Structural assessment of the medieval city wall of Seville (Spain) using low-cost accelerometers

Abstract

The remains of fortifications are commonly found embedded in the contemporary urban fabric of historic cities. Due to their typological nature, these heritage assets are usually elements of great size and impact on the urban landscape. Their location within cities exposes them to very specific risks, such as pollution, vibration, biodegradation, and anthropic factors, which must be taken into account for the proper conservation and maintenance of such remains.

The case of the medieval wall of Seville (Spain), built on rammed earth, shows specific factors of vulnerability, degradation, and erosion which prompt the need for non-destructive techniques (NDT). In the present research, vibration-based inspection is used and, specifically, when subjected to ambient vibrations. The collection of accelerations, which covered a period of nine months, was conducted in different parts of the structure under study by using mainly high-resolution accelerographs. These data allowed to perform an exhaustive dynamic characterisation of this defensive structure. Among other findings, it was determined that the filling of cracks and voids in the walls increased the fundamental frequency by about 2 %. In addition, no serious damage to the structure was found.

Knowledge of the historic buildings' condition is essential in order to determine the most appropriate intervention. To this end, a protocol for the global structural diagnosis of these buildings has been proposed. It is based on the recording of accelerations with low-cost accelerographs. Besides being an inexpensive protocol, it allows a quick decision making, since the dynamic characterization of the structure is performed by means of the fast Fourier transform.

Basically, the designed protocol is based on the variation of the fundamental frequency of the structure between two time instants. These values are used for the calculation of a damage index. By means of an exhaustive review of conventional masonry buildings tested in the laboratory, the damage index has been correlated with the state of the structure prior to collapse. Specifically, this threshold, which ranges from DS4 to DS5 according to the criteria of the European Macroseismic Scale 1998 (EMS-98), provides a valuable reference point for the structural assessment of historical box-shaped constructions with conventional masonry load-bearing walls.

This simplified procedure for structural diagnosis was applied in its initial phase at the White Tower. It was found that this protocol is able to identify the fundamental frequency of the structure with an error of <0.65 % in comparison to the results obtained through operational modal analysis methods.