I built an mother-in-law addition this summer. The home owner wanted a clear span in the basement so I went with a I- Joist system. I spanned 25' 6" with a 14" I- joist that was rated to span 29' 1" . Could have gone with a lesser series but I don't like building to the minimum. They ended up rearranging the interior walls once I got the exterior walls up and the W/D combo ended up in the middle of the floor and approx. 7' in from the end of the other exterior wall.
They told me that when the washing machine is in the spin cycle the whole floor vibrates. I went over and noticed they have a front load washer. I believe those type cause floors to vibrate because if the load is slightly out of balance it will cause the machine to hop, unlike a top load that dispurses it's load sideway instead. Anyway, I installed a double 2x6 header under the floor area of the W/D and posted it down to the concrete floor in the basement.
I get a call today that the vibration is still bad. They had the repair guy from Sear's check the washing machine. He said there was nothing wrong with the machine and the problem was shoddy construction by the contractor

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I have built many Colonials with the W/D set up on the second floor and never had a problem, all were built using 2x10 floor joists. Is the I-Joist system more susceptible to problems like this? I told her (mo-in-law) I was coming back on Saturday and building a support wall the length of the basement, which is what the homeowner (son-in-law) doesn't want.
Any input would be welcomed, meanwhile I need a drink.