Environmentally Compatible Spatial Structures

Marta Gil Pérez 1, Ju Dong Lee 1, Sanghee Kim 1 and Thomas H.-K.

Kang 1

1 Dept. of Architecture & Architectural Engineering, Seoul NationalUniversity, Korea

Abstract. Spatial structures are those that can cover a long-span spacewith a lightweight structure. This paper deals with various spatialstructures and their design and principles. Actual application and aspectof environmental compatibility are also introduced. The improvement ofthe optimization of the energy usage and the decrease of the materialneeded for the building are being developed along with the optimizationof the structure design itself and the form-finding computationalmethods. In this way, it is possible to find a wide range of examples ofinnovative buildings constructed using some spatial structure such astension cable, membrane or hybrid structures that have alreadyimplemented aspects of environmental compatibility for better energyefficiency.Keywords: Spatial structures, environmental compatibility, case study

1.IntroductionSpatial structures are those that can cover a long-span space with a

lightweight structure. It is for this reason that they are used for awide variety of public buildings such as concert halls, stadiums,exhibition pavilions and so on. Different construction systems andmaterials can be used and computational simulation methods have been usedfor the design of these lightweight structures in the last years. Thetypes of spatial structures discussed in this paper include tension cablestructures, membrane structures, hybrid structures with tensioning chord,suspend-dome or lattice shell, and curved beam or surface structures. Thefollowing sections present the discussions and several case studies.

2.Various Spatial Structures2.1. Tension structures

The power of tensile structures is that while under compression a thinstructural member will escape the direct force line by buckling sideways,under tension there is only one way, the straightway. High strengthmaterials can be used and make members thin. Their stability is generated

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ISBN xxx2014 International Conference on Geological and Civil Engineering

(ICGCE 2014)Macau, 24-25 January, 2014, pp. xxx-xxx

by having two intersecting force surfaces stressed against each other(Fig. 1).

Fig. 1: Raleigh Arena cable diagram.[1] Fig. 2: Frei Otto tensedstructures: Munich Olympic stadium.

Fig. 3: Computational experiments that investigate the behaviour and form ofprestressed cable nets.[2]

Form-finding methods have always formed a central part of the designof lightweight structures. In earlier years these were carried out asanalogue experiments employing physical models of membranes and elasticmaterials, whereas nowadays computational simulations have taken over,allowing for quick calculations and manipulation of the parametersinvolved in form generation. Frei Otto’s famous form-finding experimentshave been used for several years as one of the major architecturalreferences for understanding the intrinsic relation between the form andthe distribution of forces and material (Fig. 2).

In a digital form-finding approach the geometry is revised in aniterative process, until the equilibrium is reached. A spring-particlesimulation, being one of the simplest possible approaches to digitalform-finding, follows an iterative logic by computing forces and pointcoordinates. The coordinates of each point and their connectivity as well

as the properties of particles and springs are needed for initial input.The simulation is constrained by the points that are fixed in space. Ineach step all forces applied to all particles due to gravity, springforces, etc. are accumulated. The acceleration and velocity arecalculated to get the next position of each particle by solving Newton’ssecond law (i.e., F = ma).[2] A dynamic relaxation method is also usedfor form-finding, which can lead to a “real time” discovery of structuralform encouraging the morphogenesis of optimized structures rather than apost-designing optimization. The behavior and form of prestressed cablenets obtained through dynamic relaxation simulations are emergent, asthey depend on the local interaction between particles. The exact shapeof the overall form and distribution of particles cannot be predictedbefore simulation; instead, it is obtained as a result of the simulation.There are numerous parameters that influence the forms obtained fromspring-particle simulations, some in greater extent than others (Fig. 3).The boundary conditions (anchor points and suspension points) are themain parameters that influence the global form, while the type of gridand its orientation mainly influence the pattern in local scale. Thedensity of the grid affects the surface curvature.[2]

2.2. Hybrid structuresIn many cases, spatial structures do not follow just one specific

structural system but they can be composed by several of them, and theyare called hybrid structures. The variety of possible structures createdby the combination of structural system is very wide and it is still afield in full development. Some examples of those structures could be thefollowing ones.

There is at least dozen of air-supported membrane structures used forgymnasium, tennis court or warehouses. The building is usually in barrelshape with semi-spheres at two ends. The transverse span is in the rangeof 30 to 70 m with a large height. By adjusting the shape of themembrane, pressing of the stiffening cables and increasing the inner airpressure, the membrane structure can resist the effect of rain, snow andwind effectively.[3]

Another field of wide application of cables is on hybrid structuresnamed “Structure with Tensioning Chord” or “String Structure”. The upperchord is usually formed by arch-shaped lattice truss with single-plane ortriangular section. By pre-tensioning the lower chord cables, the latticetruss with intermediate struts will act integrally with lower cables andthen resist much more loads. For structures with tensioning chord, theupper chord may also be arranged in two ways. The construction iscomplicated by the curved configuration of the roof and different typesof connecting nodes. In addition to the assembly of a large number ofmembers of the lattice grids, it is also necessary to pretension thechord cables. An innovative method of two-directional sliding techniquecan be employed.[3] For buildings with circular or elliptical plan, if alattice shell is served as the upper chord, then “Lattice Shell withTensioning Chord” or “Suspend-dome” is obtained. “Single-layer Kiewit”type lattice shell is used as the upper chord combined with three coursesof ring cables. Compared with ordinary lattice shell, the stability ofthe structure increases to a large extent. The forces in the members tend

to be about only 1/3 of the original shell and are more uniformlydistributed.

A surface net is defined as a hybrid system that combines the behaviorand characteristics of a surface component assembly with those of aprestressed cable net. Based on the established knowledge in cable nets,surface nets take the discourse one step forward by integrating surfacecomponents. Considering that the form of nets relates to the forcesacting upon them it is evident that the design of such systems cannot bedisconnected from the adopted structural strategy.[2] Additionally,deployable umbrellas are used that protect pedestrians from directsunlight during the day and radiate heat stored during the day up to thenight sky.

2.3. Permanent & convertible membrane structures with intricate bending-active supports

The term “bending-active” describes curved beam or surface structuresthat base their geometry on the elastic deformation of initially straightor planar elements.[4] Highly efficient structures can be realized withthe use of elastic bending. The use of elastically bent (bending-active)beam elements, as an intricate support system in membrane surfaces,offers a great potential for new shapes and highly efficient structuralsystems for mechanically prestressed membranes. Incorporating elasticbeams in a membrane surface enables free corners to be created which arestabilized solely by the beam which in turn is restrained by the membranesurface. Such elastic beams may also be used to generate new types ofmembrane geometries that are based on an elastic system of arches andclosed loop configurations. Owing to its elasticity, the beam partiallyadapts to the curvature of the surface, but can carry compressive forcesbecause it is restrained against buckling by the membrane. Suchintegration of elastic beams in a mechanically prestressed membrane hasbeen a challenge for numerical form-finding methods.

In most membrane structures the approximate shape is already reachedlong before the entire prestress is induced. Since the curvature of thebeam is largely dependent on the prestress of the membrane, it could beobserved that large wrinkles disappeared abruptly during the final stageof the prestressing. This may be another indication for the tightinterdependence between the bending and form-active part of thestructure. The active use of bending in the supporting structure ofprestressing membranes may lead to very slender and lightweightsolutions. It could be proven that an intelligent placement of theelastic bending members within the tension prestressed system maydrastically increase structural stability under external loads.

2.4. Case studiesFoshan Century Lily Stadium in Guangdong province: The maximum span of the

cantilevered canopy is 92.5 m. Between the inner and the outer rings isthe prestressed cable system composed of ridge cables, valley cables,middle radial cables and inclined cables. A skin made of PTFE (Teflon)fabric is tensioned over these cables (Fig. 4).

Fig. 4: Foshan Century Lily stadium.[3]

Fig. 5: Shenzen Bao'an stadium.[3]

Fig. 7: DoKEEP tower computer model.[5] Fig.8: Anchorage and edge catenaries.[1]

Shenzen Bao'an Stadium: The roof is formed by outer ring beam and two-layer inner cable rings with interconnecting cable trusses. Fabrics areestablished on steel arches spanning across the trusses (Fig. 5).

DoKEEP tower in Mecca (tent structure): The minimal shaped double curvedmembrane structure follows the footprint of the building and consists of8 pointed membrane surfaces with each two of them connected along a ridgecable.[5] These membrane structures are spanning between curved boundarycables and the central high points (Fig. 7). Two different structuralsystems are used. First, a steel frame structure is introduced supportingthe high points. This steel frame structure follows the conical curvedmembrane shape and is arranged non visible in the void between the outermembrane roof and the inner membrane ceiling. Second, a three freesanding mast is used to support the high points.

Jeppesen terminal in Denver airport (fabric structure): This regular building isenclosed in translucent fabric and glass. With 10% of daylightpenetrating the translucent fabric, little artificial light is needed.[1]

Also the fabric reflects 75% of sunlight and heat from the sun. Thisreduces the heat gain of the building as does the fact that in the nightthe translucent skin radiates out heat. The structural fabric spansbetween ridge and valley cables. The valley cables hold the structuredown and are anchored outside (Fig. 8).

3.Aspects of Environmental CompatibilityWhen spatial structures are used as multiuse facilities covering huge

spaces such as dome stadiums or concert halls, they can shelter manypeople at the same time and might use lots of energy temporarily. Forthis reason, it is encouraged to implement aspects of environmentalcompatibility for better energy efficiency. This following chart shows 3overall categories and the different strategies or methods for developinga more eco-friendly spatial structure.

Fig. 6: Three categories of eco-friendly spatial structures.

Fig. 7: Eden project.[6]

Fig. 8: Great glasshouse of national botanic garden of Wales.[7]

Fig. 9: Park dome.[8]

Fig. 10: Millennium dome.[9]

The following is a few examples that incorporate the eco-friendlyaspect of spatial structures. The Eden project is a series of biomesindoor gardens covered by domes that consist of hundreds of hexagonal andpentagonal, inflated, plastic cells supported by steel frames (Fig. 7).The foil material used in this spatial structure allows much more UVlight to pass into the domes and also provides good heat insulation.Besides, it minimized the weight and amount of steel needed for theconstruction reducing also the amount of CO2 emitted to the atmosphere. Asimilar example of energy efficiency is the great glasshouse of thenational botanic garden of Wales where the aluminium glazing system andits tubular-steel supporting structure are also designed to reducematerials and maximize light transmission (Fig. 8). In addition, tooptimize energy usage, indoor and outdoor conditions are monitored by acomputer-controlled system.

In the Park dome stadium, the dome is constructed with a doublepneumatic-film structure (Fig. 9). The design utilizes and conservesnatural energy by a swirl flow natural ventilation system, undergroundthermal tunnel and underground heat storage air-conditioning system usingreduced-rate night time electric power. In the case of the MillenniumDome, the membrane is being supported by a dome-shaped cable network fromtwelve king posts (Fig. 10). A water-cycle project was developed with theroof surface of the building, becoming one of the largest recyclingschemes in Europe. It is designed to supply up to 500 m3/day of reclaimedwater.

4.Summary and ConclusionIn this paper, a wide array of spatial structural systems are

presented, including tension cable structures, membrane structures,hybrid structures with tensioning chord, suspend-dome or lattice shell,and curved beam or surface structures. The design process and case

studies of said systems are briefly discussed. Additionally, the aspectof environmental compatibility is emphasized in this paper.

5.References[1] Berger, H. (2005). Light Structures-Structures of Light. 2nd ed. AuthorHouse.[2] Symeonidou, I. (2012). “From Cable Nets to Surface Nets: A case study.” Proc.,

IASS Symp., From Spatial Structures to Space Structures, IASS, Seoul, Korea, May 2012 (No.FF-151).

[3] Lan, T. (2012) “Advances of Spatial Structures in China – Recent Developments and Applications.” Proc., IASS Symp., From Spatial Structures to Space Structures, IASS, Seoul, Korea, May 2012 (No. FF-485).

[4] Lienhard J. and Knippers, J. (2012). “Permanent and convertible membrane structures with intricate bending-active support systems.” Proc., IASS Symp., FromSpatial Structures to Space Structures, IASS, Seoul, Korea, May 2012 (No. FF-028).

[5] Schoene, S., and Michalski, A. (2012). “Structural Engineering of Membrane Structures on High-Rice Buildings.” Proc., IASS Symp., From Spatial Structures to Space Structures, IASS, Seoul, Korea, May 2012 (No. FF-190).

[6] http://ecobrooklyn.com/wp-content/uploads/2012/07/Eden-Project-London.jpg[7] http://www.gardenvisit.com/assets/madge/wales_botanic_glasshouse/600x/

wales_botanic_glasshouse_600x.jpg[8] http://www.taiyokogyo.co.jp/wc_stadium/stadium_e/eng/camp/kumamoto/img/

photo02.jpg [9] http://wwp.millennium-dome.com/images/millennium-dome/millennium-dome-at-

dusk.jpg