Space and Form have always been the old dilemma in architecture, but nowadays with the advances in technology and computer softwares, materiality has become an essential catalyst that offers solutions for contemporary architecture. One of the most intimate materials to the human body is fabric; it has always been the primary shelter for humans starting from clothes to tensile structures. Together with light steel structures and cable supports, membranes play an essential role in creating the skin for structures. While most of the buildings are compression structures, tensile structure provides another way to construct buildings using light materials and membranes depending on forces of tension rather than gravity and weight. From nomadic tensile structures to high performance polymers, Form plays an important role in how fabrics function. This paper will investigate the different forms of tensile structures and how form affects their performance creating new trajectories for fabric applications. Through the work of Horst Berger and PTW architects in their projects the Hajj terminal in Jeddah and the Water Cube in Beijing, I will try to identify how form and choice of material affects the performance of both buildings using two different applications of fabric in two different climates.
Due to the ability of tensile structures to cover large spans, it has been associated with the construction of gathering spaces and public facilities, where tensile structures can be used either as a temporary structure or a permanent structure. Temporary structures are commonly used for religious events, entertainment such as circus, and for social events such as receiving condolences or weddings. Other temporary structures used for housing army and temporary disaster housing. However, tensile structure are famous for its permanent structures that are used in airports, halls and to cover sports facilities, such as stadiums and aqua centers.
There is a wide scope for the applications of fabric. In some cases, it acts as one holistic system where it offers both the roof and the cladding systems in one structural element. In other cases, it becomes one element in a building, for example, when used as a shading device. Fabrics are cost efficient when compared to glass. Although it requires maintenance, it can cover larger spans than other compression structural systems and it is more economical.
One of the greatest benefits of tensile membranes is its ability to become transparent or translucent, as well as the possibility of using different coatings to it in order to improve its properties especially the abrasion resistance and water absorption. The woven fabric using the appropriate coating allows 10% of light to be transmitted through it. This provides a very comfortable level of illumination which reduces the need for artificial lighting and saves energy in a lot of projects. In addition, the high reflectivity of the tensile membranes enhances the lighting systems especially by night and creates an aesthetic effect. This natural diffused light is allowed, but with reduced heat load and with the least absorption of solar energy.
History of Early Tensile Structures
One might think that tensile structures is a new form of building technologies, but the fact is that fabric was used as a construction material thousands of years ago. In some areas, even before compression structures were known tension structures were a primary form of building structures. Where no building materials were available, tents were used to shield people from weather and wild animals. Due to its ease of construction and flexibility, tents were constructed where a nomadic form of life existed. Although the tent is the primitive form of shelter and may seem simple in design, its sophistication comes from the way it deals with the climate changes.
In cold climates such as Siberia and Iceland, the old tents were made from animal skins and large bones or tree branches if found. Since building materials at that time were biodegradable materials, it was hard to trace how specifically these tents were made. In other regions such as Russia and Mongolia, Yurts were built as a different form of shelters, were designed as a circular lattice wall frame with a felted wool covering made from horsehair and sheep's wool. The roof had a circular opening to exhaust smoke from the ceiling.
Among the best known are the American tipis which is formed as a conical shape with an oval ground and is constructed of a double layer liner and a cover to maintain thermal insulation. The original tipis were made from animal skin wrapped around the structure; later woven fabrics were used to construct recent tipis. With the tipi facing winds, the aerodynamic shape of the oval base and the cone form act together to reduce the force of airflow on the structure. In addition, it has door flaps to exhaust smoke that act as vents too.
Not only in cold climates were tents used, but also in hot climates. In the Middle East the black tents were weaved from black goat hair. The woven membrane shaded the tent and allowed air to cool the covered space. During the day, the black color absorbed infrared rays and prevented its penetration to the covered space which maintains temperature difference of about 30 degrees between inside and outside. While at cold nights the fabric released the stored heat to maintain a warm weather inside the tent. The woven fabric was also designed to shelter from rain, so in humid days the fabric swells and block out rain. The black fabric was structured using ropes and which were anchored on stakes. Not all early tensile structures were buildings, sailboats, air balloons, bridges and even early airplanes all used fabric in different ways and inspired architects later. [2]
Pioneers of Tensile Structures and Early Forms
In their search for form, Vladimere Shukhov is one of the early pioneers who studied the free form and tried to implement it in his early eight thin-shell structures pavilions for the All-Russia Exhibition in Nizhny Novgorod of 1896. In his early work he studied the hyperboloid structures and the lattice shell structures, in parallel with tensile structures and built his first double curvature lattice steel shell and first steel tensile structure. Shukhov not only was the first to develop light weight structures and structural free plans, but he also provided mathematical analysis for his work. His work remained as a master piece and inspired a lot of architects later. [3]
Despite all precedents, fabric architecture was not recognized until after the World War II. Frei Otto was inspired by Shukhov and was one of the first to leave simple geometric solutions to investigate free forms in order to respond to complex planning and structural requirements. Otto's success lies in his study of the self-forming processes of nature such as soap bubbles, crystals, plants, animal life, and branching systems. He discovered that a natural object creates forms that are efficient and use minimum of materials. In 1967 he created his famous German pavilion which inspired a lot of designers and inspired his Olympic stadium project in Munich later. Due to the lack of technological advances in the fabric industry at the time, Otto was not able to find strong enough fabric to withstand the tension required for such a structure. As an alternative, he decided to design a network of interconnected cables to form the structure and suspended a fabric membrane below the cable net. [4]
Although Otto published a lot of studies about pneumatic structures, Walter Bird was first to put the idea into practice. Bird used low curvatures and high strength cables but they faced mechanical problems of deflations. In 1948, Bird was assigned by the army to develop thin non-metallic shelters, avoiding interference with the radar signal. Bird and his team achieved the construction of a 15m diameter pneumatic dome, as a prototype for a series he called "Radomes". [5]
Form and Fabric
In order to understand how tensile membranes are structured it is important to identify the basic shapes of tensile structures. Tension structures are present in two primary shapes, the Anticlastic structures with two double curvatures in opposite directions and synclastic structures with two double curvatures in the same directions. The anticlastic structures are present in a variety of free forms such as hyperbolic paraboloid, the cone and the
arched vault form, while the inflatable structures are synclastic forms.
Those four forms present the basic forms to establish a fabric structure. Other forms are derivatives of these basic forms either multiples or combinations of more than one form. In order to understand the difference between each form it is important to define the basic forms first. The Cone are forms that are tent like structures and either have fixed edge or cable edges around the parameters with variations in height. The arched vault is constructed of parallel and crossed arches. On the other hand, the hyperbolic paraboloids are crossed arches of two opposing high points that sustain the loads and two opposing low points that resist wind. While inflatable structures depend on constant air pressure that gives it its curvature form. Each form differs structurally and in performance, but in all cases pretension structures are used to maintain the form of the membrane. In case of anticlastic surfaces the membrane remains stabilized as long as the loading or prestressing tension forces are in equilibrium. While synclastic surfaces are stabilized when the membrane tension forces are in equilibrium with the internal pressure tension forces. [6]
Types of Fabric
Not only form affects the performance of fabrics but also the type of material used. There are three basic external fabric types most commonly used in tensile fabric structures. The first one is the PVC (polyvinyl chloride) coated polyester whichis the least expensive and has a life span that reaches up to 20 years. The second well known material is the silicon coated glass, and finally the Teflon coated glass P.T.F.E. (poly tetra fluro ethylene) which has a higher tensile strength than PVC coated polyester, but is slightly more brittle. Other structures make use of ETFE films, either as single layer or in air cushioned form, which are inflated to provide good insulation properties and solar control when inflated under different pressure and also it has an aesthetic effect.
As technology advances, other composite polymers are developed creating more responsive and efficient materials. Other new developed materials are light fabrics with trapped aerogel layer in between. For example, Tensothermâ„¢ with Nanogel®, a registered material developed by Bridair incorporate, the material offers the same architectural aesthetics of PTFE fiberglass tensile fabric membrane, but with the added benefit of a light insulation layer that traps air to prevent heat loss and solar heat gain. This layer of aerogel, has an air content of 95 percent, making it the lightest solid material in the world. Auxetics are another developed materials, when stretched they become thicker in the direction perpendicular to the exerted force. One type of it is Zetixâ„¢, a regestired material developed by Auxetix Ltd, which is designed to be used for military defense applications to protect against explosions. With all the advances in fabrics, one can expect in the future a lighter, more sustainable, and more resilient tensile membranes that can enhance the performance of buildings, secure it and save energy.
I chose the Hajj Terminal project and the Water Cube Project because both represent two completely different applications of tensile structures. With the openness of the Hajj Terminal in a hot climate and the use of the traditional tent structure supported in tension, a cultural dialogue is created within the context. On the other hand, the spectacle appearance of the water cube with its enclosed inflated sustainable envelope in a colder climate struggling to find that cultural relevance. Both buildings considered environmental solutions and used recent materials to enhance the performance of the buildings, but with different approaches. Through the use of tensile structures, both buildings created an aesthetic notion of structural revealing.
Hajj Terminal
"The absence of weight, the mixture of material with widely different properties, the non-linear behavior of the structure, and the large surfaces being dragged through the air, all combined to dampen any dynamic build-up." [7]
The Hajj terminal building in Jeddah was designed of multiples of conical form tents to host 700,000 pilgrims on the Hajj season to their way to Mecca. In 1977, the New York office of Skidmore, Owings &Merrill was asked to design the new terminal; later Horst Berger was asked to join the team as he was able to mathematically study and to determine the shape of tensioned structures. The area of the terminal is about 5.4 million square feet with 210 conical tent structure distributed in 10 modules each has 21 tent units. While the structure is supported by 150 ft. pylon of tubular steel that is located between tents on a unit grid of 150 ft. in between. Each tent is suspended by four cables from the top of the pylons and has an edge cable to stabilize it and to transfer the loads to the pylons and to the foundation. [8]
With such a huge size of a project, tensile structure was the most practical solution to cover that area. Due to the high temperature of the region, the tents were designed to protect against the heat and the sun rays. Berger used PTFE coated glass fiber fabric that reflects 70 % of the sun's heat and radiates it out during the night. Because of the high temperature in Jeddah, any typical concrete or metal structure would heat up and would require mechanical ventilation that costs a fortune to cool that space. In this notion, the coated fiber glass kept the temperature underneath the structure less than that of the ambient air.
The efficiency of the system is clearly seen in simple technical issues that saved a lot of energy and money by using tensile structures in this climate. Due to the translucency of the fabric, electrical lighting is eliminated, in addition at night the fabric reflects the electrical lights which enhances the aesthetics of the structure. Mechanical fans are used to ventilate the space and circulate the air between the pylons, while exhaust fans were used to exhaust smoke from buses.
This project resonates with the Mina Tent City in Mecca, which is another
stage in the Hajj rituals journey. The Tent City was built to accommodate the pilgrims during the hajj. Unlike the Hajj Terminal, the city is a temporary relocatable structure that is constructed during hajj time and then demolished to leave an open event space. With its white squared tents the city becomes another traditional stop in the journey. The city was designed by SL, Rasch in 1998 with PTFE coated glass fabric to cover an Area of 5000 000m². With both projects located in the same area the designers gave presence to the urban area through their choice of material and form and proved that the original tent form is still a valid solution in hot climates.
The choice of fabric structure in this area not only provided a practical and efficient solution in this region but also creates an analogy that symbolizes the traditional building materials in that region and creates an expression of the cultural ideals of the inhabitants. The tent as the shelter and as means of transportation is rooted in the nomadic desert life of the ancestors. The tents as form of shelter take them through a journey in the past, where pilgrims start their hajj rituals with. The repeated square tents recall a memory of the old city, while the white fiber glass presents the purity of the desert and the holiness of the journey.
Water Cube
Water Cube project is designed by the Australian architects PTW in collaboration with ARUP. The project is located in Beijing and was built to host the 2008 Olympic games. The aquatic center has a capacity of 17,000 seats and covers an area of 70,000 m2.
The building consists of a block of ETFE bubble like shape. The ETFE (Ethyle tetra flouro ethylene) air filled cushions is a tough and abrasion resistance material used as an insulation material. ETFE became famous after the Eden project designed by Nicholas Grimshaw dsigned in the united kingdom.in this project, Grimshow wanted to create something new to amaze the future genetrations.The project consisted of 3 bioms that promotes relationship between people and plants in order to educate visitors about the environment.because of the intreaging form and the creative use of material the projec became a tourist attraction.
In addition, the ETFE's proved its strength, transparency and durability and has spread fast as a building material. [9]
With the same conceptual approach, ETFE was used to cover the Water Cube center to create that intreaging expression. Although the design is transparent and modern, the desingners claimed that it symbolizes the traditional Chinese square houses, which in my point of view is apart from the traditional chinnese house.The notion of revealing the structure and the openness of th envelope makes it part of the contemporary architecture in china. The use of the transparent material and the unrepeated pattern of the bubble symbolize the advances in technology, while the traditional chinese sqare house remains detached that context.
The building consists of a 3 layer shells where the cells are arranged in between, which makes each steel component of a different size. The infinite array of the translucent fabric is created by welding 22,000 steel sections of different sizes together with spherical nodes to produce more than 100,000m2 of transparent ETFE envelope to become the world's largest ETFE building. However each individual bubble is connected to a pump connection to maintain the pressure inside the cushions. The designed roof allows harvesting water and uses around 80% of it to pool backwash system. [10]
This form of inflated cushions allows controlling solar absorption by adjusting the pressure between ETFE layers, in this notion one can easily create a green house effect and trap solar energy for use later. In this project the trapped solar energy was used to reheat the pool and the interior. "We've considered the impact of solar rays hitting the building and capturing the energy created by the solar rays and reusing that within the building. The energy savings may be second to none in the world." [11]
The designers succeeded in developing a sustainable approach to the building and implemented it through the use of material abd the design of the envelope. The cell arrangement of the soap bubbles created a 3 layered envelope that ventilated the space and controlled the solar gain during day. The ETFE allowed the building to perform effeciently compared to glass, glass could have allowed more light and heat absorption. In dealing with water they designed the roofs to allow water harvesting and to recycle it back to the system.
Since the early ages. certain form of tensile structres were associated with different regions of certain climates.This proves that the performance of tensile structures depends on the form and choice of material.With the advances in technology, materials became an inspiring element in design that compliments form. It enhances creativity and provides efficient solutions that are energy and time consuming. On the other hand, each geometrical form creates a new opportunity for the material and opens a new trajectory for discovery. Both discussed buildings succeeded in addressing technical aspects and performance issues through the use of different forms.The Hajj terminal project respected the cultral context and created effecient energy solution with the minimal material and cost. while the Water Cube presented how the advances in materials can open new environmental trajectories through the use of tensile structure. But with new trajectiories comes limitations that defines the role of tensile structures in the future. The question is could light membranes withsatnd severe weather in cold climates ?. The failure of the Metrodome stadium in Minnesota in 2010 due to a severe snow storm raised a lot of questions about certain materials such as ETFE. in this notion, limitations and conceptual approaches of tensile structures needs to be revisisted in order to be able to define where technology is going and if it could be applied successully in variuos climates.
