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SUSPENSION BRIDGES

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The suspension bridge building method SBEF, is actually a spin-off from the development of the PEF offshore lifting method, and uses the same types of tools.

The traditional suspension bridge building method is the following:

  1. Pylons and wire anchors are cast in reinforced concrete on both sides of the crossing
  2. The pylon top is prepared with a steel saddle for the main span cables and tie-back cables
  3. The pylon tops are tied back to the wire anchors in the ground
  4. Two wires are pulled across between the pylon tops and a temporary bridge, a so-called Catwalk is established.
  5. The main span cables are pulled across the catwalks and secured to the wire anchors
  6. The main span cables are fitted with hanger clamps and hangers and clad or spun and painted.
  7. The girder is fabricated in sections, normally with the length of 12 to 36 meters making up an arch. 
  8. Starting in the center, the sections are lifted up by a crane running on the main span cables or by a floating crane.
  9. During installation, the sections are temporarily connected like a bicycle chain ( vertical flexible but stiff sideways) since the geometry of the main cables will change during the process of hanging up the girder sections.
  10. When about 80 % of the sections are in place, they may be pulled together, adjusted and welded.
  11. After completing the welding process of the sections, sandblasting and painting may start.
  12. The inner ventilation system, instrumentation, railing and lighting is fitted, and at the end the concrete / asphalt is laid.
  13. The local mayor arrives to cut the ribbon, saying something about tying people together, while the newspapers demand an investigation into the building budget overrun.

The cost distribution:

Suspension bridge cost distribution curves.   The cost of suspension bridge building distributed on various activities clearly show where savings will have most effect.  Erection, prefabrication and surface protection activities are obvious candidates for improvement.

The erection activity, which means to fit the main span cables, catwalk, girder hangers and the girder sections may be improved by two actions : Reduction of site work, and simplifying the remaining necessary site work.

Prefabrication may also be improved by fabricating more finished and longer bridge girder sections in a yard. Actually, in some cases it might be possible to prefabricate the complete girder and install it into the span in one unit using the SPEF technology.

Surface protection is a very demanding task in bridge building in that most work has to take place on site with varying ambient conditions. Moving such work to an in-house situation will certainly reduce cost.

The SBEF method:

The method developed and patented by TRC takes advantage of the possibilities of installing very long girder sections or complete girder spans by the use of a controlled lifting system.

This opens up for yard fabrication of complete girders in steel or extruded aluminum with railing, lighting, instrumentation, ventilation and asphalt or epoxy ceramics. 

The installation sequence may be described like this:

Picture of first step of the girder installation. The main cables are erected and special hangers are fitted hanging down to the sea level. The main cables are erected and special hangers are fitted hanging down to the sea level.

Step two. The center section, brought to the site floating or by barge, is connected to the hanger wires by hydraulic linear pulling winches, and lifted up under the control of a computer system. The computer monitors and controls such factors as load distribution, bending moments, main cable geometry and lifting height. The center section, brought to the site floating or by barge, is connected to the hanger wires by hydraulic linear pulling winches, and lifted up under the control of a computer system. The computer monitors and controls such factors as load distribution, bending moments, main cable geometry and lifting height.

Step three. The two side- girders are brought in, and may be lifted simultaneously to ensure a balanced geometry during lifting. By using the computer system, the transition zone between the girder sections is controlled in order to make a smooth curve of the main cables in that area. The two side- girders are brought in, and may be lifted simultaneously to ensure a balanced geometry during lifting. By using the computer system, the transition zone between the girder sections is controlled in order to make a smooth curve of the main cables in that area.

Step four. The girder is completed by lifting the two end sections. If the end section(s) are situated over the river bed or land, the sections may be moved sideways from their pick-up zone in the water. The girder is completed by lifting the two end sections. If the end section(s) are situated over the river bed or land, the sections may be moved sideways from their pick-up zone in the water.

This bridge type is normally designed with a so called orthotropic hollow girder. The girder will float in the water on approximately one ft, and may be towed to the site. 

In order to reduce the amount of painting on a steel girder, the hollow girder is fitted with a ventilation system with a de-humidifier system reducing the humidity to less than 40%. The steel does not corrode in this atmosphere, and the interior of the girder may remain unpainted, a saving of 20% of the surface protection cost.

Aluminum bridge girders do not need surface protection other than on the lanes, where an epoxy / ceramic layer may do the trick.

 

Advantages :

The bridge girder may be constructed in a workshop, hence the productivity will be increased, time will be saved and the quality increased.
The bridge girder may be fitted with rails, lighting and asphalt before it leaves the yard. 
Reduced site team for hook-up reduces time and cost
The bridge girder is "self elevating", and may be lifted directly up from the sea within a day.

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TR Consulting, Joh. Falkb.gt. 10, 5538 Haugesund, Norway. Tel: +47 52729124, Fax: +47 52724196, C.E.O. Cell phone.  +47 90 616969  e-mail: trcons@online.no or trcons@start.no