Civil Engineering Reference
In-Depth Information
Figure 17.7
Byker Tunnel under construction (Photo: Robert Benaim)
haunches, where there is a greater weight of earth. Over most of its length it rested on
abutments cut into massive sandstone. As the hard rock fell away, it was replaced by
unreinforced concrete walls, either dressing the soft rocks of the coal measures, or for
short lengths acting as cantilever retaining walls.
The arch was cast in-situ on a set of steel travelling shutters, Figure 17.7. As the
slope of the arch at the springings exceeded 18°, a partial top shutter was necessary.
Such an arch needs only light reinforcement, and the bars should if possible be of small
diameter such that they will lie to the arch radius without pre-bending, preferably
under their own weight.
Two alternative designs carried out by Benaim demonstrate the economy of
reinforced concrete arches. Both carried the M74 motorway in Scotland on very skew
crossings of twin-track railways, Figure 17.8. The New Cowdens Bridge was 190 m
long with an arch span of 21 m and crossed the electrifi ed main line from England
to Scotland, while the Maryville Bridge had a span of 16 m and crossed a lesser
line. The arch thickness for New Cowdens was 450 mm, and 300 mm for Maryville.
The greater thickness of the former was adopted to allay concerns about the impact
resistance of the structure in the event of a derailment. These alternatives consumed
approximately one-third of the materials of the conforming designs they replaced,
Figure 17.9. The precast arches were erected in a matter of days, and once in place
they protected the railway from the remaining construction activities. This minimised
possession times and reduced the risk to the railways. An additional advantage of
such arch bridges is that they require less maintenance; in particular they do not need
roadway expansion joints.
The arch profi les consisted of two radii; a tighter radius is required over the lower
part of the arch to accommodate the increasing weight of earth and lateral pressure.
