So, as it stands now I am hoping to build a hot tub from lumber. I've done ponds and makeshift pools and such, but this I want to be a lasting thing. It'll be framed and then lined with a thick rubber.
The hot tub will be rectangular or square, on a professionally poored concrete pad. I don't think I'm worried about weight of the tub pushing onto concrete, correct me if I'm wrong. I plan on using lumber in the 2×4 to 4×4 range. But could go bigger.
My concern is would the weight of the water put to much force pushing out? Thinking like either 6'x6'x3'. Or 8x8x3'. Maybe a 5'x10'x3'. I may increase depth to four foot.
Any of you guys see an issue with the weight of the water if I fully frame and support the outer structure? Or anything else?
Best Answer
You need to calculate the outward force.
Assume 8' x 8' x 3' hot tub, filled to the top.
3 ft of head * 0.424 * 1(specific gravity of water) = 1.302 psi
That's the pressure at the bottom. Obviously the pressure at the top is 0. If I were to sketch the pressure gradient, it would be a triangle, so we can easily calculate the average pressure as 1.302 /2 = 0.651 psi.
That pressure is acting over an area of 8x3x 144 = 3456 si.
So the outward force on the side wall is 0.651 lb/si * 3456 si = 2250 lbs.
For tipping purposes, take this force to be acting as a point load at a height of 1 ft from the ground (Centroid of the pressure gradient triangle).
So, whatever you build will need to support an outward push of 1 ton on each wall. You might be able to get away with it using some sort of steel banding, but I wouldn't consider it with just lumber -- your corners are going to rip apart.
Look into buying a pre-formed fiberglass hot tub liner. That will support the outward forces, and all you have to do is build the appropriate frame to support it.