THE ROLE OF FABRICS IN TENSAIRITY - Eawag-Empa Library

THE ROLE OF FABRICS IN TENSAIRITY
Rolf H. LUCHSINGER and René CRETTOL
Empa, Center for Synergetic Structures, CH-8600 Duebendorf, Switzerland
E-mail: rolf.luchsinger@empa.ch rene.crettol@empa.ch
ABSTRACT
The new structural concept Tensairity ® is a synergetic combination of a pneumatic structure with a
cable-strut structure. The beam or shell structures have interesting properties. They are very light,
compact transport and storage is possible as well as fast and easy deployment. Furthermore, interesting
lighting possibilities and forms can be realized with Tensairity. Thus, Tensairity is ideally suited for a
variety of applications ranging from roof structures and foot bridges to temporary structures as
advertisement pillars. Recent applications include a roof over a parking garage in Switzerland and a
skier bridge in the French Alps. The basic models of Tensairity so far presented have treated the
membrane as an inextensible hull. While this approximation is justified to understand the basic
concepts of this new structure, the fabric properties do have an impact on the detailed behavior of
Tensairity. Thus for a deeper understanding of Tensairity, the role of fabrics in Tensairity has to be
investigated. Here we present first experimental studies highlighting the fabric nature of Tensairity. An
initial response in the load-deflection diagram similar to the behavior of fabrics under biaxial testing is
found.
1. INTRODUCTION
Pneumatic structures have attracted engineers and architects since several decades. The combination of
a membrane and air has some very attractive features. The set up and dismantling of pneumatic
structures is fast by simple inflation or deflation of the structure and the transport and storage volume
of inflatable structures are minimal. Pneumatic structures have found their application in civil
engineering as airhouses. Inflatable emergency tents make use of the deployable nature. Despite of
these remarkable properties one of the major drawbacks of pneumatic structures is the very limited load
bearing capacity of airbeams. Substantial loads can only be carried with very high pressures in the
airbeam. This leads to very high fabric tensions demanding for high strength and expensive fabrics.
With increasing pressure airtightness of the hull becomes a serious issue and safety questions have to
be addressed. Thus, high pressure airbeams can be used only in very special applications e.g. as
military tents.
The problem of the limited load bearing capacity of pneumatic structures is solved by the new
structural concept Tensairity ® [1]. The basic Tensairity girder consists of an upper and a lower chord
separated by an airbeam, where depending on the load situation one chord is purely under tension and
can thus be a cable while the other chord has to withstand compressive forces and therefore acts like a
strut. This combination of an airbeam and a cable-strut structure leads to a strong and very light beam
element with compact transport and fast set up options as well as a low air pressure. Ideal applications
of the patented technology are roof structures, footbridges and temporary structures as emergency
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idges and tents. In the roof for a parking garage in Montreux, Switzerland with 28 m span intensive
use of the interesting lighting options of Tensairity was made (Fig. 1).
Figure 1. Tensairity roof structure for a parking garage in Montreux, Switzerland with 28 m span, 2004
(Luscher Architectes SA).
Recently, a skier bridge in the French Alps with a span of 52 m was completed (Fig. 2). During the
winter season, a ski slope runs over the bridge and thus the bridge deck is covered with a thick layer of
snow leading to high loads. The bridge is an impressive demonstration of the potential of Tensairity for
wide span structures with heavy loads.
Figure 2. Tensairity skier bridge with 52 m span in the French Alps, 2005 (Charpente Concept SA,
Barbeyer Architect & Airlight Ltd).
2. SYNERGETIC STRUCTURES
Tensairity is a combination of an airbeam and a cable-strut structure. This fusion of two so far
separated technologies leads to a new structural element with improved properties. For example the
load bearing capacity under bending load of a Tensairity girder is by factors higher than the sum of the
load bearing capacity of the airbeam and the compression strut. The whole is greater than the sum of its
parts. Tensairity is a synergetic combination of an airbeam and a cable-strut structure.
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To strengthen the research of Tensairity structures is the main focus of the recently founded Center for
Synergetic Structures, a private public partnership between Empa and prospective concepts/Festo. For
this purpose, new test facilities will be set up at Empa to study the behavior of Tensairity structures.
These studies will help to deepen the understanding of Tensairity and to further optimize the
technology and its applications. A special focus will be set on the mechanical characterization of the
fabrics, as the fabric with its non-linear material behavior is a relevant part of Tensairity.
Important properties of Tensairity are shown in Figure 2. Each property is related to the underlying
airbeam or cable-strut structure, where it is inherited from. Little weight and small transport volume are
properties of both the pneumatic structure and the cable-strut structure. The capacity to carry heavy
loads goes back to the cable-strut structure, while fast set up, the temporary nature, thermal insulation,
lighting options, adaptiveness and so on do have their roots in the pneumatic structure. The low air
pressure is an emergent property of the combination of both structures and therefore shown with
dashed arrows.
pneumatic
structure
Tensairity ®
Tensairity ®
Tensairity ®
- light weight
- heavy loads
-low pressure (~100 mbar)
- small transport volume
- fast setup
-temporary
- thermal insulation
- transparency
- lighting
-floating
- adaptable
-safe
Figure 2. Properties of Tensairity, a synergetic combination of a pneumatic structure with a cable-strut
structure.
An important issue is the safety of Tensairity. The vulnerability of the airbeam is an undesired property
of Tensairity taken over by the pneumatic structure. An injury of the hull of a pneumatic structure leads
to a loss of air and the resulting drop of the air pressure endangers the integrity of the structure. The
situation is different in Tensairity due to an inherited property of the cable-strut structure. The struts or
chords in Tensairity have a resulting bending stiffness. They can be designed such that the dead load of
the structure is carried by the chords without the need of the stabilisation of the airbeam. The air
pressure is needed for sustaining the additional live loads [2]. In a lightweight Tensairity structure as
the roof over the parking garage in Montreux, the live load is roughly ten times higher than the dead
load. Stabilization by the over pressure is crucial for the live load, but the integrity of the structure is
not jeopardized with zero pressure solely under dead load. It also enables a pressure release in a
Tensairity structure when e.g. people have to enter the airbeam for service reasons. In structures like
the roof in Montreux high live load events are rare. The risk of a complete air loss in a Tensairity girder
with a high live load at the same time is very small. Furthermore, all larger Tensairity structures are
connected to an external fan, which starts blowing when the pressure drops below some threshold value.
This fan can easily compensate undesired air loss caused e.g. by a gun shot of vandals. And the
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cable-strut
structure

integrity of Tensairity structures can be easily checked by a simple pressure sensor or even by the
naked eye. The combination of an airbeam and a cable-strut structure enables new safety concepts for
Tensairity far outreaching the safety of conventional pneumatic structures.
3. BASIC TENSAIRITY
The basic principles of Tensairity have been described by Luchsinger et al. [3]. As a structural analogy,
a Tensairity girder can be compared with a truss girder. Both structures have an upper and a lower
chord. In Tensairity, however, the airbeam takes over the role of the vertical and diagonal struts of the
truss [4]. The struts in the truss have essentially three functions: load transfer from the upper to the
lower chord, reduction of the buckling length of the chord under compression and transfer of shear
forces by the diagonal struts. In Tensairity, the load transfer from the upper to the lower chord is
accomplished by the pretensioned fabric of the airbeam. Note, that a compressive force is transmitted
by a fabric. The buckling length of the chord under compression is reduced due to the stabilization by
the airbeam in Tensairity. The chord can be viewed as a beam on an elastic foundation. The buckling
length is then a function of the bending stiffness of the chord and the spring constant of the foundation,
which in the case of Tensairity is determined by the over pressure [5]. A proper choice of the bending
stiffness and the over pressure can lead to a buckling load which is higher than the yield load of the
chord, a property we call buckling free compression. Thus, load transfer from the upper to the lower
chord and reduction of the buckling length can be well taken over by the airbeam. The transfer of shear
forces in Tensairity is less obvious and subject of further studies.
In the basic Tensairity model the fabric is treated as an inextensible hull. The non-linear and
non-isotropic fabric behavior is not taken into consideration in these models. This approximation is
justified to understand the basic concepts of Tensairity. However, the fabric properties do certainly
influence the behavior of Tensairity structures. Understanding the fabric specific behavior of Tensairity
is important to gain deeper insight into the potential of this new structure. With increasing size of the
applications of Tensairity, where testing of a 1:1 prototype is not feasible and a reliable numerical
analysis is crucial, a detailed physical model including the fabric specific properties based on
experimental and theoretical studies is needed (Fig. 3).
Figure 3. The experimental set up for the study of the behavior of Tensairity beams.
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3. FABRIC EFFECTS IN TENSAIRITY
Tensairity beams have essentially the shape of a cylinder or a spindle [6]. The fabric is stressed both in
the hoop and longitudinal direction due to the internal over pressure. The ratio of the hoop stress and
the longitudinal stress in a cylinder is 2:1. For slender spindles, approximately the same ratio is found.
The magnitude of the stress is determined by the over pressure and the radius of the cylinder. Since the
fabric in Tensairity is in a biaxial stressed state, biaxial tests are important to investigate the fabric
properties for Tensairity. In Figure 4 left, the force-strain response in the warp direction is shown for a
biaxial test of a polyamide based fabric [7]. A force controlled load profile with a ratio of 2:1 in warp
and weft direction is applied for five load cycles. The total applied force varies between 1.25 kN and
7.5 kN in the warp direction and half of that value in the weft direction. The fabric shows an initial
response in the first cycle and gradually converges in further cycles.
The load-displacement response of a Tensairity beam under bending load is shown in Fig. 4 right. A
spindle shaped Tensairity girder with 5 m length and a central diameter of 0.5 m is used for this
experimental investigations in the laboratory (Fig. 3). The fabric of the Tensairity girder is identical to
the fabric of the biaxial test. Since this fabric is not airtight, a thin PU foil is embedded. An over
pressure of 150 mbar is applied for this test. The upper and lower chords of the Tensairity test beam are
made of aluminum. In the experiment, a central bending load is applied by means of a hydraulic piston
in three load cycles ranging from 0 kN to 0.9 kN. The fabric adapts to the new load condition during
the first load cycle. The load-displacement responses of the second and third load cycle are almost
identical. The Tensairity girder also shows a pronounced hysteresis indicating energy dissipation. This
hysteresis has to be attributed to the air beam, since the aluminum chords of the Tensairity girder have
an elastic behavior. It can result from the fabric or from energy dissipation into the compressed air. The
details behind the hysteresis are not yet understood and are subject of further studies. It is in any case
an interesting aspect regarding the dynamic properties and damping behavior of Tensairity girders.
force [kN]
8
6
4
2
Biaxial fabric test
0
0 1 2 3
strain [%]
4 5 6
load [kN]
Figure 4. Experimental data of a biaxial fabric test (left) and a Tensairity beam under central bending
load (right).
5
1.2
1
0.8
0.6
0.4
0.2
0
Tensairity beam p = 150 under mbar bending load
-0.2
0 10 20 30 40 50
displacement [mm]

4. CONCLUSIONS
Tensairity is a synergetic combination of an airbeam and a cable-strut structure or, in other words, of
fabrics, compressed air, cables and struts. The fabric plays an important role in Tensairity. It is
pretensioned due to the compressed air. Therefore it can transfer compressive forces from the upper
chord to the lower chord. The fabric stabilizes the chord under compression against buckling. And the
fabric has to take care of shear forces. The fabric is the key to the outstanding properties of Tensairity
such as light weight, fast and simple setup, compact transport and storage volume, interesting lighting
options, adaptiveness and so on. The fabric has a complex non-linear material behavior. Part of this
behavior is transmitted to the Tensairity structure, as can be seen by the initial response to a new load
condition of the fabric under biaxial testing and the Tensairity beam under bending load. Thus a deeper
understanding of Tensairity is strongly connected to a detailed analysis of the material properties of the
involved fabrics. Research along these lines is underway. As a matter of fact, the fabric is more than the
compartment of the compressed air. It is an important part of the interesting and rich behavior of
Tensairity structures.
REFERENCES
[1] Tensairity ® is a technology of the Swiss company Airlight Ltd developed in close collaboration with
prospective concepts ag. The research activities of prospective concepts have been recently transferred
to the Center for Synergetic Structures at Empa, the Swiss Federal Laboratories for Materials Testing
and Research. For further information about Tensairity see www.airlight.biz .
[2] R.H. Luchsinger, R. Crettol (2006), “Adaptable Tensairity”, F. Scheublin, A. Pronk, A. Borgard and
R. Houtman (eds.), Adaptables’06, Proceedings of the joint CIB, Tensinet and AISS international
conference on adaptability in design and construction, Eindhoven.
[3] R.H. Luchsinger, A. Pedretti, P. Steingruber, M. Pedretti (2004), “The new structural concept
Tensairity: Basic Principles”, A. Zingoni (ed.), Progress in Structural Engineering, Mechanics and
Computation, A.A. Balkema Publishers, London.
[4] R.H. Luchsinger, R. Crettol, P. Steingruber, A. Pedretti, M. Pedretti (2005), “Going strong: from
inflatable structures to Tensairity”, E. Onate and B. Kröplin (eds.), Textile composites and inflatable
structures II, CIMNE, Barcelona.
[5] R.H. Luchsinger, A. Pedretti, P. Steingruber, M. Pedretti (2004), “Light weight structures with
Tensairity”, R. Motro (ed.), Shell and Spatial Structures from Models to Realizations, Editions de
l’Espérou, Montpellier.
[6] A. Pedretti, P. Steingruber, M. Pedretti, R.H. Luchsinger (2004), “The new structural concept
Tensairity: FE-modeling and applications“, A. Zingoni (ed.), Progress in Engineering, Mechanics and
Computation, A.A. Balkema Publishers, London.
[7] Biaxial tests were made by the Laboratorium Blum, Stuttgart, Germany.
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