A method of manufacturing a sheath for a structural cable

Abstract

For manufacturing a sheath for a structural cable of a construction work, a method is proposed using two sheath segments (21) comprising respective thermoplastic tubes having a same outer shape. The method comprises heat welding respective axial ends of the sheath segments (21). During the heat welding step, a protrusion (40) is formed at the axial ends of the sheath segments (21). The protrusion (40) protrudes radially beyond the outer shape of the thermoplastic tubes.

Description

A METHOD OF MANUFACTURING A SHEATH FOR A STRUCTURAL CABLE

[0001] The present invention relates to sheaths used to contain structural cables in construction works, designed in consideration of climate conditions to which the work is exposed.

[0002] Typically, it applies to stay cables used to suspend structures such as roofs or bridge decks, or to stabilize structures such as towers or masts.BACKGROUND

[0003] Known structural cables comprise a bundle of tendons disposed parallel to each other and contained in a collective sheath.

[0004] A typical issue when dealing with this type of cable arises when the weather is cold and humid. Ice, snow or frost may accumulate on the sheath and become relatively thick. When the temperature rises after an icing episode, ice chunks may detach and fall from the cable, causing risks of damages to vehicles or to the infrastructure or risks of hurting people.

[0005] Several strategies have been developed to cope with this issue.

[0006] WO 2018 / 196966 A1 combines a conventional composite sheath, having active heating elements, with a helical ridge pattern as disclosed in WO 2014 / 001514 A1. The ridges on the sheath are expected to retain the ice, so as to limit the risk of ice falling in periods when the heating elements are not activated. The improved retention of ice and snow by the ridge pattern allows targeted lane closures on the cable-stayed bridge for the active de-icing, thus reducing the impact on traffic flow once a significant accumulation of ice is observed on the stays. The document notes that the ridge pattern causes weaknesses in the ice layer when the active system is powered, so that the ice falls as smaller fragments. However, these fragments are still fairly large (several tens of cm) and thick. The fragments are typically not smaller than the pitch of the helical ridge pattern and the diameter of the sheath. They fall quickly once the surface of the sheath starts heating upon turning on the active system, because the weaknesses of the ice layer are localized at the ridges and promote indentation of fairly large pieces before a substantial thickness of ice has molten. Such fragments may still cause damage or injury when falling. For this reason, special protective measures such as traffic closures are required.

[0007] WO 2020 / 144489 A1 discloses a passive solution to this issue. This document describes a sheath for a structural cable whose outer surface has a roughness texture with dimensions in a range of 0.1 mm to 2 mm perpendicular to the outer surface of the sheath to promote retention of frozen water.

[0008] Other solutions are needed to deal with ice or snow accumulations on the sheaths of structural cables in order to keep in place or fragment chunks of ice, snow or frost on the cable surface despite an adhesion failure at the interface between the sheath surface and the accumulated ice or snow and thus allowing to delay the fall of chunks while reducing their size.SUMMARY

[0009] The present document discloses a. method of manufacturing a sheath for a structural cable of a construction work. The method comprises:- providing two sheath segments comprising respective thermoplastic tubes having a same outer shape; and- heat welding respective axial ends of the sheath segments.

[0010] During the heat welding step, a protrusion is formed at the axial ends of the sheath segments. The protrusion protrudes radially beyond the outer shape of the thermoplastic tubes.

[0011] The method provides protrusions that may be designed for efficient prevention of risks due to frozen water accumulation on cable sheaths. The protrusions compartmentalize accumulation areas of ice on the sheath, thus delaying the fall of frozen water and reducing the size of any falling chunks. The parameters for forming such protrusions can be controlled relatively easily to ensure a high quality of the sheath. It is not necessary to design and mount protrusions around the sheath post-manufacturing.

[0012] Embodiments of the above-defined method further include one or more of the following features: the protrusion is in the form of a ridge around the sheath segments; the ridge extends perpendicularly to the sheath segments; the ridge protrudes from a base part at an outer surface of the thermoplastic tube to a distal end and has a portion in which a width taken parallel to a longitudinal direction of the thermoplastic tube increases towards the distal end; the ridge protrudes from a base part at an outer surface of the thermoplastic tube to a distal end and has, at least in a part adjacent to the distal end, a constant width parallel to a longitudinal direction of the thermoplastic tube; the protrusion protrudes from a base part at the outer surface of the thermoplastic tube to a distal end by a height in a range of 2% to 15% of an outer diameter of the thermoplastic tube; the method further comprises arranging a mold around the axial ends of the sheath segments before heat welding the axial ends of the sheath segments, the mold having a shape complementary to the protrusion to be formed; the mold comprises a pair of collars each having an end surface having a groove formed therein, wherein arranging the mold comprises fitting the collars around the axial ends of the sheath segments, with the respective end surfaces opposing each other, wherein heat welding the axial ends of the sheath segments comprises bringing together the end faces of the collars to let fused thermoplastic material flow into the grooves; the fused thermoplastic material flowing into the grooves is thermoplastic material that was part of the thermoplastic tubes; the two sheath segments are aligned with each other, with a first gap between respective axial end surfaces of the sheath segments, the mold being arranged around the axial ends of the sheath segments with a second gap between the opposing end surfaces thereof, the second gap being wider than the first gap; heat welding the axial ends of the sheath segments comprises inserting a heating plate into the first gap to fuse thermoplastic material at the axial end surfaces of the sheath segments;the method further comprises removing an internal weld bead formed at an inner face of the sheath segments when heat welding the axial ends of the sheath segments.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Other features of the present disclosure will become apparent from the following description of non-limiting embodiments, with reference to the appended drawings, in which:Fig. 1 is a schematic side view of a stay cable;Fig. 2 is a perspective view of a sheath comprising protrusions according to a first embodiment;Fig. 3 is a top view of the sheath shown in Fig. 2;Fig. 4 is a longitudinal section view of a first example of a protrusion in an embodiment as shown in Fig. 2;Fig. 5 is a longitudinal section view of a second example of a protrusion;Fig. 6 is a perspective view of a sheath comprising protrusions according to a second embodiment;Fig. 7 is a longitudinal section view of a protrusion in an embodiment as shown in Fig. 6;Figs. 8-15 are longitudinal section views of sheath segments at different steps of an example of manufacturing method of a sheath; andFig. 16 is a longitudinal section view of another example of a protrusion.DESCRIPTION OF EMBODIMENTS

[0014] Fig. 1 shows a structural cable 10 having a sheath 20 configured according to the present disclosure.

[0015] The cable 10 is, for example, a stay extending along an oblique path between first and second parts 12, 14 where it is anchored using respective anchoring devices 16, 18. The stay cable is used to suspend the second part 14 (e.g., a bridge deck) from the first part 12 (e.g., a pylon), or to stabilize a tall structure including the first part 12 from the ground or some lower structure forming the second part 14.

[0016] The structural cable 10 comprises a bundle of tendons 22 disposed parallel to each other and contained in a collective sheath 20. For example, the bundled tendons may be steel strands each protected by a substance such as grease or wax and individually contained in a respective plastic sleeve.

[0017] The collective sheath 20 forms a protective cover for the bundle of tendons 22. It is in the form of a duct which internally defines a cavity running along the length of the cable 10 and within which the bundle of tendons 22 is arranged. The cross-section of the sheath 20 is typically circular. Other shapes, e.g. polygonal, elliptical, etc., are possible. The collective sheath 20 extends along an axis A (Figs. 2-3).

[0018] The cable 10 may have a length of up to several hundred meters. Without limitation, the bundle may include a few tens of tendons 22.

[0019] The sheath 20 is typically made of plastic material such as high-density polyethylene (HDPE).

[0020] The sheath 20 may be formed by connecting a plurality of segments 21 one after the other. For connecting two adjacent segments 21 to each other, a known technique is mirror welding. It consists in locally heating and fusing the plastic material of the sheath at the ends of two adjacent segments 21 and bringing those two ends together for welding the two segments 21 . Another possibility is to have a telescoping interface between two adjacent sheath segments 21.

[0021] Each segment 21 may be formed by assembling two or more shells together. In such a case, the sheath 20 can be installed on the bundle of tendons 22 after the tendons have been mounted and anchored to the structure.

[0022] Alternatively, each segment 21 (or the whole sheath 20 if it is made of one piece of plastic material) is provided as an integral duct section. There are different possible mounting techniques for such a sheath 20.

[0023] In one technique, the plastic sheath 20 is laid on the ground, or bridge deck and, after threading the tendons 22 therein, the upper end of the cable thus assembled is hoisted to be connected to the upper anchoring device 16 at the first part 12, and the lower end is connected to the lower anchoring device 18 at the second part 14.

[0024] In another technique, the sheath 20 is first mounted along the oblique path of the cable 10, and the tendons 22 are subsequently threaded, one after the other or all together, into the sheath for connection to the anchoring devices 16, 18.

[0025] In yet another technique, the tendons 22 are first connected to the upper anchoring device 16 at the first part 12 and the sheath segments 21 are pushed up one after the other from the lower end of the cable to form the sheath 20 before connecting the first (supporting) tendons 22 to the lower anchoring device 18.

[0026] The outer surface 24 of the sheath 20 is exposed to the environment. When the weather is cold and humid, ice, snow or frost (hereafter referred to collectively as ‘frozen water’) may accumulate on the sheath. In the high parts of the cable, at least, it is preferable to take measures to minimize the risk that chunks of accumulated frozen water fall, in order to avoid damages or injuries.

[0027] Such measures include protrusions formed on the outer surface of the sheath 20. The protrusions are designed to enhance retention of frozen water on the sheath.

[0028] In the embodiment illustrated in Figs. 2-3, the protrusions 26 are in the form of pins and distributed in a discrete pattern on the outer surface 24 of the sheath 20. Each pin 26 extends radially from a base part 28 at the outer surface 24 of the sheath 20 to a distal end 30.

[0029] In the example of Fig. 4, an individual pin 26 has a frusto-conical shape. Perpendicularly to the radial direction, it has a width w which increases from the base part 28 to the distal end 30. The width w is shown to increase linearly from a minimum value w1 at the base part 28 (e.g., 5 mm < w1< 20 mm) to a maximum value w2 at the distal end 30 (e.g., 6 mm < w2 < 25 mm). It will be appreciated that the pins 26 may also have curved side surfaces (concave or convex).

[0030] The pins 26 have a height h, from the base part 28 to the distal end 30 perpendicular to the outer surface 24 of the sheath, which may be in a range of 2% to 15% of an outer diameter D of the sheath 20. For example, the height h is between 4 mm and 20 mm.

[0031] The pins 26 shown in Fig. 4 have rotational symmetry around the radial direction, i.e. their cross-section is circular. Other cross-sectional shapes, such as polygonal or elliptical, are also possible.

[0032] Fig. 5 illustrates an alternative example in which only a portion of the height of a pin 26 has the width w increasing towards the distal end 30. Beyond that portion, the pin 26 has a convex end portion 32.

[0033] The pins 26 have shapes and dimensions adapted to maintain the aerodynamic properties of the sheath 20, so as to limit drag forces acting on the profile of a cable covered with such a sheath 20.

[0034] The sheath 20 shown in Fig. 2 also has a pair of parallel helical ribs 36 configured to increase the resistance of the sheath 20 to the combined effects of rain and wind.

[0035] The helical ribs 36 may be conventionally formed by affixing two HDPE beads to the outer surface of the sheath 20.

[0036] Typically, the helical ribs 36 have a height measured perpendicularly to the outer surface 24 of the sheath 20 that is in a range of 1 mm to 3 mm. Typically, the helical ribs 36 have a width taken parallel to the outer surface of the sheath 20 in a range of 2 mm to 5 mm.

[0037] The helical ribs 36 may have a constant pitch P. The pitch P may be between 3 to 6 times the outer diameter D of the sheath 20. In particular, the pitch P is between 30 cm and 100 cm. In Fig. 2, a spacing between the two helical ribs 36 along the axis A of the sheath 20 is half of the pitch P of the helical ribs 36.

[0038] In an embodiment shown in Fig. 2, each pin 26 sits on a helical rib 36. The pins 26 are evenly spaced along the helical rib 36.

[0039] Referring to Fig. 3, a longitudinal spacing S1 between two consecutive pins 26 along the axis A of the sheath 20 may be is a multiple of the half-pitch P / 2 of the helical ribs 36. A longitudinal spacing S2 between two consecutive pins 26 along a helical rib 36 and projected on the axis A is typically S1 / 2. A radial spacing S3 between two adjacent pins 26 along a helical rib 36 and projected on a straight line perpendicular to the axis A is typically between S2 / 2 and S2.

[0040] Adapting the density of the pin pattern allows controlling the individual mass of frozen water chunks 38 (shown within a dashed line in Fig. 3) in the melting phase.

[0041] The pins 26 may be formed by welding to the outer surface 24 of the sheath 20.

[0042] In the above-described embodiments of Figs. 2-5, the protrusions 26 are pins at discrete locations on the sheath 20, and both of their dimensions parallel to the outer surface 24 increase towards the distal end 30.

[0043] Alternatively, only one dimension of a protrusion parallel to the outer surface 24 increases towards the distal end 30. In particular, if the protrusion is pin-shaped, it may be the dimension parallel to the longitudinal axis A of the sheath.

[0044] Another option, discussed below with reference to Figs. 6-7 involves using protrusions 40 that have some extension along a first direction parallel to the outer surface 24 of the sheath 20, and that have their dimension increasing, towards their distal ends, along a second direction perpendicular to the first direction.

[0045] In the second embodiment of Figs. 6-7, the protrusions are in the form of annular ridges 40 perpendicular to the longitudinal axis A of the sheath 20. It is also possible to arrange the ridges with some non-square angle (e.g., between 70° and 90°) with respect to the longitudinal axis A.

[0046] The ridges 40 may be formed at regular intervals along the longitudinal axis A. The interval between two consecutive ridges restricts the size of frozen water accumulations at the outer surface 24 of the sheath 20.

[0047] Each ridge 40 has a height h defined from its base part 28 to its distal end 30, perpendicular to the outer surface 24 of the sheath 20 (Fig. 6), which is typically between 2% and 15% of the outer diameter D of the sheath 20.

[0048] The ridge 40 may have a width w parallel to the outer surface 24 of the sheath 20, which increases towards the distal end 30. Typically, the width w1 at the base part 28 is between 5 mm and 18 mm and the width w2 near the distal end 30 is between 6 mm and 20 mm. Similar to Fig. 5, there can be a convex end portion (not shown) at the distal end 30.

[0049] The annular ridges 40 in Fig. 6 are arranged at regular intervals P’ along the outer surface 24 of the sheath 20. For example, the interval P’ is between 15 cm and 100 cm. Alternatively, the annular ridges 40 are arranged at variable intervals along the outer surface of the sheath 20.

[0050] The ridges 40 allow compartmentalizing accumulation areas of ice on the sheath 20. Their width increasing towards the distal end provides a lock function promoting retention of frozen water on the sheath 20. Therefore, the fall of chunks of frozen water is delayed and the size of any falling chunk is reduced.

[0051] The annular ridges 40 have shapes and dimensions adapted to maintain the aerodynamic properties of the sheath 20. Thus, drag forces acting on the profile of a cable covered with such a sheath20 are limited when the weather conditions do not generate ice.

[0052] A sheath as shown in Figs. 6-7 may be manufactured according to a method illustrated in Figs. 8-15, which makes use of a heat welding technique.

[0053] The sheath 20 is made by assembling sheath segments 21 end to end. Each sheath segment21 may consist of a thermoplastic tube 23 made, for example, of HDPE. The thermoplastic tubes 23 have the same outer shape, for example circular with the same diameter, to ensure continuity of the outer profile of the sheath 20. They may have helical ribs 36 as described above on their outer surface.

[0054] At an assembly station where mirror welding equipment is available, axial ends 25 of two adjacent sheath segments 21 are aligned, with a gap g of a few centimeters between their respective axial end surfaces 46 (Fig. 8).

[0055] A mold 50 is brought around the axial ends 25 of the two sheath segments 21 . The mold 50 has two collars 52 that fit around the outer surface of the thermoplastic tubes 23. Each collar 52 may be made of two halves for assembling around the sheath segments 21. The collars 52 are made of a material having a higher melting point than the HDPE, for example polytetrafluoroethylene (PTFE). The opposing end faces 48 of the collars 52 have ring-shaped grooves 56 whose profile matches the desired shape of the annular ridge 40 that will be formed. Once the collars 52 are disposed around the axial ends 25 of the sheath segments, the gap g’ between their opposing faces is wider than the gap g between the axial end surfaces 46 of the sheath segments 21 . By virtue of this difference between the gaps g, g ’, the thermoplastic material of the sheath segments 21 that extends beyond the end faces 48 of the respective collars 52 will form the annular ridge 40 after the heat welding operation.

[0056] A heating plate 58 is inserted into the gap g between the sheath segments 21 (Fig. 9). The heat of the plate 58 causes fusion of the thermoplastic material at the axial end surfaces 46 of the sheath segments 21 . Then, the sheath segments 21 are pulled away from each other and the heating plate 58 is removed (Fig. 10). At this point, an amount of fused plastic material 47 is present at the axial ends 25 of the sheath segments 21 .

[0057] Then, the two sheath segments 21 are pressed axially against each other. The rings of fused plastic material 47 at both axial ends of the sheath segments are combined when they contact each other (Fig. 11 ), and the axial pressure is maintained until the opposing end faces 48 of the collars 52 come into contact. The fused plastic material flows between the two collars 52 and fills the annular cavity defined by the grooves 56, thus forming the ridge-shaped protrusion 40. A weld bead 60 may additionally be formed at the inner cylindrical face of the assembled sheath segments 21 due to overflowing of the fused plastic material, as shown in Fig. 12. The collars 52 are removed once the plastic material has solidified (Fig. 13). If necessary, cooling may be applied to accelerate the solidification. The heat welding operation is then completed. The internal weld bead 60 may be removed to facilitate subsequent threading of the tendons of the structural cable. This is done by cutting or abrading the internal weld bead 60 using a suitable tool 62 introduced into one of the sheath segments 21 (Fig. 14).

[0058] The two sheath segments 21 assembled by such a method (Fig. 15) are shifted, and another sheath segment is added at the assembly station for heat welding. The process is repeated until the full length of the sheath 20 is completed.

[0059] The above-described method produces a sheath 20 of high quality with precise dimensional tolerances, both internally and externally. The welding operation is preferably performed at the factory, which is more easily controlled than when it is done on the construction site.

[0060] It is noted that such a manufacturing method is also applicable to protrusions of different shapes on the outer surface 24 of the sheath 20.

[0061] Different shapes of the grooves 56 in the mold can be used to obtain various profiles of the ridge 40 at the interface between two sheath segments 21 . This includes a straight profile as shown in Fig. 16, in which the width w of the ridge 40 is substantially constant over its height h.

[0062] The heat welding method with a mold 50 can also be used to make protrusions that do not extend around the whole circumference of the outer surface 24 of the sheath 20, but only part of it. It may also be used to make discrete pins 26 such as those illustrated in Figs 4-5.

[0063] Making sheath 20 by heat-forming the end portions of straight thermoplastic tubes 23 is relatively inexpensive since it avoids costs associated with separate provisioning of the rings that will form the protrusions 40. It also avoids risks of insufficient adherence of the protrusions. Other possibilities include using tubes 23 having thickened end portions 25 or disposing one or two rings around the end portions 25 of the sheath segments 21 to provide the thermoplastic material that will form the protrusions.

[0064] Using the same plastic material for the thermoplastic tubes 23 and the protrusions 26, 40 reduces risks of premature damage to the protrusions due to different mechanical and / or thermal behaviors of the materials. It also ensures a homogeneous pigmentation of the sheath 20.

[0065] 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 sheath 20 may be provided with a combination of different kinds of protrusions, such as the above-described pins 26 and ridges 40.

Claims

Claims1. A method of manufacturing a sheath (20) for a structural cable of a construction work, the method comprising:- providing two sheath segments (21) comprising respective thermoplastic tubes (23) having a same outer shape; and- heat welding respective axial ends (25) of the sheath segments (21 ), wherein, during the heat welding step, a protrusion (40) is formed at the axial ends (25) of the sheath segments (21), wherein the protrusion (40) protrudes radially beyond the outer shape of the thermoplastic tubes (23).

2. The method as claimed in claim 1 , wherein the protrusion is in the form of a ridge (40) around the sheath segments (21).

3. The method as claimed in claim 2, wherein the ridge (40) extends perpendicularly to the sheath segments (21 ).

4. The method as claimed in claim 2 or claim 3, wherein the ridge (40) protrudes from a base part (28) at an outer surface (24) of the thermoplastic tube (23) to a distal end (30), and wherein the ridge (40) has a portion in which a width (w) taken parallel to a longitudinal direction of the thermoplastic tube (23) increases towards the distal end (30).

5. The method as claimed in claim 2 or claim 3, wherein the ridge (40) protrudes from a base part (28) at an outer surface (24) of the thermoplastic tube (23) to a distal end (30), and wherein the ridge (40) has, at least in a part adjacent to the distal end (30), a constant width (w) parallel to a longitudinal direction of the thermoplastic tube (23).

6. The method as claimed in any one of the preceding claims, wherein the protrusion (40) protrudes from a base part (28) at the outer surface (24) of the thermoplastic tube (23) to a distal end (30) by a height (h) in a range of 2% to 15% of an outer diameter (D) of the thermoplastic tube (23).

7. The method as claimed in any one of the preceding claims, further comprising:- arranging a mold (50) around the axial ends (25) of the sheath segments (21 ) before heat welding the axial ends (25) of the sheath segments (21 ), wherein the mold (50) has a shape complementary to the protrusion (40) to be formed.

8. The method as claimed in claim 7, wherein the mold (50) comprises a pair of collars (52), wherein each collar (52) has an end surface (48) having a groove (56) formed therein, wherein arranging the mold (50) comprises fitting the collars around the axial ends (25) of the sheath segments (21 ), with the respective end surfaces (48) opposing each other, wherein heat welding the axial ends (25) of the sheath segments (21 ) comprises bringing together the end faces (48) of the collars (52) to let fused thermoplastic material (47) flow into the grooves (56).

9. The method as claimed in claim 8, wherein the fused thermoplastic material (47) flowing into the grooves (56) is thermoplastic material that was part of the thermoplastic tubes (23).

10. The method as claimed in claim 9, wherein the two sheath segments (21 ) are aligned with each other, with a first gap (g) between respective axial end surfaces (46) of the sheath segments (21), wherein the mold (50) is arranged around the axial ends (25) of the sheath segments (21 ) with a second gap (g’) between the opposing end surfaces (48) thereof, wherein the second gap (g’) is wider than the first gap (g).

11. The method as claimed in claim 10, wherein heat welding the axial ends (25) of the sheath segments (21 ) comprises inserting a heating plate (58) into the first gap (g) to fuse thermoplastic material at the axial end surfaces (46) of the sheath segments (21 ).

12. The method as claimed in any one of the preceding claims, further comprising: removing an internal weld bead (60) formed at an inner face of the sheath segments (21) when heat welding the axial ends (25) of the sheath segments (21 ).

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