building process)
However you may be worried over nothing.. If the flooring more than meets the code but still seems just a little springy once the wood dries out and gets to it's 7% moisture content it will stiffen up a fair amount.
Bob,
How about cutting your span in half with a beam?
Tim