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8 CHAPTER TWO STAY CABLE SYSTEMS AND MATERIALS STAY CABLE SYSTEMS ropes have been popular in the United Kingdom. The spiral wires reduce the modulus of elasticity and strength of cable An overview of various stay cable systems is presented in compared with equivalent parallel wire cables, but are much this section to familiarize readers with the terminology and easier to handle (Ito 1999). The locked coil cable and spiral technical aspects of various cable designs, and the issues re- strands are examples of applications of suspension cable tech- lated to inspection and maintenance of stay cables in cable- nology to stay cables. stayed bridges. In very general terms, a stay cable can be described as a tension element composed of a single or The single or multiple bar system typically consists of multiple longitudinal MTEs, which is connected at one end one or more thread bars with a diameter of 26 to 36 mm to the bridge pylon and anchored at the other end at the (11.375 in.). The Dame Point Bridge in Florida and four bridge deck. pedestrian bridges in Calgary include bar cables. Worldwide it is believed that three other cable-stayed bridges with bars Over the years, there have been two fundamentally dif- have been built; one each in Malaysia, Germany, and Chile. ferent and competing philosophies regarding design of stay The parallel wire cables are typically made of 5 to 7 mm cables. In the first approach, which dominated early German, (0.190.27 in.) wires. Unlike the main suspension cables, British, and Japanese designs, the stay cables were designed the parallel wire stay cables do not include closely wrapped based on the well-developed suspension bridge technologies; external spiral wires to maintain the shape of cable. The that is, those of the main suspension cables and the hanger PascoKennewick Bridge in Washington State and the Hale cables, and wire rope technology from industrial applications. Boggs/Luling Bridge in Louisiana are examples of parallel In the second approach, which more or less began with the wire cables in the United States. Brotonne Bridge in France and dominated the U.S. stay cable designs until late 1990s, the cables were designed based on The parallel seven-wire strand system is the most com- the post-tensioning tendon technologies. There were also mon MTE used in the United States. The survey results indi- variations in each of the two main philosophies. The concepts cated that 75% of U.S. bridges included parallel seven-wire underlying these philosophies and their significant evolution strands (see Figure 6). In contrast, only one bridge in the over the last 30 to 40 years will be discussed later in this chap- Canadian survey had seven-wire strands. The only Canadian ter. The motivations for these system evolutions were based bridge with parallel strands is also the newest one (opened to on the field performance of the systems, technology develop- traffic in 2003), pointing to a possible move toward parallel ments and, above all, economic factors. strands. The majority of the Canadian bridges surveyed (54%) have steel wires. There are however four bridges with Main Tension-Resisting Elements parallel bars in Canada. The guaranteed ultimate tensile strength of seven-wire strands is 1860 MPa (270 ksi). There are several different arrangements of the MTE compo- nents in the free length of the cable. The free length refers to The wire and strand stays are continuous from anchorage to areas of the cable that are not in the vicinity of the anchorages. anchorage because they are produced in long lengths and trans- The MTE could be a single bar, multiple parallel bars, multi- ported on reels, but bar systems require splicing with couplers, wire helical strands (wire ropes or bridge strands), a bundle of because the maximum length of individual bars is on the order parallel wires, or a bundle of parallel seven-wire strands. Fig- of 18 m (60 ft). Figure 7 shows a bar cable with couplers. ure 5 shows some of the MTE systems. The factors that typically drive the decision on the choice of The locked coil cable was very common in early European MTEs have generally included the geographic preferences of and Japanese cable-stayed bridges. There is a central core of the designers, suppliers, and owners (based on adopted design parallel round wires surrounded by spirally wrapped layers of philosophies and available materials), perceived notions of interlocking z-shaped (and in some cases trapezoidal) wires. long-term durability (i.e., potential for corrosion and fatigue), This arrangement makes a denser more compact cable (with and cost. More recently however, issues related to inspectabil- reduced voids), with a smooth outer surface and less sensitiv- ity, feasibility for nondestructive evaluation (NDE), and possi- ity to side pressures (Walton 1996; Ito 1999). Helical wire bilities for MTE replacements and additions have also entered

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9 (a) (b) (c) (d) FIGURE 5 Various MTE cross sections: (a) locked coil, (b) helical strand, (c) parallel wire, (d ) parallel seven-wire strands (Gimsing 1998).

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10 80.0 Percent of Bridges 70.0 60.0 U.S. Canada 50.0 40.0 30.0 20.0 10.0 0.0 7-wire steel wire steel bar other no steel or answer strand threadbar MTE Type FIGURE 6 Types and frequency of main tension elements used. the decision process. The arguments that are generally made manufacturer. In bar systems, threaded nuts (matching the for, or against, one MTE system or another involve some of large threads on the bars) and anchor plates are used for the factors listed here: anchorage. In this section, a brief discussion of generic cate- gories of cable anchorages for parallel wires and strand cables Cost, is presented. For the sake of brevity, only anchorage systems Implications of corrosion exposure including surface- common to the United States are discussed. However, such to-volume ratio, systems are commonly used worldwide. In the cable free Fatigue performance including implications of crack length, the parallel wires or strands are bundled together, thus propagation, making contact with each other. As the cable approaches an Redundancy, anchorage, the wires or strands must separate from each other Interwire fretting, to achieve proper anchorage. The distance from the point that Notching at anchorages, the strands (or wires) splay out to the anchorage point is gen- Stiffness, erally referred to as the anchorage length. Tightness of MTE bundle (void areas), Implications of vibrations, There are three fundamental approaches to cable anchorage Ability to adjust MTE force, and design. The first is to individually anchor each splayed wire or Ability to remove and augment MTE. strand at a single point on an anchorage plate. That anchorage point would exclusively carry all dead and live loads imposed. Anchorage Systems The second is to transfer all loads through a conical steel socket. The force in the MTE transfers by bond through a filler mater- There is a great variety of anchorage systems used for stay ial inside the conical socket. The third is a combination of the cables, depending on the choice of the MTE and the cable first two approaches; that is, transfer dead loads through the anchorage point and carry live loads through the socket action. Point Anchorages Figure 8 shows the point anchorage concept. Typically, a two- or three-piece conical wedge with a toothed center hole grabs on the outside of the seven-wire strand and anchors it. This is essentially a modified version of the wedge system used in post-tensioning applications. Examples of this type of anchor- age include the Charles River Bridge in Boston and the C&D Canal Bridge in St. Georges, Delaware. When individual wires are used, they are generally terminated at a "button head" that is formed at the ends. In the multistrand system with point anchorages, the cable can be assembled in the field by stressing all strands at the same time, or it can be stressed one strand at a time (using a system to ensure equal distribution of stress). The grip- ping wedges create notches on the strands, which could FIGURE 7 Typical bar couplers in stay cables. become fatigue initiation points. However, stay cable systems

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11 Ring Nut Anchor Plate Anchorage Length Free Length Stay Pipe ~ Filler or empty MTE End Cap Anchorage Pipe FIGURE 8 Point (wedge) anchorage concept. go through fatigue qualification tests that they must meet. Clark Bridge in Illinois (with grout-filled socket) and the The other consideration regarding point anchorage systems Cochrane Bridge in Alabama (with epoxy-filled socket). When is the performance of such systems in a rapidly detensioning cement grout is used, the wedges carry the initial stresses on cable during an earthquake. The concern is that in such a case the cable before grouting (or epoxy filling) operations are com- the wedges could potentially exit the anchorage plate, result- pleted (similar to point anchorage). Following grouting, the ing in the loss of anchorage. There have not been however any socket would be expected to resist changes in cable load. reported cases of such an occurrence, and there is no informa- Therefore, the intent of this system is to minimize stress tion available regarding cable performance in such scenarios. changes at the point anchorages. This type of anchorage can be assembled in the field if the grout or epoxy compound is "Hi-Am"-Type Anchorages injected after the initial installation of strands and stressing. Figure 9 shows a "Hi-Am"-type socket. The strands or wires Figure 11 shows the results of the survey as related to the splay out at the entrance to a steel socket that is cylindrical on types of anchorage systems. In the United States, the conical the outside and conical on the inside. The socket is typically socket with wedges and the point (wedge) system were dom- filled with epoxy and small steel balls as well as zinc dust. The inant. It is clear however that the respondents did not simi- MTEs terminate at a locking plate. An example of this type of larly understand the anchorage characterizations, and some design is the Luling Bridge in Louisiana. misidentifications may have occurred. This type of anchorage has to be assembled to the right Recent Trends in Anchorage Design length at a plant and brought to the site, usually on reels. The load transfer between MTE and socket occurs over the length In recent years, the differences between the approaches to of the socket and not at a single point. The cable must be anchorage design of various cable manufacturers' have nar- stressed as a whole. rowed to some extent. Currently (in 2005), all of the major stay cable manufacturers in the United States have at least one sys- Bond Socket-Type Anchorages tem that more or less falls within the point anchorage system described in Figure 8. Figure 10 shows a bond socket anchorage. In this type of anchorage the strands are terminated at an anchorage plate Shop or Field Cable Fabrications with wedges, but there also exists a conical pipe (conical out- side and inside) that is filled with either cement grout or epoxy There are two different approaches regarding the assembly compound. Examples of this type of anchorage include the and erection of stay cables. In one approach, the stay cables "Hi Am " Socket Stay Pipe ~ ~ Locking Plate Filler or empty MTE Epoxy-Steel Ball Compound FIGURE 9 "Hi-Am" type anchorage.

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12 Anchor Plate Ring Nut Grout or Grease Grout or Epoxy Compound Stay Pipe ~ ~ ~ Filler or empty MTE End Cap Bond Socket FIGURE 10 Bond socket anchorage system. are shop-fabricated and installed in the field as a unit. This is given is that the pylon can be smaller (narrower) when there typical of the Hi-Am-type anchorages. However, field fabri- are no anchorages (Figure 13). However, large transverse cations have become more common in the United States, forces are generated on the cable and individual strands in the especially in the last decade. For field assembly, the cable saddles, especially at the entrances to the pylon. As the strands sheathings are first welded together on the bridge and lifted enter the saddle, they begin to move to the bottom of the pipe into place (Figure 12a). Then, strands are typically individu- and large interstrand forces can develop, particularly when ally inserted into the anchor plates at the bottom anchorage bare strands are used. Changes in cable tension can result in and fed through the stay pipes towards the top anchor plate fretting and fatigue. Such fatigue fractures have been ob- (Figure 12b). served on at least one qualification test of a saddle system (Tabatabai et al. 1995). In that test, bare strands were used The strands can be collectively stressed with large hydraulic and fractures were initiated at oval-shaped fretting marks at jacks or, as is more commonly done today, they are individu- interstrand contact points. ally stressed with small jacks as they are inserted in the cable. Different cable suppliers have their own procedures and meth- To address these issues and reduce interstrand contact, ods to achieve equal force in all strands. Shop fabrications are coated strands (such as epoxy-coated) have sometimes been still very common in Japan. The Alex Fraser (Annacis) Bridge used. In the case of the Maumee River Bridge in Ohio, the in British Columbia has shop-fabricated cables (Saul and engineer designed a "cradle" system in which each strand Svensson 1991). The Burlington Bridge in Iowa and the Lul- passes through its own stainless steel sleeve within the cradle ing Bridge in Louisiana are examples of shop-fabricated assembly (Harris 2002). cables in the United States. An FHWA Technical Advisory released in 1994 ("Cable Stays . . ." 1994) discussed a number of factors related to sad- Saddles dles and discouraged the use of saddles at that time. However, the use of saddles has continued in the United States. Among The costliest components of a stay cable are the anchorages. the factors cited by the FHWA advisory were: Therefore, some designers elect to eliminate anchorages at the pylon by providing a continuous cable through the pylon. The Stressing of cables with saddles requires simultane- curved saddle at the pylon is typically a steel pipe that re- ous stressing from both anchorages (during and after directs the cable force through the pylon. Another reason construction); 100.0 Percent of Bridges 80.0 U.S. Canada 60.0 40.0 20.0 0.0 wedges conical cylindrical "Hi-Am" other not known no answer socket w/ sockets w/ Type wedges wedges Anchorage Type FIGURE 11 Type and frequency of anchorage used.