STRENGTHENING OF STONE MASONRY ARCH BRIDGES THREE ...

Common Problems
4 Defective Masonry Arch Bridges
The sheer age of the masonry arch bridges means that virtually all such bridges can be
deemed to be defective in one or more respects, whether it is spalling masonry or lack
of waterproofing. However, from a structural point of view, the only defects which are
of concern are those which will have a significant impact on the ability of a given bridge
to successfully support foreseeable applied loading.
The causes of structural defects have been categorised into four groups:
1) Construction
2) Long-term loading
3) Transient loading
4) Environmental
The majority of the defects found in practice will arise from a combination of some or
all of the above.
The ultimate construction defect in the case of an arch bridge is presumably one which
is sufficient to cause collapse immediately after decentring, e.g. the arch being of the
wrong shape to carry the dead loads. Well designed bridges, built to carry
contemporaneous loading, are of course classed as being defective if it is perceived that
they are unable to carry modern traffic loads.
There are several very common structural defects which affect stone masonry arch
bridges.
4.1 Scour of foundations
Scour is probably the most common cause of collapse of masonry arch bridges. The
foundations are generally shallow and therefore susceptible to scour. Scour is difficult
to detect because it is likely to be at its worst when the river is in flood and access is
impossible. It is likely to be made worse by fallen trees and other debris catching in the
Leonardo da Vinci Program 27