Topographic Monitoring of the Operational Behaviour of the Longest Cantilever Viaduct in Romania

Over time, bridges have been built to fulfil different roles. With the development of industry came the need to build new transit routes linking industrial areas to cities or freight routes. The peak period of industrial development in Romania was in the 1960s, when a large number of bridges were built and put into operation. One of these bridges is the Sidex viaduct, located in Galaţi. The construction has a total length of 1,070 m, with 15 spans of different lengths (47.40 m + 13 * 75.00 m + 47.40 m). The resistance structure is made of 7 prestressed concrete frames linked together by joints located at the level of the cantilevers, the structure being the largest cantilever viaduct in operation in our country. The present article focuses on the presentation of the implementation of a special monitoring program of the viaduct’s in-service performance. For this purpose, special technologies have been used, such as topographic monitoring of the evolution of structural displacements at the most representative points of the structure, an implementation that we develop in detail.


Introduction
When in use, buildings in general are constantly subjected to the aggressive action of both the environment and the conditions of use [1].Over time, these actions directly affect the physicochemical properties of the materials from which the structure is made, leading to changes in structural response that can even impair or endanger the safety and comfort of users.
Bridges are also among the most affected by environmental action.They are also subjected to very high loads, both from their own considerable weight and from the action of cars.Given their specific nature, bridges must be capable of supporting the traffic they serve for a service period of around 120 years, a challenge accepted by managers.
In order to ensure optimal operating conditions [2] but also to be able to intervene in the shortest possible time in the event of substantial changes in structural behavior, various special construction monitoring technologies have been developed over time [3].
Special monitoring is the activity of monitoring the operational behavior of a structure by periodically measuring, recording, processing and interpreting the values of the parameters that define the extent to which the bridge maintains over time the strength, stability and durability requirements necessary to ensure safe traffic [4].
There is a very wide range of bridge technologies on the market for monitoring the most important parameters [5].One of the most commonly used bridge tracking technologies is topographic tracking.This allows regular monitoring of the evaluation of vertical and horizontal movements of various characteristic points, which can be done at a distance from the structure without hindering traffic in the area.
This technology has been successfully used for the Sidex viaduct, located in the industrial area of Galați, which provides the link between the city, the industrial platform of the SIDEX S.A. steel complex and the country road DJ 252.The structure was chosen for research and for the present study due to its particular static scheme, the viaduct being the longest cantilever bridge in operation in our country.1304 (2024) 012022 IOP Publishing doi:10.1088/1757-899X/1304/1/012022 2 Noting the high degree of degradation and advanced age, in 2019, the City Hall of Galati has commissioned a technical expertise to facilitate the process of starting the design and execution of rehabilitation and modernization of the passage.Analysing the structure and knowing the significant time periods required for an administrator to start the execution of the works, but also due to the specificity of the recommended extensive works, works that impacted the resistance structure, an extensive topographic follow-up program was implemented.This programme was carried out over an extended period of time, making records before, during and after the execution of the recommended works.
The article aims to present the application of a modern special tracking system for the SIDEX Viaduct.The technology used in the case under analysis monitors the evolution of the degradation of the structure by means of topographic measurements at specified time intervals, continuously comparing the new data captured with a set of reference data.Any change in the readings compared to the standard values was analysed and different threshold values of displacements were set, the exceeding of which triggers a strong alarm signal regarding structural safety.

Technology for topographic tracking of the operational behavior of a structure
Structural deformation monitoring of a bridge is part of the broad field of engineering condition monitoring (referred to as SHM in the literature), a field that is in full development thanks to the latest technological developments.
According to [6], [7] and [8], SHM includes all systems mounted on any construction, whether it is a building, a bridge, a wind turbine or an oil platform, whose main purpose is to inform the responsible personnel about the occurrence of any changes between the parameters recorded for the structure in good or very good working condition and the actual condition.
One such technology is the tracking of how a structure responds to permanent and live loads through topographic means.These methods [9] consist of the periodic measurement of points materialised on the structure by means of landmarks arranged on the visible faces of the component elements.There are two types of landmarks: • Movable landmarks, which were placed on the surface of the elements, i.e. on the heart of the beams and on surfaces that were not touched by the rehabilitation works before the works started.At a later stage, additional fixings were installed on the parapet beam, on the kerbs and in the longitudinal axis of the structure • Fixed landmarks installed on the ground, outside the area of influence of the construction, i.e. at the side of the structure for monitoring landmarks on the heart of the beam, and immediately after the access ramps, on an area that is stable in terms of terrain and from which landmarks installed at path level can be viewed A total station located at points considered fixed landmarks and targeting the moving landmarks is used to track the position of the moving landmarks.The height at which the mobile landmark is located is read and automatically recorded in the database.This reading is compared with a reading recorded when the monitoring starts, considered as a reference reading, the difference between them representing the change in position of the element or the whole structure.If these changes reach or exceed the limits imposed by the expert, the results of the stability and strength calculations are taken as soon as possible, and the necessary measures are taken, which may go as far as prohibiting traffic on the structure.

Sidex viaduct structure
The Sidex viaduct connects the centre of Galati to the SIDEX S.A. industrial platform, ensuring the continuity of the county road DJ 252.This viaduct was a first for the field of bridge construction when it was inaugurated in 1969, being the longest cantilevered road viaduct in Romania.
The viaduct is built from a prestressed concrete structure, with the superstructure forming the cantilever frames together with the infrastructure.This resulted in 15 spans of different lengths (47.40 m + 13 * 75.00 m + 47.40 m), the total length measured at the level of the splice covers on the slabs 3 being 1,070 m (Figure 1).In order to allow the trams to travel in optimal conditions, the longitudinal slope of the passage is 0.8 ‰.The superstructure consists of two boxes (Figure 3) with variable heights, with a minimum height of 2.00 m in the middle of the openings, which increases towards the reservoir area, reaching up to 4.80 m at the piers.
In cross-section, the two boxes are 4.35 m wide and positioned 8.00 m apart.The walls of the boxes are 35 cm thick in the central section of the openings, rising to 40 cm at the bases in order to take up the cutting force properly.For strength reasons, the bottom plate of the cassette also varies in thickness, from 18 cm in the centre of the opening to 50 cm in the areas where the ruler joins the frame posts.A particular feature of the structure was the way it was built, considered revolutionary for the time.Thus, the frame girders were made of 3.00 m long cassette sections, built of reinforced concrete in a special polygon located in the immediate vicinity of the site.The assembly of the sections was the main challenge of the execution, being cantilevered and fixed with pre-stressed cables.The two boxes are connected transversely to each other by means of a massive reinforced concrete slab.In addition, given the large spans, it was found necessary to lay out two other trusses in the field.In the longitudinal direction, the two boxes are also connected by means of a reinforced concrete slab, precompressed after curing.
The viaduct superstructure transmits loads to the foundation via the infrastructure.These consist of 2 reinforced concrete piers and 14 piers with massive elevations.The piers consist of two reinforced concrete columns with a box section (Figure 4), with a variable shape and height due to the specific terrain of the site, starting at 14.40 m for pile P14 and reaching a maximum height of 39.42 m for pile P4.
The foundation of the infrastructures is indirect, the good foundation ground in which the piles are embedded being at depths of between 11.50 m and 14.50 m.The piles are of the Franki type with a diameter of 600 mm, with an impressive 1,516 piles being built for all 14 piles.At the top, the piles are encased in reinforced concrete rafts between 3.50 and 4.50 m thick, varying for each pile depending on the plan dimensions and number of piles.
The bridge abutments are of the 4-post, sunken type, the bank with turned walls and guard walls being constructed of reinforced concrete.The foundations of the bridge abutments are made of 3 rows of 16 Franki piles embedded at 12.24 m (bridge abutment towards the combined) and 14.90 m (bridge abutment towards the Galati municipality) in the good foundation ground, where the loads are discharged.

The need and the purpose of monitoring
The viaduct was designed in 1966 and commissioned in early 1969.Analysing the design standards of the time and the specific traffic served at that time, it appears that the bridge was designed for Class I loading convoys (A30 truck convoy and V80 special wheeled vehicle), to which electric tram convoys were added.Over time, the payloads underwent changes, greatly increasing the values of the stresses they gave, which affected the strength structure that was already "weakened" by the occurrence of degradations.
These degradations occurred during the operating period and were mainly caused by the action of the environment.They were not dealt with in good time, and it is easy to see that maintenance work was not carried out at the intervals recommended by the standards.
In addition, during the period of operation, the structure was subjected to the action of several earthquakes with an intensity greater than 6.1 on the Richter scale, which seriously damaged the resistance structure.The damage caused by the earthquakes includes: • severe damage to infrastructure elevations and expansion joints, the most severe being due to the 7.2 Richter earthquake of 4 March 1977 • piers damage resulting from the cumulative stresses of the August 1986 and May 1990 earthquakes Following these earthquakes, the administrator intervened by reinforcing the infrastructure, carrying out exterior pile lining, caulking and crack injection.
In recent years, following a technical expertise which drew attention to the large number of degradation processes observed and their degree of extension, but also due to the particular constructive composition, it was decided to implement an extensive programme of special monitoring, aimed at achieving the following objectives: • ensuring the operational safety and durability of the structure by early detection of dangerous degradation processes and affected areas • monitoring the evolution of identified degradation processes • recording of operational behaviour and identification of ranges corresponding to normal operation for various parameters considered representative One of these parameters we will focus on in the article is the monitoring of superstructure displacements.By analysing their evolution, a programme has been developed to warn the administrator of the imminence of degradations with an increased impact on traffic safety.Depending on the displacement values identified and the structural element affected, the monitoring team created 3 warning thresholds, colour-coded.The administrator already knows how to intervene according to each threshold, thus minimising the reaction time needed to implement the necessary remedial measures.Table 1 gives an overview of the reference parameters used for monitoring the operational behaviour with topographic means and the warning values according to the 3 chosen thresholds.

Installation of topographic tracking devices and measurements
The equipment used to monitor the viaduct consisted of a LEICA TCA 1201 high-precision total station (Figure 5) and a LEICA DNA 03 high-precision level (Figure 6).These devices have the most appropriate characteristics for the specific works and can be easily used by the administrator's staff during the current maintenance of the works.Figure 6.LEICA DNA 03 level [11] The measurements also required the installation of tracking targets made of reflective sheeting (Figure 7), so that they could be easily seen from convenient distances.These markers were mounted in characteristic sections, i.e., in the central section and in the area of the bearing apparatus for each opening.

Figure 7. Reflective topographic target [12]
In order to be able to follow the behaviour in operation, the most representative points on the bridge were chosen, points whose displacement characterises the whole structure.At the same time, account was also taken of the period of perspective for which the topographic monitoring system is to be used.
The installation of the whole monitoring system is uncomplicated and easy to install, which helps managers who want to carry out monitoring at minimum cost.
Therefore, as a first step, tracking targets were installed on the right side of the beam heart, in the central sections of the spans and in the resemination sections.Given that rehabilitation and upgrading works were started during the period set for monitoring, and at the request of the administrator who wished to be able to easily continue the current monitoring of the structure, additional topographic benchmarks will be installed at the parapet beam level, on the kerbs and in the longitudinal axis of the viaduct upon completion of the rehabilitation works.Topographic markers have also been installed at the expansion joint covers for spans 3, 5, 9 and 13 (where the structure's joints are located) to monitor their behaviour under the action of road traffic or in the event of an earthquake.
The fixed reference points, where the total station or level is installed during the measurements, have been chosen so that all coordinates of these points are known, and all measuring points on the structure can be targeted.
After completion of the rehabilitation works and definition of the new measuring points, it was found necessary to modify the fixed landmarks as well.Thus, other points of known coordinates were identified, this time in the immediate vicinity of the access ramps, points that allow the visualization of the characteristic points on the structure.
In order to achieve the most accurate monitoring possible, particular attention has been paid to how measurements are taken.Thus, at the time periods recommended in the special monitoring project, the team of surveyors carried out repeated measurements on all points in the network.This was calculated in a local projection system and determined as a constrained grid.The offsets required for the reading were carried out using the indirect measurement method, i.e. the least squares method.The methods used were chosen because of the small degree of error in the determination of the points that can occur, an error that was quoted towards the minimum of the range recommended by the specific quality requirements of the work.
Also, with the LEICA TCA 1201 total station at hand, using the laser beam function provided, combined with the erasure method, measurement targets were targeted from at least two known coordinate monuments, reducing the risk of errors to near "0".For verification it was recommended that for at least one third of the targets the measurements should be taken from three different fixed monuments.

Evolution of the in-time structural behaviour
Following a current review, in 2015 structural deterioration was found to have progressed, endangering the safety and comfort of users.At that time, the administrator commissioned the preparation of Design Documentation in order to obtain the European funds necessary to start the rehabilitation works.One of the stages of the documentation included determining the elevations of the most important points of the structure.At that stage, it was concluded that the uncovered weak point was located between joint 4 and joint 5, where 8 points were chosen for reading the elevations as shown in Figures 8 and 9.The absolute coordinates, determined on 20.06.2015, are shown in Table 2.
When the administrator decided to start a new monitoring program in early 2021, the responsible technical staff considered it appropriate to use the data measured in 2015 as baseline data.Then, from 26.03.2021, regular readings of the most important points were taken, which were reported to the initial reading.
Following the first set of measurements, a subsidence of the analysed area could be observed, which accumulated in more than 4 years, an average of about 7.7 cm, the breakdown of subsidence for each analysed point can be seen in Table 3 below.The responsible staff, together with the technical expert, decided to continue monitoring for about 1 year.Thus, a consolidation of heaps was observed during the warm season (Figure 10), but also an increase with the arrival of autumn.This dynamic is due to the site's specific soil and bedrock characteristics.Also, if we compare the measurements of the 4 sections of the analysed area, we can observe that the maximum displacements are found in the right of the points 7 and 8 of measurement.This observation draws a strong alarm signal regarding a possible occurrence of local subsidence phenomenon only in the area of the pier P4, a phenomenon that may endanger the structural safety.
It should be noted that during the measurements on 14.04.2022 it was found that the materialisation element of measurement point 4 had been destroyed, which made it impossible to continue measuring it.
Analysing the graph of cumulative landings (Figure 11 and Table 4) we can see a relatively linear increase over time.By extrapolating this evolution assuming that the rate of increase is maintained in the coming years, the administrator can estimate the time remaining before the alarm values shown in Table 1 are reached.This can help the administrator to underline the need for rehabilitation works, which is one of the strongest arguments needed to obtain European funds.

Conclusions
The widespread implementation of modern systems for monitoring the operational performance of bridges is an important step towards knowing the state of viability of the entire road network in

Figure 1 .
Figure 1.Side view of the viaduct.The viaduct's resistance structure consists of seven frames made of prestressed concrete, joined at the top by joints.Given the specific traffic characteristics of the county road and the road serving the Iron and Steel Works, in cross-section, the roadway on the bridge has a total width of 18.70 m, consisting of a carriageway serving two tram lines and two car lanes totalling 14.00 m, plus the width of the two pavements, i.e. 1.90 m each (Figure2), a considerable width given the location of the viaduct within the town.

Figure 2 .
Figure 2. View of the path on the bridge.

Figure 3 .
Figure 3.View of the box beams of the superstructure

Figure 4 .
Figure 4. View of the P4 pile

Figure 8 .Figure 9 .
Figure 8. Location of measuring points in side view

Figure 10 .
Figure 10.The settlement compared to the previous reading, in mm.

Table 1 .
Alarm limit of reference parameters.

Table 3 .
Settlement compared to previous reading, in mm.