Struc­tures as light as air

In­flat­able con­struc­tions are ef­fi­cient, light­weight, strong, eco­log­i­cal and safe — in­tro­duc­ing a high-tech yet play­ful, rounded and sen­sual ar­chi­tec­tural lan­guage

Domus - - ARCHITECTURE - Text by Mau­r­izio Mi­lan

Air is a light and im­pal­pa­ble el­e­ment able to sup­port heavy bod­ies such as aero­planes, means of trans­port that make part of our ev­ery­day lives. Build­ing with air may seem para­dox­i­cal, but let us see why this isn’t the case.

The safety of a road ve­hi­cle rests on the cer­tainty of hav­ing well-in­flated tyres. By the same rea­son­ing, air helps pro­vide sup­port, sta­bil­ity and re­sis­tance to pneu­matic struc­tures. And as with road tyres, the dif­fer­ence be­tween the in­ner pres­sure and the out­side at­mo­spheric pres­sure cre­ates the “balloon” ef­fect.

These are sim­ple single-mem­brane struc­tures, like the ones we are ac­cus­tomed to see­ing over ten­nis courts. Ac­cess is pro­vided along a cor­ri­dor with a dual or re­volv­ing door to avoid los­ing the slight over­pres­sure in­side, which is im­per­cep­ti­ble to even the most sen­si­tive people. Of all the struc­tures in ex­is­tence, this is the light­est. With no size lim­its, it be­haves like a soap bub­ble, with iden­ti­cal ten­sion at all points in­side the mem­brane. To en­sure ef­fi­ciency, air must be blown into it and it is warm in win­ter. Very lit­tle en­ergy is needed to main­tain the over pres­sure, al­though it must be mod­u­lated to with­stand the ex­ter­nal stress of the wind.

Sim­ple con­struc­tion also guar­an­tees safety. The pres­suri­sa­tion sys­tem com­pen­sates for small losses of pres­sure, while in the event of a large tear, the light­weight shell (weigh­ing ap­prox­i­mately three to four kilo­grams per square me­tre) is ren­dered vir­tu­ally in­of­fen­sive, thanks to its soft­ness and a slow-col­lapse mech­a­nism, elim­i­nat­ing any source of se­ri­ous dan­ger to the oc­cu­pants.

The most evolved sys­tems have dou­ble walls and are also known as “pil­lows” on ac­count of their quilted ap­pear­ance. A good ex­am­ple is the Al­lianz Sta­dium in Mu­nich de­signed by Her­zog & de Meu­ron for the 2006 FIFA World Cup.

The fore­run­ner, how­ever, was David H. Geiger, who de­signed the roof of the Hu­bert H. Humphrey Metrodome foot­ball sta­dium in Min­neapo­lis, Min­nesota, in 1982. The open-air sta­dium had to be pro­tected with an al­most weight­less struc­ture that would not re­quire re­in­force­ments from the stands or work on the foun­da­tions. The roof needed a few re­pairs in the first five years but then col­lapsed in 2010 be­neath an ex­ces­sive build-up of wet snow. In 2011, it was re­con­structed sim­i­lar to the pre­vi­ous one but with more mod­ern and re­li­able ma­te­ri­als, us­ing 40,000 square me­tres of mem­brane. The sta­bil­ity of this ty­pol­ogy stems from the over­pres­sure of the air trapped be­tween two fine walls: an in­ner and an outer mem­brane. The space below the pneu­matic struc­ture does not need to be pres­surised, mean­ing that no sealed ac­cesses are nec­es­sary and it can also cover spa­ces with no walls. It does not re­quire air com­pres­sors and the pres­sure in­side the “pil­low” sim­ply has to be mon­i­tored.

To un­der­stand how these struc­tures func­tion we must ex­plain the con­cept of re­straint. The prin­ci­ple en­tails cre­at­ing prior stress, and in pneu­mo­static struc­tures the air pres­sure main­tains the mem­brane taut. The mem­brane ten­sion is then mod­i­fied within cer­tain lim­its, with­out ever be­ing an­nulled as a re­sult of ex­ter­nal loads. Pre­stressed con­crete of­fers a good ex­am­ple of a struc­ture in a state of re­straint, along with tough­ened glass and taut ca­bles.

The fine poly­mer mem­branes em­ployed for these struc­tures have no ca­pac­ity to with­stand com­pres­sion and are highly re­sis­tant to trac­tion. The air blown in­side can reach high in­ten­si­ties of a few bars, but hu­mans will never be in con­tact with this. The in­ter­nal pres­sure cre­ates trac­tion in the mem­brane and this ten­sion is re­duced or in­creased by ex­ter­nal stress — snow, wind, heat changes, sus­pen­sion, etc. — but the trac­tion ten­sion must al­ways be main­tained.

To be ef­fi­cient and en­sure a sta­ble form, i.e the de­sired ge­om­e­try, low-stretch mem­branes are adopted, gen­er­ally made from PTFE (poly­te­traflu­o­roethy­lene, also known as Te­flon) and re­in­forced with fi­bre­glass. Ex­cel­lent per­for­mance is also achieved with

PVC-coated polyester fab­ric, or the lat­est fi­bre­glass tex­tiles with sil­i­cone coat­ing that al­low up to 40 per cent translu­cence. A trans­parency of 95 per cent, mean­while, can be achieved with ETFE, eth­yl­ene tetraflu­o­roethy­lene.

These dual-layer struc­tures con­sist of two par­al­lel mem­brane lay­ers, which are linked to each other by walls or con­nec­tion points to pre­vent the in­ter­nal pres­sure in­flat­ing them like a balloon. Hence the struc­tures dis­play the typ­i­cal “quilted pil­low” pat­tern. It is also sig­nif­i­cant that the dry air blown be­tween the two mem­branes pro­vides good ther­mal in­su­la­tion. Fur­ther­more, if the mem­branes are trans­par­ent, they can be anti-UV treated and pro­vide ex­cel­lent nat­u­ral light dis­tri­bu­tion in­side the space.

Some years ago, I was work­ing on Renzo Pi­ano’s project to up­grade the Baia di Sis­tiana, near Tri­este, where a large wa­ter space was to be pro­tected with a light­weight roof, thus cre­at­ing an “in­door sea”. I was in­spired by the parafoil, a parachute with a wing cross-sec­tion. With its multi-cel­lu­lar struc­ture, the so-called “fly­ing mat­tress” fills with air when opened and re­mains rigid through­out the de­scent to ground, where for­ward ve­loc­ity is an­nulled and it de­flates. The light­weight struc­ture de­signed for Sis­tiana adopted the same prin­ci­ple: many cells com­bined to form an im­mense “pil­low” ap­prox­i­mately 50,000 square me­tres in size and with a dou­ble trans­par­ent Ted­lar mem­brane.

In self-erect­ing struc­tures, air pres­sure is used to lend sta­bil­ity to a pres­so­static frame

Fur­ther development in the sus­tain­abil­ity of this con­struc­tion tech­nique brought the in­tro­duc­tion of a fine in­ter­me­di­ary layer con­tain­ing pho­to­voltaic cells be­tween the two mem­branes, of which at least the up­per one is al­ways trans­par­ent. This sys­tem was adopted for the roof over the court­yard of the new Lom­bardy Re­gional Coun­cil build­ing in Mi­lan.

An­other type, which we can call in­ter­me­di­ary, uses air pres­sure to lend rigid­ity and sta­bil­ity to a pres­so­static frame. This is the con­struc­tion

In the event of nat­u­ral dis­as­ters, in­flat­able struc­tures have ex­traor­di­nary prop­er­ties due to their light­ness

form pre­dom­i­nantly adopted for self­er­ect­ing struc­tures. The con­fig­u­ra­tion is sim­i­lar to the stan­dard struc­ture for tra­di­tional build­ings, but the beams and pil­lars are tubu­lar and in­flated by a single com­pres­sor. They are in­ter­con­nected by a pneu­matic cir­cuit with safety valves to pre­venta lo­cal tear caus­ing the whole build­ing to col­lapse. The walls and the ceil­ing are mem­branes stretched be­tween the frame.

A space can be cre­ated sim­ply and in just a few min­utes to serve as emer­gency shel­ters, civil pro­tec­tion ac­com­mo­da­tion, first-aid ar­eas, op­er­at­ing the­atres or sim­ply shel­ters. When they are no longer needed, they are de­flated and packed up ready to be de­ployed some­where else.

The di­verse re­quire­ments dic­tated by the use of these struc­tures do not nec­es­sar­ily call for the stan­dard “soap bub­ble” form that best ex­presses the char­ac­ter­is­tic mem­brane shape, as rep­re­sented by Rem Kool­haas in his Ser­pen­tine Gallery pavil­ion. The func­tional needs, pe­cu­liar­ity of the ma­te­ri­als, con­struc­tion ca­pac­ity and de­sign flair, ac­com­pa­nied by the tech­nol­ogy and struc­tural-anal­y­sis in­stru­ments that sim­u­late con­duct in all con­di­tions of use, al­low cre­ations that dis­tance them­selves from typ­i­cal mem­brane ge­ome­tries. The re­straint cre­ated by air pres­sure of­fers de­sign­ers am­ple free­dom, even for more de­mand­ing forms such as the par­al­lelepiped which have lit­tle in com­mon with the tra­di­tional mem­brane con­fig­u­ra­tion.

As a fur­ther note on the sub­ject of safety and, more im­por­tantly, in the event of nat­u­ral dis­as­ters such as earth­quakes, these struc­tures have ex­traor­di­nary prop­er­ties gen­er­ated by their re­mark­able light­ness. With their small mass, the force ap­plied by earth­quake ac­cel­er­a­tion is ex­tremely lim­ited and, given their great sup­ple­ness, even strong tremors will never trig­ger a struc­tural cri­sis or sit­u­a­tion of dan­ger as oc­curs in more cus­tom­ary rigid struc­tures. Any dan­ger of col­lapse can be ex­cluded.

Mau­r­izio Mi­lan, struc­tural en­gi­neer, is the founder of Mi­lan Ingeg­ne­ria. His prin­ci­pal ac­tiv­ity in­volves sup­port­ing ar­chi­tects, with whom he has com­pleted over 1000 projects, in­clud­ing com­plex com­mis­sions us­ing un­con­ven­tional ma­te­ri­als and al­ter­na­tive tech­nolo­gies.

Above: In para­chutes with a wing cross-sec­tion, the ad­join­ing cells fill with air and re­main rigid dur­ing de­scent

Above and next page: the biomes rep­re­sent the sec­ond phase of Ni­cholas Grimshaw’s Eden Project, form­ing a se­quence of eight in­ter­con­nected trans­par­ent ge­o­desic domes. The cladding pan­els con­sist of triple-lay­ered high-per­for­mance ETFE film pil­lows; left: in­flat­able struc­ture in the shape of a par­al­lelepiped

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