Curved walls are not very affordable for wood construction, but they are a somewhat natural choice if you are working with shotcrete, particularly in an earth sheltered application. But even if the curved walls are easy, many other aspects, from surveying to carpeting, are much trickier. Tricky translates into expensive. I will keep track of curve related issues as I build and put them here.
If you try to balance a playing card on its edge, it will fall over, but curve that card and it will stand. When building a concrete wall, a curved wall is self bracing. Damns are curved because it allows them to hold back much more water pressure. Earth sheltered homes with flat roofs need at least 12 inches of concrete, but curved roofs can be as thin as 4 inches.
Curves also look and feel very appealing. Humanity has only been living in wooden boxes for a few hundred years.
“The idea of putting a square home under the earth made no sense. Caves are not boxes, and the box is not a shape that lends itself to the immense load of earth above.” ~William Lishman
On the other hand, curves bring lots of issues. While laying out our curvy house, we had lots of issues making things work. It is just not as easy to lay out a wedge shaped room as it is to lay out a square one. It also added lots of difficulty for the architect. In fact, if my house were rectilinear I might not even have needed an architect.
Looking ahead to construction, I am sure that siting the house will be much more difficult due to the curved walls. I can’t just string a rope to make a straight line. Digging a curved trench will be more difficult than digging a straight one. Framing will require curved track which costs more per ft than straight track costs. Placing floor joists is more difficult because each joist is a different length and angle from the wall. Some constructions systems (like pre-stressed concrete planks) are out of the question, and others (like curved ICFs) just get a lot more expensive.
Rectilinear homes rely on the principles of the post and beam, but when the beam between two posts is curved horizontally, it results in torsion. the weight above the beam doesn’t just push the beam down, it applies a rotational force to twist it off its posts.
In a curved wall, window lintels are not just a simple beam. They need to be specially fastened to the posts to resist this torsion. I only have windows in the curved front wall of the house. Here the curve is really just to be continuous with the other earth sheltered walls. To reduce this torsion effect, I have kept the wall above the windows as light as possible. It is the only section of wall made of steel studs without shotcrete. I also put beams between each 40 inch wide window so that the max deviation at the mid point between supports is only about an inch.
The Formworks earth sheltered homes all use curved back walls to support the earth. But most have very simple layouts and simple flat front walls. I am treading into dangerous territory with my curves, but at least I am aware of it.