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950 gallon Fishtank over 2×10,tongue and groove subfloor & hardwood floor

Jxpz | Posted in Construction Techniques on July 8, 2010 08:23am

Hi

I do custom work for a few clients and have been asked to research the installation of a 950 gallon rectangular plexi fishtank in a wood framed house in Milford NJ.  The home is around 15 yr/old and the joist below the are where the tank is to be installed is 2×10, 16 o/c , the level directly under the tank is the basement.  Should I build additional support underneath where the tank will be?  Can anyone offer advice?

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  1. rdesigns | Jul 08, 2010 10:18am | #1

    For every foot of water depth, you add over 62# per sq. foot of the tank's footprint. So, a 5-foot-deep tank will add over 300# per sq. ft. of floor. So, probably you need to give it some help. Things like spans, wood species, etc will affect the answer, so you will need to provide more information, and other posters may agree with me, that you ought to talk to an engineer about this.

    As a reference, today's codes require floors to be designed for 40lbs/sq.ft. live load.

  2. Shacko | Jul 08, 2010 01:39pm | #2

    Fish Tank

    That tank will hold almost 8ooo lbs. of water, I think you need an engineer.

  3. JAGQueen | Jul 08, 2010 02:53pm | #3

    An Engineer looks at the problem

    Bottom line: for that much water, you need steel or concrete.  I would recommend a steel "I" beam (Of course required to be designed by an engineer in your jurisdiction) that re-inforces the floor joists  and creates a soffit in the basement.  Here is why:

    Facts: Water wieghts 8.35 lbs per gallon, which for a 950 gallon tank works out to 7,932 lbs.  Water has a density of 231 cubic inches per gallon, or .133 cubic feet per gallon (231/12/12/12).  That means the water will occupy 126.35 cubic feet.

    Assumptions: You did not give the spans for the flooring (Not really critical, but for a look at the calculations, see: http://www.southernpine.com/spantables.shtml)  and you did not give the dimensions for the tank.  But, plexi is good for about 4 feet per side, though most tanks are only 2 feet deep.  Using those dimensions, the water tank will be 15.8 feet long and weigh 502 lbs per lineal foot. (126.35/4/2 and 4*2*8.35/.133)

    Putting in a safety factor for the gravel, filters, weight of the plexi, steel reinforcing at the corners and the weight of some sort of stand, figure 1,000 lbs per lineal foot or 1/2 ton per foot of tank.

    Conclusion: You have to check three modes of failure.  Shear (Crushing the wood fibers where they bear on the wall), Torsion (Assuming you are next to a wall, most of the wieght will be directly carried to the wall below and not act upon the span) and support (What is between the support and the ground).  Looking at he wood tables, you will note that they deal in live loads of the 40-100 lbs per square foot category, not 1,000.  So you are out of those tables at this point, just on sheer magnitude.

    But for the sake of argument and to point out some other things to consider...  The shear is the mode I would be most worried about.  The problem is the wood needs 8 inches or so of bearing area not to be crushing the wood, and if there is only a 2x4 under there, that is only 4.75 square inches of bearing.  So, assuming that the basement wall is decent concrete (worth checking though) the wall will keep the wieght up, but the wood may get crushed and start casusing the floor to shift downward.  In addition, after the first foot or so, the water will start trying to bend the joists toward the center, which will require a steel beam at 2-3 feet off the basement wall.

    For a quick comparison, my hot tub out back holds 625 gallons and is a six person tub.  This tank will be larger than that.  I recently reinforced a floor, going from 2x8, 16 in OC with a 14 foot span to 2x12, 16 in OC with a 14 foot span, the new beams sistered in, to hold a claw foot tub that "only" holds 100 gallons.  Claw foot tubs are heavy, but still in teh wood range.  you are out of the wood range.

    1. Clewless1 | Jul 10, 2010 10:29am | #4

      "for that much water, you

      "for that much water, you need steel or concrete. "

      I beg to differ ... a general statement like that is a bit much. That's like saying use a steel structure if you want better fire protection. A common misconception that one material is inherently better than another.

      I've seen garage floors done in a wood structure (recently, not the old fashioned ones).

  4. Clewless1 | Jul 10, 2010 11:17am | #5

    That's roughly 4 waterbeds; about 4 tons ... hmm ... time for a reality check, I agree. That's a very large fish tank. About 2 hot tubs worth. Check your load per square foot and talk w/ e.g. an engineer or a building official.

    Is your basement unfinished? Putting e.g. columns in to add support may not be easy or desireable (the location of the columns may be awkward in the basement). Also columns generally can't just sit on the slab ... they need a footing. that may mean cutting e.g. a 16 inch square (or two) out of the slab and pouring a thickened slab. A quick visit and fee to an engineer would surely set you at ease.

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