Membrane Structure - A Modern and Aesthetic Structural System

Seminar dan Pameran HAKI 2011 – “Konstruksi Indonesia Melangkah ke Masa Depan”

MEMBRANE STRUCTURE: A MODERN AND AESTHETIC STRUCTURAL SYSTEM

FX Supartono 1)

Li Zhongli 2)

Wang Xiujiang 3)

ABSTRACT This paper presents various examples on the application of membrane structures in Indonesia and abroad, as well as to present the membrane structural design concept. Due to its flexible behavior, the membrane structure can be calculated as a thin membrane element on which the external loads are carried by tensile stresses that are induced in the membrane surface only, called membrane stresses. These membrane stresses should form an internal and external equilibrium against the external forces within a specific boundary condition that may induce the reaction forces, which represent in fact the basic forces to design the supporting structure such as pre-tensioning cables or space truss structure. Besides, some damages and failures of membrane structure are presented and discussed in this paper as a good learning for the future membrane design, fabrication and erection. Due to its characters of being flexible, easy to form, and light weight, the membrane structure is often used for the aesthetic roofing or siding of various buildings such as stadium, theatre, swimming pool, exhibition hall, lobby’s roofing or side walk’s awning of a hotel or mall, or a rest area’s awning in a garden or beach, as well as for roofing at an entertainment center, etc. KEY WORDS: membrane, aesthetic, light weight structure, pre-tensioning.

ABSTRAK Makalah ini menyajikan berbagai contoh penggunaan struktur membran di Indonesia dan luar negeri, serta menyampaikan konsep perancangan struktur membran. Karena perilakunya yang fleksibel, maka struktur membran dapat dihitung sebagai elemen membran tipis yang tidak menahan momen dan gaya geser, melainkan hanya menerima gaya normal tarik yang bekerja mengikuti permukaan membran yang melengkung, dan yang akan menimbulkan tegangan tarik pada bidang membran yang disebut membrane stress. Tegangan membran ini, yang akan menjadi dasar pra-penegangan struktur membran, harus dapat menimbulkan keseimbangan internal maupun eksternal sesuai dengan beban luar dan kondisi batas struktur membran, yang kemudian menimbulkan gaya reaksi yang dapat digunakan untuk merancang gaya prategang kabel luar atau struktur baja ruang yang mendukung membran tersebut. Di samping keberhasilan struktur membran, beberapa kegagalan struktur membran akan pula dibahas dan diberi komentar penyebab kegagalannya agar dapat menjadi pelajaran dalam perancangan, fabrikasi dan pemasangan struktur membrane di kemudian hari. Karena perilakunya yang fleksibel, mudah dibentuk dan ringan, maka struktur membran sering digunakan sebagai struktur atap atau dinding bangunan dengan nilai estetika yang tinggi, seperti untuk stadion, teater, kolam renang, exhibition hall, atap lobby atau side walk di hotel-hotel berbintang atau mal, tenda istirahat di taman atau pantai, atap entertainment center, dan lain sebagainya. KATA KUNCI: membran, estetika, struktur ringan, prategang. ______________________________________________________________________________ 1) Director of PT. Partono Fondas Engineering Consultant 2) Director of Beijing Z&T Fabric Architecture Technology Co. Ltd. 3) Director of Beijing Xindayu Water Conservancy & Construction Co. Ltd.


MEMBRANE STRUCTURE:

A MODERN AND AESTHETIC STRUCTURAL SYSTEM

FX Supartono Li Zhongli

Wang Xiujiang 1. INTRODUCTION The membrane structure, called also fabric structure or tension structure, is a modern structural system that was developed in the middle of 20th century, with a thin and flexible surface (membrane) that carries loads through tensile stresses only (no compression or bending). This structure uses various types of thin and high strength membrane materials such as PVC, PTFE or ETFE, which is usually supported by another tension or compression or bending structures such as high strength cables or steel columns or space truss structure. By applying a tension stressing in the membrane surface, it may present an aesthetic shape of a spatial structure. Due to its characters of being flexible, easy to form, and light weight, the membrane structure is most often used as roofs as they can economically and attractively be extended to large span, e.g. for the aesthetic roofing or siding of various buildings such as stadium, theatre, swimming pool, exhibition hall, lobby’s roofing or side walk’s awning of a hotel or mall, or a rest area’s awning in a garden or beach, as well as for roofing at an entertainment center, etc. The first successful development of membrane structure in large-scale was realized by Russian engineer Vladimir Shukhov in the Nizhny Novgorod Fair in 1896 [1]. He has developed a membrane shell structure covering an area of 27,000 m2 that were composed of membrane and steel trusses (Fig. 1). For this expo, Shukhov has designed and realized successfully eight membrane structures as thin shell structure exhibition pavilions.

Fig. 2. Membrane roofing at Olympic Stadium in Munich 1972 [Wikipedia]

Fig. 1. World’s first membrane shell structure in 1896


In the 20th century, the concept was more developed by European architects e.g. Italian architect Antonio Gaudi and German architect Frei Otto for various large span membrane-covered structures in Europe and Australia such as German Pavilion at World Expo 1967 and the Sydney Myer Music Bowl. However, the membrane roof of Olympic Stadium designed by Frei Otto in 1972 Summer Olympic Games in Munich (Fig. 2) has induced extensive use of membrane roofing structures in all over the world [1]. The later successful development of membrane structures are shown as examples in Fig. 3 to 8. Fig. 3 shows the famous Jiangsu Nantong Stadium with retractable membrane roofing system (being able to be retracted for opening and shutting). Fig. 4 shows the Grand Opera House in Shenzhen with tension style membrane roofing. Fig. 5 shows the Tenggarong Madya Stadium at Kutai Kartanegara with frame membrane roofing system. Fig. 6 shows the membrane covered pedestrian street at Shenzhen. Fig. 7 shows the beautiful membrane roof of Grand Stadium at Wuhu. Fig. 8 shows the membrane-covered Pasar Kapiten (Pasar Kuliner) at Palembang. Those examples show different types of membrane structures in various purposes.

Fig. 3. Jiangsu Nantong Stadium with retractable membrane roofing system [Li Zhongli]

Fig. 4. Shenzhen Opera House [Li Zhongli]

Fig. 5. Tenggarong Madya Stadium [FX Supartono]

Fig. 6. Pedestrian street in Shen-zhen [Li Zhongli]

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during operation. To attain this condition, an automatic air pressure regulator should be installed in order to ensure the maintenance of constant air pressure inside the membrane (Fig. 15 and Fig. 16). 3. VARIOUS TYPES OF MEMBRANE MATERIALS 3.1. PVC Type Membrane Material PVC type membrane is basically composed of high strength fiber such as polyamide, polyester or polyvinyl as a base fabric, and then apply main coating of poly-vinyl-chloride (PVC). For this type of membrane, it is mostly required to apply another surface treatment by using poly-vinyl-di-fluor (PVDF) or acrylic in order to improve its self-cleaning performance and increase its durability (Fig. 17). 3.2. PTFE Type Membrane Material PTFE type membrane is based on glass fiber cloth and surface lining of poly-tetra-fluoro-ethylene (PTFE). It is no need to give any surface treatment on this type of membrane material because PTFE itself is chemically very stable (Fig. 18). In general, PTFE has better strength and durability as well as better self-cleaning performance than PVC (PVDF) type membrane, but it is more expensive in price.

3.3. ETFE Type Membrane Material ETFE type membrane is composed of thin ethylene-tetra-fluoro-ethylene layer. Due to its fineness, ETFE membrane is much more transparent (tranparancy rate ≈ 90%) than the

Fig. 15. Air-supported membrane structure in Beijing [Li Zhongli]

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4.1. Form-finding Analysis Form-finding is a step of analysis to find an equilibrium shape of the membrane that may satisfy the pre-tensioning force distribution at the given boundary condition. A correct form-finding analysis will give a good base to enter on the next steps of loading analysis and cutting pattern analysis. It should be understood that the behavior of membrane structure which depends upon pre-tensioning to attain their strength is non-linear, so that the membrane shape is generally difficult to design. For that reason, the form-finding analysis is usually done in two steps. Firstly, we may assume the membrane mass is very light, and that the pre-tensioning force will not vary on the variation of membrane form. Based on assumption of constant pre-tensioning “force” and constant “cable length” with very small elastic modulus (to attain a very flexible assumption), we may obtain a membrane curve shape that satisfies the equilibrium condition. Next step, based on the first step result, we return to the real characteristic and behavior of the materials in doing the real form-finding analysis so that we may obtain a real membrane shape that satisfies the real equilibrium condition at the given boundary condition. 4.2. Loading Analysis Loading should be analyzed in accordance with the local building code. In general, in the tropical weather zone, we should consider at least the following loading condition: • Dead load / self weight • Prestress / pre-tensioning load • Live load • Wind load • Other loads such as rain water load, temperature load, etc. Loading combination should be done in accordance with the local building code. For the frame membrane structure in particular, seismic load should be considered in the analysis. 4.3. Cutting Pattern Analysis Cutting pattern analysis should be developed according to the correct form-finding analysis result. Objective of this analysis is to obtain how to use the plane and flat membrane material without pre-tensioning in controlling and managing the real curve shape (e.g. paraboloid shape, saddle shape, etc) under pre-tensioning. In this step, economic aspect should be considered in how to obtain the most effective and efficient cutting pattern so that a minimal material waste can be achieved. 4.4. Membrane structure design concept Due to its flexible behavior, the membrane structure can be calculated as a thin membrane element on which the external loads are carried by tensile stresses that are induced in the membrane surface only, called membrane stresses. These membrane stresses should form an internal and external equilibrium against the external forces within a specific boundary condition that may induce the reaction forces, which represent in fact the basic forces to design the supporting structure such as pre-tensioning cables or space truss structure.

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Now the non-linear finite element analysis has been largely used by the designers to analyze the membrane structure, especially for the complicated membrane shape analysis (Fig. 21). Makoto [20] and his Japanese colleagues have also developed a complete CAD & CAM program for analysis and fabrication of membrane structure. 4.5. Membrane structure design procedure 4.5.1. Safety factor First should choose type of membrane to be used, and then define its mechanical characteristics. In the design analysis, a safety factor of 3.0 to 4.0 will generally be required for temporary loading condition, and 6.0 to 8.0 for permanent loading condition to calculate the maximum warp and weft stresses of membrane elements. 4.5.2. Loading 4.5.2.1. Self weight / Dead load (DL)

Membrane self weight will be defined according to the membrane type.

4.5.2.2. Pre-tensioning load (PL)

The membrane has to be pre-tensioned in order to form the required shape and geometry. A uniform pre-tensioning load should be inputted in warp direction and weft direction. Reaction force will be obtained by calculation that can be used later as action loads to the supporting structures.

4.5.2.3. Live load (LL)

Live loads will be applied for example as installation working load or snow load.

4.5.2.4. Wind load (WL)

Wind load will be defined in accordance with the design wind speed and the site characteristics.

4.5.2.5. Other loads (OL)

Other loads will be applied for example for temperature load, differential boundary displacements, seismic load, etc.

4.5.3. Loading combinations

Loading combinations will be calculated generally according to the Working Stress Design Method (non-factored combination):

Load Case 1: DL + PL Load Case 2: DL + PL + LL Load Case 3: DL + PL + WL Load Case 4: DL + PL + LL + WL Load Case 5: DL + PL + LL + OL Load Case 6: DL + PL + WL + OL Load Case 7: DL + PL + LL + WL + OL


In fact, the loading combination can be more than the above mentioned, because wind load, temperature load and seismic load can be considered for (+) and (-) depending on the loading “direction”.

5. MEMBRANE PROJECT EXAMPLES

5.1. Anhui Wuhu Stadium – Wholly tension membrane structure

Wuhu stadium has 254 meters long, and 225m wide, with membrane surface of 20,700 m2. The longest cantilever span of the membrane roof is 57m, with the highest membrane’s ridge cable level at +44m, and the highest upper ring beam level at +69.1m (Fig. 7 and Fig. 22 & Fig. 23). This stadium is a wholly tension membrane structure on which the stay cables’ tensioning is realized together with the membrane’s tensioning.

5.2. Shandong Yantai Stadium – Element tension membrane structure

Yantai stadium has a symmetric crescent moon shape of membrane roofing that is stayed by cables on a steel arch of 285m span, with membrane surface of 14,000 m2. The longest cantilever span of the membrane roof is 36m, with the highest membrane roof level at +27.6m, and the highest steel arch level at +38.6m (Fig. 24 & Fig. 25). This stadium is a tension membrane structure with tensioning applied per membrane element.

Fig. 22. Wuhu stadium in Anhui

Fig. 23. Membrane structural scheme of Wuhu stadium

Fig. 24. Membrane structural scheme of Shandong Yantai stadium


5.3. Tenggarong Madya Stadium – Frame membrane structure

Tenggarong Madya stadium has a curve shape of membrane roofing that is supported by arch steel truss of 232.93m span, with the highest arch level at +37.98m. The membrane surface area is 6,300 m2, with the widest span of 32.42m (Fig. 5, 13 & 14, Fig. 26 & 27). This stadium has capacity for 30,000 persons, and being one of the most modern stadiums in East Kalimantan. 5.4. Beijing Environment Air-Supported Membrane Structure

This membrane structure has rectangular shape at its base, 120m long and 68m wide, with the highest level at +24.2m. The total volume inside is 150,000m3 (Fig. 15, Fig. 28 & Fig. 29). The membrane has 3 exit doors, among them two doors are for emergency exits. Air pressure inside the membrane is approximately 3/1000 higher than the atmospheric pressure.

Fig. 25. Yantai stadium in Shandong

Fig. 27. Membrane structure scheme of Tenggarong Madya stadium

Fig. 26. Tenggarong Madya stadium in Kutai Kartanegara


5.5. Aero Spatial Experiment Base Camp – Air-inflated membrane structure

This membrane structure consists of two parts combination of air-supported membrane and air-inflated membrane, having dimension of 92m long, 60m wide, and cylinder with 40m diameter (Fig. 30 & 31). The dome is installed with 12 air-inflated stiffeners (3m diameter each) in meridian direction and 3 air-inflated stiffeners (2.5m diameter each) in transverse direction; while the cylinder is installed with 5 air-inflated stiffeners (2.5m diameter each) in perimeter and 3 air-inflated stiffeners (2m diameter each) in longitudinal direction (Fig. 31).

Fig. 28. Beijing Environment Air-supported Membrane Structure

Fig. 29. Air-supported membrane pattern

Fig. 30. Air-inflated membrane structure


6. LEARNING FROM MEMBRANE FAILURE CASES Following the large utilization of membrane for the building structural system, some accidents have occurred in the past on part or whole membrane structures, particularly due to improper installation process or by mistakes in the membrane structure design as well as the membrane cutting pattern design. Due to its flexible behavior, it is easy to induce instability in the membrane surface or part of the surface due to strong wind or rain water during installation before pre-tensioning. For example when it is raining, if the membrane doesn’t have a good sloping pattern for water flow, it is possible to induce a water basin inside the membrane area that may exceed the designed service load or allowable stress. The membrane may then be cracked or even failure. Membrane installation is also important step that should be done with a lot of care. For example sometime the membrane surface had been poked by a metallic equipment during installation and become slightly damaged. In this case, special treatment should be done immediately. Missing to do that, the membrane may decrease its stress bearing capacity and possible become failure when it is tensioning. Mistakes in membrane structure design or cutting pattern design may induce high stress concentration in part of the membrane surface that may also cause cracks or failure of the membrane. Sometime it is difficult to design the whole membrane surface in a homogeny stress in service state. When this condition appears in the design analysis result, the membrane area that represents higher stress concentration should be verified with care. If necessary, a local strengthening treatment should be given in that particular area. Fig. 32 here-in below shows some damage or failure cases of membrane structures.

Fig. 31. Air-inflated stiffeners inside membrane structure


7. CLOSING REMARK In the past civil engineering history, the civil engineers have tried to find and develop a structure that may decrease or eliminate the action of bending moments and shear forces through an arch or dome structure for obtaining a state of near compression only. The membrane, due to its character of being flexible, will have that advantage to carry loads through tensile stresses only (no compression or bending moment or shear). This will facilitate the easy application of membrane to be used for large span roofing structure. In addition, due to its light weight, we believe that the membrane might become one of the favorite construction materials in the future.

Fig. 32. Some damage or failure cases of membrane structures


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