Washer on weak floor - advice needed

Hello all, I have a 2nd floor laundry room, and a front-loading washer. In an unfortunate bit of engineering, my laundry room was placed over my garage (a 20+ foot span of floor with no support underneath). In the final spin cycle, you can feel the floor shaking throughout the second floor, and it's pretty loud downstairs too. I wasn't real worried about it till I started getting cracks in drywall joints, both upstairs and down.
After some research (including Google searches of this group), I thought of three solutions to this problem: 1) Add extra subloor under the washer & dryer 2) Put an adjustable jack post (lally column) directly under the washer 3) Move the washer to the garage where it can sit on concrete slab.
I have already tried #1. I cut two pieces of 3/4" plywood to fit the area under both washer and dryer. I screwed the first piece down, into the joists, using 3" screws. Then I screwed the second piece over that, down into the joists. It didn't seem to help at all.
Now I'm getting ready to try my #2 solution - the jack post. Here's where I need advice. My garage ceiling is drywall. After I determine where to position the column, so that it's directly under the washer, how exactly should I attach it to the ceiling? Can I just attach it to joists, or do I have to put a beam up there, and attach to that?
And here's another (probably more important) question: Will this jack stop my house from shaking? My #1 solution was a complete strike-out, so now I'm not all that confident in solution #2. I'm thinking that the (27" wide) washer is sitting on, at most, two joists. If I could somehow get the post to support these two joists, then it seems the spin cycle could not possibly shake the floor.
Thanks for any advice. John
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John,
Use the jack to support a 2" x 12" x 4', placed directly against the garage ceiling, beneath the washer. Crank in enough force to stop the vibration but stop short of distorting the floor structure. This WILL stop the house from feeling the vibration.
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I assume you mean right against the drywall? No cutting out to expose the joists?

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In designing a structure, the first consideration is the ability of the structural members to carry the designed loads. A second consideration, however, is deflection; although that is more of a useability consideration as opposed to safety. Your 20' joists are obviously strong enough to carry the impact forces from the shaking washer, but they are probably at the limit for allowable deflection. (Relatively new house, right?)
Your adding extra subfloor did little to increase the section properties of the floor joists, and obviously didn't have much effect as the calculations would predict. Option 2 should work more effectively because you will not only shorten the span, but will also move the load over a support. That will immensely reduce the deflections (and the shaking). I am hesitant to recommend it, however without knowing the specifics of the construction. There are several important engineering considerations.
1. The column, if it is to truly pick up the mid span load, would end up carrying more than half the total load of the joists affected. That's not just the washer, but everything else in the room along those and adjacent joists. 2. Any member transferring the joist loads to the column should be designed for the applicable shear and bending stresses that would result. 3. The bearing surface between the joists and the new beam would need to be adequately sized to prevent a crushing failure of the bottom flange of the joist. 4. The garage slab may not be (and probably is not) adequate to prevent a punching shear failure. To do it right, you'd need to cut out a part of the slab & install a real footing. If the garage is not heated, that footing would need to be down to the frost line. 5. Even if none of the problems above are problems, the floor is still gonna shake, just not as badly. If you want true isolation from the washer motion, move the washer.
From a non-engineering perspective, it seems to me an added column in the garage would be a PITA, but that's just me. Unless convenience is a major factor, I'd vote for option 3.
$.02, Joe F.
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Any chance the washer is a Whirlpool built front loader (or equivalent Sears Kenmore HE3?). I had one with a defective cabinet and it had to be replaced due to vibration problems. Bottom of cabinet was not attached properly to rest of it.
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It is a Frigidaire Gallery. The washer runs solid as a rock on concrete.
Hey, this is going in my list of must-haves for any future house: No upstairs laundry!

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Double joists or blocking would have prevented the problem.

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Hi all,
Extra braces and extra sub floor thinkness always helps....but one thing we noticed is if the builder screws down the sub floor in the main floor laundry room we get less bounce and problems than the ones that are just nailed down.
jeff. Appliance Repair Aid http://www.applianceaid.com /
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Mine are nailed, screwed and glued. Even so with a new Sears frontloader HE3T I'm going to add some blocking though a sheet of plywood helped significantly.

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