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We will use a hexagonal plate as the modular unit from which to build larger structures, this being a fully tiling form and a fair compromise between the material economy of a circle and the construction convenience of linearly bounded shapes.
To minimize the materials cost, we would normally use the least density of cellular concrete we could structurally get away with, around .8. However, it will simplify casting to select a density of about 1.0, so a solid or flooded platform would have slight negative buoyancy in salt water. To produce the positive buoyancy needed, we will cast our platform from this material, but with a hollow hexagonal honeycomb interior. Since we have chosen a concrete density equal to that of water, we can suspend an array of water-filled plastic bags in the concrete. Confining the bags with hexagonal fences of wire mesh will result in a concrete honeycomb, after the concrete has cured, and the bags are emptied.
We will need someone knowledgeable in structural engineering calculations to help us decide how little cell wall thickness we can get away with here. Something like 4” thick interior walls would give us some useful storage and living spaces, and reasonable bouyancy and materials cost. In addition, we have selected a platform thickness of 10 feet, because these dimensions yield rooms of a practical height. We will cover the outer surface with a layer of normal concrete. More than half of the total weight will be due to this outside layer. It’s thickness is a crucial compromise between strength and buoyancy. Four inches is the current figure, giving a platform that floats, unloaded, with only 3 of 10 feet below water.
As for the diameter, we have selected 50 feet for now. I needs to be large enough to be useful as a residence. But anything larger would get pretty awkward to handle. A 50 foot module is already 116 tons. Getting it into the water will be difficult enough.
It’s 5 rooms in diameter, yielding 25 10x10X9.5 ft rooms, 12,300 cubic feet of storage and habitat space, and1,300 square feet of inside floor area. Some of that will be consumed by cells that remain closed and inaccessible to provide safety floatation in case of flooding. 3 cells yield 8 tons flooded bouyancy and .6 ft freeboard. 5 cells yields 38 tons bouyancy, 3ft freeboard. Since the wall material can be cut and drilled like wood, it is easy to remove walls to form hallways and larger rooms.
For what amounts to an ocean-going vessel, bouyancy is a critical number. A hex module will support 380 tons of 'cargo' without sinking. Flooded, it weighs 38 tons (with no safety floatation cells). For comparison, the dry weight of the unfinished module is 116 tons.
92 CubicYards of concrete (before air) are required. At contractor prices, we would need about 25 tons of cement at a cost of about $2,000 to make a hex platform of these dimensions. Adding the other components, the cost of materials is $3,700. Plus another $2,000 equipment rental and labor gives a total cost of $5,700.
Iwe have selected a ¼ scale for our experiment. This gets us 2 feet thick by 12 feet diameter, requires 2.5 CY (un-aerated) concrete, and weighs 3.3 tons. The total cost of materials and equipment is $340. This is a manageable cost, size, and weight.