I-Joist Strongbacks and Floor Vibration Control 1

[phamENG]

Anyone have any neat tricks for limiting vibrations and unpleasant deflections in an I-joist floor? I have a 12" deep floor system spanning a little over 20'. It meets code for strength and minimum serviceability, but it's a wide open floor above and below with virtually nothing to damp the vibrations and I can foresee an issue. Using Forteweb a roughly equivalent Weyerhauser I joist (I spec APA performance rated I-joists for wider applicability) lands in the 'meh' category for floor performance.

I have finished space above and below and no wiggle room on floor depth. Since it meets code I know I'm 'ok' but I like to give my clients a little more than just 'ok' - otherwise they could just have the guy at the lumber yard size everything. Most of the methods I'm familiar with require dropping below the bottom of the joist, which isn't an option here. Blocking between the joists doesn't do much, but is better than nothing - maybe a continuous coil strap at the blocking on the bottom of the floor?

One of my more outlandish ideas is to cut a 4" round hole in the middle of the joist around the 1/3 points of the span (manufacturer guidance dependent) and string 2x4's through with a vertical 2x4 at each joist nailed into the flanges. Should work, but to say it's abnormal is probably an understatement.

 

[XR250]

Might be easier to just decrease the joist spacing or increase flange width if that makes a vibrational difference.

 

[skeletron]

Decrease the joist space to a non-standard value (8" o/c)
Otherwise bridging and strapping seems to make the most effect in span tables.
The hole idea is interesting, but seems like more labour than necessary. KISS

 

[phamENG]

Thanks. I'll probably go with the bridging and strapping and hope it doesn't turn into a problem. I'm limiting LL deflection to L/480, but I know that's not always enough.

Reducing the spacing more will create an expensive floor system that the contractor will jump on to convince the owner to "value engineer" out. Since we don't really have a good wood floor vibration reference apart from "look what this manufacturer says about their cool performance rating", I find it harder to argue the point successfully.

I think the strong back through holes could work well if the I joists have factory punch outs, but while I've heard of such creatures I haven't seen them in the wild around here. I'll keep it in the back of my mind for remedial work.

 

[XR250]

In these cases, I put, "Use this model joist or install XXX" O.C. for a stiffer floor"
That way it is on them if they want to value engineer it.

 

[skeletron]

Glue and nail as well will improve vibration
Extra row of blocking instead of at 7'0" max

 

[bones206]

Some good info here:

https://www.weyerhaeuser.com/woodproducts/engineered-lumber/resources/tj-pro-rating/

https://www.weyerhaeuser.com/woodpr...ry/document_library_detail/svc-4006/#download

https://www.weyerhaeuser.com/woodproducts/document-library/document_library_detail/tb-104/

One thing I heard anecdotally is that joist mfr's dislike use of bridging in their floor systems because of increased likelihood of squeaking (i.e. callbacks). So you may run into a warranty issue if it isn't explicitly approved by the mfr.

The framing of my bedroom is 12" deep I joist spanning about 20'. There is a little perceptible bounce, but much less compared to shorter spans of 2x12's in other parts of the house. Also much quieter than the 2x12 framing with 1x wood cross bridging. I noticed the bedroom furniture helped dampen some of the vibration as well.

Edit: I believe the performance ratings are built into their proprietary design software. So you can select your desired performance level, which is correlated with their customer satisfaction surveys.

 

[phamENG]

Thanks, bones. Good point on the warranty. I was planning to use a detail similar to one in the last link - blocking with gobs of glue everywhere to minimize friction during movement and the resulting squeaks.

Thanks for the anecdote. Always good to have perspective. I was in a hotel a few weeks ago...similar floor span, but I'm not sure if it was I joists or trusses and I didn't stop to try to figure out the floor depth. My 4 year old walking across the room made the lamps shake on the tables and made it feel a bit like I was on a boat. Drove me nuts.

 

[phamENG]

Yep. Sadly they don't participate with APA's performance ratings...so you have to sort of guess based on EI values...but that's usually where I start (Weyerhauser products are in oddly short supply around here - we're mostly Roseburg, Boise Cascade, and PowerJoist - PSLs can occasionally be found and of course they can special order Weyerhauser if you really want it). That's what I meant by the "Using Forteweb a roughly equivalent Weyerhauser I joist...lands in the 'meh' category for floor performance." I just couldn't get it into a performance rating with much confidence.

 

[jdgengineer]

With that span, we would commonly use 14" deep joists minimum.

If you are restricted to 11 7/8", I would highly recommend 1 1/8" plywood and LVL joists rather than I-joists. If you need to use I-joist, I would use the 560 series. All of these would give you TJ Pro Ratings >50 which is what we use as a minimum for custom residences (high-end market).

Blocking / Bridging, IMO, will do minimal to decrease vibration and may introduce additional sources of noise.

I wouldn't recommend joist spacing <16" oc. Smaller joist spacing always seem to making lighting problematic.

 

[phamENG]

jdg - thanks for your input. I agree on the 14", but this is an addition, so we're trying match certain elevations and get desirable ceiling heights.

Good comment on the lighting issues - hadn't run into that before, but I can see it happening. I also imagine there'd be more issues with plumbing and trades hacking apart the webs around the bathrooms.

 

[JLSE]

Glue and ringshank nail. (edit - Block all edges. )L/480, with a check on manuf. performance rating, is all I would do... and I thought that was beyond standard..... (feeling the plywood edges is the worst)

 

[XR250]

If you are restricted to 11 7/8", I would highly recommend 1 1/8" plywood and LVL joists rather than I-joists.

I tried spec'ing 1 1/8" plywood once and the contractor told me he had to order an entire unit of it.
Is it readily available in pieces in your location?

 

[jdgengineer]

I tried spec'ing 1 1/8" plywood once and the contractor told me he had to order an entire unit of it.
Is it readily available in pieces in your location?

For high end, custom residences in our area it is becoming the typical specification. It's used frequently in our area. I'm not sure of how it is sold, but I've never heard a complaint about it when we have specified it. Several contractors over the years have asked to substitute it when we have specified 3/4" for higher end homes.

In Forte's software, if joist depth is limited, using 1 1/8" plywood is the best way I have seen to significantly improve the TJ Pro Rating as it significantly improves the ability to engage multiple joists in the floor system.

 

[XR250]

Interesting. Is that because the joists are 24" O.C.in these high-end gomes? 3/4" seems to perform fine @ 16" O.C.

 

[jdgengineer]

Interesting. Is that because the joists are 24" O.C.in these high-end gomes? 3/4" seems to perform fine @ 16" O.C.

The expectations for floor performance are very high on some of these homes, and the spans can be very large so the 1 1/8" plywood can help keep the TJ Pro Rating in the 50-55+ category. We don't do this on all homes, but it's becoming more common. Often times the joists are 16" oc but other times they can also be at 24". For medium to high end custom residences, we target L/720 Live load, L/360 total load, and TJ Pro Rating 50 minimum. There are often areas of stone, tile, etc. that have varying floor performance requirements and the specific finishes are not always well defined at the time of our design.

Where there are basements below a first floor, a somewhat recent trend is to use 24"-30" deep floor trusses @ 24" oc. This allow the contractor to go wild with HVAC, plumbing, wiring, etc without needing to furr down the ceilings.