Already at early stages of the design process the designer has to pay attention to the design of the structural details. Structural details are fittings, saddles and anchorages. Fittings are attachments used to grip the cable at the ends or along its length. They can be classified, in accordance with the type of application, as the friction or clamp type, the pressed or swaged type, and the socketed type. Saddles are used when the cable has to run continuously over masts and other supports.
In self-supporting systems, cables are anchored into structural members, such as a concrete ring or an arch. In other systems the cable forces are resisted by anchors in the ground [57]. A comprehensive survey of structural details is given by Chaplin et al. [23].
2.6.1 End fittings
An end fitting (terminal) is an attachment, which transmits the cable force to the supporting system. To be totally effective, the end fitting must withstand the full breaking force of the cable without significant yielding, endure dynamic loading without risk of fatigue failure and not induce fatigue failure of the cable. For ap-plications where large forces are to be transmitted to the supporting structure two different end fittings are accepted [125]: the socketed type and the swaged type, Figure 2.18.
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2.6. STRUCTURAL DETAILS
(a) Socketed type (b) Swaged type
Figure 2.18: Cable end fittings with pin connectors. Reproduced from [16].
The most reliable, but also the most expensive, of the end fittings is the socketed type. It is manufactured by splaying the end of the cable a prescribed length and cleaning the individual wires. When the wires are cleaned and dried the conical socket of machined or casted steel is positioned on the splayed cable section. Then molten socketing material is poured into the socket, hardens and forms a cone, Fig-ure 2.18(a). As tension is applied to the cable the cone is drawn into the socket and wedging forces are developed which grip the wires. As socketing material either of zinc or resin is used. Pure zinc has been used for over a century and it offers a cathodic protection for the cable, but it is sometimes criticised for impairing the fatigue resistance of the cable in this region. Another, more important, disadvan-tage with sockets filled with pure zinc is that they are prone to creep effects under high stresses. Therefore zinc alloy, with improved creep resistance, is often used.
Polyester or epoxy resin has better creep resistance. As the resin is casted at low temperature the fatigue resistance of the cable will not be impaired. Socketed end fittings can be used for all cable sizes but cables of smaller diameter, approximately less than 38 mm, can be terminated by means of hydraulically compacted fittings called swaged end fittings. Swaged end fittings are cheaper than socketed types but they are only guaranteed to resist 95 % of minimum breaking load of the ca-ble. All end fittings are manufactured, installed and rigorously tested by the cable manufacturer [16, 125].
2.6.2 Intermediate fittings
Intermediate fittings are used to connect cables to other cables. These fittings are usually not standard appliances and their behaviour depend on the frictional force between the cable and the clamp. To prevent sliding of the clamp, the clamping force must be large and thereby high radial stresses are induced. Cables are more prone to fatigue when the pressure between adjacent wires is high and it is, therefore, important to use fittings where the clamping force is evenly distributed over the cable. The resistance of a spiral strand and a locked coil strand to clamping forces, where the latter has the higher resistance, can be found in Eurocode 3 [35]. When the cable is tensioned the diameter will decrease and consequently the clamping force. It can therefore be necessary to retension the clamp bolts to prevent sliding.
CHAPTER 2. LITERATURE REVIEW
To avoid abrasion between the clamp and cable under cable movements, which can result in fatigue failure, the ends of the fittings must be radiused. Different types of intermediate fittings are shown in Figures 2.19–2.20.
(a) Clamp connection (b) Swaged clamp connection
Figure 2.19: Cable connections for dual-strand cable nets. Reproduced from [16].
(a) Single U bolt connection (b) Double U bolt connection
Figure 2.20: Cable connections for two-way cable nets. Reproduced from [16].
In the search for the best economical solution one key is to use few types of structural details, as the number of fittings in, for example, a cable net can be quite large. A way to achieve this is to use a fitting which can be adjusted for different angles between cables. The fitting shown in Figure 2.19(b) can be mounted in a factory and thereby it is possible to reach a high accuracy. As mentioned above, accurate assembly of the fittings is necessary in order to obtain the desired internal force distribution in a cable net.
2.6.3 Saddles
When the cables have to run continuously over supports like columns and masts, they have to be supported by saddles, Figure 2.21. When designing a saddle one has to take the bending stiffness of the cable into account. Two factors have to be checked:
• the tensile stress in the outer wires, and
• the pressure between the cable and the saddle.
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