2.1 Geometry and dimensions
The brick arch bridge over the Cauca River in Popayan, is 153 m long and has a half-point main arch with a 19.06 m diameter, which crosses the variable current river. It also has 3 half-point leveling arches, leading to the slope on the north side; while on the southern side the main arch is directly supported upon a natural rock support vertically fitted to the ground (Fig. 2).
The structure has a constant 5.84 m width throughout its whole length. It is entirely made of solid clay brick joined with lime mortar, although beneath the main arch there are some rows of etched stone -construction material often also found on the tympanums of the bridge and on the clefs of the vaults. A sole cutwater with a diamond-shaped base is found under the extreme north section of the main arch and it is also entirely built with brick. The dimensions and dimensional ratios are given in Table 1.
The genesis of the structure of the bridge over the Cauca River is kept in several dispersed documents, which allow its partial reconstruction. In the Historical Archives at Universidad del Cauca (Popayan) there is a document dated in 1753, which registers that priest Simon Schenherr, member of the Company of Jesus and master builder in charge of the construction of various religious buildings in the city and participating member in the design phase of the bridge construction. He authored two blueprints each corresponding to the same bridge.
The first of the blueprints (Fig. 3) bears a drawn of a covered wooden bridge, raised over four pillars with triangular cutwaters on each face, two of them with foundations in the water. The bridge roadway surface was made up of 3 spans and reached a total approximate length of 34 m, assembled over longitudinal beams supported above the pillars through beams at 45° angles. Next to the drawing, there are also indicated the amounts of construction materials necessary for its execution.
The second blueprint (Fig. 4) reveals a 3 arched masonry bridge: the two arches of the extremes with 24 m diameters and the central arch with a 6 m diameter, supported on 4 pillars with cutwaters of equal geometry, two of them set at the river bottom. The bridge roadway surface has a double slope and according to a notation on the drawing, it was 4 m wide.
For unknown reasons, the bridge could not be built following any of Schenherr's designs. Additionally, he did not participate in directing the work.
In another document kept in the Nation's General Archives (Bogotá) there is testimony relating how, during a first stage of the work begun in 1769, the bridge collapsed at the precise moment of closing one of the brick arches - due to the lack of technical direction. The final construction was left to Francisco Basilio de Angulo and Josep Hidalgo de Aracena.
Once in service and for decades, the bridge over the Cauca River was always considered one of the best in the region and the nation; for example, English colonel J. P. Hamilton briefly mentions it in his book (Hamilton, 1827) as part of his journey between Popayán and Cali, just prior to the earthquake of 1827 that affected the region and destroyed several temples and caused severe damage to the bridge's main arch, rendering it useless.
There is evidence (Zawadsky, 1929) that on at least two occasions local construction workers tried to rebuild it, but with no satisfactory results given that in both cases, upon removing the wooden trusses placed under the vault, the materials fell and were dragged away by the river. It was not till 1840 that Polish engineer, Estanislao Zawadsky, directed the definite reconstruction work of the bridge.
A visual inspection carried out in 2006 permitted verifying that the bridge has not been altered by recent structural interventions, except for the restitution of the roadway surface in asphalt. A masonry plasterwork made with Portland-type cement mortar during the first half of the 20th century is in poor state of conservation (Fig. 5) and only remains on the upper portion of the bridge's tympanums. On the lower part of the rings there is evidence of an accelerated degradation process of the bricks by weathering of the clay-like material due to prolonged exposure to environmental humidity and to the presence of the type of vegetation found in
tropical regions, which contributes to the concentration of distinct living organisms upon bridge surfaces (Fig. 6).
The macro structure is still in service to vehicular traffic up to 5 Tons, as indicated to users. Maintenance work is necessary, as is the replacement of ceramic pieces that have disappeared by using new materials with similar physical and mechanical properties to those used in its construction.
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