It is today generally agreed that cables should have two barriers against corrosion.
For spiral strands, wire ropes and locked coil strands the second barrier consists of filling the interstices between the wires with a blocking material and coating the outer surface. The primary purpose of the blocking material is to prevent ingress of moisture [42]. Suitable blocking materials are synthetic waxes and compounds based on petrolatum (petroleum jelly), which are hydrophobic and have good adherence.
The final coating can be ordinary paint or, if necessary, a more displacement resistant compound [125].
If the cable is exposed to an aggressive environment it is normally sheathed with a tube made of steel or polyethylene. The space between the tube and the cable is filled with a suitable compound such as polymer cement grout or petroleum wax [42].
Sheathing is the most effective method for corrosion protection and it is considered as impermeable. Materials used for sheathing must be ductile and if polyethylene is used it must be resistant to ultraviolet radiation. An alternative sheathing method is to extrude polyethylene directly onto the cables (no filling) [35].
2.5 Cladding
In analysis of a prestressed cable structure the cladding is usually assumed not to add any contribution to the structural stiffness. Some contribution will in any case be added to the performance of the building, which cannot be neglected. Especially the damping properties of the roof will be enhanced, which have significant importance for the dynamic behaviour of the structure.
There are two main categories of cladding: continuous membranes and unit cov-erings. Membranes can be made of fabric, foil or metal sheet. Unit coverings are panels of metal, wood or plastic [23]. The choice of cladding material depends on the type of structure (e.g. its shape), the expected lifetime, static and dynamic be-haviour, security and maintenance. What type of cladding to be used should be decided upon at an early stage in the design process in order to avoid large changes, which might effect the cable spacing and the design of structural details [16].
2.5.1 Fabrics and foils
Fabric is today the most common cladding material used for lightweight tension structures. As a structural element, the fabric must have the strength to span between supporting elements, carry wind and snow loads, and be safe to walk on.
To comply with these requirements, the fabric must be prestressed, since it has a negligible bending stiffness. The amount of prestress and the patterning of the membrane, i.e. how the membrane should be cut and assembled, is given by the structural analysis of the roof. Besides the structural requirements, the fabric must meet the requirements which affect the environment inside the building; these are air tightness, water protection, fire resistance, heat insulation, light transmission, acoustic properties, maintenance and durability [10].
CHAPTER 2. LITERATURE REVIEW
Fabric membranes are composite materials. Inside the membrane there are filament fibre yarn, designed to resist tensile forces, woven in different directions forming an anisotropic surface. For permanent buildings with expected long lifetimes only two types of fibres can be used: glass and aramid (Kevlar2) fibres, of which glass fibre is the most common. The mechanical properties of these fibres compared to the properties of a steel wire are shown in Table 2.1. To protect the fibres from environmental degradation, they are coated with some resin. Resin used are PTFE3 (Teflon2), silicone and PVC4 [99].
Table 2.1: Comparison of filament yarn characteristics [42, 130].
Property Glass Aramid Steel
(E-HTS glass) (Kevlar 49) (Cold drawn wire)
Density (g/cm3) 2.55 1.44 7.86
Young’s modulus (GPa) 69 124 205
Tensile strength (MPa) 2410 2760 1570
Max. elongation (%) 3.5 2.5 4.0
Temp. resistance (◦C) 350 250 500
Fibreglass coated with PTFE has found the broadest use for permanent buildings.
PTFE is a clear material which is chemically inert, so all dirt washes off without damaging the coating. It is also resistant to abrasion and highly reflective, absorbing little light as well as heat. The fact that Teflon comes in two forms, PTFE and FEP5, with different melting points makes it possible to heat weld seams, which enables a fast installation of the roof cladding. In addition to its high initial cost, PTFE-coated fibreglass has two disadvantages: the material is brittle and requires considerable care in the packing, shipping and installation of panels, and it has little elastic forgiveness and must therefore be accurately patterned [99].
Fibreglass coated with silicone is more flexible than PTFE-coated fibreglass, so it is less likely to be damaged during shipment and installation. With a silicone coating, the fabric can be made more translucent than with PTFE and the need for artificial lightning during daytime can be almost eliminated. Fabric joints are chemically bonded or glued. The self-cleaning properties of silicone rubber are not yet as good as those of PTFE; it is recommended to clean the membrane once a year [99].
Fabrics of Kevlar have high tensile strength, high stiffness and very low weight.
These properties make it possible to span large distances with Kevlar fabrics without a supporting cable net. One major disadvantage with fibres of Kevlar is that they are highly susceptible to ultraviolet radiation and cannot be coated with translucent resin. The fibres must be shielded with an opaque carbon black coat. Due to the sensitivity to ultraviolet radiation the joints of Kevlar fabrics cannot be heat welded with clear Teflon. The seams must instead be sewed, but it is impossible to develop
2Kevlar and Teflon are registered trademarks of E. I. du Pont de Nemours and Company
3Abbreviation for Polytetrafluoroethylene
4Abbreviation for Polyvinyl Chloride
5Abbreviation for Fluorinated Ethylene Propylene
22
2.5. CLADDING
the full strength of the fabric through the joints due to the high strength of the base material [40].
The newest membrane material is EFTE6 foil, which is not a woven fabric but a polymer film sheet. From a structural viewpoint, EFTE foil is interesting because of its high tear resistance. In addition to the structural properties, the foil has many properties that make it work well as enclosure material. For example, it can be considered as incombustible, impervious to ultraviolet radiation and most chemicals, and it can be manufactured with a translucency of over 90 % [99].
2.5.2 Metal sheets
Instead of a fabric membrane a metal membrane can be chosen. Sheets of aluminium or steel sheets with thicknesses of 1 to 5 mm are found to be suitable for this appli-cation. Due to the low bending stiffness of the sheets, it is necessary to prestress the membrane to prevent buckling. Prestressing is achieved by applying the membrane before the roof is fully erected. When the roof is raised to the final position the membrane is pretensioned. The metal membrane is composed of small accurately cut sections jointed by welding, gluing or bolting. Metal sheet membrane is a fea-sible choice for long-life structures and can be designed with openings covered with glass to provide natural lightning. Heat loss is prevented by attaching insulation material internally [23]. In [131], Yeremeyv and Kiselev describe the manufacturing and erection of a number of large projects in Russia where metal sheets are used as covering.
2.5.3 Panels
A cable net with cable spacing of around half a meter is ideal for small elements (panels). The elements are either shape-cutted or jointed in such a way so that they will conform to the shape of the structure. The panel system is most economical if it is made of light material, not to impose extra weight on the cable structure. Panels of fibreboard, aluminium and plastic are appropriate to use for covering roofs [23].
For the Olympic Stadium in Munich, a system with translucent plastic panels (Plex-iglas7) with thickness of 4 mm and size of 2.90 m × 2.90 m was used. The panels were fastened to the supporting cable net with shock absorbing flexible connections to prevent cracking of the panels under roof movements. The joints between the panels were sealed with continuous neoprene profiles, as seen in Figure 2.17 [63].
However, it should be mentioned that many architects, e.g. Philip Drew [31], find the Plexiglas cladding of the Olympic Stadium ugly. Therefore, it will probably not be used again.
6Abbreviation for Tetrafluoroethylene
7Plexiglas is a registered trademark of AtoHaas Americas Inc.
CHAPTER 2. LITERATURE REVIEW
Figure 2.17: Acrylic panels for the Olympic stadium in Munich. Reproduced from [57].