Preliminary design of a network arch

In this chapter the design of the railway network arch bridge is discussed and guidelines or examples are given. For a systematic design process the bridge is divided into separate elements:

- Arch

- Lateral bracing

- Main girder

- Deck

- Hangers

The aspects mentioned in this paragraph can be used for an efficient determination of a preliminary design, or in a variant study.

Conceptual choice

Geiβler et al. [7] determined the following span range for economic application of network arches. These ranges correspond to the overview of network arches already built (Figure 9).

- Road bridges (LM1) 55m – 300m

- Railway bridges (LM71 met α=1,0; SW/2) 80m – 300m

Per Tveit [3] gives the following conditions for which network arches will also provide an economic solution:

- Bridges for which high stiffness is needed.

- Areas with bad soil conditions, network arches are beneficial because of light weight.

- Areas where labor is cheap compared to material cost. Because of low steel weight and relatively high number of connections.

- Bridges crossing water, here the bridge can be completely lifted into place.

Arch and lateral bracing.The structure of an arch bridge is generally built up by 2 arches connected with lateral bracing to provide horizontal stability. All kinds of arch variations have been developed over the years see Figure 10. The choice of the arch cross section and the type of lateral bracing mostly depends on their appearance but other factors that have a big influence are:

- Span length

- Required stiffness

Span length.Per Tveit [3, 4, 5] recommends a standard H-profile to be applied for spans up to 100m. This would provide the most economical solution for double arches because of simple fabrication and relatively simple hanger connections see Figure 11. With larger spans a box-section would become more economical, because of the higher bending- and torsional stiffness. Other examples of arch sections are: hat- or tubular-section.

Other advantages of fabricated sections are that a variable cross section can be applied, leading to a more economic cross section. For instance, the required stiffness in plane of the arches could be much less than out of plane, leading to material savings.

Required stiffness.Compared to vertical or diagonal hanger arrangements, the network arrangement provides more support in plane of the arch. This allows the arches of a network arch to be more slender. When considering the out-of-plane stiffness, no differences are found between vertical and diagonal hanger arrangements. The type and size of lateral bracing determines the out of plane stiffness. Regular solutions for lateral bracing are shown in Figure 13. Also a combination between basket handle shape and

lateral bracing is possible, which leads to large horizontal stiffness.