Structural Concerns of Natural Buildings

Dan Chiras has been involved in natural and alternative building since 1994 and lives in an off-grid passive solar/solar electric home in the foothills of the Rockies. His house is built from straw bales, rammed earth tires, and numerous green building materials and is powered entirely by wind and solar energy. Dan is author of The Natural House: A Complete Guide to Healthy, Energy-Efficient, Environmental Homes, published by Chelsea Green; and numerous articles on natural building and sustainable design, which have appeared in Mother Earth News, Natural Home, and The Last Straw. Dan embraces a comprehensive systems approach to building that offers a wide range of benefits to people, the planet, and our economy. He will field general questions on natural building and offers consultation on project design and construction, as well as lectures and workshops on various aspects of natural and sustainable design and construction.

Questions and Answers

Q: I am researching for a thesis on whether there is a conflict in using sustainable materials for construction in regions of the world that are prone to earthquakes. It seems to me that many such parts of the world are abandoning their vernacular--which may incorporate design techniques to protect against earthquakes--for reinforced concrete. Whilst I can find plenty of info on environmental building, and plenty on earthquakes and seismic construction methods, I can't find much that considers both of these topics together. Can you suggest any books, institutions or specialists in this area? I would like to find out about both traditional and modern solutions.

A: (Kelly) What you say is true about abandoning vernacular architecture for modern reinforced concrete. This is unfortunate, because there is much of value with the traditional ways...they just need to be updated with current knowledge of how to mitigate against earthquake damage. Stone and adobe structures, for instance can be made quite safe by utilizing a well designed bond beam at the top of the wall that connects to the roof structure. Earthbag buildings have been demonstrated to be extremely resistant to earthquake damage. Most building methods can be engineered to be safe. The problem in general is the perception that the old ways and materials are faulty and that the newer methods are inherently better. It's that old perception that progress is linear, and the latest is always the best.

Q: I am planning on building a home on a steep hillside on Tortola in the British Virgin Islands. Most homes here are built on a solid concrete foundation, with a cistern, and the homes themselves are of concrete block with wood roofs. A few homes are wood, especially the older ones. I am interested in natural building techniques but am chiefly concerned with whether they will stand up to hurricane-force winds and earthquakes?

A: (Kelly) As you know, there are many different forms of natural building, and they have varying degrees of stability under conditions of stress. Most techniques will withstand considerable stress if they are properly built. I have experience with earthbag building, which should stand up to almost any kind of natural calamity, especially in the form of small domes.

Q: How big can we go? Is it possible to make a large barn-sized building with hybrid natural building methods? Which shapes would be best? Which materials?

A: (Kelly) The potential size depends on a lot of factors, but generally natural building techniques can be used for quite large structures. If you are just building walls, and the roof is made with conventional wood or steel framing, then certainly it is possible to go several stories high. For instance with rammed earth, you can go up to 10 times as high as the base thickness of the wall; a 3 foot thick wall could be as high as 30 feet, and this could taper as it rises. Two-story strawbale buildings are not unusual, if they are sufficiently braced and pinned, and the same would be true of cordwood and earthbag buildings. Often these sorts of structures would be much stronger if the walls follow a curved path.

Making vaulted or domed structures, that reduce the need for wood or steel roof framing materials is a bit trickier to accomplish on a large scale. My earthbag dome living space measures about 20'X30', and is about as big as anything that I know about that encloses a single space. Large vaulted or domed buildings usually employ a collection of smaller components that are grouped together.

Q and A (Kelly): I intend to demolish and reconstruct my vacation home in the Dominican Republic and am fascinated by cost, energy savings and friendliness of alternatives on this and other sites. However I have a slight doubt these alternatives can be used for a 3 story residential home and am unsure whether they can resists cat 5 hurricanes or seismic movements/earthquakes that are a possibility here. Last year we got a minor earthquake and tropical storms.

Resistance to these natural forces is primarily a matter of design, proper engineering, and quality of construction. Most building materials, if properly used, can be employed safely.

Sugarcane bagasse is available in Dominican Republic but found very limited information on its use in mixtures for construction.

I don't know anything about this.

Cast Earth looks like it can work for 3 levels and seems to be a developed system but I'm unsure.

This might work, but the process requires heavy equipment and a trained crew to operate it.

Rastra made of thastyrone looks like an awesome system but the company never answers mails and they don't have phone #'s.

In the US there are many manufacturers of ICF's like Rastra blocks, but none of these are particularly ecological. They do create a building that would withstand any of the natural forces that you mention, however.

Papercrete looks promising but doubt whether it will be suitable.

I also doubt if this would be suitable.

Strawbale is great but here its sunny and all a sudden you get tremendous amounts of rain out of the blue; rather avoid the trouble.

If you first build a post and beam structure with a roof, and use the strawbales as infill, then you can avoid the problems of sudden rains.

Whatever recommendations you can make as to what will be best to stick with I'll look further into, there are so many options they all sound good, but for a serious construction project it seems you cannot find all necessary details in one place.

Going 3 stories is quite an undertaking that requires heavier duty construction and engineering. You might consider putting the first floor underground, as a basement. This would have some advantage in terms of interior climate control as well.

Q: How many stories can most load-bearing, not wood-framed structures be safely built?

A: (Kelly) Most earthen or strawbale buildings rarely go over two stories. Above that, the lower walls must be very thick to be stable.

Q: Is it possible build a house of several floors with natural building techniques?

A: (Kelly) It depends on the design and the materials you want to build with how many floors is possible. Certainly with timber frame, and many manufactured systems it is possible to go quite a few floors. With some of the other natural materials, such as earthen, stawbale, or cordwood techniques, these are rarely built more than two stories high, although with proper engineering, higher is possible.