Patent Publication Number: US-2021172116-A1

Title: Sheath for a structural cable of a construction work, methods of installation and maintenance

Description:
This application is a National Stage filing under 35 U.S.C. 371 of International Application No. PCT/IB2017/001514 filed Nov. 3, 2017, which is hereby incorporated by reference in its entirety for all purposes as if fully set forth herein. 
    
    
     The present invention relates to structural cables used in the construction industry. It is applicable, in particular, to stay cables used for supporting, stiffening or stabilizing structures. 
     BACKGROUND 
     Stay cables are widely used to support suspended structures such as bridge decks or roofs. They can also be used to stabilize erected structures such as towers or masts. 
     A typical structure of a stay cable includes a bundle of tendons, for example wires or strands, housed in a collective plastic sheath. The sheath protects the metallic tendons of the bundle and provides a smooth appearance of the stay cable. 
     In certain cases, the sheath is in the form of a continuous, integral tube which extends from the lower anchoring point to the upper anchoring point of the stay cable. The tendons are threaded, usually one by one or small groups by small groups, into the sheath before anchoring them at both ends. Examples illustrating such technology are described in U.S. Pat. Nos. 5,461,743 and 7,779,499. 
     In other cases, the sheath is made of segments following each other along the cable. Each segment can be made of several sectors assembled around the bundle of tendons. 
     U.S. Pat. No. 5,479,671 illustrates the latter kind of technology. It discloses a sheath made of segments supported by a rope running parallel to the load-bearing tendons of the stay cable. The sheath segments are supported independently of each other by the rope, i.e. no segment carries the weight of the segment(s) located above it. Such independence between the sheath segments is needed due to the large difference between the thermal expansion coefficients of the materials of which the tendons and the sheath are made. By attaching each sheath segment at a point of the supporting rope (and thus to the tendons since the thermal expansion coefficients of the ropes and the tendons are similar), elongations due to temperature variations are kept homogeneous between the segments. It results in reduced friction wear and fewer risks of exposing the tendons. The sheath segments are installed after the load-bearing tendons are anchored. The sheath segments are made of several sectors that are put around the bundle of tendons at the lower part of the stay and assembled along longitudinal joints. After a segment is assembled, it is attached to the supporting rope by means of fasteners operated from the outside of the sheath, and the supporting rope is pulled up to clear the space for installing the next sheath segment. After all the segments have been installed, the supporting rope is fixed near the upper anchorage of the cable. 
     Some construction works make use of very long and/or very inclined (e.g. close to vertical) structural cables, leading to a number of challenges:
         continuous sheaths cannot support their own weight;   the relative elongations due to the difference in the thermal expansion coefficients may cause an excessive displacement at the top of the sheath;   the area needed for pre-fabricating or assembling the sheath may become too large;   handling of the sheath become complex and risky, in particular when it is lifted in windy environments.       

     While some of these challenges are addressed by the technology disclosed in U.S. Pat. No. 5,479,671, the situation could be improved. In addition, the technology has limitations since it cannot be used if the tendons are not installed before the sheath. Also, it is not advantageous that the fasteners remain visible outside of the sheath and may cause water to leak into the sheath. 
     An object of the present invention is to propose another kind of sheath design for structural cables. 
     A further object is to propose a sheath design that is well suited for very long structural cables, and/or very inclined structural cables (e.g. close to vertical). 
     Still a further object is to propose a process for installing long and/or very inclined structural cable. 
     Still a further object is that, if needed, members supporting the sheath can be made replaceable during the lifetime of the construction work. 
     SUMMARY 
     Part or all of the above-mentioned objects are addressed by providing a sheath for a structural cable of a construction work, the structural cable having a path between an upper anchorage and a lower anchorage. The sheath comprises: 
     sheath segments assembled along the path of the structural cable; 
     at least one supporting rope extending along the sheath segments and having an upper end connected to the construction work adjacent to the upper anchorage; and 
     connectors for connecting the sheath segments to the at least one supporting rope. 
     The connectors are configured to block relative upward movement of the at least one supporting rope with respect to the sheath segments and to allow relative downward movement of the at least one supporting rope with respect to the sheath segments. 
     The sheath segments can be caused to travel upwards along the supporting rope, especially when the sheath is being installed, while they are maintained in their prescribed positions by the connectors during use. The supporting rope(s) and the connectors can be used to lift the sheath segments or to provide abutments for holding them at discrete positions when they are lifted by some other means. If a supporting rope needs to be replaced for maintenance, it can be pulled down while bringing a new supported rope coupled to its upper end. 
     The at least one supporting rope and the connectors may be located fully inside the sheath segments. 
     In an embodiment, the sheath segments are connected to the at least one supporting rope independently of each other by the connectors. Accordingly, a sheath segment does not have to bear the weight of the other sheath segments located above it. 
     A way of connecting first and second sheath segments independently of each other consists in providing a telescopic coupling between the first sheath segment and the second sheath segment assembled next to the first sheath segment along the path of the structural cable. The telescopic coupling comprises a first sleeve portion belonging to the first sheath segment and a second sleeve portion belonging to the second sheath segment and inserted into the first sleeve portion. At least one of the connectors may have a connector part secured to the first sheath segment at an inner surface of the first sleeve portion so as to receive a respective supporting rope. 
     In particular, a plurality of the connectors may have respective connector parts mounted on a collar fixed inside the first sleeve portion so as to receive a respective supporting rope extending through the first and second sheath segments. 
     To have a smooth aspect of the sheath, the first sleeve portion may have a same outer cross-section as main portions of the first and second sheath segments. 
     In an embodiment, each of the sheath segments has a duct in which at least one longitudinal channel is formed for housing the at least one supporting rope, the channel being separated by a wall from a main space of the duct provided for receiving load-bearing tendons of the structural cable. 
     If at least one supporting rope housed in the channel has connector parts of the connectors secured thereto at discrete locations, the channel is conveniently designed with a cross-section sufficient for allowing longitudinal movement of the connector parts secured to the supporting rope housed therein. 
     If a telescopic coupling is provided between a first sheath segment and a second sheath segment assembled next to the first sheath segment along the path of the structural cable, the telescopic coupling comprising a first sleeve portion belonging to the first sheath segment as an extension of the duct of the first sheath segment and a second sleeve portion belonging to the second sheath segment as an extension of the duct of the second sheath segment and inserted into the first sleeve portion, the channel formed in the duct of the second sheath segment may be extended in the second sleeve portion in alignment with the channel formed in the duct of the first sheath segment. 
     In an embodiment of the sheath, each of the connectors has a first connector part secured to a supporting rope, a second connector part secured to a sheath segment and a third connector part configured to block relative upward movement of the first connector part with respect to the second connector part and to allow relative downward movement of the first connector part with respect to the second connector part. 
     Another aspect of the present disclosure relates to a structural cable of a construction work, comprising: 
     an upper anchorage; 
     a lower anchorage; 
     load-bearing tendons extending along a path of the structural cable between the upper and lower anchorages; and 
     a sheath as defined above, in which the load-bearing tendons are housed. 
     Another aspect of the present disclosure relates to a method of installing a sheath for a structural cable of a construction work, the structural cable having a path between an upper anchorage and a lower anchorage, the sheath having a number N of sheath segments (N≥2). The method of installing the sheath comprises: 
     mounting at least one supporting rope with an upper end adjacent to the upper anchorage; and 
     for each integer n such that 1≤n≤N:
         inserting the at least one supporting rope into the n-th sheath segment;   connecting the n-th first sheath segment to a supporting rope; and   lifting the first to n-th sheath segments along the at least one supporting rope.       

     Connectors are provided to block relative upward movement of the at least one supporting rope with respect to the sheath segments and to allow relative downward movement of the at least one supporting rope with respect to the sheath segments. 
     In an embodiment of the method, each supporting rope has first connector parts secured thereto at discrete locations, and each sheath segment has at least one second connector part secured thereto. Each of the connectors is formed by associating a first connector part secured to a supporting rope, a second connector part secured to a sheath segment and a third connector part configured to block relative upward movement of the first connector part with respect to the second connector part and to allow relative downward movement of the first connector part with respect to the second connector part. 
     Connecting the n-th first sheath segment to a supporting rope for an integer n such that 1≤n≤N may comprise forming at least one connector (by associating a first connector part secured at a lowermost discrete location of a respective supporting rope, a second connector part secured to the n-th sheath segment and a third connector part. 
     Lifting the sheath segments for an integer n such that 1&lt;n≤N may comprise forming at least n−1 connectors by associating, for each integer j such that 1&lt;j&lt;n, a first connector part secured to a supporting rope at a (n−j+1)-th discrete location, starting from the lowermost discrete location, a second connector part secured to the j-th sheath segment and a third connector part. 
     An embodiment of the method comprises, for an integer n such that 1&lt;n≤N: 
     lowering the supporting rope while preventing downward movement of at least the (n−1)-th sheath segment; 
     inserting the at least one supporting rope into the n-th sheath segment; 
     forming an n-th connector by associating the first connector part secured at the lowermost discrete location of the supporting rope, a second connector part secured to the n-th sheath segment and a third connector part; and 
     pulling back up the supporting rope, thereby forming the at least n−1 connectors. 
     Supporting ropes mounted with respective upper ends adjacent to the upper anchorage may comprise: 
     an active rope that is lowered and pulled back up; and 
     a static rope used to prevent downward movement of the sheath segments when the active rope is lowered and pulled back up. 
     In such an embodiment, the sheath segments have second connector parts arranged to form connectors with first connector parts secured to the active rope and additional second connector parts arranged to form connectors with first connector parts secured to the static rope. 
     Assembling the sheath segments before installation of the load-bearing tendons makes it possible to use sheath segments having an integral cross-section. It may then be appropriate to hold the (n−1)-th sheath segment to restrict lateral movements thereof while inserting the at least one supporting rope into the n-th sheath segment and forming the n-th connector, for each integer n such that 1&lt;n≤N. In addition, it may be appropriate to tension at least one supporting rope to restrict lateral movements of the sheath segments while lifting the first to n-th sheath segments. 
     Another aspect of the present disclosure relates to a maintenance method for a structural cable of a construction work, the structural cable comprising: 
     an upper anchorage; 
     a lower anchorage; 
     load-bearing tendons extending along a path of the structural cable between the upper and lower anchorages; and 
     a sheath in which the load-bearing tendons are housed. 
     In that maintenance method, the sheath comprises: 
     sheath segments assembled around the load-bearing tendons along the path of the structural cable; 
     at least two supporting ropes extending along the sheath segments, each having an upper end connected to the construction work adjacent to the upper anchorage; and 
     connectors for connecting the sheath segments to the at least one supporting rope. 
     The connectors are arranged to block relative upward movement of the at least one supporting rope with respect to the sheath segments and to allow relative downward movement of the at least one supporting rope with respect to the sheath segments. 
     The maintenance method comprises replacing a first one of the supporting ropes by a second supporting rope while the assembled sheath segments are supported by at least another one of the supporting ropes. Replacing the first supporting rope by the second supporting rope comprises: 
     disconnecting the upper end of the first supporting rope; 
     coupling a lower end of the second supporting rope to the upper end of the first supporting rope; 
     pulling a lower end the first supporting rope to remove the first supporting rope while installing the second supporting rope; and 
     connecting an upper end of the second supporting rope adjacent to the upper anchorage. 
     In an embodiment of the maintenance method, each of the first and second supporting ropes has first connector parts secured thereto at discrete locations, and each sheath segment has second connector parts secured thereto. Before replacing the first supporting rope by the second supporting rope, connectors are formed by associating a first connector part secured to the first supporting rope, a second connector part secured to a sheath segment and a third connector part configured to block relative upward movement of the first connector part with respect to the second connector part and to allow relative downward movement of the first connector part with respect to the second connector part. After replacing the first supporting rope by the second supporting rope, new connectors are formed by associating a first connector part secured to the second supporting rope, a second connector part secured to a sheath segment and a third connector part configured to block relative upward movement of the first connector part with respect to the second connector part and to allow relative downward movement of the first connector part with respect to the second connector part. The first connector parts secured to the first and second supporting ropes travel through the second connector parts secured to the sheath segments when the lower end of the first supporting rope is pulled downward for replacing the first supporting rope by the second supporting rope. 
    
    
     
       BRIEF DESCRIPTION THE DRAWINGS 
       Other features and advantages of the invention disclosed herein will become apparent from the following description of non-limiting embodiments, with reference to the appended drawings, in which: 
         FIGS. 1 and 2  are schematic side views of a stay cable; 
         FIG. 3  is a sectional schematic view of a possible arrangement of connectors between sheath segments and supporting ropes; 
         FIGS. 4 and 5  are sectional schematic views of particular embodiments of connectors usable in some sheath arrangements; 
         FIGS. 6 and 7  are perspective schematic views showing cooperating parts of such connectors; 
         FIG. 8  is a perspective view of assembled sheath segments in an embodiment; 
         FIGS. 9 and 10  are cross-sectional views of a sheath segment along planes IX-IX and X-X shown in  FIG. 8 ; 
         FIGS. 11 and 12  are perspective views showing part of  FIG. 8  in more details; 
         FIGS. 13-17  are diagrams illustrating different steps of an installation method of a cable sheath in an embodiment of the present invention; and 
         FIG. 18  summarizes installation steps in a single diagram. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a stay cable  10  which is a structural cable extending between two parts  11 ,  12  of a construction work. The first part  11  is at a higher position than the second part  12 . For example, the first part  11  belongs to a tower, while the second part  12  belongs to a foundation to stabilize the tower. Alternatively, the first part  11  may belong to a pylon, while the second part  12  belongs to some structure suspended from the pylon  11 . 
     The construction work typically includes a number of stay cables  10 , only one of them being shown in  FIG. 1 . 
     The structural cable  10  has a load-bearing part  15  which consists of a bundle of tendons disposed parallel to each other. For example, the load-bearing tendons may be strands of the same type as used to pre-stress concrete structures. Each strand may optionally be protected by a substance such as grease or wax and individually contained in a respective plastic sheath (not shown). 
     Each stay cable  10  may have a length of up to several hundred meters, and include a few tens of tendons. 
     The load-bearing tendons are anchored at both ends of the bundle  15  using an upper anchoring device  16  mounted on the first part  11  of the construction work and a lower anchoring device  17  mounted on the second part  12  of the construction work. Between the two anchoring devices  16 ,  17 , the bundle of tendons  15  follows a catenary curve due to its own weight and the tensile force maintained by the anchoring devices. The anchoring devices  16 ,  17  are positioned on the first and second parts  11 ,  12  by taking into account the pre-calculated catenary curve of each stay cable  10 , that defines its path. 
     The bundle of tendons  15  is contained in a protective sheath  20  typically made of plastic or metallic material. 
     To ensure good dynamic properties of the stay cable  10 , it may be useful to give the sheath  20  a regular profile, typically with a circular cross-section. The sheath  20  may also be provided with specific surface structure, known in the art, e.g. double helical ribs, to improve its behavior in the presence of a combined action of rain and wind. 
     The sheath  20  is made of a plurality of segments  21  assembled along the path of the structural cable  10 . The length of each sheath segment  21  is selected as a function of the design of the stay cable structure. It is possible to use segments  21  having a nominal length L, e.g. of the order of 10 to 100 m or more, for building different stay cables  10  of the construction work. One of the segments can then be cut depending on the length of the particular stay cable. Alternatively, the length L of the different segments of a given stay cable can be selected according to the total length set for the stay cable. 
     In the example illustrated in  FIG. 1 , the lower end of the sheath  20  is adjacent to the upper end of a guide tube  25  through which the bundle of tendons  15  passes near the lower anchoring device  17 . The upper end of the sheath  20  penetrates into another tube  26  disposed on the first part  11  of the construction work, through which the upper end of the bundle of tendons  15  passes to reach the upper anchoring device  16 . The second end of the first sheath  20  is not connected to the tube  26 , so that it can slide therein when the tendons  15  and the upper sheath segment  21  undergo different expansion or contraction on account of the thermal expansion coefficients of their materials. The arrangement prevents run off water from flowing inside the upper sheath segment  21 . 
     The weight of the plastic sheath  20  is taken up by one or more supporting ropes  30  which are shown in the diagram of  FIG. 2  where the lateral dimensions of the stay cable  10  are exaggerated to show more clearly how the sheath segments  21  (shown with broken lines) are suspended. 
     Each supporting rope may be made of stainless steel. It extends along the series of sheath segments  21 , and has an upper end connected to the construction work at or near the upper anchorage  16  where the loadbearing tendons  15  are anchored. 
     In an exemplary configuration discussed here, the supporting ropes  30  are located inside the sheath segments  21 , as well as connectors  32  shown diagrammatically in  FIG. 2 . 
     Each connector  32  forms the interface between a respective rope  30  and a respective sheath segment  21 . It is configured to block relative upward movement of the rope  30  with respect to the sheath segment  21  and to allow relative downward movement of the rope  30  with respect to the sheath segment  21 . The connector  32  may also be made of stainless steel. 
     The sheath segments  21  are connected to the supporting rope  30  independently of each other by the connectors  32 . By way of example, each 100 m segment may be submitted to a compression effort lower than 2.0 MPa. 
     In the example shown in  FIG. 3 , each connector  32  has a first part  32 A secured to a supporting rope  30  and a second part  32 B secured to a sheath segment  21 . 
     When the sheath  20  is assembled, the second connector part  32 B is in abutment against the first connector part  32 A, so that the sheath segment  21  to which the second connector part  32 B is secured is supported by the rope  30 , as shown on the left-hand side of  FIG. 3 . 
     However, as shown on the right-hand side of  FIG. 3  (arrow F), the second connector part  32 B does not prevent downward movement of the first connector part  32 A secured to the supporting rope  30 . The first connector part  32 A is also allowed to travel downward through the second connector part  32 B. 
       FIGS. 4 and 5  illustrate possible arrangements of connectors  32  that have such mechanical behavior. In those examples, the first connector part  32 A is a metallic sleeve which is fixed to the supporting rope  30 , for example by swaging, while the second connector part  32 B secured to the sheath segment  21  has a through hole  33  receiving the supporting rope  30 . The cross-section of the through hole  33  is large enough to let the supporting rope  30  and the first connector parts  32 A travel through it. In order to block relative upward movement of the rope  30  with respect to the sheath segment  21 , the connector  32  includes a third connector part  32 C that is mounted on one of the first and second parts  32 A,  32 B to interact with the other one of the first and second parts  32 A,  32 B. 
     Different arrangements of the third connector parts  32 C are possible. In the example shown in  FIG. 4 , the third connector part  32 C includes a pair of pawl members  34  articulated near the top of the second connector part  32 B and pushed towards the supporting rope  30  by springs  35  near the bottom of the second connector part  32 B. When the supporting rope  30  moves down with respect to the sheath segment  21  (or the segment  21  moves up with respect to the rope  30 ) and the first connector part  32 A reaches the second connector part  32 B, the first connector part  32 A pushes the pawl members  34  outwardly against the springs  35  so that it can travel further down as shown by the arrow F. On the other hand, when the first connector part  32 A reaches the second connector part  32 B from its bottom side, the end surfaces of the pawl members  34  provide an abutment that locks the supporting rope  30  which is thus prevented from traveling further up (arrow F′ in  FIG. 7 ). 
       FIG. 5  illustrates an alternative arrangement of the connector  32 , in which the pawl members  34  forming the third connector part  32 C are articulated on the first connector part  32 A secured to the supporting rope  30 . One or more springs  35  push outward the top end of the pawl members  34 . When the supporting rope  30  moves down with respect to the sheath segment  21  and the first connector part  32 A reaches the second connector part  32 B, the second connector part  32 B pushes the pawl members  34  inwardly against the springs  35  so that it can travel further down as shown by the arrow F. On the other hand, when the first connector part  32 A reaches the second connector part  32 B from its bottom side, the end surfaces of the pawl members  34  are spread out and provide an abutment that locks the supporting rope  30  which is thus prevented from traveling further up. 
       FIGS. 6 and 7  illustrate connector parts in an arrangement as shown diagrammatically in  FIG. 4 . The swaged first connector part  32 A can have a beveled lower surface  36  to facilitate its penetration into the through hole  33  when the supporting rope  30  travels downward and reaches the second connector part  32 B, and a straight upper surface  37  for abutment on the pawl members  34  forming the third connector part  32 C. 
     It will be appreciated that many other connector arrangements providing the required functionality can be considered. It is possible, in some particular embodiments, that the connector  32  does not have any part secured to the supporting rope  30 . For example, locking the supporting rope  30  against relative upward movement with respect to a sheath segment  21  can be performed by (conical) jaws interacting with a connector part secured to the sheath segment to lock by a wedge action. Some mechanism may then be needed to unlock the conical jaws when the supporting rope  30  has to travel down with respect to the sheath segments  21 . 
     It may, however, be preferred to provide first connector parts  32 A fixed in advance on the supporting ropes  30  at discrete locations. The intervals between the discrete locations correspond to the lengths L of the individual sheath segments  21 . 
     It is advantageous that the connectors  32  have no part protruding out of the sheath  20 . This avoids impacting the visual aspect of the stay cable  10 . It is also preferable for water tightness of the sheath. 
     The diagram of  FIG. 3  shows a telescopic coupling of two adjacent sheath segments  21 . In this example, the lower end of a first sheath segment located above a second sheath segment has a first sleeve portion  38  that is widened in order to receive a second sleeve portion formed by the upper end of the second sheath segment to provide the telescopic coupling. The first and second sleeve portions can have a relative movement along the axis of the cable in order to absorb thermal expansion or contraction of the sheath segments  21 . Advantageously, the second connector parts  32 B are located in the sleeve portion  38 , in its upper region where the cross-section of the sheath segment is widened (see also the diagram of  FIG. 2 ). 
       FIGS. 8-12  illustrate another embodiment of the telescopic coupling. Here, each sheath segment  21  includes a duct  40  that makes up most of the length of the sheath segment, a lower sleeve portion  41  and an upper sleeve portion  42 . The example includes four supporting ropes  30  extending parallel to the load-bearing tendons of the structural cable. 
     In the embodiment of  FIGS. 8-12 , the lower sleeve portion  41  of a sheath segment  21  has the same outer cross-section as the main portions (i.e. ducts)  40  of the two adjacent sheath segments, while the upper sleeve portion  42  of the underlying segment  21  has a smaller outer cross-section and is inserted in the sleeve portion  41 . Such a sheath design has no bulging part, which may be preferred for aesthetic reasons. 
     In the example of sheath segments of 100 m, the sleeve portions  41 ,  42  may be dimensioned to provide a 1.5 m stroke. For HDPE ducts, this is enough to accommodate thermal expansion or contraction in a temperature range of about 80° C. 
     Two longitudinal channels  48  are formed within the circular inner cross-section of the duct  40 . Two of the supporting ropes  30  are housed in each channel  48 . It will be observed that this is merely an example. There could be only one channel  48 , or more than two channels. There could also be one supporting rope  30  per channel  48 , or more than two. A symmetrical configuration of the channels  48  and supporting ropes  30  will generally be preferred to minimize moments when the sheath segments are lifted. 
     A wall  49  of each channel  48 , which may be co-extruded with the duct  40 , prevents contacts of the supporting ropes  30  with the load-bearing tendons  15  received in the main space of the duct  40 , in order to avoid potential damage of the tendons  15  or their individual sheaths. 
     In order to allow relative (upward) movements of the sheath segments  21  along the supporting ropes  30 , the cross-section of the channel  48  must be sufficient to allow the first connector parts  32 A swaged on the ropes  30  to circulate. 
     As shown by numeral  48 ′ in  FIG. 8 , the channel  48  formed in the duct  40  of a sheath segment  21  is extended in the upper sleeve portion  42  of the underlying sheath segment  21 , in alignment with the channel formed in the duct  40  of the underlying sheath segment. The channel  48  is, however, interrupted in the lower sleeve portion  41  in order to allow the telescopic action of the sleeve portions. In that limited interval, no contacts between the supporting ropes  30  and the load-bearing tendons  15  are possible. 
     The sheath according to the present invention may incorporate cavities for mounting equipment such as, e.g., light sources, as described in the international patent application No. PCT/IB2017/000214 filed on Feb. 3, 2017 and published as WO 2018/130271 A1. Such cavities may be formed together with the above-mentioned channels  48 . It will also be noted that a sheath according to the present invention may have a double-walled structure as disclosed in the international patent applications Nos. PCT/IB2016/001314 filed on Jul. 27, 2016 and published as WO 2018/020288 A1 and PCT/IB2016/001978 filed on Nov. 18, 2016 published as WO 2018/020289 A1. 
       FIGS. 8 and 10-12  show pawl boxes forming the above-mentioned second connector parts  32 B and third connector parts  32 C. The pawl boxes  32 B/C are secured to the lower sleeve portion  41  of the sheath segment  21  by means of a collar  50 . The collar  50  is fixed inside the lower sleeve portion  41 , at its upper end. The pawl boxes  32 B/C are mounted on the collar  50  so that the through holes  33  described with reference to  FIGS. 4-7  are aligned with the channel  48  of the duct  40 . 
       FIGS. 13-18  illustrate an exemplary method for installing a cable sheath  20  of the type described above. In the example, two supporting ropes  30  are shown. It will be appreciated that the number of ropes  30  is not a limitation. For example, if there are four ropes as shown in  FIGS. 8-12 , they can be operated in pairs with the same method. 
     In the following description, one of the two supporting ropes  30  is referred to as an active rope  30 A, while the other one is referred to as a static rope  30 S. When there are two ropes per channel  48  ( FIGS. 9-10 ), one of them can be an active rope while the other is a static rope. 
     During the installation, static ropes will stay fixed, while the active ropes will make synchronized trips back and forth to grab the sheath segments  21  one by one and lift them together. To do so, during the installation, the static ropes  30 S are directly connected to their final upper anchorage ( 60  in  FIG. 18 ), while the active ropes  30 A are connected to a winch  61  which is used to move them back and forth. 
     Before the lifting operation, the supporting ropes  30  (with their connector parts  32 A installed at factory) are inserted inside the first sheath segment  21  by its top side until the lower connector part  32 A of each rope goes through the pawl box  32 B/C at the other side of the duct  40 , thus forming a connector  32 . To facilitate threading of the ropes  30  into the through holes  33  of the pawl boxes  32 B/C, it is convenient to provide the sheath segments  21  with temporary links extending along the length of the segment and inserted into the through holes  33  and the channels  48  before assembling the lower sleeve portion  41  with the duct  40 . When the sheath segment  21  is installed, the end of the temporary link on the upper side of the segment is coupled to the lower end of the supporting rope, and the supporting rope is slid into the channel and the through holes  33  by pulling on the temporary link. 
     The initial step of mounting the first sheath segment  21  on the supporting ropes  30  ( FIG. 13 ) can be performed on the ground. Then, the upper ends of the supporting ropes  30  are lifted and attached to the upper anchorage  60  (for the static rope(s)  30 S) and to the winch  61  (for the active rope(s)  30 A). 
     At this point, the first sheath segment  21  is also lifted along the ropes  30 , with its connectors  32  locked, to the position illustrated in  FIGS. 14 and 18 ( a ), which is referred to as an assembling position since it is the position where the sheath segments  21  will be successively juxtaposed. The assembling position is noted ‘A’ in  FIG. 18 , where ‘G’ denotes the ground level. 
     The winch  61  can be operated to reel and unreel the active rope  30 A. 
     In order to withstand the induced catenary force and to ensure secure conditions for installing the sheath  20 , even in windy weather conditions, some tension is applied to the supporting ropes  30  by pulling the sheath segment  21  parallel to the direction of the segment at the assembling position A. This can be done, for example, by means of one or more tensioned slings  62  or legs connected to the ground and to a collar or some other means that grabs the bottom part of the sheath segment  21  located at the assembling position A. Pulling down the segment  21  by means of the sling  62  applies tension to the static rope  30  through the engagement of the pawl box  32 B/C with the first connector part  32 A of the static rope  30 S. 
     It is then possible to bring and assemble the next sheath segments  21  by moving the active rope  30 A. In the following, ‘N’ denotes the number of sheath segments  21  of which the sheath  20  is made. Each of the second to N-th sheath segment  21  can be assembled in the same manner, illustrated in  FIGS. 15-17 . The steps shown in  FIGS. 15-17  for n=2 are readily generalized to any integer n such that 1&lt;n≤N. 
     While the (n−1)-th sheath segment  21  is at the assembling position A and pulled by the sling  62 , the ground area G is available for inserting the free ends of the supporting ropes  30  into the next (n-th) sheath segment  21  by its top side, for example with the help of temporary links ( FIGS. 15 and 18 ( b )). The active rope  30 A is unreeled from the winch  61  (and possibly pulled from its lower end) until its first connector part  32 A secured at the lowermost discrete location reaches the pawl box  32 B/C of the n-th sheath segment  21  ( FIGS. 16 and 18 ( c )), which connects the n-th sheath segment  21  to the active rope  30 A. During this process, the first connector part  32 A′ of the active rope  30 A, which is at the second discrete location starting from the lowermost one, has also traveled through the pawl box  32 B/C of the (n−1)-th sheath segment  21 . 
     Once a new connector  32  is formed by associating the lowermost first connector part  32 A of the active rope  30 A with the pawl box  32 B/C of the n-th sheath segment  21 , the active rope  30 A is reeled back up by activating the winch  61 . When the connector part  32 A′ of the active rope  30 A reaches the pawl box  32 B/C of the (n−1)-th sheath segment  21 , that (n−1)-th segment starts moving up, along with the n-th segment. The first to (n−2)-th segments, if n&gt;2, are lifted at the same time, new connectors  32  being formed with each of them. Before that point is reached, the static rope  30 S is connected to the ground and the sling  62  is removed, as shown in  FIGS. 17 and 18 ( d ). The rising movement of the (n−1)-th sheath segment  21  clears the assembling position A which is then occupied by the n-th sheath segment  21 . 
     While the first to (n−1)-th sheath segments  21  are lifted by operating the winch  61 , n−1 connectors  32  are formed with the active rope  30 A and also with the static rope  30 S by associating, for each integer j such that 1&lt;j&lt;n, a first connector part  32 A secured to a rope  30 A or  30 S at the (n−j+1)-th discrete location and a pawl box  32 B/C of the j-th sheath segment  21 . 
     The reeling action of the winch  61  may lift the n-th sheath segment  21  slightly beyond the assembling position A, as shown in  FIG. 18( d ) . If this happens, the winch  61  is activated again move down the active rope  30 A until the first connector parts  32 A of all the ropes  30  are aligned. 
       FIGS. 18( e ) and 18( f )  correspond to  FIG. 18( c )  and  FIG. 18( d ) , respectively, for n=3.  FIG. 18( g )  corresponds to  FIGS. 18( c ) and 18( e )  for n=4. The above-described process is repeated up to the N-th sheath segment  21 . 
     Once all the sheath segments  21  have been installed, the active rope  30 A can be fixed in its final position by removing the winch  61  and replacing it by a permanent anchorage  60 . In certain cases, the active rope(s)  30  may be used only temporarily, or as a tool to successively install different stay cables of the construction work. In such a case, the active rope  30 A may be removed after installation of the N sheath segments  21  by pulling it from its lower end. 
     In the above-described exemplary embodiment of the installation process, the first to (n−1)-th sheath segments  21  are lifted along the supporting ropes  30 , so as to clear the assembling position A, at the same time as the n-th sheath segment  21  is brought to the assembling position A. The lifting and bringing actions are completed simultaneously by means of the active rope  30 A. Variations of the method where the two actions are separated can also be considered. 
     The above-described installation method is suitable when the sheath segments  21  have an integral cross-section. However, other methods are applicable within the scope of the present invention, including methods in which the sheath segments are made of sectors assembled together on site. For example, such sheath segments made of sectors can be assembled around the bundle of load-bearing tendons  15  which has been installed and anchored beforehand (like in U.S. Pat. No. 5,479,671). In such an embodiment, it may be sufficient to use only one supporting rope for independent suspension of the sheath segments. 
     A nice feature of the above-described sheath arrangement is that it makes it possible to replace one or more of the supporting ropes  30  during the lifetime of the construction work, if needed. 
     Such a replacement phase may be performed as follows:
         the old rope  30  to be replaced is disconnected from its anchorage  60 ;   its upper end is coupled to the lower end of a new rope having first connector parts  32 A distributed at the prescribed locations along its length;   the lower end the old rope is pulled to remove it while installing the new rope;   the upper end of the new rope adjacent is connected in place of the old rope.       

     The connectors  32  that were formed by associating first connector parts  32 A secured to the old rope with pawl boxes  32 B/C of the sheath segments  21  disappear in the replacement phase as the first connector parts  32 A of the old rope travel through the pawl boxes  32 B/C while the lower end the old rope is pulled. Instead, new connectors  32  are formed by associating the first connector parts  32 A secured to the new rope with the pawl boxes  32 B/C of the sheath segments  21  after the first connector parts  32 A of the new rope have travelled through the pawl boxes  32 B/C when the new rope has been pulled all the way down. 
     It will be appreciated that the embodiments described above are illustrative of the invention disclosed herein and that various modifications can be made without departing from the scope as defined in the appended claims. 
     For example, the invention is applicable to structural cables other than stay cables.