Steel strip hangers

This type has, up to now, only been applied in German network arches, specifically railway bridges [7]. The main advantage of this hanger type is the relatively simple hanger connection. This is done by supporting and aligning the strips before connecting them by a simple butt weld. This results in good fatigue performance. For more about the construction method see paragraph 2.4.

Another advantage is the availability of steel strips. These are cut from steel plates and can be delivered in almost any size, length and steel grade, this results in a relatively cheap hanger type. One major aesthetical disadvantage is the rough mesh that is created by the relatively large strips. The vibrational effects that could affect these rectangular cross sections are:

- Flutter

- Galloping

- Vortex-shedding

- Structural vibrations (parametric excitation)

Steel rod hangers

The steel rod hangers can be connected in two different ways: by means of welding or with the use of special connectors, as is shown in Figure 22. For welded hanger connections, the same advantages as for strip steel hangers could be achieved. Except that the connection plate requires careful fabrication, in order to achieve the same fatigue performance.

When the steel rods are connected by bolts or connectors, a hanger stressing procedure is required. See paragraph 2.4 for more information. Steel rod hangers have good fatigue properties.

Steel rods in combination with welded connections can lead to relatively cheap hangers. Compared to strip steel they are less economic, because rods have a maximum length of 13m. This means that for longer hangers special coupling welds are necessary. When steel rods are used in combination with special connection elements, the price is assumed to be higher, because high quality products are used. The vibration effects that could affect steel rod hangers are:

- Vortex shedding

- Rain and wind induced vibrations

- Structural vibrations (parametric excitation)

Cable hangers

Cables can be connected by special anchorages which are fixed to the structure (see Figure 22), or by a set of adjustable fork connectors, as shown in Figure 23. Both cable systems require a stressing protocol for the assembly of the hangers. For cable systems, three types of cables are available: locked coil, spiral strand and parallel strand. The parallel and spiral strand types need an additional protective duct that encases the whole bundle. This bundle of wires is connected to the bridge through an anchorage device, see Figure 16 and Figure 27. The locked coil strand is a combination of parallel wires that form the core with outer rings of interlocking Z-shaped wires, providing corrosion protection (see figure Figure 24) 

The vibrational effects that could affect cable hangers are the same as for steel rod hangers, because both hangers have circular cross sections. The costs for a cable system will be relatively high because of the extremely high yield strength and the specialized hanger connections.

Hanger arrangement

An optimal hanger arrangement could seriously enhance the structural performance of the bridge. S. Teich [1] developed a guideline to determine the optimal hanger arrangement. This is the most recent and extensive research on optimal hanger arrangements performed up to now. The arrangements are optimized for the following structural parameters:

- Reducing the bending moments in arch and main beam

- Sufficient resistance against hanger relaxation (compression)

- Equal force in all hangers and optimal utilization of the cross section

- Reducing maximum forces in hangers and thereby reducing the cross section

- Reducing the force variation in the hangers to improve fatigue resistance

- Aesthetic appearance of the bridge

The results of this research are translated into a step by step design guide. Based on the number of hangers and the length of the span, three optimal arrangements are given. These combinations are all provided with a score to indicate their structural performance. The guideline does not provide any insight in the magnitude of the optimized structural behavior. It is therefore impossible to generate a hanger arrangement for which only one structural parameter is optimized.

All optimal arrangements obtained by the guideline result in no hanger relaxation in the SLS. In the ultimate limit state, hanger relaxation is almost inevitable.

For this research a double track railway load was used, represented by load model 71 (LM71). This load was applied on a full steel network arch. For a more detailed description of the network arch that was applied, see [1].

Number of hangers

To determine the amount of hangers, Teich advises not to use more than 50 hangers. In Teich’s design guide, also guidance is given to determine the number of hangers. But a trade-off has to be made between costs per hanger and efficiency.