Do we have any structural engineers here on WAMAS?

[madweazl]

The issue isn't that the floor will suddenly collapse from the weight of a big tank.  The issue is that these calculations tell you how much weight the floor can bear with a deflection (sag) that's "acceptable" for the typical resident.  But how much deflection is acceptable before an aquarium cracks?  Your tank will crack long before the floor breaks. 

 

A 1/4" deflection in a floor is probably acceptable for a refrigerator, or an entertainment unit, or a waterbed.  But would you put a big tank on a stand that had a 1/4" sag in the middle?

 

Floor loading calculations for aquarists should identify (1) how much deflection will there be, and (2) how much deflection you think your tank can handle.  If you just ask whether the floor will safely support the weight of the tank, you're only asking part of the question.

 

Or you can just wing it.  Lots of people do without splitting a seam. 

A well made stand shouldnt have any deflection.

Edited February 23, 2015 by madweazl

[dave w]

Here's a simple solution.  Glue a 2x6 to the bottom of your plywood I-beam, turning it into a much stronger T-beam.  For $30 you will double the strength of your floor.  I built my house and followed manufacturers recommendations for my living room floor.  Bad idea, the middle of that 24' span felt like a trampoline.  Even though it was up to code, it had too much deflection for my taste.  So I increased the bottom chord with 2x6s.  

 

The two engineering concepts to understand are: one, the strength of a beam is related to the square of it's depth (double the depth and you have 4 times the strength), and the 2x6 adds another 1.5" to the depth of your beam.  Two: all the stress on a beam occurs at the extremes of the top and bottom chords.  The top micron (which is in compression) and the bottom micron (which is in tension) are all that matters.  You can cut holes in the middle of a beam without affecting strength.  So a wide top and bottom chord make a strong beam, which is why I-beams give the best strength with the least amount of material.  The wider the I portion, the stronger the beam.  

 

When the builder put plywood on your floor he undoubtedly used glue, so your plywood floor actually became the top chord of your I-beam and strengthened it enough to pass code.  But the bottom chord is your weakness.  Glueing a 2x6 as a bottom chord increases that last micron in tension by 3-4 times.  You increase the chord from 2" to 5 1/2".  

 

So here is the a simple method to double your floor strength for $30.  Pick out your 2x6s to be as knot free as possible.  They don't have to span from one wall to the next, they can stop a foot short.  Put enough Elmer's wood glue on the part of the 2x6 that will contact the I-beam that it squeezes out the sides, raise the 2x6 into place and put only ONE nail or screw in the center of the span to temporarily hold it.  Then get a hydraulic car jack, put a 4x4 post under the center nail, and jack up the floor a good 1/2" to 3/4".  Once the center of the beam is raised, go down the 2x6 nailing it into the I-beam above.   You don't need screws, the nails are just meant to hold the wood in place until the glue dries.  You could even remove them after everything is cured if you wanted to save 2 cents.  

 

The raised beam creates a curve or camber.  When weight is put on the camber  the beam becomes STRONGER under load because stress (up to a certain point) increases the beam strength.  I know this sounds counter intuitive but it is true.  Look at the flatbed trailers being driven down the road.  Empty flatbeds have a camber, loaded ones do not.  There is a lot of camber in the length of the beams on a 40' trailer.  When the bed is loaded it straightens out and becomes stronger.

 

Once you've nailed the rest of the 2x6 into place, let the jack stay in place at least 1-2 days to cure.  Then let off the jack pressure and you've doubled the beam strength.  If you get on a ladder and look across the bottom of the beam, you'll see a camber just like the flatbed trailer.  Repeat this process for each beam under the tank.  For $30 worth of wood you'll not have any load problems and you'll not have to pay any structural engineers.  If it makes you feel better to receive this advice for free, give me a fish some day.  Just kidding, I don't want anything.

Edited February 28, 2015 by dave w

[s2nhle]

Wow, very good information. I may have to do this to support my tank when I decide to go bigger. Thank you for sharing.

[howaboutme]

The method mentioned above is a way of prestressing the beam. It is done in concrete mostly using steel tendons. In some occasions, it's done in wood. Prestressing is done before the member is put in place and before any loads are applied. My understanding is you cannot apply camber after the fact. Wood cambers are created by either adding steel into the engineered wood and stressing that tendon to force the beam to camber in the opposite direction of the gravity load. It can also be done by stressing individual wood tendons. These are done at the factory.

 

Also, the plywood being added to beams does minimal to gravity load. It is for lateral loads and stability. It works the same for walls too.