The earth systems structural forming system

The Earth Systems Structural Forming System Warren Eberspacher R6sum6--Les thkoriciens ont depuis longtemps reconnu la supkrioritk inh~rente du dome hkmisph~rique en bkton de ]aible kpai~seur pour des applications de constructions protkgkes de terre. Cet article dkcrit un systkme brevetk de structure modulaire qui, dans sa [orme de kit prkIabriquO, a rendu le dome monolithique en bkton de Jaible kpaisseur a la ]Dis pratique et kconomique. Aux Etats-Unis, les constructeurs amateurs et proJessionels ont construit ces types de structure depuis 1980.

Abstract--Theorists have long recognized the inherent superiority of the thin-shell hemispherical concrete dome for earth-sheltered applications. This paper describes a patented modular structural forming system which, in prefabricated kit form, has made the thinshell monolithic concrete dome both practical and economical. Both amateur and proJessional builders have been constructing these types oJ structures throughout the U.S. since 1980.

Background espite the name of the company, the founders of Earth Systems, Inc., did not set out to develop earth-sheltered homes. Their original intent was to develop energy-efficient, maintenance-free, economical housing within the constraints of readily available technology, skills, material, and equipment. Earth sheltering emerged as an obvious option, with thin-shell hemispherical concrete domes the superior configuration within the earth-sheltered option. A l t h o u g h theorists had long recognized the superiority of the thin-shell concrete dome for earth-sheltered applications, they could not readily foresee how the complex hemispherical shape could be formed on a practical and economical basis using conventional concrete forming techniques. Because the founders of the c o m p a n y had not previously been involved in the housing industry, they were not handicapped by the constraints of traditional and comm o n l y accepted b u i l d i n g practices. Instead, they eliminated conventional forms and went directly to a kit-controlled, "free-form" approach in which concrete is shot into a hemispherical steel grid. Further, by m a k i n g the kit m o d u l a r and offering four different sizes of domes (24-, 32- and 50-ft diameters); a wide variety of versatile

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configurations could be provided easily for diverse applications. Those unfamiliar with modern earthsheltered structures hold a variety of misconceptions, largely based on oldstyle basements and outdated box-type earth-sheltered configurations. A common misconception is that earth-sheltered structures will be dark, d a m p and crowded, with no outside view. I must admit to feeling a certain degree of pleasure when a skeptic walks into a 12.19-m (40-ft) Earth Systems model and looks u p to a soaring cathedral ceiling 6.09 m (20 ft) above, bathed in natural light. While these structures can be designed to be compact and efficient, the most p o p u l a r models built to date are open and roomy. Because Earth Systems designs meet all the same glazing requirements of the

T h e structural forming system has four main part: (1) T h e foundation kit. (2) Steel beams for the frame. (3) Fabric panels. (4) A rebar grid.

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Present address: Warren Eberspacher, Marketing Director, Earth Systems, Inc., P.O. Box 3270, Durango, CO 81302, U.S.A.

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F i g u r e 1. F o u n d a t i o n l a y o u t f o r an E a r t h S y s t e m s m o d e l .

Tunnelling and Underground Space Technology, Vol. 2, No. 3, pp. 299-305, 1987. Printed in Great Britain.

The Structural Forming System

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This article first appeared in Advances in Geotectural Design (L. L. Boyer and R. L. Sterling, Eds), the proceedings of the Second International Earth Sheltered Buildings Conference. It is reprinted here with permission of the Department of Architecture, Texas A&M University.

Uniform Building Code, their openended and open-sided models, atriums, and above-grade cupolas provide all the light and views found in conventional homes. T h e cupola, in particular, provides a 360-degree v i e w - - a n advantage that one customer exploited for his art studio. Figures 1-6 provide a sample of the variety possible today using thinshell hemispherical domes for modern earth-sheltered structures.

0886-7798/87 $3.O0 + .O0 © 1987 P e r g a m o n J o u r n a l s Ltd.

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kits pernlits a veJy vcrsatilc at Jaugcm(.ul ()f stl t l ( ' l t t l a l layouts. ~()IIIU ()1 | ] l ( ' {()llll n o n options o p e n t o ns('1% o [ | h u ~,}'Sl¢'Ht lue

LARGER DOME 2ND FLOOR SUPPORT BEAMS IN PLACE

THE STRUCTURE IS DESIGNED FOR ONE SHIM PER VERTICAL SEAM & Is PLACED ON THE SIDE OF THE BEAM WHERE IT WiLL MARE THE SEAM VERTICAL & PLUMB NEVER USE MORE THAN 2 SHIMS WITH A SEAM

(1) Each of the lour sizes oi d o m e ~ : . l be e h m g a t e d to any length desired, m 1.82-m (6-ft) in¢rements, l ' h e hmgesl two-story structure investigated to date t)y a prospective client has been 914.,t m (3000 ft) hmg, for the purt)ose o[ undelground records storage. (2) Each size of d o m e can be b u i h as an o p e n - e n d e d or open-sided conligurat ion. (3) T h e thiee largest domes can be b u i h w i t h above-grade cupolas for enhanced internal l i g h t i n g or natural air circulation u s i n g the b u i h - i n "solar c h i m n e y " effect. (4) T o connect structures, 1.82-m- (6ft-), 3.65-m- (12-ft-), and 7.31-m- (24-ft-) wide tunnels can be used. O n e c o m m o n e x a m p l e is the use of a tunnel both to connect the living space with a garage, and to serve as a utility room. (5) Structures of the same size or different sizes can be connected to form annexes, alcoves, a line of o p e n - e n d e d structures, or structures formed into an L shape, a U shape, or to c o m p l e t e l y s u r r o u n d a central open courtyard.

THIS FIGURE ALSO APPLIES GENERALLY TO EXTENDED DOMES AS WELL

Figure 2. Pure dome Jrame.

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Construction Process

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Figure 3. Elongated dome Jrame. An illustrated construction manual accompanies the kit. T h e f o u n d a t i o n kit includes steel a n c h o r plates that are e m b e d d e d in the slab f o u n d a t i o n before it cures. T h e steel beams are bolted to these plates in order to a n c h o r the frame. Plastic tubes are also embedded in the concrete a r o u n d the p e r i p h e r y of the shell to locate the vertical rebar. T w o types of steel beams are used for the frame: I-beams, for the 12.19-m (40ft) and 9.75-m (32-ft) domes; and box beams, for the 7.31-m (24-ft) domes. T h e two larger domes have 3.05-m- (10-ft-) wide c o m p r e s s i o n rings to w h i c h the u p p e r ends of the beams are bolted; the

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smallest d o m e uses a single ridge I-beam for the connections. Prefabricated fabric panels serve two purposes: (1) the o u t w a r d facing fabric (a material similar to burlap) serves as a backstop for the shotcrete stage; and (2) the i n w a r d - f a c i n g 15.24 x 15.24 cm (6 x 6 in.) concrete screen provides a foundation for the interior plaster coat. T h e steel rebar grid uses 1.90-cm (3/4in.) and 1.27-cm (1/2-in.) elements in a pattern prescribed for each structure. T h e first horizontal row is locked to the steel b e a m frame u s i n g preset steel tabs welded o n t o the beams. All rebar is tied in place with c o n v e n t i o n a l tie wire. T h e m o d u l a r i t y designed into all the

TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY

E x c a v a t i o n . Excavation is a m a j o r part of the construction process and m a i n l y varies d e p e n d i n g on whether the h o m e is built on flat g r o u n d or into a hillside. O n flat ground, the excavation typically is deep e n o u g h to accommodate the lower floor. T h e earth removed from the excavation is usually sufficient to cover the r e m a i n d e r of the structure above grade to a m i n i m u m depth of 0.91 m (3 ft) over the top of the dome. Builders are c a u t i o n e d to leave a m i n i m u m of 1.52 m (5 ft) of clearance all a r o u n d the d o m e ' s footing to ease the m o v e m e n t of scaffolding, rebar, shotcrete hoses, etc. F o u n d a t i o n . T h e first step in laying the f o u n d a t i o n is to survey accurately both the a n g u l a r and radial distance locations of the vertical beams, using the d o m e ' s vertex as the reference p o i n t (Fig. 7). These reference points then become tile basis for constructing a c o n v e n t i o n a l concrete footer and slab floor. T h e f o u n d a t i o n s are designed for a worst-case soil load-bearing c o n d i t i o n of 7320 k g / m 2 (1500 psf). Soils exhibiting a worse case condition, e.g. expansive clay, can be, and are being, handled by m o d i f y i n g the standard design to be c o m p a t i b l e with the specific conditions. W h i l e the concrete is still setting up, the vertical beam a n c h o r plates and vertical rebar location tubes are embedded in the soft concrete u s i n g the same a n g u l a r and radial distance reference points (Fig. 7). F r a m e Erection. Standard scaffolding is most c o m m o n l y used for this step,

V o l u m e 2, N u m b e r 3, 1987

Figure 4. Open-end Jrame with lower [abric panels in place.

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Fzgure 6. Steel rebar grid ]or an open-ended home. Volume 2, Number 3, 1987

and even the longest beams are light enough for four people (two u p and two down) to handle without specialized equipment. T h e steel beams are bolted at the bottom and top. Figure 8 shows a pure dome; Fig. 9, an elongated dome. T h e prefabricated fabric panels are then fitted over the steel frame and tied in place (Fig. 10). A three-layer steel rebar grid composed of 1.27-cm (1/2-in.) and 1.90-cm (3/4in.) rebar is then attached in a pattern specified by engineering for the specific shell design. T h e first horizontal layer is attached to the vertical beams using prefabricated tabs to accurately locate the base layer. As the layers of rebar are built u p horizontally and vertically and tied, the inherent stiffness automatically creates the c o m p o u n d hemispherical shape (Figs 11 and 12). Rebar is also used to tie the curve shell surface into adjacent retaining or atrium walls. Inasmuch as the rebar grid between the vertical beams is still flexible at this stage, m i n o r internal bracing is added to help support the point loads of workmen w a l k i n g on the grid and the dynamic loads of wet concrete until the whole shell cures sufficiently. Builders typically use roughed-in interior walls and subflooring as anchor points for this temporary bracing (Fig. 13). Special attention is paid to bracing around openings in the shell where there is a discontinuity in the rebar tension since these are "soft spots" in the shell until the conrete cures sufficiently. Shotcrete. This generic term is used to describe the use of both the shotcrete and gunnite processes. T h e final cured strength specified by Earth Systems is 27.58 MPa (4000 psi). While a seven-bag mix is adequate in theory by ACI (American Concrete Institute) standards, Earth Systems specifies an eight-bag mix to compensate for any field variations in concrete mixing, application, a n d / o r cutting. For shotcrete, the recommended m a x i m u m aggregate size is 0.95 cm (3/8 in.), a size compatible with most shotcrete equipment pumps. Earth Systems also specifies a sequence for shooting the concrete into the grid in order to load the steel skeleton as equally as possible. The shotcrete crew works completely around the base of the dome and applies shotcrete up to a comfortable working height (approx 2.44 m, or 8 It). By the time the shotcrete crew gets back to the starting point, the first course has set up sufficiently to give the next higher course something to build on. This process continues until the top surface of the dome is shot (Fig. 14). Crews have used a variety of "work platforms" to reach the upper surface, e.g. standard scaffolding, ladders, hydraulic lifts. T h e result of this process is a monolithic shell without any joints or breaks in the surface. Concurrent with shooting the concrete, a backup crew follows along

TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 301

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behind, trowelling tile surta(e into a smooth finish. This process is essential to ensure a good surface bond fol d~c waterproofing coat. Aftra {onsiderabh. research and field experience with various types of waterproofing material, Earth Systems selected "Watertite T P M " (made by PennKote of Canada). T P M retains its resilience widl age, is able to bridge small imperfections in the ~oncrete surface, has sufficient permeability to resist subsurface water v a po| under massive earth pressure (0.54 × 10-3 perms, metric), and has long-term resistance to the effects of moistme, ozone, uhraviolet, salts, diluted a~id and alkali solutions, bacteria, and fungi. Backfilling. Th e comtete shell is allowed to cure for 28 days to achieve its full strenght before backfilling (ommemes. Earth Systems spe(ifies ba{kfilling unilormly to common levels all around the dome before going to the next higher level, in order to load lhe shell as uniformly as possible. Rigid board insulation is placed against tile shell as backfilling takes place. There are two reasons for this placement of insulation: (1) Graduating the thickness from 2.54cm (1 in.) at the base to 10.16cm (4 in.) over the top provides additional interior temperature buffering against changes that take place above grade. (2) Th e placement provides a physical buffer against rocks and other sharp objects during backfilling to protect the waterproof (:oat. Unlike the backfilling process on outdated box-type earth sheltered structures, backfilling of dome-type structures places very little restriction on the size of the equipment; ill some instances, equipment as large as a D-6 Caterpillar tractor has been used for backfilling. Interior and Finish. Concurrent with the curing a n d / o r backfilling, tile interior is finished. Conventional construction is used on interio! walls, second floors, etc. Th e interior of the shell normally is coated with commercially available plaster, although wood panelling has been used. Entry and interior decor are finished to suit the tastes of the owner and have ranged from contemporary to modern, rustic, Spanish and Victorian styles.

Factors Influencing Life-Cycle Costs

Figure 8. Shooting the top surJace with shotcrete. 302

TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY

Th e Earth Systems founders disagree with the commonly accepted--but shortsighted--emphasis on buy-in ol movein cost, because it emphasizes shortterm expediency rather than long-term durability. A far more important factor, in the estimation of Earth Systems management, is the structural life-cycle cost, because it represents the costs that the owner must support end-to-end in order to be able to keep the sm~cture that Volume 2, Number 3, 1987

was bought. Factors included in the lifecycle cost analysis are discussed below.

Reduced Land Cost Because excavation and backfill are major steps in the construction process, houses can be built on sites that are considered unsuitable, e.g. too uneven, too close to objectional noise sources, or cost prohibitive, e.g. too hilly or too steep, for conventional above-grade construction. Typically, these sites can be purchased at a lower price than conventionally acceptable land.

More Structure for the Money Much like the shell of a chicken egg, the thin-shell earth sheltered hemispherical dome achieves its superior strength through form rather than great mass. This p h e n o m e n o n translates into a more economical structure in two - respects: (1) Shell strength is achieved with 10.16-cm- (4-in.-) thick walls, as opposed to 2.32-cm- to 30.48-cm-(8-in.- to 12-in.-) thick walls and with 35.56-cm- to 50.80-cm- (14-in.- to 20-in.-) reinforced roofs on outdated box structures. This reduces labor and concrete costs. (2) A sphere (or hemisphere) encloses more volume for a given surface area-typically, half again as much as a box with equivalent surface area. The ovenbird intuitively understands this and builds her nest as a sphere. The spherical shape enables the owner to buy equivalent interior volume for less shell cost, or greater interior volume for the same shell cost as a box-type house.

Figure 9. Solar-augmented open-end model o/an Earth Systems home in New York.

Minimized Maintenance With only a negligible part of the durable Earth Systems structure exposed to the deteriorating elements, maintenance/upkeep manhours and material costs are drastically reduced, if not eliminated. The inevitable repainting, roof replacement, etc. on a conventional above-grade home are virtually a thing of the past. This durability protects the investment. Appraisers have already granted 85-year economic lives on new Earth Systems construction for loan purposes. Even this figure is very conservative, considering that pre-1900 concrete structures are still in use and WW II military bunkers withstood the heaviest bombardments that could be delivered.

Figure 10. Pure dome house in Arizona.

Inherent Invulnerability T h e basic configuration of the Earth Systems structure, the materials used, and the construction process create an inherent invulnerability to common external hazards such as heavy snow loads, snow slides, hail, ice accumulation, brush fires, high winds, falling trees, tornadoes, vandalism, etc. This vulnerability protects not only the owner's investment in the structure, but the internal contents as well.

Volume 2, Number 3, 1987

Figure 11. Pure dome house in Arizona. TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY 303

Energy Efficiency

Figure 12. Open-end Victorian model in Colorado.

Th e energy efficiency benefits gamed from the tons of concrew and earth thermal mass in an earth-sheltered home are well known to anyone familiar with the literature. The token efforts to increase the energy performance ot above-grade homes, e.g. through the use of Trombe walls, rock beds, water tanks, sah trays--while helpful, cannot begin to compare with the energy savings from the tons of thermal mass that are a natural consequence of the earth sheltering construction process--especially when above-grade systems today typically use energy-consuming control systems to "help save" energy. Earth Systems home owners consistently report that even in sub-zero weather, their homes use only 10-20% of the energy used by their above-grade neighbors in modern, well-insulated homes. Some of the earth-sheltered home owners report thai they have never used any supplemental heating or cooling. Given the steadily es(alating costs of energy, there arises a serious question of whether people in tonventional above-grade homes will be able to continue to afford energy 10 or 20 years from now, especially after they retire on reduced incomes. From the Earth Systems home owner's standpoint, an occupied structure remains liveable under the worst condition of zero energy input, typically stabilizing at 15.5518.33°C (60-65°F). A sweater, a few candles, small amounts of passive solar heating, etc., make the interior comfortable.

Reduced Insurance Costs Maintenance elimination, long-term durability, and inherent invulnerability all translate into less risk for the insurer; and, hence, lower insurance rates. Earth Systems owners typically pay lower, if not the lowest available, insurance rates on their structures. As with energy, the steady escalation of insurance costs shows no sign of leveling off, let alone decreasing.

Current Cost and Performance Data

Figure 13. Second-floor kitchen area. 304

TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY

Based on experience gained in construction o[ many homes throughout the U.S., the closed-in shell ready for backfill and interior finish typically takes approximately nine days from the time the foundation is ready, at an average cost of SUS161.46-215.28 per m 2 ($15-20 per ft2). Owners doing their own building or subcontracting can expect a turn-key completion cost of $301.40-376.75 per m z ($28-35 per ftz). Totally contracted homes have averaged $376.75-538.21 per m 2($35-50/ft2). Earth

Volume 2, Number 3, 1987

Systems is now moving into commercial, industrial, municipal, and agricultural applications and is starting to collect equivalent statistics for these types of structures. The Earth Systems structures have achieved the energy efficiency that theorists have long predicted. Unoccupied, unheated structures typically stabilize at 10-12.78°C (50-55°F), even in sub-zero weather. This feature attracted a local fire district to build an earthsheltered fire station because, even if a total power failure occurred, the vehicles would always be ready to start and water in tankers would never freeze. The performance of occupied structures has been noted above. Wherever Earth Structures owners live throughout the U.S., they consistently pay substantially less than their neighbors for heating and cooling. A 2300-ft z, two-story open-ended model with a greenhouse, in Durango, Colorado, provides a typical example. The local elevation is 1981.2 m (6500 ft). Typical winter temperatures range from -15 to -12.2°C (5-10°F), with a week or two ranging from -26.1 to -31.6°C (-15 to -25°F). Over the past three winters, the owner has never used his electrical backup heat and has burned no more than one cord of wood each winter; yet his home maintains an interior temperature range of 18.33-21.22°C (65-70°F), due to normal household activities, e.g. cooking, clothes washing and drying, showers, etc., of a family of four. F'I

Figure 14. Second-floor dining area.

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