I figured that the Science/Engineering conference could use a
thread about domes.

One side effect of pursuing clutter reduction is that you find
things you didn't realize were there, and get a diversion...
Pursuant to an article I found, I took a break from the current
writing project and started checking out a concept of Bucky
Fuller's.  Back in 1957, he invented a way of making a geodesic
dome from a series of flat panels attached to each other at the
corners.  Unlike the "traditional" scheme where the structural
members are the struts which go directly between vertices and
the skin does not carry much of the load, the bent-panel dome's
vertices are empty space; the "struts" are bends in the panels
themselves, which carry the entire load.

I've done a partial mock-up model out of 3x5 cards, and the
thing works!  Even guessing at the proper angles results in a
form which is clearly an approximation to a sphere and is far
more rigid than anything made of such paper has a right to be.
I expect that it would have been even better had I known the
exact angle to use at the pentagon vertices (I used 70 degrees
for the included angle; it should have been slightly less, to
achieve a greater concavity at the pentagons and less at the
triangle/hexagon vertices).

One of the recurrent problems with conventional domes is that
they use butt joints between skin segments, which leak.  The
plydome avoids this by overlapping the skin segments, which
allows them to be arranged like shingles to shed water.

The model made from 3x5 cards is roughly the size of a soccer
ball.  Right now I'm trying to find a way to build a model
which is tent-sized.

5 responses total.

Very interesting.

As I mentioned in Agora, a friend of mine lives in a steel-reinforced
concrete "key dome" with extensions.  It makes a rather nice home for
two people, though it would be awkward for more than that.  The dome
shape means you really have to design the floorplan around the idea of a
large, central common area or end up with a lot of wasted space.

It's probably the sturdiest and lowest-maintenance house I've seen. 
Every couple of decades it gets a new coat of waterproof paint, and
that's about it.

Here's a picture of it.  Unfortunately I don't have any good interior
shots. http://www.gull.us/~gull/dome.jpg

He had a really hard time getting financing when he bought it, though. 
Banks are highly conservative and won't finance anything that isn't a
"conventional stick-built home."

Banks have to consider that their mortgage money is secured by
the property, and if the owner defaults the bank has to find a
buyer to recover their investment.  The more unusual something
is, the harder it is likely to be to sell.  I'm sure that some
banks simply don't want to touch anything risky.  (I suppose the
alternative is to get a mortgage on the value of the land and
build or buy the buildings for cash.)

Interesting photo.  The dome looks rather small, so I'm not
surprised that it only works for two people.  Is it laid out
purely ranch-style inside, or is there a loft area?

There's another concrete dome style I've seen, pushed by a company
called Monolithic (http://www.monolithic.com).  One of their promo
pictures shows a 2-story dome with large cutouts for what appear to
be very appealing second-floor porches, sadly without overhangs to
keep things out.  I doubt that this would work well in places which
get much rain or snow.  The one owner-builder testimonial I've seen
had photos looking like it was very short on window area.

All the concrete domes I've seen appear to be half-spheres.  I'm
not sure why; maybe the tension in the below-center fraction of
a 5/8 sphere wouldn't work with concrete, and a 3/8 sphere has
too much wasted area with insufficient headroom (or outward pressure
on the foundations doesn't work well with concrete).

The plydome idea seems to have possibilities, but I'm not sure how
easy it would be to take care of doors, windows and things people
desire like second-floor decks.  I'm going to have to stop by a
lumberyard and price the materials for a shell, just for fun.

There's a loft area, in the center, where the skylights are in the
picture.  The ceiling gets very low at the edges of the loft, much like
a "half story" attic room on a conventional house.  If I remember right
the loft is suspended from the dome roof with steel rods connected to
some of the vertexes.

Those Monolithic domes do look very short on window area, which I guess
is understandable given the challenges of putting windows in one.  The
dome I was in had three small skylights near the peak; the rest of the
windows were in the straight extensions at the sides, avoiding the
problem of trying to fit them into the triangular faces.

I've heard of the air-form construction method that Monolithic is using
before.  It's cheap and effective but it has to be *very* carefully
managed -- it's not something an unskilled worker can do.  The air
pressure is absolutely critical because it sets the contour of the dome.
 Too high or too low an internal pressure and the shape will be
incorrect, making a seriously weakened structure.  Temperature or
barometric pressure changes during the concrete curing process can
change the air pressure (and hence the contour of the form) enough to
flex and weaken the concrete while it sets.  Some domes constructed this
way have "snapped through" and collapsed shortly after the form was
removed because of this problem.  (I think one collapse like this is
discussed in the book _Why Buildings Fall Down_.)

I think the reason for the dome being a half sphere is simply because
you get the most usable area that way.  Any other configuration is going
to have low ceilings at the edges.

It is much more difficult to put a refrigerator or bathtub or bed into
something without long straight sides.  Lots of space gets wasted.  There is
one dome house in Ann Arbor that stuck a plain cube type garage on it.

Yup.  That's probably what inspired the extensions on this dome.  The
kitchen is in one of them, taking advantage of the straight walls.  The
dome area itself is mostly taken up by a loft and a central living room.
 Part of it (along with one of the extensions) is walled off for a
master bedroom and bathroom.

Like I said, it's a great layout for two people, but privacy would be
lacking for more than that.

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