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22 and bottom neoprene boots. They recommended using sili- cone filler under the boot before clamping. Responses to the questionnaire for at least three bridges indicated problems with neoprene boots. Table 7 shows the survey responses related to problems with neoprene boots. Four bridges in the United States and one in Canada reported problems with neoprene boots. However, two of the four U.S. cases referred to neoprene boots that are not as described above. STAY CABLE DESIGN CHALLENGES FIGURE 27 Failure of the keeper ring and dislocation of Aside from structural strength, the design of stay cables also neoprene washer (Telang et al. 2000). must address the challenges of corrosion, fatigue, vibration, inspectability, and maintainability. More recently, considera- Figure 28 shows the results of the survey with respect to the tion of extreme events such as fire, ice, blasts, impacts, and use of neoprene rings. Most cable-stayed bridges in the United earthquakes are attracting more attention in the design of stay States (64%) use neoprene rings, whereas 31% of Canadian cables. In this section, the mechanisms for corrosion, fatigue, bridges have neoprene rings. and vibrations (including rainwind vibrations) are first dis- cussed, followed by a discussion of the challenges of design- Responses to the questionnaire indicated that seven bridges ing stay cables for inspectability and maintainability. The PTI in the United States and two bridges in Canada had problems recommendations, including qualification tests, are reviewed. with movements of the rings out of position for various rea- Finally, a brief outline of issues related to extreme events is sons, indicating that this is a relatively common problem (see discussed. Figure 29). It should be noted that although these issues are presented At least one cable supplier has developed a proprietary vis- separately, they are highly interrelated and cannot be con- coelastic damping system that also serves the purposes of the sidered independent. For example, corrosion and vibrations neoprene washer. The topic of vibration damping is discussed could have major negative influence on fatigue performance. later in this report. The ability to inspect and maintain also influences durability of cables in all areas. These major structures must safely carry traffic for a long time. Therefore, a clear understanding of the Neoprene Boot durability limits and issues is very important. Neoprene boots are generally used to cover the gap between the cable sheathing and the end of the guide pipe near the neo- Corrosion prene ring. Figure 30 shows a typical neoprene boot that is in good working condition. Typically, hose clamps are used to Corrosion protection for stay cables is understandably one tighten the boots against the sheathing and the guide pipe. In of the primary concerns of designers, suppliers, and owners some cases, it has been observed that the clamps become dis- involved in cable-stayed bridges. According to the PTI Rec- placed and rainwater can enter the guide pipes. Bloomstine ommendations for Stay Cable Design, Testing, and Installa- and Stoltzner (1999) reported on water intrusion into the top tion (2001), a minimum of "two nested qualified barriers" 80.0 70.0 U.S. Canada Percent of Bridges 60.0 50.0 40.0 30.0 20.0 10.0 0.0 yes no not known not applicable no answer Neoprene Rings FIGURE 28 Use of neoprene rings.
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23 80 Percent of Bridges 60 U.S. Canada 40 20 0 yes no, not known not applicable no answer Neoprene Ring Problems FIGURE 29 Movements of neoprene rings out of position. must be provided for the corrosion protection of the MTE. By inclusion of additional features such as vibration control and clearly specifying minimum acceptable levels of protection force measurements as part of the cable design. This consti- and setting performance requirements with respect to corro- tutes a "system approach" to the design of stay cables. Embed- sion, these provisions are a major departure from earlier prac- ded corrosion monitoring systems can also be considered as tices. A two-tier system is established in which the individual technology develops further. barriers must first be qualified through testing, followed by the testing of the nested barriers as a system. As the number of There have been a number of debates over the years on nested qualified barriers (that are compatible with each other) the issues of corrosion and the overall health of stay cables. increase, the system redundancy and reliability is expected to In a 1988 article, "Cables in Trouble," Watson and Stafford improve. (1988) presented an alarming picture of the condition of stay cables, indicating that cable-stayed bridges were in serious It is important to realize that corrosion can be either inter- danger as a result of corrosion. The authors pointed to corro- nally or externally driven. The primary mode of protection has sion (of the Kurt Schumaker Bridge in Germany), vibration naturally and rightly been against externally driven corrosion (of the Brottone Bridge in France), intersliding of wires, and (i.e., moisture and other harmful substances entering from out- long-term creep behavior of cables as evidence of serious side). However, internally driven corrosion mechanisms have challenges for cable protection. In response, in a 1991 article, also been observed and must be addressed in design and main- "Cables Not in Trouble," Grant (1991) countered that cable- tenance. Examples of these corrosion mechanisms include stayed bridges were not in danger of failure from corrosion corrosion resulting from the presence of free grout water in of cables. Grant reported tests on the Sitka Harbor Bridge in different components of cables. Alaska involving removal of six galvanized structural strand cables and their examination by magnetic, ultrasonic, radi- Design of cable components for corrosion resistance should ographic, and X-ray methods. All cables were reported in consider, when applicable, the effects of extreme tempera- "nearly new condition." Tests were also performed on the tures, solar radiation, shrinkage or expansion of fillers, age, Meridian Bridge in California and the PascoKennewick vibration, and fatigue on the effectiveness of the system. As Bridge (Washington State), and the steel elements were will be seen later in this report, recent trends have been toward reportedly found to be without corrosion. Saul and Svensson (1991) discussed some of the damage observed on cable-stayed bridges. In the case of the Kohlbrand Bridge in Germany, they reported on the detection during inspections of 25 broken wires on the nongalvanized locked coil cables that were protected with red lead and linseed oil TABLE 7 SURVEY RESULTS--PROBLEMS WITH NEOPRENE BOOTS (Question 4.24) U.S. % U.S. Canada % Canada % total Yes 4 14.3 1 7.7 12.2 No 19 67.9 6 46.2 61.0 Not known 1 3.6 0 0.0 2.4 Not applicable 1 3.6 3 23.1 9.8 Other 2 7.1 0 0.0 4.9 No answer 1 3.6 3 23.1 9.8 FIGURE 30 Neoprene boot (courtesy: Indiana DOT).
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24 (Figure 31). The bridge was under construction between 1969 and 1974, and the wire breaks were found in 1976. The authors attributed failures to many factors including misalignment of cables, missing protection at the sockets, cable vibrations, and possible effects of deicing salts. The Lake Maracaibo Bridge in Venezuela also suffered corrosion of its galvanized locked coil cables after fewer than 18 years of service, and all of its cables were subsequently replaced in 1980 ("Cable Stays . . ." 1994). Saul and Svensson (1991) reported that the causes included inadequate mainte- nance and painting in the hot marine climate and a mistake made in not reinstalling neoprene boots during inspections, which resulted in a humid microclimate. Figure 32 shows the fracture of wires on the Lake Maracaibo Bridge. Sarcos-Portillo et al. (2003) reported that inspections car- ried out in 19971999 revealed "corrosion in both cables and sockets, as well as considerable settling in the sockets" of the new cables. A "significant" amount of water was also found in most sockets. Vibration-based tension force measurements indicated major force changes. Deck profile changes were also noted. The cables were retensioned, and they recom- mended painting the cables and waterproofing and lubricat- ing the sockets. FIGURE 32 Corrosion of locked coil cables of the Lake Maracaibo Bridge in Venezuela (Frank and Breen 2004). The response to the questionnaire for the Fox Hollow pedestrian bridge in Calgary, Canada, indicated failures of two galvanized bars used as MTEs, and replacement of a third bar. There were no sheathings or grout used on these cables. On further inquiry, the respondent reported that the failures were without any sign of prior problems. An evalu- ation has reportedly been performed by outside experts and the failure mode was reported as "corrosion induced fatigue." The remaining bars were examined and a third bar was iden- tified with a corrosion pit and replaced. Wire rope cross cables were installed after the failures. No further information was available at this time. As discussed earlier, Saul and Svensson (1991) reported on the cracking of the grouted HDPE pipes on the Luling Bridge in Louisiana and the twin ZarateBrazo Largo Bridges in Argentina. The longitudinal cracks in the pipes were attributed to high strains owing to grouting during hot temperatures. Sub- sequent cooling against hardened grout creates stresses in the pipe. Both bridges used shop-fabricated cables that were deliv- ered on reels. In the case of the Argentine bridges, they were left on reels for up to 3 years. In the case of the Luling Bridge, failures of the butt welds between HDPE segments were also noted, which were attributed to malfunctioning welding equip- ment and uncoiling at low temperatures. Repair of HDPE in both of these bridges included filling cracks with polyurethane FIGURE 31 Corrosion and rupture of locked coil cable on the grout and wrapping them with filament tape and PVF tape Kohlbrand Bridge in Germany (Frank and Breen 2004). (Saul and Svensson 1991).
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25 Telang et al. (2004) reported on inspections of the cables of the Luling Bridge in 2002 and 2004. They reported that, at least in one location, exposed and rusted stay cable wires were detected. The original "epoxy repair" had deteriorated and resulted in the rupture of the protective tape and filler grout and corrosion of wires. Extensive water leakage inside sock- ets of deck level anchorages was observed. Water dripping from the split rings and shims was observed at most locations. It was suggested that rainwater entered the steel box at the cable exit locations through gaps in neoprene washers. It should be noted that the neoprene washers on the Luling Bridge are different from those described earlier. They sur- round the sheathing and are caulked to the opening at the top of the box girder (Figure 33). FIGURE 34 Water exiting the end cap of one anchorage The caulk that was used around the washers was weathered, (Telang et al. 2004). cracked, or missing at some locations. Neoprene washers were removed and a video boroscope (videoscope) examination was performed. Accumulated water was found surrounding the coming out of one end cap as bolts are loosened. Various cable inside the box. The end caps of sockets were removed degrees of corrosion were noted in the cables (see Figure 35). to expose the button end of the wires. Figure 34 shows water Telang et al. (2004) concluded, based on vibration-based measurements of cable forces, that "the cables have not suf- fered any significant damage." They do not however discuss whether corrosion damage would necessarily result in global stiffness changes in grouted cables resulting in force changes. Further testing is planned for the Luling Bridge. In the case of the ZarateBrazo Largo Bridges, Saul and Svensson (1991) stated "five years after the repair the cables were inspected by the Argentine Federal Highway Adminis- tration and found in good condition." It is estimated that the inspection was probably performed around 1987. In November 1996, the first ever rupture and complete fail- ure of a parallel wire stay cable occurred on the Guazu Bridge in Argentina, one of the two ZarateBrazo Largo Bridges (Andersen et al. 1999). These bridges were built in the early FIGURE 33 Neoprene washers on the Luling Bridge at the exit FIGURE 35 Corrosion at the end plate of one socket with wire point of cables from the steel box (Telang et al. 2004). button ends (Telang et al. 2004).
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26 1970s. The cable consisted of grouted nongalvanized parallel There has been some work done on the corrosion and wires within HDPE pipes and anchored within Hi-Am-type embrittlement of high-strength wires for suspension bridges, sockets (Andersen et al. 1999). According to the authors, "a which can be relevant to stay cables as well. Laboratory work combination of corrosion and fatigue has been found to be the by Barton et al. (2000) reported that "corrosion degradation of cause. The corrosion has taken place due to insufficient cor- high-strength wires exceeds mere loss of load-bearing mate- rosion protection of the non-galvanized wires. The likely rial." Wire strength was reduced more than the cross-sectional cause is that the cement grout, which was supposed to be the area suggesting that "cracking or pitting effects may be pres- main corrosion protection, was insufficient in the anchorage ent, whether induced by corrosion or by hydrogen interaction, zone due to the presence of a non-protecting epoxy tar." They or both." Their studies indicated that hydrogen was absorbed also stated that "following intrusion of water through defects into the corroded wire, with hydrogen retention being higher in the PE pipe or due to condensation of water inside the PE in galvanized wire. Corrosion results in higher embrittlement pipe, corrosion has been initiated." A complete rehabilitation of both galvanized and nongalvanized wires. of the bridge was planned for 1999/2000. Mayrbaurl and Camo (2004) reported on a study of struc- tural safety of suspension bridge parallel-wire cables. They dis- The cable had failed in an area near the entrance to the bot- cussed issues related to corrosion of galvanized wires, in- tom anchorage. Subsequent ultrasonic testing on other anchor- cluding categorization of wire corrosion in four stages. They ages revealed damage to other cables, with up to 62% wire also presented cable strength models based on field assessments breaks. The cable with 62% wire breaks had adjacent cables of wire data. However, unlike stay cables, the primary tool for with 41% and 20% breaks. Damage to bottom anchorages was inspection in main suspension cables is the removal of outside significantly greater than to top anchorages. Cable force mea- wrapped wire and the physical opening of the cable (insertion surements reportedly indicated that forces in the cables had of wood wedges) to visually inspect the interior of the cable. changed by as much as 20% when compared with forces at the Despite some similarities, suspension main cables and stay inauguration of the bridge. This however appears to have in- cable have major differences in design, materials, inspection cluded the effect of the lost cable, and it is not clear whether the processes, deterioration mechanisms, and anchorage systems. forces at the inauguration of the bridge were actually measured However, information related to long-term deterioration of gal- or estimated by the designer. Large amplitude cable vibrations vanized wires is still valuable to the stay cable community. (reportedly not rainwind vibrations) had taken place on this bridge. During emergency repairs, 13 cables were replaced. In 1992, the U.S. Patent and Trademark Office issued Patent No. 5,173,982 to inventors T.G. Lovett and S.L. Stroh Prato et al. (1997, 1998) reported on the replacement of for a corrosion protection system for stay cables ("Immersion all locked coil cables of the ChacoCorrientes Bridge in of Stays . . ." 1993). It is designed to keep the stay cable Argentina. The locked coil cables had external galvanized immersed in a lightweight, corrosion-resistant fluid within the wires. This bridge was built in 1973. Failure of several cable sheathing. It is not known if this concept has been used z-shaped wires (in the external layer of wires) on four cables on any actual stay cables. occurred in 1986 and the cables were replaced in 1996. Kitagawa et al. (2001) reported on a dry-air injection sys- Reinholdt et al. (1999) reported on the replacement of all tem used to reduce humidity levels inside the main cables of wire rope cables of the Luangwa Bridge in Zambia in 1997. the Akashi Kaikyo Suspension Bridge and other bridges in The bridge was built in 1968. The shop-fabricated cables Japan. The system includes salt filters to remove chlorides. were originally made longer than required resulting in a dip Humidity measurements inside the cable reportedly show the in bridge deck surface. This was addressed by installation effectiveness of the system. of "cable clamps" to reduce cable length by approximately 135 mm (5.3 in.). Severe corrosion and pitting of cables was Figure 36 shows survey responses with respect to moisture noted in 1997, resulting in replacement of all cables. found inside the stay cable components. Respondents for 25% 60.0 Percent of Bridges 50.0 U.S. Canada 40.0 30.0 20.0 10.0 0.0 yes no not tested not known not no answer applicable Presence of Moisture FIGURE 36 Occurrences of moisture inside stay cables.
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27 of bridges in the United States indicated that moisture has been fatigue failures were observed after 10 million cycles for a found inside cables. Another 21% either have not tested or do ±0.9° angle change. not know if moisture exists. Figure 37 shows survey responses with respect to MTE corrosion. Only one U.S. bridge and two Frank and Breen (2004) discussed stay cable bending Canadian bridges were reported to have evidence of MTE cor- fatigue test results in which performance of grouted and rosion. The corrosion status of five other U.S. bridges and ungrouted stay cable specimens were compared. Bare strands three Canadian bridges was reported to be unknown. As will were used and the two cable types were identical except for be discussed later, assessments of MTE corrosion in cable- grouting. The number of wire breaks recorded was much stayed bridges are, in many cases, very difficult. higher in the grouted specimens. The authors suggest that the grout acts as an abrasive that reduces fatigue life resulting from fretting. Fatigue Prato and Ceballos (2003) studied dynamic bending stresses The PTI Recommendations for Stay Cable Design, Testing, near anchorage sockets for grouted cables with HDPE pipes, and Installation (2001) provide detailed fatigue and static but with bituminous epoxy replacing grout just before the qualification testing requirements for stay cables. Three cable anchorage (Figure 38). The authors show that the dynamic specimens are typically tested for each bridge. These tests stresses in wires are higher, and stress concentration occurs, include two million cycles of loading, with a stress range of when such a discontinuity is present (i.e., grout is replaced by 28 ksi (159 MPa) and a maximum stress equivalent to 45% of bituminous epoxy). They noted that shear deformations in such the cable's nominal strength. The number of wire breaks dur- cases would not be negligible, and the dominant discontinuity ing fatigue tests should not exceed 2% of the total number of would be that of shear stiffness and not bending stiffness. Fig- wires in the cable. After fatigue tests, cables are loaded stati- ure 39 shows the results of the survey with respect to fatigue. cally to achieve a target load of 95% of the nominal strength or 92% of the actual strength of the strands. Some European codes such as the SETRA/CIP require fatigue tests that include Vibrations a small angle change (rotation) induced at the anchorages. The PTI requirements do not have this provision at this time. The Since the mid-1980s, bridge owners and researchers have PTI recommendations also specify procedures for axial and reported large-amplitude stay cable vibrations with increas- flexural tests involving cable saddles. It should be noted how- ing frequency. This has resulted in increased concern for the ever that the PTI qualification tests do not specifically address fatigue performance of cables. Figure 40 shows vibrations fatigue issues related to cable vibrations. recorded on the Cochrane Bridge in Alabama, and witnessed by this writer. In response to the observed rainwind vibrations on two bridges in Texas, Dowd et al. (2001) began a research proj- ect aimed at developing a set of procedures for evaluating Categories of Vibration fatigue damage in stay cables resulting from large amplitude and rainwind-induced vibrations. This effort includes test- The primary types of stay cable vibrations are as follows ing of cable specimens in the laboratory as they are subjected (Irwin 1997): to axial loads and simultaneous cyclic lateral loads at the mid-point of the cable. The authors reported that similar tests · Rainwind induced vibrations, were done in Japan on cables with 163 parallel and galva- · Sympathetic vibration of cables with other bridge com- nized wires with Hi-Am-type sockets and PE pipes (without ponents excited by wind (parametric excitation), grout). In the Japanese tests, angle changes of ±1.35° pro- · Inclined cable galloping, duced fatigue failures at 0.26 million cycles, whereas no · Vortex excitation (single cable or groups of cables), 80.0 70.0 U.S. Canada Percent of Bridges 60.0 50.0 40.0 30.0 20.0 10.0 0.0 yes no not known not applicable no answer Corrosion FIGURE 37 Incidence of MTE corrosion.
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28 FIGURE 38 Deformations near cable anchorages with discontinuous grout (Prato and Ceballos 2003). · Wake galloping, and also reported that the rivulet changes its position with wind · Buffeting by wind turbulence. speed and also cable motion. Miyazaki (1999) reported that the lower rivulet is formed at lower wind speeds, and both The rainwind-induced vibrations are by far the most rivulets appear at higher speeds. This is consistent with what widely reported, large-amplitude (up to a few feet) vibration this writer observed on the Cochrane Bridge in 1998. In this phenomenon in stay cables. It was first reported on the Meiko case it was the lower rivulet that appeared first; however, it West Bridge in Japan in 1986 (Matsumoto 2000), and has was the subsequent formation of the upper rivulet that initi- since been reported on many bridges worldwide. This phe- ated large amplitude vibrations. Also, the rivulets appeared nomenon occurs in moderate wind and rain conditions, and to oscillate up and down within a "wet" band as they moved is believed to be caused by an aerodynamic instability result- down the cable (Figure 41). ing from the formation of water rivulets on the surface of the cable. However, uncertainties still exist regarding this phe- Larose and Wagner Smitt suggest that the "wetability" of nomenon (Matsumoto 2000). the cable surface is important in the formation of rivulets. They noted that a slightly eroded surface with dust particles When vibrations are occurring, the speed of the wind is is more "wetable," and thus can form the rivulets more eas- sufficient to maintain the upper rivulet within a critical zone ily. This may be the reason why some bridges do not experi- (Irwin 1997). Larose and Wagner Smitt (1999) discussed ence rainwind vibrations for the first few years of their ser- the results of their wind tunnel studies and reported that vice. According to Swan (1997), "a very smooth surface may rainwind vibrations were reproduced in the laboratory for a initially avoid the problem, until atmospheric deposits allow single cable and for cables in tandem configuration. They just enough roughness to hold the rivulet." 100.0 Percent of Bridges 80.0 U.S. Canada 60.0 40.0 20.0 0.0 yes no not known not applicable no answer Fatigue FIGURE 39 Incidence of fatigue of MTEs.
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29 were attributed to vortex-induced vibrations. Tabatabai et al. (1998a) and Lankin et al. (2000) have reported on vibration measurements and mitigation efforts for the Cochrane Bridge in Alabama. In these studies, the level of damping in all cables was measured and studies were performed to determine and optimize mitigation solutions. Irwin (1997) recommended the following equation for controlling rainwind vibrations: m Sc = 10 Eq. 1 D where Sc = Scruton number, m = mass per unit length of cable, = damping ratio, FIGURE 40 Large amplitude vibrations of the Cochrane Bridge = density of air (1.225 kg/m3), and (Alabama) (Telang et al. 2000). D is the cable diameter. This equation has been adopted in the PTI Recommendations Jones and Porterfield (1997) reported on the instrumenta- for Stay Cable Design, Testing, and Installation (2001) for tion and long-term vibration monitoring of the East Hunt- control of rainwind vibrations. ington Bridge in West Virginia. They reported random buf- feting response, locked-in vortex-induced vibration, and rain Tabatabai and Mehrabi (2000) used cable information wind oscillations. They noted that significant displacement from 16 cable-stayed bridges to determine the level of damp- responses are in the lower modes of the structure. High ing required based on Eq. 1. Figure 43 shows a histogram of acceleration values at higher modes do not mean high dis- required cable damping for all stay cables in those 16 bridges. placements at those frequencies (acceleration amplitudes These data indicate that 90% of the cables would meet the are equivalent to displacement amplitudes multiplied by fre- requirements of Eq. 1 with a damping ratio of 0.7%. The quency squared). Main and Jones (2000) also reported on authors suggested that the typical first mode damping ratios the instrumentation and long-term vibration monitoring of for cables are in the range of 0.05% to 0.9%. Similar data for the Fred Hartman and Veterans Memorial Bridges in Texas. control of inclined cable galloping is also provided. Figure 42a shows a sample histogram of dominant modes for one stay cable, and Figure 42b shows vibration amplitudes Incidences of large amplitude cable vibrations have also versus wind speed for the same cable. been reported when there is no rain, and typically at higher wind speeds. There is debate and uncertainty regarding the Main and Jones (2000) concluded that the highest ampli- exact nature of all of the events that fall under this category tude responses (which occurred during rainfall) were in the of vibrations. It is known that cable vibrations can occur when lower modes and "seemed to `lock-in' to a specific mode of deck or tower vibrations are occurring at frequencies close vibration over a wide range of wind speeds." High-frequency to the cable frequency (Stubler et al. 1999; Wu et al. 2003). vibrations over narrow wind ranges were also observed, which This is also called "parametric vibrations" or "local parametric vibrations" by some investigators. Wu et al. (2003) reported that parametric vibration has been confirmed on three bridges Top Rivulet in Japan, including the Tatara Bridge. Irwin (1997) discussed the possibility of inclined cable galloping based on the work of Saito et al. (1994) in Japan. Although circular cross sec- Wind tions do not gallop when aligned normal to wind (Starossek 1994), Irwin provides a possible explanation in that the wind would "see" an inclined cable as an elliptical section, and thus Bottom Rivulet be able to gallop. This phenomenon has been investigated in wind tunnel tests and it was determined that separate require- ments to address this phenomenon are not necessary. FIGURE 41 Position and movements of water rivulets during Until recently, there were no vibrations reported on the rainwind vibrations. Sunshine Skyway Bridge, which joins St. Petersburg and
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30 Hartman Stay AS16 Number of occurrences as dominant mode (5-minute RMS in-plane acceleration > 0.25 g) 120 100 80 60 40 20 0 1 2 3 4 5 6 7 8 Mode (a) Hartman Stay AS16 (Dominant mode indicated by symbol) 3 In-plane acceleration 5-minute RMS (g) 2.5 mode 2 2 mode 3 mode 4 1.5 mode 5 1 mode 7 0.5 0 0 2 4 6 8 10 12 14 Deck-level wind speed 5-minute mean (m/s) (b) FIGURE 42 Vibration data from Fred Hartman Bridge: (a) histogram of modes, (b) vibration amplitudes (Main and Jones 2000). Bradenton in Florida, whether rainwind or otherwise. This spheric Administration records, a sustained wind of 72 kph bridge was opened to traffic in 1987, has grouted parallel (45 mph) and gusts of up to 96 kph (60 mph) were present strand cables with steel sheathing, and has two-dimensional in the area. Wind was blowing at 90 degrees to the struc- viscous dampers (shock absorbers) installed on each cable. tures (perpendicular to cable plane). There was no rain, and On April 12, 2004, Florida DOT personnel noted small- estimated vibration amplitudes of up to 75 mm (3 in.) were amplitude vibrations on one of the longest cables of the bridge reported. It should be noted that the reported amplitudes in (personal communication, S.D. Womble, April 14, 2004). It this case are far smaller than amplitudes typically reported was reported that, according to National Oceanic and Atmo- for rainwind vibrations in other bridges.
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31 400 120% 350 100% 300 Cumulative Percentage 80% Number of Cables 250 200 60% 150 40% Frequency 100 Cumulative Percentage 20% 50 0 0% 0.1 0.2 0.4 0.5 0.7 0.8 1.0 1.1 1.3 1.4 1.6 1.7 1.9 2.0 Damping Ratio (Percent) FIGURE 43 Histogram of required damping ratio for controlling rainwind vibrations (Tabatabai and Mehrabi 2000). Vortex excitation is likely the most common form of cable ing of bridge to traffic. They noted vortex shedding and vibration, with the cables vibrating at lower displacement wake-induced effects. However, they also reported a first mode amplitudes and higher frequencies (mode 5 and higher) (Main response of stays to "either galloping or bridge deck motion." and Jones 2001). Therefore, this mode of vibration is not as significant a risk to stay cables as rainwind vibrations. The responses to the questionnaire indicated that a sizable Vortex-induced vibrations have been noted on the Tatara number of cable-stayed bridges included in the survey Bridge in Japan (Yamaguchi et al. 1999). have experienced rainwind vibrations. These bridges are the Cochrane Bridge in Alabama; Talmadge Memorial over When cables are positioned in the wake of towers or other the Savannah River in Georgia; Clark in Alton, Illinois; cables, they can have large amplitude wake galloping vibra- Burlington in Iowa; Veterans Memorial between Bridge tions. However, the wake galloping that could occur in stay City and Port Arthur in Texas; and Fred Hartman in Houston, cables is typically characterized by very small cable spacing, Texas. In Canada, the Prince's Island and Fox Hollow bridges on the order of six cable diameters (Miyazaki 1999). (Alberta, Calgary), and the Hawkshaw, Longs Creek #1, and Nackawic River bridges (New Brunswick) have reportedly Bruce et al. (1987) reported on the aerodynamic monitor- been affected. Figure 44 shows the results of the survey as ing of the Luling Bridge in Louisiana 3 years after the open- related to rainwind-induced cable vibrations. It is interesting 70.0 Percent of Bridges 60.0 U.S. Canada 50.0 40.0 30.0 20.0 10.0 0.0 yes no not known not no answer applicable Rain-Wind Vibrations FIGURE 44 Rainwind induced cable vibrations.
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32 to note that the survey response for the Fox Hollow Bridge to forces that would push the wedges out of their positions indicated rainwind vibrations even though the stay cable is within the anchorage plates. This issue may be considered made of exposed threadbars, which would not likely promote by the stay cable community and studied further. However, the formation of water rivulets. It may be that, in this case, other there have not been any reported cases where this scenario vibration types have been mischaracterized as rainwind. has materialized. Figures 45 and 46 show survey results with respect to the During an oral presentation at a stay cable seminar, Zoli use of viscous dampers and cross cables. It appears that the and McCabe (2004) reported on issues related to fire, ice, most popular method of vibration control is the use of cross and impact on stay cables. They reported that there have not cables. Nearly one-third of the bridges in the United States and been major fire incidents involving cable-stayed bridges. about one-quarter of the bridges in Canada have cross cables However, six major Interstate highway fires have occurred, for vibration control, either installed from the beginning or resulting in significant cost and extended closures of major retrofitted later to control vibrations. arteries. Zoli and McCabe suggest that wedge anchorage systems would be more resistant than some other anchor- Viscous dampers are used in the United States by six ages. Zinc-filled sockets are temperature sensitive and con- bridges (21.5%) and in Canada by three bridges (23.1%). In tain materials with low melting points. Possible mitigation some bridges such as the Fred Hartman Bridge in Texas, both measures include utilization of fire-resistant cable sheathing viscous dampers and cross cables are added (retrofitted) to near deck level, intumescent paints, ablative coatings, ceram- control vibrations. In the Cooper River Bridge in South Car- ics and composites. According to Zoli and McCabe, there are olina, viscous dampers will be installed, but provisions for currently no code provisions in the United States address- future installation of cross cables are made in case they are ing fires on bridges, although the Eurocode includes some needed. Figure 47 shows the survey results with respect to provisions. the use of other types of dampers. Regarding the effects of icing on cables, Zoli and McCabe noted that ice formations on a major suspension bridge have Extreme Events been periodically removed as a safety precaution. They There are a number of extreme or unusual events that could reported on research being done on the issue of icing of affect the performance of stay cables including earthquakes, cables, including assessments of sheathing performance and fire, blasts, impacts, and ice build-up. The earthquake design icing wind tunnel tests. The effects of icing on galloping vibra- issues are generally handled through a global analysis of the tions of stay cables need to be studied. They discussed "ice- entire cable-stayed bridge. However, during the fall 2004 phobic" coatings and ultrasonic deicing systems. meeting of the PTI cable-stayed bridge committee, David Goodyear noted that there potentially are cases when during Regarding impact, Zoli and McCabe discussed a number an earthquake the tension force in a cable can rapidly decrease of approaches including cable "armoring" involving hybrid to zero or even compression. This impact loading, in a direc- ceramic FRP materials. tion opposite to how the cable anchorage is designed to resist may result in permanent dislocation and damage to some cru- Inspectability and Maintainability cial anchorage components, potentially rendering them in- effective and resulting in failures. Specifically, wedge sys- Question 11 in the survey questionnaire asked agencies the tems could be affected where there is no significant resistance following: what do you see as the single most important prob- 90.0 80.0 U.S. Canada Percent of Bridges 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 yes - from the yes - retrofitted no no answer beginning to correct vibrations Viscous Dampers FIGURE 45 Percentage of bridges using viscous dampers.
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33 90.0 80.0 U.S. Canada Percent of Bridges 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 yes - from the yes - retrofitted no no answer beginning to correct vibrations Cross Cables FIGURE 46 Percentage of bridges using cross cables. lem in stay cable maintenance? The great majority of answers · Inspecting the cable anchors and grout-filled cables. mentioned accessibility and inspection problems, especially in · Hidden nature of the system. the anchorage areas. The general consensus of the respondents · Access for inspection. points to a concern by the owners about difficulties in access · Integrity of the stays. Grouted cables are impossible for inspections and maintenance. It should be noted that to inspect with a nondestructive technique (i.e., one although there is general agreement by the stay cable commu- that does not require removal of sheathing and grout); nity about the need to address the stay cable maintenance issue therefore, it is impossible to identify corrosion prob- and problems, there is no universal consensus on this issue, lems early. especially with the characterization of the subject as a "prob- · The largest "problem" with stay cables is that they are lem" as indicated by one respondent. The following are some widely perceived of as "a problem" rather than just of the answers provided by respondents: another bridge member with specific needs and charac- teristics. Stay cables have been placed unnecessarily "on · Access and rainwind induced oscillation. a pedestal." Although they are a very important bridge · Access to upper anchorage. member, in current designs they are highly redundant, · Inspection and condition evaluation of anchorages. overtested, and (relatively) easily replaced. There is no · Effective corrosion barriers that do not interfere with the other major bridge member that fits into all three of these ability to adequately inspect and assess the health of the categories. Let us not promote the feeling that stays are cable stay system on a regular interval and within practi- "a maintenance problem." cal means. · Provide end caps that are easily removed and fully pro- · Accessibility for inspection and maintenance. tected against corrosion. · Access to the cable anchorages. · Ability to determine the effectiveness and remaining · Uncertainty of cable condition and anchorages. life of corrosion protection systems for main tension · Inspection, access, testing, and cost. elements. The configuration and construction techniques · Inability to inspect the elements inside the cable and make evaluation and inspection using nondestructive anchorage areas. techniques almost impossible. 90.0 80.0 U.S. Canada Percent of Bridges 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 yes - tuned yes - other no not known no answer mass dampers dampers Other Dampers FIGURE 47 Percentage of bridges using other types of dampers.
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34 · Lack of familiarity with this type of construction by the department's staff, which requires assistance from the consultant community in the inspection of these elements. · Cannot inspect cables without pulling strand every 10 years. · Access for inspection and actual testing. · Detection of corrosion in cables; maintenance of sheaths and boots. · Corrosion at the anchorages. · Migration of water into cable strands. · Fatigue. · Inspectability in the anchorage area. The anchorages are typically unreachable except from the deck by special "reach-all" trucks (see Figure 48). Some newer bridges (such as the Cooper River Bridge) incorporate anchor- ages that are at about deck level. The end caps are generally difficult to remove, especially when filled with grout or epoxy. Even when the end caps are removed, the condition of MTEs within the anchorage area and beyond cannot be examined visually. If moisture were to enter the cable along its length, gravity would likely force it down to the bottom anchorage. There is currently no easy way to check for the presence of moisture or corrosion in the bottom anchorage, except through removal of the cap. Massive reinforced concrete or steel superstructure elements that are designed to resist anchorage forces typically surround the anchorage zones. Therefore, the FIGURE 48 Access to cable bottom anchorage for ultrasonic sides of the anchorage zones are generally neither visible nor testing. accessible all the way up to the top of the neoprene rings and boots. Some recent anchorage designs (such as the 6th Street and adequate. Figure 49 summarizes these responses. The bridges in Wisconsin) have incorporated individually coated U.S. respondents were far less certain than their Canadian and sheathed strands that reportedly allow for future replace- counterparts, with less than 40% believing that they have ments of individual strands (one by one). Some recent bridges effective and adequate methods available. One of the respon- also include additional strands in the cables that are designed dents indicated that for cables with steel sheathing the for removal at 10 to 15 year intervals for inspection. In some inspection methods available are limited. Another respon- cases, allowance is made in the cables to add new strands, if dent referred to problems in inspection of anchorage areas needed. Permanent access platforms for use by inspectors are and expressed the need for a technological breakthrough to also an important consideration. address this problem. Question 5 in the survey asked whether the current inspec- One important question in the maintainability of a cable- tion, testing, monitoring, and repair methods were effective stayed bridge is whether the cable (or individual strands) can 100.0 Percent of Bridges 80.0 U.S. Canada 60.0 40.0 20.0 0.0 yes no not known Methods Effective? FIGURE 49 Respondents reporting that inspection, testing, monitoring, and repairs are effective and adequate.
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35 90.0 80.0 U.S. Canada 70.0 Percent of Bridges 60.0 50.0 40.0 30.0 20.0 10.0 0.0 yes no not known no answer Replaceable? FIGURE 50 Can the cables (or strands) be replaced? be replaced, if needed. In the opinion of respondents for 79% · Transparent outer pipe, eliminate grout. of U.S. bridges and 62% of Canadian bridges, the answer to · Current grouted and sheathed systems do not allow for this question is "yes." Figure 50 summarizes the survey results visual inspection. New stay systems (perhaps ungrouted, for this question. unsheathed systems consisting of bare corrosion- resistant tension members) need to be developed that Another question in the survey asked whether there is an allow for inspection of the entire stay length. Research inspection and maintenance manual for the bridge. Figure 51 is also needed to develop rapid, economical nondestruc- shows the results of the survey for this question. The great tive evaluation (NDE) methods to determine conditions majority of U.S. bridges (71%) have maintenance manuals; of stay cables. however, more than 92% of Canadian bridges do not. As will · Access is a very sharp two-edged sword. If you can more be discussed later, there is a wide variation in topics discussed easily access the cable, so can corrosive elements (not to in individual maintenance manuals. mention potential terrorist/security considerations). · Include a maintenance manual with clear instructions for both specific wires or full cables. Survey question 10 asked whether an up-to-date resource · Perhaps a permanent load cell that would permit real- such as a national database of information on stay cable time readings of cable forces at any time during the life inspection, repairs, and testing would be a useful tool. Fig- of the bridge. ure 52 summarizes the responses. An overwhelming major- · Our cables are reasonably accessible, inspectable. Pos- ity of responses (approximately 90%) in both the United sibly a closeable drain at the lower end of the cable to States and Canada responded in the affirmative. allow visual inspection, sample collection, testing for corrosion product of any water in the cable sheaths. There was a wide variety of answers provided to the survey · Different corrosion protection system at the anchorages question on what the cable suppliers should incorporate into that permits easier visual inspection. Removable sec- their systems to make them accessible and inspectable. The tions of the HDPE and Vandal Tubes would make it following are some of the suggestions: easier to inspect strands near the anchorages. 100.0 90.0 U.S. Canada 80.0 Percent of Bridges 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 yes no not known no answer Maintenance Manual? FIGURE 51 Bridges with an inspection and maintenance manual available.