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 2×6 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 2×6 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:
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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?
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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?
Best Answer
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).
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.