In Guadalajara, the oldest surviving structure is the viaduct that carries traffic across the Henares River. It's noteworthy since it's one of the few surviving Andalusian bridges in Spain.
Although this viaduct was originally attributed to the Romans by historians writing in the 17th century, it has now been linked to 'Abd al-Rahman III's building projects.
Ashlar stone is used in the viaduct's rope-and-timber building technique. It is supported by four angular pilasters that run counter-current and are rounded in opposing directions, creating four big spans and one smaller span on the western side. One of its pillars is shaped like a horseshoe and serves as a spillway.
In his investigation of the bridge, architect Basilio Pavón produced the first planimetric surveys (including floor plans and elevations) and those of the structure's composition, pinpointing the many changes made at different times.
Therefore, beginning at the city's shoreline, arches 1-2, pilasters A, B, and D (the latter only at its base), and the massif that goes into the alluvial terrace would be of Caliphate construction; arch 5 would date to the mediaeval Christian period; arches 3-6, and pilaster C would date to the 18th century. He also suggested a restitution of its section, proposing to split the roadway in two parts that each reached their zenith at an intermediate point: arch 2, where it reaches a maximum height of 10 metres above the sheet of water, and subsequently sloping downwards towards its ends until it reaches a length of more than 117 metres.
He determined that the tower that the Christian stonemasons had built to pinpoint the Bridge's entrance should be set atop Pillar B because this was the pillar that had been most fortified by them in the 13th century.
The photographs of the city by Anton Van den Wyngaerde (1565) and the elevations by Bernardo Martnez (1628) and José de Arce (1742) were not yet available when Pavón presented these hypotheses, therefore he did not take them into account. For example, he was unable to analyse the final arches across from the city because of "the buildup of alluvium and barriazales." Neither could he pinpoint the exact placement of the tower that rested atop pillar C or locate the hollows that fed the caz of the Guadalajara mill.
Until 1940, the Guadalajara Bridge was thought to be a Roman engineering feat. However, in an article titled "El puente de Guadalajara" (The Guadalajara Bridge) published in the journal Al-Andalus, Arabist architect Leopoldo Torres Balbás argued that the bridge was actually built during the Caliphate period, during the transition from the 10th to 11th centuries. He based this on the You may see these traits of Andalusian architecture in the public buildings of Roman Hispania.
The atmospheric phenomenology in the middle of the 18th century was characterised by plentiful and frequent rainfall, which in Guadalajara manifested as repeated flooding of the Henares River with detrimental repercussions on the bridge. Of particular note are the extraordinary flows of 1739 and 1757, years in which the thread of the central arches, the ashlars of the cutwater on which the tower rested, the retaining wall and connection with the Salinera bridge, and the zampeado were all severely damaged by the force of the extraordinary flow.
Mateo José Barranco, José de Arce, and other architects of the time period contributed reports. Although it was planned by Marcos de Vierna, construction didn't start until Charles III's reign. The tower was taken down and the pillar it rested on as well as the nearby arches were reconstructed during this operation. The inscription on the monolith that stands as a reminder of the project's completion reads, "MARCO VIERNA OPUS DUCENTE / M D CC LXXVI." This dates the work to 1776.
To prevent water erosion of the foundations, the grass had to be meticulously maintained. The river bed next to the viaduct has been paved with huge stone blocks and reinforced with a grid of strong wooden beams. After the publication of the Royal Order on 19 July 1901 approving the works, the final major intervention in this sense was carried out in the summer of 1901.
In 1922, the engineer Landelino Crespo implemented a plan to modernise the viaduct in order to increase the bridge's width to 10 metres. This was accomplished by demolishing the existing stone parapet that restricted its width, replacing it with concrete forms big enough to provide a smooth running surface, and installing wrought-iron railings and electric street lamps. In that year, on November 4th, a load test was accomplished that would eventually allow the bridge to open to traffic.
The bridge was designated as a Historic-Artistic Monument on June 3, 1931, and it remains today as an Asset of Cultural Interest, a special category of protection for historic and artistic structures.
A new concrete bridge was opened in 2008 adjacent to and parallel to the original bridge, allowing visitors to see the landmark for the first time. This new bridge, designed by Carrascal Fras and Aguado Roca, is shorter than the original bridge from the 10th century, but it features a single arch with maximum spans.