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I know there are a lot of questions on here questioning the weight bearing capabilities of dimensional lumber but this seems like such a specific scenario since the load is static and completely even.

This is a stand made entirely of 2x6 SPF lumber to support two 55 gallon aquariums right next to each other. The span from leg to leg is 10ft. There is are 4 through bolts connecting the horizontal beams to the legs. The spans are connected in the middle by short sections of 2x6 which are also there to support the short edge of the tank.

Each tank when fully loaded is 500lbs or more. From what I have been seeing this shelf is not strong enough but nothing quite matches the scenario. Here are 2 photos of the shelf:

55g on shelf The basic structure of it

  1. Will this be able to support another 55g across the ten foot beam? Of not, would adding a middle support to cut the beam length in half help?

  2. If there are 2 beams and the aquarium sits on both, does that mean each beam takes half the weight of the aquarium. For example in this case there are 2 beams supporting the bottom of the tank, does that mean, if the tank were 500lbs, that each beam is supporting 250lbs?

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  • I agree with Isherwood completely, double beams & middle legs. Additionally, I'd go with either much bigger & thicker washers or ideally plates on the fronts & backs to get rid of the sinking-in & therefore weak connections (Joist Hanger type products). I'd also strongly suggest an X-brace in the back, even in the front if the bottom is to be enclosed or could be smaller end cabinets for the tank equipment & supplies. Finally, think about anchoring the setup to the wall since the footprint's so narrow. If not, then side X-braces on the area beneath the tanks..
    – Iggy
    Mar 3, 2016 at 23:05
  • Personally, I would use 2x10 for that span, and add more bracing (Xs) between members.
    – user4302
    Mar 4, 2016 at 5:59

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  1. No. It may support two aquariums without collapsing, but it will sag substantially. Adding legs in the middle would resolve that problem, but I'd also consider doubling the beams, front and rear. I'd have confidence in that setup as well (though you'll still probably see some sag).

  2. Yes, the front and rear beams each carry half the load. This is a rare example of a very evenly-distributed mass.

Other notes:

It appears as though your upper bolts are very near the top edge of the beams. SPF (spruce/pine/fir) lumber of the last few decades is very soft and prone to splitting. It won't take much to rip those bolts out of the upper edge of the lumber. I'd leave at least 1" between the edge of the lumber and the bolt holes, or add two more in between.

SPF lumber can vary quite a lot in terms of strength and stiffness. By using a single beam front and rear you incur some risk that either the front or rear beam will sag more than the other, resulting in a twist in the aquarium. That's what breaks glass. With something like this, rigidity is key, not just strength.

Here's my 55 setup. notice that it really only has 4 primary points of contact, but they're in key locations and provide a flat plane of support.

enter image description here

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2 2x6's spanning 10' 12" o.c will safely support about 1000lbs. You will be over that. Another set of legs will work in middle. I would also attach it to wall or add bracing. Like stated before your top bolts aren't holding much. I would add 2x6 blocking in-between 2x6 beams on legs.

With this much weight you have to consider your floor weight limit. If floor joists are running parallel with tanks all weight will be on 1 or 2 floor joists and they could break.

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