STRENGTHENING OF STONE MASONRY ARCH BRIDGES THREE ...

Masonry Arch Bridge Construction
thickness as that visible on the exposed face. The thickness of the arch barrel may also
increase from the crown to the springings, and this appears to be more common with
arches made from uncut voussoirs and for longer span arches.
The arch shape will change during construction when the centring is removed. To
ensure that the line of thrust remained within the middle third, recommended a German
practice that three or more joints are inserted of a material such as lead covering the
middle third of the joint. When the centring had been removed and the spandrels etc
completed (but presumably before any fill was put in place) the joints were filled with
cement.
Ashlars masonry may have been built with or without mortar. Having mortar between
voussoirs reduced the stress in the stone by 30%. Also when removing the centring
before the mortar is fully set stops loss of mortar by crushing which would otherwise
occur.
2.5. Spandrel Walls and Parapets
Spandrel and wing walls retain the fill and carry the parapets. The spandrel walls also
stiffen the arch ring at its edges and may have a considerable strengthening effect on the
vault as a whole. They are commonly thickened towards their base to increase their
stability and for the same reason many wing walls are buttressed or built with a sloping
outer face. Wing walls can add to the strength of a bridge by restraining the in-plane
displacement of the spandrels. Many walls were built curved on plan such that the
bridge was at its narrowest at mid-span and this may make some contribution to their
ability to resist the outward pressures from the fill. Masonry spandrel walls may consist
of a relatively thin layer of dressed stone backed by a thicker layer of rubble masonry.
A problem with masonry arch bridges is that their parapets do not meet present day
requirements for containment of errant vehicles.
2.6. Fill Material
Leonardo da Vinci Program 21