Monday, July 18, 2011

Air supports masonry?

Air is one of the lightest materials, whereas masonry is one of the densest.   It seems improbable that air can be effectively used to support and enforce masonry.  But that’s what I’m looking at today.
A number of innovative approaches to using air as a structural support member have been developed and introduced over the past decade or so.  Some of the most intriguing of these applications involve bridges.

For example, textile composites are used as a framework for inflatable structures.  As discussed here, these inflatable elements are used to provide arches in a concrete bridge.  The arches are made of a membrane inflated with air which serves as a support form or scaffolding while the concrete bridge is constructed.   This approach means that no rebar is necessary; no wooden formwork or other difficult, expensive scaffolding material is required.  The inflated tubes shown above are treated with resin and solidified, then filled with concrete.  These cast curved tubes (or toroidal sections) of concrete are especially ductile, yet very strong.
A number of approaches have been used to provide high-strength inflatable bridges, such as those shown below:

There is an elegant simplicity to using an inflatable element as scaffolding for massive masonry structures.    An inflated surface, or bubble, represents a least energy surface.  There is a balance between the force of the inflated –or pressurized- air, and the tensile strength of the membrane, or “bubble”.  This balance results in a least energy surface.

The simplest of these surfaces is a simple round sphere.  These surfaces can also be tubes, cylinders, and combinations of these elements.  As we look at more complex shapes, we quickly enter the mathematician’s and geometer’s realm of topography.  There are saddle shapes, ‘monkey saddles,” parabolic and hyperbolic curves, even catenary configurations.  Each of these represents a least energy surface.

I am currently experimenting with an inflatable bladder to be used as scaffolding for constructing a masonry sphere.  I am assembling a rubber sphere, which is configured like a ‘beach ball.’   This will be used to assemble masonry spheres below ground, for water storage tanks, septic tanks, and other similar applications; around 8 feet in diameter.  For this use, the sphere will be inflated to a relatively low pressure (~20 psi) and will support block as they are laid.  Once the sphere is complete, the inflatable bladder is deflated and removed.  It is incredible (to me, at least) how much weight a relatively low air pressure can support.  25 psi can hold up tons of block!
Although my test sphere will be relatively small, it will be scale-able.  If the proportions remain intact, then a much larger sphere can be made using the same approach.  As discussed earlier, Galileo tried to impose his square cube law on masonry structures, and he was WRONG, the ancient masons had it right:  scale-ability is one of the defining features of masonry design.  The same system used to build an 8 foot diameter sphere can also be used to build an 80 foot (or 800 foot!) diameter sphere.

Other dome manufacturers use inflatable bladders for their systems; notably Monolithic Domes, as I discussed earlier, here.  Their use is for spraying shotcrete though, not for support and assembly of masonry construction.

As I use my inflatable bladder for masonry scaffolding I’ll post updates and pictures on this blog.  In the past I’ve used wooden scaffolding, I expect this to be much, much easier, faster, safer and cheaper.