The An Dong Bridge in Vietnam was the first project to benefit from the new multihole saddle design.

580 m long, this extradosed bridge has five spans each 140 m long and its four towers are equipped with two parallel planes of six cables which are continuous through the cable saddle. In total, 48 cable saddles and 152,000 linear metres of Cohestrand were supplied to the project.

  • Name of the owner and client
    Ninh Thuan Project Management Unit
  • Delivery date of the project
    May 2015
  • Design consultant
    TEDI South

The use of saddles as an alternative to cable anchors in bridge towers has become commonplace over recent decades, especially as the adoption of extradosed bridges has risen. Such components can be beneficial as they allow bridge towers to be slimmed down and reduce the amount of working at height that is required during construction.

Non-slip saddle

Without anchors to actively secure the cables at the tower tops, a suitably high friction coefficient in the saddle is crucial to prevent stay cables from slipping when adjacent spans experience unbalanced live loads, and to efficiently transfer loads to the structure. The durability of the saddle and the cables within it are also key considerations that impact on the longevity of this arrangement.

Freyssinet’s ongoing product development has driven continuous improvements from the first generation of ‘mono-tube’ saddles, in which all the stays were grouted together within a single pipe, to the latest version of the ‘multihole’ saddle, featuring individual holes for each strand, and cast in ultra-high-performance fibre-reinforced concrete.

Multi-strand saddle

Redefining the first generation multihole saddle design

The first generation multihole saddle design incorporated galvanised tubes, one for each strand, cast in concrete inside a box. This was subsequently refined so that the steel tubes could be omitted, eliminating another potential maintenance issue and allowing the cables to bear directly onto the concrete.

Holes that accommodate each strand have a cross-section in the shape of an inverted water droplet, designed so that under tension, the strand will be pulled into the narrow channel at the bottom of the hole, where it fits snugly. The wider diameter at the top of the void assists with threading of the cables during construction, and subsequent removal if replacement is required.

Specially-designed rubber gaskets are used to form the voids at the required radius

The rubber is textured on the underside to produce a series of transverse grooves in the concrete around the lower part of each hole. These boost the friction coefficient to a minimum of 0.65 and provide a certain element of mechanical interlock with the strand.

Once the UHPFRC has been cast, the rubber gaskets are simply pulled out of the saddle from one end and can be reused. The saddle is then shipped to the site for installation.

Since the first use of the new multi-tube saddle, it has been deployed on several bridge projects every year and with cables of up to 127 strands. It is also applicable for larger cables, even up to as many as 192 strands, depending on the radius of curvature required.

Ultra-high-strength cables of 2,160 MPa are proposed for use on the Sam Cheon extradosed bridge that is currently under construction in South Korea, and the UHPFRC multi-tube saddle will be employed on this scheme too.

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