Digital photogrammetry, GPR and computational analysis of structural damages in a mediaeval bridge
Introduction
The interest of the study of traditional architectonic heritage lays in the fact that it is witnesses of the ways of life and the history of the modern societies, and characterizes the landscape of a region, as it is one of its main elements. Nowadays it is assumed that the cultural heritage and landscape are fragile and irreplaceable resources. Nevertheless Spanish heritage protection policies have revealed to be frequently inefficient and the course of time often develops into a progressive deterioration of materials and the degradation of some structures; moreover, the damages caused by successive reforms frequently consisting on the addition or substitution of materials end up leading to the destruction and loss of the historical buildings.
Any research process aimed at the planning of preservation and/or restoration interventions in architectural heritage monuments might be based on an accurate updated documentation of all what concerns the geometric shape, the architectural characteristics, the characteristics of materials and the structural analysis of the construction [1] in order to locate highly stressed areas were fractures might emerge and/or identify likely causes of current cracks. This information should be taken as a decision tool to plan strengthening interventions or restoration actions.
Unfortunately, many technicians actually involved in heritage conservation still work on the documentation of monuments in a rather traditional way. They use hand-measured survey methods and transfer these data to 2D paper drawings. The main drawback of this approach is that it is not accurate and all information is distributed in different types of documents (2D drawings, texts, photographs, etc.), resulting that getting an overview of the information available becomes a difficult task, as well as ex-changing this material with other technicians or researchers, or the distribution to the public. Commonly the geometric information source for structural analysis has been obtained through these hand made measurements, which are cost and time consuming and poorly accurate, or the original planes when they were available, even when it is well known that real structures use to differ from them. Furthermore, traditional devices used in crack measuring are contact demanding, such as those made out of paper, plaster, glass or plastic. They involve bulky risky systems for its placement in large high structures, and do not allow detecting stressed areas were cracks might occur in the future [2].
These considerations about the state of the art of the conservation discipline has leaded to conservationist’s and restoration’s to assert the importance of adopting advanced non contact (non-destructive) surveying techniques and rigorous scientific analysis methods to get the real complete knowledge of the current cultural heritage properties and state of decay [1].
In this paper, a multidisciplinary approach is presented which integrates close range digital photogrammetry, ground penetrating radar and finite elements analysis in the documentation of a masonry bridge. Close range digital photogrammetry and GPR techniques are used in the geometric survey and building material homogeneity analysis; they are also employed in cracks detection and mapping, since cracks are the external appearance of severe structural problems. Resulting information is used to properly define a finite elements based structural model (FEM), which is used to model the structural behaviour of the bridge in several load hypotheses. Results are compared to the cracks mapping. This comparison allows inferring the likely causes of the current state of decay in the bridge.
This methodology has been tested in the Fillaboa Bridge, a masonry monument which first construction date back from the roman period. Further reconstructions took place between the 13th century and 1605. Today, the bridge shows a crack distribution considered to be the external appearance of severe structural problems (see Fig. 4). The bridge is placed over the Tea River, in the Salvaterra de Mino Council, Northwest of Spain. Some studies conclude that the bridge is placed where an old roman bridge was built before (Alvarado et al. [3]). Several roman bridges are situated in the region of Galicia. All these bridges were useful to the roman gold mining exploitations present along the Mino Valley. The Fillaboa bridge includes four masonry arches of varying sizes (Fig. 1, Fig. 2). Its total length is approximately 75 m and it is 4.20–4.70 m width. The main arch is a roundheaded arch with a span of 16.34 m. The other three arches are pointed arches. All the arches, as well as the external walls of the bridge, are made of relatively small and medium ashlars joined by mortar. These masonry bridges contain a backfill that does not have a resistant function. This means that, during the bridge construction a continuous support from below the arches is needed. An arch is in compression throughout, and it cannot stand except as a whole. It therefore requires temporary support, or formwork, until it is complete. This type of formwork is called centring.
Section snippets
Close range digital photogrammetry
Image-based measurement techniques play an increasingly important role in virtually all natural sciences and engineering disciplines since they can provide amount of qualitative and quantitative information and knowledge about observed objects in a global, non-contact way with high spatial resolution. In the last years some interesting approaches have been developed involving the application of close range photogrammetry in the heritage documentation field. Some examples can be found in Alby et
Instrumentation
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Digital calibrated non-metric camera, Canon EOS 10D, 6,291,456 pixels CCD resolution. The calibration process was performed without zoom lens for the minimum diaphragm (maximum field of view). Calibration parameters are shown in Table 1.
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Circular paper targets with cross points.
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Monoscopic digital photogrammetric station. This system is based on the software package Photomodeler Pro 5.0. It is used for the orientation and restitution processes. The results are obtained in a graphic format (DXF; 3D
Method
The documentation of the Fillaboa Bridge has been sequenced in three steps, which are explained in detail in this section. The photogrammetric process is aimed at the geometric survey through the obtaining of the corresponding 3D accurate wire-frame model of the bridge and at the crack detection and mapping. The second step consisted on a non destructive test through GPR aimed at the analysis of the homogeneity of the interior material in the bridge and the detection of internal holes or
The photogrammetric results
In the data collection step several difficulties for keeping theoretical overlapping and convergence conditions arose from the narrowness of the bridge on one side and the impossibility of taking shots from mid river bed on the other side. The accessible locations were just the upper path and the river sides. The upper path was used for photographing the handrails, the own path and the cutwaters up and downstream; but given the small width of the path 114 were needed for recording it ensuring
Conclusions
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The photogrammetric surveying carried out, has allowed to define exactly the actual bridge geometry and its damage state (crack position, size, depth, etc.). An innovative aspect of this study has been the use of photogrammetry in order to provide a graphic pre-process data needed to apply a structural analysis by using the finite elements method.
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The present geometry of the vintage monuments usually differs from the original one, due to changes in the original structure, restorations and
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