Lune Arch Bridge
This reinforced concrete arch, dwarfing the lone rower in the second picture above, takes the M6 motorway over the river Lune in Lancashire. It has a separate arch for each carriageway, reducing the tunnel effect below. The arch seems to be slightly flattened at the top.
Since the load is taken to the arch through well separated spandrel walls, in principle the arch could have been built in segments with slight angles between them. Some bridges have indeed been made with such segments, and if the deck provides the stiffness, the segments may even be straight and quite thin. No doubt the ratio of self weight to live loads is a consideration. The arch is cellular, providing stiffness without excessive weight and thickness. The appearance was considered to be particularly important for this attractive river valley.
|Openings in arches, whether to relieve the pressure of flood water
or to reduce weight, have been used since early times. In the 20th century, Robert
Maillart, seeing that certain cracks in a reinforced concrete bridge were not harmful,
realised that he could remove material, and this resulted in some very elegant
In the Lune bridge the arch provides both the rigidity and the support. You only have to look at a fairly flat arch to see that thrust is generated at the foundations, requiring sound rock at those points with sufficient volume to transmit the forces into the ground.
The open design, with arch and spandrel walls, is an almost exact analogue of a suspension bridge, with all the tensions and compressions interchanged. The span is 230 feet, with a 44 ft rise. The bridge was completed in 1959.
What is the correct shape for an arch? If the arch has uniform weight per unit length along the curve, and it carries no load, it should be a catenary. If the arch is infinitely light compared with a load which is uniformly distributed horizontally, it should be a parabola. In practice, neither of these conditions occurs, especially with a masonry arch.
|Again, in theory, for a bridge like the Lune arch, the segments between the spandrel walls should be in the form of a catenary, with slight kinks at each support. In practice, however, it is not usually worth the expense and trouble of doing this. Even with a deck-stiffened arch, with a thick deck and a thin arch, the segments are often straight.||Nobody would ever bother to make anything but a circular arc or a straight segment for a short span between the spandrel walls. Christian Menn has in fact made a segmented arch over the Hinterrhine in Viamala Gorge which looks form the photograph in Leonhardt's book as though the segments are very slightly curved.|
|In the first picture below, the man fishing gives some idea of the size of the bridge.|