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Ferienakademie 2006 Kurs 4: Numerische Optimierung und Formfindung – Realisierung an einem Membrantragwerk

Convertible Roofs

Vortrag von Matthias Walter


Ferienakademie 2006 - Convertible roofs

List of contents

1. Historical convertible constructions…………………………………………….. 3 2. Convertible roofs of today……………………………………………………….. 4 2.1 Classification of convertible roofs…………………………………………… 4 2.2 Essential points to be considered in the design of convertible roofs…………. 5 2.3 Examples for rigid retractable roofs………………………………………….. 8 - Fukuoka Dome………………………………………………………….. 8 - Ocean Dome …………………………………………………………….. 9 - Gerry Weber Centercourt……………………………………………….. 9 2.4 Examples for convertible membrane roofs…………………………………… 10 - Arena in Zaragozza……………………………………………………… 10 - Town hall in Vienna …………………………………………………….. 11 - Architectural Umbrellas………………………………………………….12

3. Prospects in the design of retractable roofs……………………………………. 14 4. Literature…………………………………………………………………………….. 15


Ferienakademie 2006 - Convertible roofs “Convertibility refers to the variability of the building itself. We must distinguish between external and internal convertibility: 1. External convertibility means: variability of the outer shell of the building. This includes roofs which can be extended or retracted, enlarged or made smaller, raised or lowered as well as variable outer walls. 2. Inner convertibility means: variability of the building interior, say, by moving or transporting walls, raising or lowering levels; open-plan rooms with freely-variable functional possibilities can be convertible.” Frei Otto

1. Historical convertible constructions Convertible roof constructions have been built for centuries. There are various forms of historical retractable roofs – from the Roman vela to umbrellas and the Tipi Tents of the Sioux Indians.

Fig. 1-1 Reconstruction of the Roman Theater Velum. The basic type of the vela consists of wooden poles and vertical rods that are connected flexibly by ropes. The sails could be moved parallel between the poles. By pulling the ropes, the angle of the roof could be adjusted according to the sun.

Fig. 1-2: The oldest known picture of an umbrella is from the 13th century B.C. It shows the Assyrian ruler Assurbanipal with a sun umbrella; Umbrellas have been signs for power and wealth for a long time. The construction principle has not changed over centuries: A tension loaded membrane, a central rod and compression loaded ribs form one constructional unit.


Ferienakademie 2006 - Convertible roofs

The fabric lies on a cone shaped structure made from wooden rods. At the top of the tent, the smoke vent can be adjusted according to the wind direction. Moreover, the membrane can be bunched to an individual height. Fig. 1-3 shows the Tipi Tent from the Sioux Indians.

2. Convertible roofs of today 2.1 Classification of convertible roofs In 1964 the German architect Frei Otto founded the “Institut für Leichte Flächentragwerke” that worked systematically on retractable roofs. Frei Otto an his team developed the following classification for convertible roofs. Thus it can be distinguished between rigid constructions and (soft) membrane constructions. Together with the direction of movement, the possible forms of convertible roofs can be summarized in a movement matrix.

Fig. 2-1 movement matrix -4-

Ferienakademie 2006 - Convertible roofs

2.2 Essential points to be considered in the design of convertible roofs Buildings with a moveable roof that can be rectracted partly or completely, are very different from permantly covered buildings in several respects. First of all, two different states have to be considered in planning the interior of the building. Thus it can be necessary to design the interior wheatherproof. Moreover the mobility of some parts of the building requires the use of mechanical components from the field of mechanical engineering. The cooperation between architects, civil engineers and mechanical engineers is one of the big challenges during the planning of a rectractable roof. The following points have proofed essential, especially for the construction of moveable membrane-roofs. ƒ Geometry of the transport tracks Rigid constructions often use the technology of moveable cranes with regard to transport tracks, control and drive. Thus most rigid retractable roofs run on rails. Membrane roofs run on cables in most cases, mainly because of the light weight and the flexibility of cables. In rigid structures, the transport tracks should run parallel in order to avoid tension and distortion in the construction. But in soft constructions, it can be very useful when the distance between two cables or tracks decreases towards the point of retraction. Like this, the membrane gets less tensioned during retraction and can be moved more easily.

Fig.2-2 Geometry of the transport tracks

ƒ Securing of the membrane in the extended position - Tensioning The membrane has to be secured against wind in the closed state. Normally this is achieved by pretensioning the membrane when it is fully extended. Besides, large areas have to be curved anticlastically to provide sufficient stability. The tensioning can be achieved by the drive systems itself or by special tensioning devices at the edge of the roof. Another possibility is to raise masts and lower points. Other methods to secure the membran against flapping in the wind, are placing weigths or rigid rods on the membrane. ƒ Securing of the membrane during transport During the movement of the roof, the membrane is not tensioned and thus very prone to wind. In order to prevent damage of the membrane, it should not be moved during strong wind or it can be secured by attaching weights or rigid elements. ƒ Securing and stowing of the membrane in the retracted position In order to extend the working life of the membrane, it is recommended to protect it with a little roof or housing when it is retracted. This also prevents the formation of water or snow sacks, that could damage the fabric. -5-

Ferienakademie 2006 - Convertible roofs

ƒ Folding of the membrane According to the selected form of the roof, it is possible that the membrane is sagging during the movement. It is import to know the space, the folded membrane requires, in all phases of the movement. Otherwise the roof surface could come in contact with fixed structures and tear.

Fig. 2-3: Folding of the membrane; the left image shows lines of equal height in the folded membrane.

ƒ Rain and snow Most often, flexible rain gutters have to be used in retractable structures, since rigid systems cannot adapt to the movement of the roof. The forming of rain and snow sacks has to be prevented by draining the lowest points in the membrane surface. ƒ Introduction of forces Membranes are unsuited for point-wise loads. Loads and forces can be introduced at the edge cables or at enlarged points.

Fig. 2-4: Suspension points have to be enlarged in membrane constructions. ƒ Membrane drive systems The driving mechanism has the task to move the roof in the required time and it is also the connection between the roof surface and the supporting structure. In most cases, convertible membrane roofs are moved with electric motors. The motors can be either installed stationary and move the roof by a system of tow cables, or they can move along the cables or tracks themselves on socalled tractors. Both systems have been realized and proofed reliable.


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Power by tractors The driving forces have to be transmitted from the tractor to the transport track by friction, when tractors are used to move the membrane. Several methods have been developed to increase the friction force between cable and tractor.

Fig. 2-5 shows a tractor that is in use at an open-air theatre in Bad Hersfeld. The friction between the tractor and the cable is increased by a pair of caterpillars.

The biggest disadvantage of powered tractors is the heavy weight of those constructions. Especially when the tractors are also used to tension the membran, a very high contact pressure is required, to transmit the occuring forces. Besides, it is difficult to synchronize the movement of several tractors and thus membranes can be damaged. o

Stationary drive systems Stationary drive systems are another way to move the roof surface. Each motor has a fixed place and moves the membrane via a system of cables. The membrane is attached to non-powered pulleys or slides, that are connected to the tow cables. Such constructions weigh less than powered tractors. The system of tow cables has to operate in both directions for extension and retraction. Stationary systems can be used for membrane roofs as well as for rigid constructions. Fig 2-6 Principle retraction mechanism of the rigid retractable roof over a Centercourt in Halle / Germany.

It is important, that the movements of more than one electric motor are synchronized to prevent tensions in the membrane. Therefore various sensors, like rotary sensors on wheels, velocity sensors or acceleration sensors, can be used.

Besides those points, facts like the shadow of the retracted roof ( especially in stadiums ), the desired time of opening and closing, the degree of openess etc. have to be considered, too. -7-

Ferienakademie 2006 - Convertible roofs

2.3 Examples for rigid retractable roofs ƒ Fukuoka Dome ( Japan ) The Fukuoka Dome in Japan is a baseball stadium for 40,000 spectators. The retractable roof is shaped as a sphere divided in three parts. One part is fixed, the other two parts can be retracted in a rotary movement and parked above the fixed part. The diameter of the roof is about 200m. Each of the three roof parts is an independent framework. It takes approximately 20 minutes to open and close the roof. Fig. 2-7 Fukuoka Dome in Japan

Fig. 2-9 Photo of the driving bogie

Fig. 2-8 Roadbed section and driving device

Fig. 2-10 Roof framing


Ferienakademie 2006 - Convertible roofs ƒ Ocean Dome ( Japan ) In 1993 the Ocean Dome in Miyazaki City, Japan was finished. Under the large rectractable roof, a huge wave pool, an artificial beach and various amusement facilities are located. The convertible part of the roof consists of four structurally independent panels with a maximum span of 110m. A steel frame that is covered with membranes, forms each arched panel. By moving the four parts of the roof in a parallel movement, the roof can be opened by about 65 % in approximately 10 minutes.

Fig. 2-11 Ocean Dome

Fig. 2-12 Detail picture of driving device

ƒ Gerry Weber Centercourt ( Halle, Germany ) For the Centercourt in Halle, a very lightweight structure was chosen to cover the lawn. Thus, the convertible part of the roof can be closed in only 1.5 min. It consists of two moveable panels ( each 700 m² ) that are moved by a stationary driving system with tow cables. A filigrane steel structure combined with pneumatically stabilized cushions forms the panels

Fig. 2-13 Gerry Weber Centercourt

Fig. 2-14 Open and closed state -9-

2.4 Examples for convertible membrane roofs ƒ Arena in Zaragozza, Spain ; Centercourt in Hamburg Rothenbaum Built towards the end of the 19th century, the Arena in Zaragozza has always been used as a bullfighting ring. In 1990 it was covered with a combination of a fixed and a convertible, central bunchable, membrane roof. The construction does not have any columns over a diameter of 88m. As structural principle, a double spoked wheel was chosen. The compression loaded outer rim and the tension loaded inner hubs (36 m in diameter) are connected by tension loaded, radial spokes (cables up to 36mm in diameter). The hubs are ring cables that are held apart by rods. From the hub to the central node, another set of radial cables is attached. The convertible membrane is suspended from slides, that can be moved on the inner cables by a stationary drive system. In the central node, the upper and lower spokes are connected by a electric spindle. By rotating the spindle, the spokes are moved towards each other and the membrane is tensioned. So the process of moving the membran, where small forces and large ways are required, and the process of pretensioning with large forces and small ways, are seperated in this construction. Thus, the 16 electric motors could be designed much smaller.

Fig. 2-16 View on the membrane from above

Fig. 2-17 View on the Arena

Fig. 2-18 Central Node with spindle

During opening and closing, the syncronization is controlled by a number of contactless measuring sensors.

Fig. 2-15 Unfolding of the membrane

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Even bigger than the Zaragozza roof, is a bunchable membrane-roof over the centercourt in Hamburg Rothenbaum. The principle is quite similar to the roof realized in Spain, but the diameter of the retractable part in Hamburg is 63m. The fabric with a surface of approximately 3000m² can be withdrawn in less than 5 minutes. Different to the spindle system in the bullring, the membrane in Hamburg is tensioned like a drum by 18 sets of hydraulic rams at its edge.

Fig. 2-19 The membrane in Hamburg Rothenbaum is tensioned by hydraulic ra ƒ Town hall in Vienna, Austria Vienna´s town hall is a historical building with an inner courtyard, that was covered with a foldable membrane roof in the year 2000. On two sides of the courtyard, longitudinal girders are fixed to the building and serve as tracks for four cross-girders and several cables. Both, the cross-girders and the cables that support the membrane are assembled on axial and radial roller bearings and run on a T-shaped profile, welded to the longitudinal tracks. The main function of the cross-girders is to pick up the horizontal forces from the cables, resulting from the weight of the membrane. Thus the building has to bear only compression loads but no tension loads. The roof is moved by electric motors that are installed on three of the four cross-girders. The torque of the motors is transmitted to toothed racks, welded to the edge beams. The cables that carry the membrane are pushed or pulled ( according to the direction of movement ) by the cross-beams. The membrane is spanned like a concertina between cables and linear ballast in the “valleys”. The opening and closing takes approximately 4 minutes. Fig. 2-20 Foldable roof in the town hall of Vienna

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Ferienakademie 2006 - Convertible roofs

Fig. 2-21 basic structure of the foldable roof

Fig. 2-22 The moveable cables are connected to the edge beam with a combination of axial and radial roller bearings.

Fig. 2-23 Electric motors

ƒ Architectural umbrellas Since the 1950´s various umbrella constructions have been built as convertible roofs. The umbrella principle has been known for centuries: A tension loaded membran, together with roof ribs, bars and a central rod make up one constructional unit. In the 1950´s Frei Otto developed a new type of architectural umbrella, based on the minimum surface principle. The funnel shaped membrane is now attached under the compression-loaded bars. For the Cologne Garden Exhibition in 1971 Bodo Rasch realized the first convertible umbrellas with a diameter of 19 meters that was built according to this principle. For the American tour of Pink Floyd, Frei Otto designed ten convertible umbrellas to cover the stage. The largest convertible umbrellas that have been built so far, were installed in two courtyards of the Prophet´s holy Mosque in Madinah. They are open during the day and closed in the night and thus contribute to the regulation of the climate in the building. After a few days the shaded surfaces are significantly cooler than their surroundings.

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Ferienakademie 2006 - Convertible roofs Twelve large umbrellas with an area of 17m x 18m and a height of 14m in the open state were installed in the courtyards. A hydraulic cylinder, fixed centrally on the mast, moves the arms of the umbrella during opening and closing. The moving mechanism has been developed from a German mobile crane specialist. The protection of the membrane in the closed state is realized by very light carbon flaps that are connected to the arms of the umbrella. During the closing procedure, they support the folding of the membrane and when the construction is fully closed, they provide a solid cover for the fabric.

Fig. 2-25 Fully opened state

Fig. 2-24 Umbrellas in Madinah during opening process

Fig. 2-26 shows a cross-section through a diagonal arm. The carbon flaps are open in the upper picture and closed in the lower picture. That way they provide a solid cover for the membrane.

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Ferienakademie 2006 - Convertible roofs

3. Prospects in the design of retractable roofs For the future, several new projects with convertible roofs are planned – rigid constructions as well as membrane constructions. It is planned to built retractable membrane roofs in stadium size, what has been realized only once up to now in the Montreal olympic stadium, but has caused quite a lot of problems. A completely different attempt is pursued by some other scientists: The realization of large retractable structures, that consist of a foldable lattice of beams, connected by cylindrical joints, to which rigid covering panels or membranes are attached. Thus the supporting structure becomes convertible itself and can be folded towards its perimeter. The principle behind those structures is shown in Fig. 3-1. Two sets of parallel, straight rods are connected by cylindrical joints. Under certain circumstances, this structure can be converted to a plane, circular structure that can be folded towards its perimeter as shown in Fig. 3-2. In a further step, each inner ring can be raised with respect to the previous ring. Thus, a rectractable dome structure is created. All in all, the development of convertible roof is still at its beginning and there will be for sure further interesting constructions in the future.

Fig. 3-1 Principle structure

Fig. 3-3 Model of a retractable roof based on the principle of the foldable lattice

Fig. 3-2 circular, foldable structure - 14 -

Ferienakademie 2006 - Convertible roofs

4. Literature [1]

Kazuo Ishii, Structural Design of Retractable Roof Structures, Southampton, 2000


Institut for Leightweight Structures, Univ. Stuttgart, IL5 Convertible Roofs, 1971


Frei Otto, Bodo Rasch, Finding form, Deutscher Werkbund Bayern, 1995


Werner Sobek, art of engineering, Basel, Boston, 1999


Lehrstuhl für Tragwerksplanung, TU München, Zugbeanspruchte Konstruktionen – Seminarbericht Vertiefungsfach Tragwerkslehre 2, München 2003


Christoph Gengnagel, Rainer Barthel, Bewegliche Dächer, Detail 5/2001, S. 841-854


Schlaich Bergermann & Partner, Stuttgart, Bewegliche Überdachung für die Arena in Zaragozza – Stahlseil, Membran, Detail 6/1994, S. 813-818


Silja Tillner, Wien, Mobile Hofüberdachung in Wien, Deutsche Bauzeitschrift 4/2003, S. 48-53


Silja Tillner, Fahrbares Membrandach im Wiener Rathaus, Detail 6/2000, S. 983


Bodo Rasch, Architectural Umbrellas, Architectural design 9/1995, S. 22-25


Bodo Rasch, Aufgespannt – Bodo Rasch: Zur Entwicklung von Schirmkonstruktionen, Deutsche Bauzeitung 9/1993, S. 68-71


Kassabian, P.E., Retractable roof structures, Proceedings of the Institution of Civil Engineers, Band 1, Jahr 1999, S. 45-56

Pictures Front page:

[4], page 89

Fig. 1-1: Fig. 1-2: Fig. 1-3:

[6], page 841 [2], page 20 [2], page 14

Fig. 2-1: Fig. 2-2: Fig. 2-3: Fig. 2-4: Fig. 2-5: Fig. 2-6: Fig. 2-7: Fig. 2-8: Fig. 2-9: Fig. 2-10: Fig. 2-11: Fig. 2-12:

[2], page 45 [2], page 46 [1], page 38 [2], page 113 [1], page 41 [1], page 116 [1], page 126 [1], page 130 [1], page 131 [1], page 130 [1], page 139 [1], page 146 - 15 -

Fig. 2-13: Fig. 2-14: Fig. 2-15: Fig. 2-16: Fig. 2-17: Fig. 2-18: Fig. 2-19: Fig. 2-20: Fig. 2-21: Fig. 2-22: Fig. 2-23: Fig. 2-24: Fig. 2-25: Fig. 2-26:

[1], page 114 [1], page 115 [4], page 88 [4], page 41 [7], page 813 [7], page 816 [4], page 96 [8], page 49 [8], page 50 [8], page 52 [8], page 53 [3], pages 208 + 209 [3], page 193 [3], page 194

Fig. 3-1: Fig. 3-2: Fig. 3-3:

[12], page 51 [12], page 50 [12], page 49

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Convertible roofs