The Three Arch Bridge
Once the Flowering Reflection Pond is drained, the base of the bridge will need to be evaluated and a plan devised for reconstruction and restoration. We don't know the actual work that will be needed until the footings and substructure can be visually examined. The stonework that has fallen from the bridge will need to be retrieved, cleaned and evaluated.
A little about the construction of an arch bridge ...
Stone, brick and other such materials are strong in compression and somewhat so in shear, but cannot resist much force in tension. As a result, masonry arch bridges are designed to be constantly under compression, so far as is possible. Each arch is constructed over a temporary falsework frame, known as a centering. In the first compression arch bridges, a keystone in the middle of the bridge bore the weight of the rest of the bridge. The more weight that was put onto the bridge, the stronger its structure became. Masonry arch bridges use a quantity of fill material (typically compacted rubble) above the arch in order to increase this dead-weight on the bridge and prevent tension from occurring in the arch ring as loads move across the bridge. Other materials that were used to build this type of bridge were brick and unreinforced concrete. When masonry (cut stone) is used the angles of the faces are cut to minimize shear forces. Where random masonry (uncut and unprepared stones) is used they are mortared together and the mortar is allowed to set before the falsework is removed. Traditional masonry arches are generally durable, and somewhat resistant to settlement or undermining. However, relative to modern alternatives, such bridges are very heavy, requiring extensive foundations.
A view of the bridge from the
Looking across the bridge down the Main Road. Walls confine traffic to the center of the bridge surface. The bridge fill of gravel provides a safe and durable surface
Where the arches are founded in a stream bed the water is diverted and the gravels excavated to a good footing. From this the foundation piers are raised to the base of the arches, a point known as the springing. Falsework centering is fabricated, typically from timbers and boards. Since each arch of a multi-arch bridge will impose a thrust upon its neighbors, it is necessary either that all arches of the bridge be raised at the same time, or that very wide piers are used. The thrust from the end arches is taken into the earth by footings at the stream bed walls, or by large inclined planes forming ramps to the bridge, which may also be formed of arches. The several arches are constructed over the centering. Once the basic arch barrel is constructed, the arches are stabilized with infill masonry between the arches, which may be laid in horizontal running board course. These may form two walls, known as the spandrels, which are then infilled with loose material and rubble. The road is laid and parapet walls protectively confine traffic to the bridge.
|A view of the Flowering Pond||The Pond with the Bridge in the background|