Nature is the ultimate utilitarian. She combines function with form, using the simplest strategy to get the highest level of structural integrity with the least amount of materials. Take a fresh raw egg, place the ends in both palms and squeeze with all your might. This thin, seemingly fragile membrane will resist your effort. Like an egg, a dome is designed using a double curvature wall; it curves in both the horizontal and vertical plane at the same time.
To better understand the dynamics of a double curvature wall, let's compare a dome with a cylinder and a cone.
A cylinder (a vertical round wall) curves in one direction, horizontally, while the sides remain vertically plumb. A cone also curves in one horizontal direction and, although decreasing in circumference at one end, it still maintains a linear profile. The dome curves both horizontally and vertically, producing a spherical shape. Technically, a dome's profile can be many shapes, varying from a low sphere to a parabolic shape, like the opposite ends of an egg (Fig 11.2).
Earthen domes rely on two opposing natural forces to hold them together: gravity and tension.
This balancing act of downward pressure meeting perimeter resistance is a very sophisticated engineering technique and has been employed for millennia. Domes built from individual units such as
single curvature cylinder wall single curvature cone wall double curvature dome wall adobe block, stone, and, in our case, earthbags (or tubes) use gravity and resistance as integral structural devices. These forces differentiate them from a geodesic or cast concrete dome that rely on a monolithic framework to hold them together (Fig. 11.3).
11.2: Comparison of three shapes: A cylinder, a cone, and a dome.
11.3: Dynamics of a dome: Gravity tries to pull all the mass of the earth overhead down (compression), while the outer perimeter of the dome rests on a "ring of tension" that resists the spreading of the walls.
Like an arch, a dome is only as strong as its buttressing. A dome, however, is self-supporting and does not need any structural formwork other than the arched window and door openings. A dome is an arch built in the round. The point where the springline begins is where buttressing needs to be present.
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