WO2012052796A1

WO2012052796A1 - Structural cable with fire protection

Info

Publication number: WO2012052796A1
Application number: PCT/IB2010/003026
Authority: WO
Inventors: Erik Mellier
Original assignee: Soletanche Freyssinet SA
Current assignee: Soletanche Freyssinet SA
Priority date: 2010-10-19
Filing date: 2010-10-19
Publication date: 2012-04-26
Anticipated expiration: 2013-04-19

Abstract

The structural cable (12) comprises a tension unit (1 ) having one or more metallic tendons, and a thermally insulating tube (10) arranged around the tension unit. A channel (20) is formed along at least part of the structural cable by an air gap between the thermally insulating tube and the tension unit. The channel has lower and upper ends both open to the outside such that air is allowed to circulate along the channel.

Description

STRUCTURAL CABLE WITH FIRE PROTECTION

BACKGROUND OF THE INVENTION

[0001] The present invention relates to structural cables used in construction works. [0002] While it was primarily designed for suspension cables such as stay cables, the skilled person will appreciate that the invention is applicable to other kind of cables used in construction, including pre-stressing cables in certain configurations.

[0003] A concern in the design of cable-stayed structures is the risk of fire. A cable-stayed bridge has a deck usually with a road and/or railway. A typical hazard is the case of fire in a truck or other vehicle. The load of the deck is supported by the stay cables extending along the deck up to one or more pylons. The stay cables are made of metallic tendons such as strands whose strength falls dramatically if the temperature exceeds about 1 ,000 °C, a temperature range which may be reached in case of a vehicle on fire. If this happens, the cable cannot perform its primary function which is to carry the deck.

[0004] In most cases, safety constraints require that the suspension should remain effective during a minimum time needed to evacuate the deck and/or the surroundings, e.g. from 30 minutes to 2 hours.

[0005] Some protection measures have been proposed to keep such cables functional long enough in case of fire. In general, their principle is to provide around the cable a thermally insulating blanket which delays the temperature increase of the metallic part of the cable. WO 2007/093703 describes an example of such a solution with a blanket made of two layers of insulating material. To be well suited (e.g. to maintain the metal temperature below about 300 °C for 1 or 2 hours with an outside temperature of 1 ,100 °C), a protective arrangement of this kind requires an important thickness of insulating material, typically 30 to 50 mm around the bundle of metallic strands. [0006] To be effective, the known solutions are rather difficult to install due to the weight and bulk of the blankets. Moreover, they are detrimental to the aesthetics of the construction, while stay cable are expected to be slender elements. [0007] The present document addresses the need for relatively compact fire protection systems suitable for stay cables or other kinds of structural cable.

[0008] It is proposed a structural cable, comprising: a tension unit having at least one tendon; and at least one thermally insulating tube arranged around the tension unit. A channel is formed along at least part of the structural cable by an air gap between the thermally insulating tube and the tension unit, the channel having lower and upper ends both open to the outside such that air is allowed to circulate along the channel.

[0009] Protection of the cable from a strong increase of the outside temperature takes advantage of an air circulation along the channel provided around the tension unit including one or more metallic tendons. The air can circulate by natural convection when the temperature increases due to the height difference between the lower and upper ends of the channel, thus evacuating some heat which is not transferred to the load-bearing metal parts.

[0010] The thermally insulating tube enhances the fire barrier by reducing the heat conduction from the outside to the channel surrounding the tension unit. The thermally insulating material of the tube may have a thickness lower than 20 millimeters, i.e. significantly lower than what is required in prior art solutions for a comparable heat resistance behavior.

[0011] In an embodiment which is conveniently installed, the thermally insulating tube includes at least two elements assembled together along a longitudinal direction of the structural cable.

[0012] In order to maintain the air gap between the thermally insulating tube and the tension unit, the structural cable may comprises spacer members distributed between the thermally insulating tube and the tension unit. Such spacer members can be made of thermally insulating material or of another material provided that they are thin enough to avoid significant thermal conduction between the tube and the tension unit.

[0013] If the thermally insulating tube is made of a plurality of assembled elements, these elements can comprise intumescent material, i.e. which bulges if the temperatures increases, thus avoiding intervals between the elements which would be detrimental to heat insulation.

[0014] Another aspect of the invention relates to a construction work having a support structure, a suspended structure and at least one structural cable as defined above for connecting the suspended structure to the support structure. Where the construction work is a bridge, the suspended structure comprises a deck, and the open lower end of the channel provided in said structural cable is advantageously located underneath the deck, where fresh air can be picked up to be circulated along the channel in case of fire. The open upper end of the channel may be located more than ten meters above an upper surface of the deck.

[0015] The structural cable should preferably have a sufficient height difference to obtain a sufficient air convection effect between the lower and upper ends of the channel which should be located where the fire risk is to be taken into account. The most typical situation is that in which the structural cable is a stay cable mounted inclined between the support structure and the suspended structure, e.g. along a deck. However, depending on the design of a specific construction work, it may be useful to make use of the fire protection afforded by the above-defined structural cable arrangement. For example, the fire-protected structural may also be a hanger of a suspension bridge, a prestressing cable, etc.

[0016] Other features and advantages of the structural cable disclosed herein will become apparent from the following description of non-limiting embodiments, with reference to the appended drawings.

BRIEF DESCRIPTION THE DRAWINGS [0017] Figure 1 is a diagram showing a cable-stayed bridge to which the present invention is applicable.

[0018] Figure 2 is a schematic view of a stay cable equipped with an embodiment of a fire protection system.

[0019] Figure 3 is a perspective view showing components of the protected stay cable.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] The cable-stayed bridge 15 shown in figure 1 has a support structure consisting of one or more pylons 14, a suspended structure comprising a bridge deck 13 and a plurality of stay cables 12 for connecting the deck 13 to the pylon 14. The stay cables 12 have inclined trajectories and carry the load of the deck.

[0021] If an accidental fire occurs on the deck, the cables 12 may be damaged and lose their load-carrying function. In order to provide some fire protection, a system diagrammatically illustrated in figure 2 may be used. [0022] The stay cable 12 shown in figure 2 has a tension unit 1 which includes the metallic tendons which are the active parts supporting the load of the deck 13. The tendons 2 of the tension unit 1 can be seen in figure 3. In this example, each tendon 2 consists of an individually coated steel strand. The strands are assembled together into a rather compact bundle which may be surrounded by a plastic sheath 3 along the running part of the cable (between the deck 13 and the pylon 14) to protect the cable from the environment.

[0023] The tendons 2 have their ends anchored to the pylon 14 and to the deck 13. The lower anchorage 4 is shown in figure 2. It is typically located on the lower face of the deck 13, although other arrangements are also possible. The tension unit 1 including the tendons 2 passes through the deck 13 in a channel 5 and emerges on the upper side of the deck 13 to extend up to the pylon 14 along the inclined trajectory. An example of anchoring device 4 usable in the present context is described in WO01 /20098A1 .

[0024] The fire protection system with which the stay cable 12 is equipped includes a thermally insulating tube 10 which surrounds the tension unit 1 in its lower part, near the deck 13. The thermally insulating tube 10 extends around the tension unit 1 up to a height H above the upper surface of the deck 13. The height H is selected such that the upper end of the tube 10 is located out of the region which is potentially affected by a fire caused by a vehicle on the deck 13. The minimum height H is typically of the order of 10 meters. A thermally insulating tube 10 can be arranged at the lower part of each of the stay cables 12 in order to protect the whole suspension system of the bridge.

[0025] The arrangement of the thermally insulating tube 10 is such that an air gap is present between the inner face of the tube 10 and the outer face of the tension unit 1 . This air gap defines a channel 20 around the tension unit 1 which is open to the outside both at its lower end and at its upper end.

[0026] Thus, air can circulate in the channel 20 along the tension unit 1 . In particular, if a fire occurs on the deck 13 and starts heating the tube 10, the convection phenomenon will cause air to circulate upwards along the channel 20, thus evacuating heat and delaying the heating of the tension unit 1 .

[0027] In the embodiment shown in figure 2, the channel 20 formed by the air gap inside the thermally insulating tube 10 is an extension of the channel 5 formed in the deck 13 for guiding the cable to the anchoring device 4. An air inlet 21 is formed between the channel 5 and the lower face of the deck 13 to let cool air penetrate from the outside into the channel 20.

[0028] At the upper end of the thermally insulating tube 10, a cover 22 may be provided to prevent water or objects from entering into the channel 20, while permitting evacuation of the air flowing up along the channel 20. The cover 22 may, for example, be skirt-shaped as illustrated in figure 2, with an upper part connected to the sheath 3 of the cable and a lower part surrounding the upper end of the thermally insulating tube 10 with a gap providing the air outlet.

[0029] The shape of the channel 20/5 may be adapted depending on equipment which may be provided at the lower part of the stay cable 12. For example, the channel 5/20 may accommodate a sealing tube connected to the rear face of the anchoring device 4 while keeping the air gap to make sure that the air can flow along the channel 20. In certain cases, the lower part of the stay cable 12 is fitted with a damping system to improve the dynamic behavior of the stay cable. The thermally insulating tube 10 is then shaped depending on the damping system and still accommodates the channel for the circulation of cooling air in case of fire.

[0030] Figure 3 illustrates a possible structure of the thermally insulating thermally insulating tube 10. In this example, the tube 10 includes a cylindrical support grid 25 on its inner face and a layer of thermally insulating material 26 attached to the outside of the support grid 25. [0031] The layer 26 may be made from various thermally insulating materials, for example silicone loaded with a refractory filler, a fire-resistant resin, ... These materials are typically provided in the form of two or more elongated elements 26A, 26B assembled together along the longitudinal direction of the stay cable 12. At least near the longitudinal edges of these elements 26A, 26B, the thermally insulating layer 26 can contain intumescent material. Conventional intumescent materials used in fire protection applications can be used. The volume of the intumescent material increases when the temperature raises. Thus, any interval at the joint between two adjacent elements 26A, 26B is closed to create a continuous insulating barrier all around the cable.

[0032] Several sections of the thermally insulating layer 26 can be arranged along the cable. In this case, intumescent material is also provided at least near the upper and lower ends of adjacent elements.

[0033] For mounting the thermally insulating tube 10 around the tension unit 1 , spacer members 30 are distributed between the sheath 3 and the support grid 25 in order to maintain a substantially constant distance between these two cylindrical parts.

[0034] In the embodiment shown in figure 3, the support grid 25 is fixed to a number of rings 31 spaced from each other along the cable. At the level of each ring 31 , several spacer members 30 are attached to the outside of the sheath 3 and to the inside of the ring 31 . The spacer members 30 can consist of thermally insulating materials, for example in a form of pads made or ceramic or the same material as the thermally insulating layer 26. Alternatively, they may be made of metallic links which are thin enough to prevent substantial heat transfer from the thermally insulating tube 10 to the tension unit 1 . The overall cross section of the spacer elements 30 is kept low so as not to disturb the air flow along the cable.

[0035] The thickness of the thermally insulating layer 26 can be kept relatively low by virtue of the cooling effect provided by circulation of air along the channel 20. Such thickness can be lower than 20 millimeters. A thickness of about 10 millimeters is typical.

[0036] It will be appreciated that the embodiment described above is an illustration of the invention disclosed herein and that various modifications can be made without departing from the scope as defined in the appended claims.

[0037] For example, a plurality of concentric thermally insulating tubes separated by concentric air circulation channels 20 can be provided around the tension unit 1 to further improve the heat resistance effect if there is room enough for such multiple tubes.

[0038] The fire protection system described above is applicable to other kinds of structural cable, provided that there is a significant height difference between two parts of the cable when a region of the cable located between two such parts need to be protected in case of fire. A thermally insulating tube and an air circulation channel can then be provided around such region of the cable, the channel having its two ends open to the outside in the two parts located at different heights. Thus, air circulation by convection will evacuate heat to which the cable may become exposed.

Claims

C L A I M S

1 . A structural cable, comprising:

- a tension unit (1 ) having at least one tendon (2); and

- at least one thermally insulating tube (10) arranged around the tension unit,

wherein a channel (20) is formed along at least part of the structural cable by an air gap between the thermally insulating tube and the tension unit, the channel having lower and upper ends both open to the outside such that air is allowed to circulate along said channel.

2. The structural cable as claimed in claim 1 , wherein said tube comprises a thermally insulating material (26) having a thickness lower than 20 millimeters.

3. The structural cable as claimed in any one of the preceding claims, wherein the thermally insulating tube (10) includes at least two elements (26A, 26B) assembled together along a longitudinal direction of the structural cable (12).

4. The structural cable as claimed in any one of the preceding claims, further comprising spacer members (30) distributed between the thermally insulating tube (10) and the tension unit (1 ) to maintain the air gap.

5. The structural cable as claimed in claim 4, wherein the spacer members (30) are made of thermally insulating material.

6. The structural cable as claimed in any one of the preceding claims, wherein the thermally insulating tube (10) includes a plurality of assembled elements (26A, 26B) comprising intumescent material.

7. A construction work having a support structure (14), a suspended structure (13) and at least one structural cable (12) as claimed in any one of the preceding claims for connecting the suspended structure to the support structure.

8. The construction work as claimed in claim 7, wherein the suspended structure comprises a deck (13), and the open lower end of the channel (20) provided in said structural cable is located underneath the deck.

9. The construction work as claimed in claim 8, wherein the open upper end of said channel (20) is located more than ten meters above an upper surface of the deck (13).

10. The construction work as claimed in any one of claims 7 to 9, wherein the structural cable is a stay cable (12) mounted inclined between the support structure (14) and the suspended structure (13).