Geodesic Dome of Polystyrene and Concrete_tcm45-341686

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Cut 6-inch-thick triangles of e x t ruded polystyre n efoam, assemble a geodesicd o m e, and encase it with

re i n f o rced concre t e. This method of homebuilding re q u i res no heavy equipment, uses locally fabri c a t e dmaterials, and delivers a strong, en-e rg y-efficient dwelling suitable formany climate zones. Shell costs aremoderate for the space enclosed.

Geodesic domes:What and why?

A geodesic dome may be de-s c ribed as a part of a sphere con-s t ructed of multiple straight barsa r ranged to form tri a n g l e s. The barsa re called geodesics and the tri a n-gles are facets. St ru c t u ra l l y, the geo-desic dome is a space truss and isthe strongest known stra i g h t - l i n ef o rm enclosing space. It is efficientas well as strong; there is nos t raight-line geometric form that re-q u i res less surface skin to enclose ag i ven amount of space than a geo-

desic dome.In the concrete and polystyre n e

dome described here, folds in thec o n c rete shell replace the stra i g h tb a r s. The folds, howe ve r, act just asthe bars would to transmit loads tothe foundation.

The concrete geodesic dome

The triangular facets of this con-

c rete dome are arranged in hexa-gons and pentagons to approx i m a t ea spherical surf a c e. Take a close look at a soccer ball and yo u’ll see thesame pattern .

Dome designs are available in 32-and 40-foot base diameters. Eachdome is about three-eighths of acomplete sphere. The aero d y n a m i c

shape helps it resist strong winds,and snow tends to form its own arc hover the dome stru c t u re. The domeis designed for a safe snow load of 50pounds per square foot and for a wind velocity of 120 mph.

The shell of concrete and poly-s t y rene has an R value of 27. This in-sulating capability can readily be in-

Concr ete in housing

Geodesic dome of polystyrene a nd concr ete

Saves money, space, and energy

BY M. K. HURD



c reased with selected interior fin-ishing materials to R-35 or more.

Foundation requirements

A 32-foot-diameter dome we i g h sabout 25 tons. That weight usually can be carried by an edge beam 12inches wide and 110 feet in peri m e-t e r, resulting in soil pre s s u re lessthan 500 pounds per square foot.Local soil conditions should alwaysbe investigated to be sure the soil is

s t a b l e, with adequate bearing ca-p a c i t y. Where the soil is subject to

f re ezing, the footing must be placedb e l ow the frost line, with peri m e t e rinsulation to reduce heat losst h rough the foundation.

The 4-inch slab on grade with itsedge beam can be laid out as a 10-sided polygon. The slab is thickenedto 12 inches at the location of any l o a d - b e a ring interi or part i t i o n scalled for by the arc h i t e c t u ral de-

sign. Dowels embedded in the edgebeam project 1 foot into the domeshell to tie it to the foundation. Thecenter of the slab must be clearly m a rked as a re f e rence point forp o l y s t y rene panel ere c t i o n .

The dome designer re c o m m e n d susing a polyethylene film mem-b rane under the slab as a vapor bar-ri e r.

Cutting and assemblingthe triangular panels

All the components for the con-c rete geodesic dome can be fabri-cated at the site. Two sizes of tri a n-gular panels and seve ral re c t a n g l e smust be square cut from 6-inch-thick extruded polystyrene foam.Since it may be difficult to get then e c e s s a ry accuracy with a hand saw,a power saw is suggested. Cut to w h a t e ver depth you can with yo u r

p ower saw and cut the rest by hand. You can even break the panel along the power saw cut, since only onee d g e, the inside one, needs to becompletely accura t e.

The triangular panels are pre-assembled into pentagons and half hexagons before being erected inthe dome. Metal clamp plates, twofor each joint, hold the panels to-gether at the outside surf a c e. A boltt h rough the clamp plate and thep o l y s t y rene foam secures a 2x4 ri b

on the inner surface (see detaild rawing). Joints between the poly-s t y rene panels are filled with a two-component polyurethane which ex-pands after it is poured into place.

Fi ve rectangular base panels areglued in place on the foundation tos t a rt the final dome assembly. Ty p i-c a l l y, door and window openingsa re formed between these panels.

Five rectangular base panels are firsterected on the slab foundation. Thenpreassembled pentagons are mounted ontop of them and braced and shored to theslab.

Welded wire fabric in two layers is placed over the polystyrene panels toreinforce the geodesic dome. A hoop of reinforcing bars circles the dome (atlevel of worker’s foot) to reinforce a ring beam.



Then the preassembled pentagonsa re set on top of the base panels and joined with clamping plates and in-t e rior wood ri b s. Next the half hexa-gons are clamped into place, and as-sembly proceeds with more panelsuntil the dome is complete. A con-tinuing check measurement fro mpanels to the center point of the

base slab maintains the geometri-cal accuracy of the dome. Any joints not filled during the

p reassembly must be filled with ure-thane as the erection goes on. In t e r-nal struts and shores are added tos u p p o rt the dome and workers on it when they place re i n f o rcing steeland concre t e.

Why are the polystyrene panels alls q u a re cut and erected this way e ven though they join at an angle?The polystyrene is not dimensional-

ly stable enough to be glued accu-ra t e l y. It expands slightly as panelsa re cut from the manufactured bil-l e t s. Also there are nine different an-g l e s, which would make panels diffi-cult to shape. The polyure t h a n efoam is like mortar between bri c k s ;it allows for any imperfect cuts ands u rf a c e s.

When the polystyrene is fully as-sembled and braced, it should bet e m p o ra rily anchored with t ie wire sto pre vent uplift by wind.

Setting steel and applyingconcrete

Principal re i n f o rcement for thedome consists of two ove r l a p p i n g l a yers of galva n i zed welded wiref a b ric, 6x6—W10xW10. Un g a l va-n i zed mesh may be used if a water-p roof membrane is planned for thed o m e’s outer surf a c e. The two laye r sa re placed at approximately one-t h i rd and two-thirds depth, re s p e c-t i ve l y, of the 13 ⁄ 4-inch concrete shell

t h i c k n e s s. Wood spacer blocks areused as needed to maintain steel inthe proper position. Wood blocksa re also glued to the panels asgauges for the application of con-c rete to proper thickness.

Added re i n f o rcing steel is placeda round openings in the dome, and aring of five #3 bars encircles it at thel e vel of the top of the door openings.

The builder has a choice of thre e ways to apply the concre t e :

sTroweling by hand

s Sh o t c reting, either wet- or dry-m i x

sLaminating with a plaster gun

For the ow n e r-builder who trow-els the mix on by hand in smallq u a n t i t i e s, a stiff mix using one partcement, two parts sand, and thre ep a rts pea gra vel (by volume) is sug-gested. You place the concrete in 8-inch laye r s, working up from thebase of the dome. By the time yo uh a ve made a full circle around thedome the first layer has begun to setenough to support the next laye r. As

you go higher and the slope of thep o l y s t y rene is less steep, depth of the layers can be incre a s e d .

Built with shotcre t e, the dome re-

q u i res a 3000-psi mix. Or, using aplaster gun, a stucco mix of sandand cement can be applied in seve r-al laye r s, each 3 ⁄ 8 to l ⁄ 2 inch thick. W h a t e ver method of application is

chosen, 7-day moist curing of thec o n c rete is re c o m m e n d e d .

Interior and exterior finishes

Some hairline cracks in the domecan be expected because of settle-ment, cre e p, and shrinkage Howe v-e r, the designer considers the typical

dome waterproof when simply painted on the outside, if the quality of the concrete and its applicationa re satisfactory. If there are any re s e rvations about waterpro o f n e s s,the owner may select from a va ri e t y of waterproofing membrane mate-ri a l s.

Another exterior finish tre a t m e n tuses 2x2 cedar or redwood ribs toaccent the ridges of the dome. Wo o d

shakes can be applied over the con-c rete for a more traditional finish.Or textured stucco can be used.

On the inside it is possible to plas-ter directly onto the polystyre n e without use of lath or wire mesh.Perlite plaster about 5 ⁄ 8 inch thick p rovides a permanent finish that in-c reases the basic shell’s R value of 27. The plaster can be applied be-t ween the 2x4 ribs that we re needed

ESTIMATED MATERIALS FOR GEODESIC DOME BASE SLAB AND SHELL

DOME DIAMETER

32 feet 40 feet

SLAB ON GRADE WITH EDGE BEAM

3000-psi concrete, cubic yards 16 23

Wire mesh, 6x6–W10xW10, square feet 1,150 1,700

Rebar for edge beam, pounds 750 900

6-mil vapor barrier, square feet 920 1,400

DOME INCLUDING ARCHES (FIVE OPENINGS)

Extruding polystyrene(density 1 pound per cubicfoot), board feet 9,025 14,200

Galvanized wire mesh, 6x6–10Wx10W, square feet 3,500 5,500

Hoop rebar, #4, Grade 40, pounds 250 430

3000-psi concrete, cubic yards 11 16

Lumber, 2x4 framing grade, board feet 425 550

Urethane foam, two-component, gallons 16 20

Latex rubber cement, gallons 1.5 2

Galvanized 14-guage steel for clamp plates, square feet 30 30

Galvanized 1/4 -inch threaded rod, feet 158 1581/4 -inch galvanized nuts, each 420 420

1/4 -inch galvanized washers, each 210 210



for erection of the dome, or the ri b scan be re m oved to permit continu-ous application of plaster. Gy p s u mb o a rd can be installed over woodf u r ring strips if pre f e r re d .

In t e rior partitions are of conve n-tional wood framing. Typically a loftor second floor is placed in the cen-ter of the dome where 8 to 10 feet of

h e a d room is ava i l a b l e.Costs and material quantities

The designer has estimated mate-rials costs at about $10,500 for a 32-foot-diameter dome, and $16,000for a 40-foot dome. This includes aslab on gra d e, the dome shell, doorsand window s, plus rough interi o rf raming of partitions and loft. Yo ucan use the quantities given in thetable to estimate your own costs forthe slab and dome shell.

Main floor area of the 40-foot

dome is 1,300 square feet, with 500s q u a re feet more on the upper floor.The 32-foot diameter dome has 850s q u a re feet on the main level plus300 square feet in the second floorloft. Both are designed with twob a t h ro o m s.

St a n d a rd plans and step-by- s t e pc o n s t ruction pro c e d u res are ava i l-

able from the designer for 32- and40-foot domes. Larger diameter,h i g h - p rofile geodesic domes up to50 feet in diameter are available ascustom designs.

Credit

Architect: Peter M. Vanderklaauw,

Miami

P U B L I C AT I O N # C 8 8 1 0 1 8Copyright © 1988, The Aberdeen Gro u p

All rights re s e r v e d

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Geodesic Dome of Polystyrene and Concrete_tcm45-341686