Abstract

Optimal network arches are arch bridges where some hangers cross other hangers at least twice. When the arches are less than 15 m part they normally cost less if the tie is a concrete slab and the arches are steel tubes or universal columns or American wide flange beams. Network arches are best suited for spans between 80 m and 170 m, but will compete well in a wider range of spans. This results in attractive bridges that do not hide the landscape behind them. A network arch bridge is likely to remain the world’s most slender arch bridge. The transverse bending in the slab is usually much greater than the longitudinal bending. Thus the main purpose of the edge beam is to accommodate the hanger forces and the longitudinal prestressing cables. The partial prestress reduces the cracks in the tie. This is part of the reason why the first two Norwegian network arches are still in good shape after over 50 years. For load cases that relax none or only very few hangers, network arches act very much like many trusses on top of one another. They have little bending in the tie and the arches. To avoid extensive relaxation of hangers, the hangers should not be inclined too steeply. Small inclination of hangers tends to increase the bending moments due to concentrated loads. A compromise should be sought. All hangers might have the same cross-section and nearly the same decisive load. When there are no transversal beams in the tie, upper nodes should normally be placed equidistantly along the arch. The network arch can be seen as a beam with a compression and a tension zone. An increased rise in the arch will give smaller axial forces in the chords and lower steel weights. It is mainly aesthetic considerations that limit the rise of the arches. Most of the shear force is taken by the vertical component of the arch force. The hangers act like a light web. They take some of the variation in the shear force. Because there is little slenderness between the nodal points of the arch, and tension is predominant in the rest of the network arch, this type of bridge makes good use of high strength steel. All members in an optimal network arch efficiently carry forces that can not be avoided in any simply supported beam. Network arches are very stiff. This is very important when the network arch is used for railway bridges, especially in bridges for high speed railways. Compared with conventional bridges, the network arch, where the tie is a concrete slab, usually saves over ⅔ of the steel weight. See fig. 98, p. 93. The details are simple and highly repetitive. Thus the cost per tonne is not very high. The slender tie leads to short ramps. This makes it simpler to attach roads at the ends of the bridge. The network arch on page 93c is a most competitive network arch. The details are simple and the exposed surface is small. The steel weight is low, but not minimal. The arch and hangers supplemented by a light temporary lower chord can be moved when lifted near the ends. This steel skeleton can be erected on side-spans or on ice between the abutments. It can also be lifted in place by floating cranes. When the span is in place, this steel skeleton has enough strength and stiffness to support the concrete tie while it is being cast. For wide bridges, three or four parallel arches could be used to keep down the span of the concrete slab between the arches, see figs 30 to 32. For long bridges, where many spans are needed, the network arches could be made exclusively from prestressed high strength concrete. The spans should be cast on shore before being floated to the site on pontoons or by big floating cranes. See pp. 47-50, 94-94a. The local conditions will influence the type of erection. See pp. 15, 19a and 20. Sometimes the tie can be cast on a timber structure. After the arch and hangers have been erected the hangers can be tensioned till they carry the tie. See pp. 6b and 7a. Finished network arches spanning 200 m or more can be moved to the pillars by means of pontoons or big floating cranes. This is more likely to take place in coastal areas. See pp. 93c-94a. The fact that the optimal network arch uses so little materials makes it environmentally friendly in a broader sense. Unemployment is a problem in most countries. A high percentage of the cost of network arches is wages. Thus network arches would lead to more bridges, more employment and more practical training of the workforce from the same limited funds. The building of optimal network arches can bring great savings. However steel firms are usually not interested in using very little steel. Considering the great poverty in the world, it would be morally wrong not to use network arches at suitable sites. The introduction of the network arch would create extra work for bridge authorities, but it is up to them to promote it. General conservatism is probably the main obstacle to the use of this very promising structure. To some civil engineers the author’s claims may seem exaggerated, but it would be stupid to exaggerate when the bare facts seem like an exaggeration.