I had this problem with a waterbed. I simply placed a 4x4 under and across the two centermost joists and supported it with a floor jack, tightening it only until it was really tight and bouncing was gone.

By putting the aquarium there you've put a concentrated load of ~ 500 pounds at midspan. Big load, big span with pretty shallow (read not stiff) joists.

Not sure what kind of "beam" you're putting there, but as twebbz decribes if you support the area where the aquarium is with a post below, then it will work. Of course, you now have an extra post below.

Putting a 16 ft beam perpenduclar to your joists and supporting on columns will cut your 18 ft span in half and will stiffen the entire floor significantly.

Putting a 16 ft "beam" parallel to or "sistered" with your existing joists will change things slightly but likely not a noticeably and isn't worth the effort.

"Am I correct in that joist deflection means that the joist itself has dropped slightly from the subfloor?"

No deflection is sag under load. The floor is on top of the joists and the the aquarium is on top of the floor. You can't deflect the supporting joist without deflecting the floor.

Thanks for the responses. Sdello, will it be safe then to physically raise the floor slightly with a floor jack? I guess IÂm just concerned that it could shake the aquarium around. My plan is to place the 16Â beam perpendicular to the joists in order to stiffen the floor. Should my gameplan be to place a floor jack under the portion of the beam that is under the aquarium joist and raise it until the beam is level and then go from there?

slow and easy with the jack will be fine. a suden 2 inch lift would tilt the tank, but a slow rise to level should be problem free.

"My plan is to place the 16Â beam perpendicular to the joists in order to stiffen the floor. Should my game plan be to place a floor jack under the portion of the beam that is under the aquarium joist and raise it until the beam is level and then go from there?"

You say the joist span between supports is 18 ft. Does 16 ft. represent the full width of the room? I suggest at least 3, better with 4, supporting columns for the 16 ft. One within ~1ft on each end and one or two equally spaced between them. It would be most desirable to have a post below the aquarium but it isn't a necessity.

You're only adding support to the existing condition not trying to re-level the floor. Snug the new posts up tight and give a tad extra to secure it. I wouldn't "jack" anything unless you need to in order to correct some other problem.

I don't know what orientation you are planning for your new beam, but the 2 x 8's are most effective with the 8 in. dimension vertical.

Hope this helps.

I would take twebbz's approach, a 4x4, or a couple of 2x8's catching the joist under the tank and one to each side, putting a jack post in the middle. Unless, you have a problem with the entire floor.

As said above, don't over correct.

Paul

You didn't ask for a "cheap'n easy" solution, but here's a thought in case you're inclined that way:

Assuming your crawlspace has a concrete floor, just run a (probably pressure treated) 2x4 plate perpendicular to your joists, mid-span under the affected area. Nail or screw this to the concrete floor, then measure short 2x4 studs up to the joists that need stiffening. Toenail to the plate and to the joists under the aquarium and you're done! If you want to get fancy, cut a 1x2 rabbet on the joist end to give you a better nailing surface.

I would cut them nice and tight, or maybe 1/16th long and use your jack to get them in place.

This will only help the joists supported, of course, not the rest of the room.

Well, IÂm thinking that a beam constructed of two 2x8Âs run perpendicular to the joists at mid-span with the 8" dimension vertical, and spanning the entire 16-foot width of the room is going to be my best solution. The wall opposite my aquarium has a table on which the lamp shakes every time one walks past, so I would like to correct the bounce in the whole room and just be done with it. I plan to support this beam with three 4x4" posts (one each at one foot from each end and one in the center) placed into cement deck footers designed to accept 4x4Âs. IÂm assuming everyone agrees this should work? Should I have the 4x4" sections cut a little longer (maybe 1/16") so that they will fit tight up against the beam? If I do this, will I need a jack to lift the beam a bit, or can I just use a mallet to bang them in? The last thing I want to do is cause any harm to my floor. Thanks!!

Why not just use floor jack columns? Will make it a lot easier to tension up and level the sag.

I agree floor jack columns sound like a good idea, although I doubt if you'll ever need to adjust them (unless the 2x8 beams are too green and shrink).

If you go with your 4x4 post idea, I would probably go with the extra 1/16th and then use your jack to get them in there nice and snug. Lifting the floor 1/16th" shouldn't affect anything.

I thing you'll notice a big difference in that floor once you're done - a span that long has to be awfully bouncy.