Though the tensile membrane evolved in twentieth century but it has historical linkage with lightweight tents. These tents are being used in various activities for a long time in human history ranging from the use of nomadic tents to war tents as well as in disaster relief. Tents are lightweight and portable, hence less interventions are required to install them. These qualities of traditional tents have been transformed into modern Tensile Membrane Architecture. Philip Drew discussed the historic relation of tents to contemporary tensile membrane architecture in his book ‘The New Tent Architecture’. Where he compared the traditional Arab Bedouin black tents with tensile membrane structures and discussed its similarities. But unlike tents, tensile membrane has some unique characteristics such as it requires tension in fabric to be in equilibrium for its stability and to resist wind uplift force, double curvature of the fabric, translucency, optimized patterning of the fabric, fire retardancy and self-cleaning properties. Due to the lightweight character of the membrane, the tensile membrane structure is also considered as ‘Lightweight Structure’.
The development of contemporary Tensile Membrane Structure occurred in twentieth century Europe. Many architects and engineers contributed to the development of lightweight architecture. Among them most notable names are Spanish-Mexican Architect Felix Candela for his contribution in thin shell concrete structures, Italian Engineer and Architect Pier Luigi Nervi contributed in the development of innovative lightweight concrete structures such as Air craft hanger, Orvieto, Italy 1935. The German Architect Frei Otto devoted his life for the development of the tensile membrane structures. After the world war II Frei Otto began experimenting with tents for quick shelter. Frei Otto founded the famous Institute for Lightweight Structures at the University of Stuttgart in 1964, where he experimented tensile membrane structures. He coined the term ‘Form Finding’ which is a special form searching process for a equilibrium form via several iterative steps. Some of the most influential works of Frei Otto are German Pavilion of Expo ’67 and Munich Olympic Stadium 1972. In the later part of the twentieth century tensile membrane architecture spread all over the world especially in Middle East due to its resemblance to Arab Bedouin tents. One of the largest tensile membrane structures was Jeddah International Hajj Terminal. Which was inaugurated in 1981. It was designed by Bengali-American Engineer Fazlur Rahman Khan of Skidmore, Owings and Merrill (SOM) and engineered by German-American Engineer Horst Berger. The fabric was fabricated by Birdair, USA and the steel was fabricated by NKK, Osaka, Japan. Horst Berger was one of the leading tensile membrane structure designer of twentieth century. He engineered many iconic tensile membrane among them most notable membrane structures are King Fahd Stadium in Saudi Arabia, Canada Place in Vancouver, Canada, Denver International Airport in USA, and Cynthia Woods Performing Arts Pavilion in Houston, USA.
The main design challenge of a Tensile Membrane is its form itself. The form of Tensile Membrane is achieved through defining its boundary conditions and support elements. The ultimate form is found through an iterative process named as ‘Form Finding’, the equilibrium of tensile forces act on the membrane and then distributed to the ground. The mathematical concept of Form Finding is based on minimal surface of soap bubble, now a days this form finding process is done through software simulation. After form is finalized the nonlinear analysis is done to check wind load and water ponding. Pattern making is done afterwards to make the three dimensional Membrane from two dimensional flat fabric.
There are several basic forms of Tensile Membrane Structure such as Hyper shape, Wave form, Conical form, Arch form and Cushion or Pneumatic Structure. Many forms and shapes can be achieved from the combination of these basic forms. The surface of the tensile membrane structure typically has a double curvature profile. There are two types of double curvature depending on their orientation, one is anticlastic curvature another is synclastic curvature. When the surface is curved in opposite ways in two direction, it is called anticlastic curvature and if the surface is curved toward the same side in all directions, it is called synclastic curvature. The example of anticlastic shape is a simple hyper shape which is consist of two high points and two low points. And the example of synclastic shape is a cushion form.
There are various types of Membranes which are used in Tensile Structure. Among them the most commonly used are PVDF(Polyvinylidenefluoride) coated PES (polyester), PVC(Polyvinylchloride) coated PES, PTFE (Polytetrafluoroethylene) coated glass fiber. PTFE coated glass fiber coated fabrics are the most expensive and difficult to handle but the longevity of this fabric is excellent. PTFE coated glass fabric can last more than 30 years. PVDF coated PES fabric is comparatively less costly, the longevity of the fabric is medium but the cleaning maintenance and handling of the fabric is easy. On the other hand PVC coated fabric is less expensive, the life span is relatively small and the dirt is easily deposited on the fabric. The overall thermal performance of the fabric is quite comfortable. Around 70% of the incident solar radiation is reflected, 10% absorbed and rest is transmitted. The acoustical performance is not good but special fabric with higher acoustical performance is available. PVDF coated fabric is fire retardant which means it will self-extinguish and prevent the fire spread out. The PTFE fabric is fire resistant. Translucency is an important character of Tensile Membrane. Due to translucency interior space will be lit and less energy will be required during the daytime for interior lighting. The overall light translucency of these fabrics are around 10 to 12 percent but the translucency of the PTFE coated PTFE fabric can be up to 50%. The translucency of the ETFE (Ethylene tetrafluoroethylene) is extraordinary around 96 %. Due to high translucency of the ETFE harmful UV rays may penetrate interior so some controlled shading may be required for acceptable level of solar radiation.
There are variety of support elements which are used in a Tensile Structure among them most commonly used are steel members and wire ropes. Steel elements such as I beam, H beam, tubes, trusses are widely used. Other natural elements such as Wood and bamboo with sufficient load capacity are also used. Edge of Tensile Membrane can be flexible or fixed. Flexible edges consist of wire ropes or belts. And the fixed edges consist of stiff elements like pipe or beams. Cables are used as tension elements in fabric edges or as ridge or valley cable in a wave form tensile architecture or as stay cables behind the inclining columns.
The indoor environment of a Tensile Membrane Structure depends on the shape and height of the structure. During the day membrane will be heated and air near it will be thermally stratified and convective as well as radiative heat exchange will occur within the interior space. So a well-ventilated space is desirable under the tensile membrane. Ventilation space can be designed within the membrane structure. Due to translucency of the fabric, an ambient light is achieved within the membrane during day time which eliminates the use of extra artificial lighting. The acoustical performance of the membrane material typically poor, but double layer membrane with sound absorber can improve the acoustical performance. The tensile membrane with its seam joints is completely water proof but connection with supporting steel structure may leak due to improper workmanship.
In our climatic context tent structures are used in various purposes for a long time. Historically from Moghul period ‘Shamiana’ tents were used from battle field to various outdoor festival and activities in the Indian subcontinent. In recent times the largest tent structure is built for ‘Bisho Ijtema’ every year with jute fabric on the bank of Turag river in Gazipur. Unfortunately these tents lack innovation and modern technology. The new tensile structures are introduced and constructed in our country in last few years. One of the first Tensile Structure was built in BUET by students of 00 batch Dept of Architecture under the supervision of Ar Dr Khandaker Shabbir Ahmed and Ar Mahmudul Anwar Riyad. It was an outdoor shade in front of the Architecture Department Building. The entry gate of British Council was another elegant tensile structure designed by Ar Dr Khandaker Shabbir Ahmed. One of the largest and most iconic tensile membrane in Bangladesh is the entry porch of Radisson Bay View Hotel ( designed by Vistara Architects Ltd) where eight translucent tensile cone shaded the loft porch nearly 60 feet high. VSM (Village Super Market) is another largest and beautiful tensile structure in Bangladesh which is located in Dumuria Khulna, it is designed by Ar Nazimuddin Payel and constructed by ALM Tensile Membrane Structure Ltd. The nine triangular cones with cascading height crisscrossed the central court of the bazaar providing an aesthetically pleasing lofty atmosphere and a well-ventilated space with diffused natural light. The other recent notable tensile structures in Bangladesh are Bangladesh Gallery of BGB in Benapole designed by Ar Alamgir, Roof top Tensile Structure of Amari Hotel in Gulshan 2, and entry shade of American International School Dhaka (AISD) etc. In conclusion it can be said that the advantages of tensile structures over regular structures like lightweightedness, translucency and aesthetic form cannot be denied. So a well thought tensile architecture can provide quality space enclosure and dramatic architectural form with minimum obstruction.
About the Author:
Architect Golam Morsalin Choudhury Rana has obtained his B.arch from BUET and Archineer from IMS, Germany. He is the Managing Director at ALM Tensile Membrane Structure Ltd.
CONTEXT contributor: Architect Tabassum Zarin, Asst Professor , Dept of Architecture, AIUB.