Stiffening Floor Diaphragm (Longish)

[Wayne Whitney]

Hello,

Problem: My newish front loading washer shakes the house somewhat. As it speeds up during the final spin, there is a point at which it really shakes the house quite a bit; at full speed it is not too bad. I'd like to stop the "extra" shaking.

My diagnosis: I've checked that the washer is level. The washer is in a tight closet, with only 1/2" gap on one side, but it does not seem to be hitting the side wall. Since this only occurs at a narrow range of speeds, I'm assuming this is a resonant frequency of the floor system. Does this sound right?

Proposed solution: Stiffen the floor diaphragm to raise the resonant frequency above the washer's range. What is the best way to do that in this situation? The house is one story with crawl space, 26' between exterior foundation walls, with a girder at 14'. Floor joists are old 2x10s (2" x 9.5") #2 douglas fir, 26' long. Subfloor is old

1x6 (7/8" thick) T&G. The washer is basically in the middle of the 12' span.

Any suggestions appreciated. My only thoughts are to sister each of the two joists under the washer (if there is no plumbing interference) or to sheath the underside of those 2 joists (or 3 or 4?) with plywood (using screws to allow for future access).

Thanks, Wayne

[Speedy Jim]

I'm not any kind of expert in this field, but that sounds like a lot of work with a "maybe" chance of success.

If it doesn't pan out, or you want an alternative starting point, maybe consider vibration isolator mounts:

There is a lot to choose from, which may be daunting.

Jim

[George E. Cawthon]

It may be the resonant frequency of the floor but more likely the resonant frequency of the washer. A car wheel is analogous, the car hops up and down at some speed with a wheel out of balance, but the road doesn't do anything.

Easiest potential solution is to put the washer on vibration reducing pads (fairly dense material). Or you could make a secondary floating floor of

3/4" plywood on top of standard carpet pad. Then you wouldn't need to go down below.

If you want to fix it down below then put 2x4 supports under the two joists where the washer is. The support needs to be tight between the joist and concrete block but not enough to raise the floor. Easiest way is cut the 2x4 within 1/4" and then use two shingles (thin ends overlapping)under the 2x4 and tap each alternately until the 2x4 is tight.

[Michael Daly]

The standard solution for vibration problems is to _soften_ and reduce the frequency. If you stiffen it, you'll tend to get resonance but at another frequency.

The previous suggestions for vibration isolation pads under the feet will be cheaper, easier and likely more successful.

Adding weight to the bottom of the machine (after using isolation pads) may also help.

Mike

[Wayne Whitney]

I figured stiffening the floor would just change the resonance frequency, intuitively I thought it would increase it. Is that right? Since it really vibrates only during the final spin, and at a speed somewhat below the top speed, I was hoping to increase the floor's resonance frequency beyond the washer's top speed. Is that plausible? What alteration to the floor system would raise the resonance frequency most easily?

One poster suggested using a dense carpet pad with plywood on top. Is there any sort of premade product for this purpose (roughly 3' x 3') or is roll-your-own the way to go? The washer has 4 leveling feet; is one big pad better than 4 small individual pads?

Thanks, Wayne

[Warren Weber]

Wayne. Years ago when I was doing washer repair, I had a customer with this problem. Washer was OK. House was built with 4x8 joists, 4 FEET APART. Plywood flooring. Put cement block in crawl space and used short floor jack and another piece of 1 inch plywood against underside of floot below washer. adjusted jack to put tension on flooring. this cured problem. Warren

[Speedy Jim]

This stuff might do it:

3 X 3 sheet with a 3 X 3 sheet of 1/2" ply on top.

Jim

[yaofeng]

The natural frequency of the joist is very low, no more than a few Hz. Rather than trail and error sistering the joist, why not start adding a temporary stub post in the middle of the span to see if it cuts down the vibrations before devising permanent fixes.

[G Henslee]

Wayne,

Just a thought, but is the washer level when it's full?

Wayne Whitney wrote:

[BobK207]

Since you've got crawlspace access, the quickest "try it & see" solution:

cut a short length of 4xr4 to "catch" a few joists (3?) in the area of the washers support the 4x4 with a precast concrete pier block topped with a short post 4x4 & double shingle wedges to tighten the whole thing up

OR use a screw jack instead of the post

if this works..........forget about all the other stiffening / sistering etc. It's hard to beat a post at ~midspan for adding stiffness :)

cheers Bob

[Wayne Whitney]

Good suggestion. Since the washer (stacked with dryer) is in a tight closet, this would probably be easier than trying to put a vibrational-dampening pad underneath the washer.

For the concrete pier, I assume I just remove the minimum dirt necessary to level an area where I wish to put it, and then place it on top of disturbed dirt. Is that right? Is douglas fir OK for the

4x4 post, or should it be redwood/PT, since it is contact with the concrete?

Cheers, Wayne

[George E. Cawthon]

ooh, yuck. You mean the crawl space doesn't have a 6 mil layer of plastic? If not, that should be your first consideration.

To answer your question, yes just flatten it. Cover the block with 4-6 mill plastic, and yes, Douglas fir is fine. Might want to put a 1" board on top of the concrete block to make pounding the shims easier, i.e., ground, concrete block, plastic, 1" board, shims, post, joist.

[BobK207]

""For the concrete pier, I assume I just remove the minimum dirt necessary to level an area where I wish to put it, and then place it on top of disturbed dirt. Is that right?""

yes,

The piers I've used in the past came with a redwood chunk glued to them.

I'd use a treated or redwood 2x8 chunk under the post if the pier comes w/ a wood "plate"

Ideally you don't want end grain down against the pier. You could also use a steel plate; 1/4" or thicker (if you have access to a junk pile)

If you use the wedges cut the post close so you have lots of wedge "make up" in case the pier settles.

let us know how it works.

cheers Bob

I figured the "post" would be quicker than messing with the washer installation, esp now that you said it's a stacked unit.