Disaster-resistant Shells
The wall-roof cross-section details shown on this website do not necessarily represent the only methods for building a disaster-proof house. Instead, they illustrate proven solutions to the problem of building houses that are sustainable, with high resistance to powerful winds, earthquakes, fire, termites and critters. At the same time, new energy conserving methods permit easy accomplishment of significant energy conservation.
Three-dimensional Concrete Shells
The approach to disaster-resistant home construction requires that the entire exterior shell (walls, floors, and roofs) of the house be monolithic reinforced concrete in character. Well-known examples of structures that must demonstrate three dimensional structural integrity are aircraft fuselages and ship hulls, especially submarines and oil tankers. Concrete ships and barges have utilized shell technology for over half a century.
The technology for manufacturing pre-insulated precast concrete walls is generally well within the capability of most producers in the precast and tilt-up walls industry, but has not been particularly utilized for single-family homes by that industry. Similarly, the methodology for building cast-in-place concrete roofs using insulating concrete form (ICF) planks also has been available to contractors, but appears not to have been commonly utilized in the construction of homes. ICF plank roofs have been adapted to residential construction activities utilizing general and specialty concrete contractors.
Exterior walls can feature any of the attractive architectural finishes already being used in the industry. Ideas for such finishes are well illustrated on the Internet.
Disaster-resistant Concrete Shell Variations
The essential consideration for the creation of a DISASTER RESISTANT SHELL (DRS), as the term is used herein, is that the complete enclosure of a space such as a building or a house is a continuous monolithic structure made with reinforced concrete. This includes walls, floors, and roofs, all structurally connected.
Houses complying with this requirement have already been conceived and are being constructed in various parts of the world. References will be made here to houses that can be designed which can be made to respond to the DRS requirement. The list here is not all-inclusive.
Guam Houses
Non-insulated DRS houses have been constructed for fifty years on the island of Guam and are described elsewhere in this seminar.
The details that follow refer to insulated DRS houses that feature built-in capability for fabrication with integral insulation.
Insulating Concrete Forms (ICF)
The most mature of the insulated concrete construction methods uses cast-in-place reinforced concrete formed with foam insulation boards. In North America, it has become known as "ICF." The technique started out being used only for walls, but still retained the archaic wood frame roof structure. A house using a wood-framed roof structure cannot be designed for DRS performance because it does not meet monolithic shell continuity requirements. Recently, the ICF industry has been providing ICF roof elements. These enable the construction of monolithic concrete shells. A comprehensive list of ICF suppliers is compiled in the January/February 2013 issue of the ICF Builder magazine*. Most of the manufacturers listed provide Internet access to detailed construction manuals as well as personalized technical assistance.
Contrary to a general paradigm, the foam insulation products called "ICFs" are used only for forming the house structural elements. ICF forms are used solely for insulation and make absolutely no contribution to the structural strength of the shell. They provide primarily for energy conservation. Unfortunately, mythical structural qualities have been attributed to "ICFs" that truly do not exist. The structure that resists the natural disasters is fabricated with structurally designed reinforced concrete. The method used for forming the concrete elements has nothing whatsoever to do with the "disaster-resistance" of the concrete structure. Codes for designing reinforced concrete structures have been well established for decades. Unfortunately, in disaster-prone regions, single family houses have somehow been generally exempted by building codes from the requirement for structural analysis and certification by a licensed engineer.
ICF Builder magazine* carries the most up-to-date information about almost every aspect of ICF construction. Magazine issues have been archived on the Internet back to the year 2005. For a person who is seriously interested in the subject, a subscription to the magazine would be a good investment.
W Panel
Another insulated system that has seen more limited application over a number of years is one which utilizes wire cages for reinforcement and foam insulation board in the interior of the cage. Concrete is applied to the exterior and interior faces of the panels by hand or by use of shotcreting. For an example of one approach to this method of construction, one is referred to photos and additional information, see the ASPI Informational PDF.
ForeverHome™ precast concrete shell
A precast concrete entry into the DRS marketplace is a truly prefabricated concrete shell under the name of ForeverHome™. A very interesting article mentioning ForeverHome™, "Mass-produced, hurricane-resistant homes become reality," [pages 14 and 15].
Sample Structural Analysis for a Concrete Shell
A presentation was made by one of the authors at the annual conference of the Insulating Concrete Forms Association (ICFA) in November of the year 2009 in Orlando, Florida that reported on the structural feasibility of using four-inch thick conventionally reinforced concrete walls in a single family house using a concrete roof system similar to that illustrated in this website. (Structural Analysis of 790 Square Foot Residence with 4" Thick ICF Walls Subjected to 350 mph Tornado Winds and Maximum California Seismic Loads, [PDF, 2MB])
Utilizing calculations based on contemporary building codes in effect at the time, it was shown that properly designed and detailed four-inch thick reinforced concrete walls were adequate for use in single family houses that needed to resist the forces generated by earthquakes, hurricanes, and tornadoes.
In addition to the presentation, at about the same time, a magazine article on the subject was published in ICF Builder* magazine, titled Four Inch Walls.
Architectural Appearance
It is quite possible to make the exterior appearance of an attractive concrete house architecturally indistinguishable from that of a wood framed house. (Non-insulated home on the island of Guam)
Pitched roofs are the preferred architecture in North America, so references to flat roofs will not be addressed here. All details shown here will feature pitched roofs.
Exterior walls can be finished to emulate many of the preferred exterior characteristics of wood-framed construction as well, plus a number of architectural concrete finishes not available to a wood-framed house. Stamped floor techniques might be used to impart a number of attractive roofing patterns. Interior wall and ceiling finishes include conventional drywall and wood paneling, as well as plaster.
Since the entire structural disaster load-resistant requirement of the building is designed to be carried in the reinforced concrete exterior walls and roof (the shell), the open interior layout allows flexibility of floor plan. The completed exterior of such a house encloses an empty space that can be planned and built later. A commercial retail store is an example of this type of construction contract approach.
In effect, the outer shells could also well be constructed by general contractors or by precast concrete manufacturers, leaving the interior partitioning to be completed by interior specialty contractors at a later time. The exterior shell can be completed with doors and windows installed with all exterior finishes and trim finalized. Optionally, they could incorporate all utilities conventionally included in the exterior walls, including electrical and plumbing.
Interior partitions and walls should be constructed with fireproof materials such as metal wall studs. Wood studs and wood paneling should not be used when it is desired to preserve the fireproof character of the interior. The exterior shell of the house, being reinforced concrete will not burn. Use gypsum or similar non-flammable panels for closing in the walls.
*ICF Builder is the premier industry-wide magazine for people interested in energy-conservative disaster-proof houses. The addition of the structural insulated concrete diaphragm roof makes economic tornado-proof, fireproof houses an achievable reality. ICF Builder Magazine, 884 East 700 North, Mapleton UT 84664, ATTN: Mr. Clark Ricks