Supporting Historical Structures Technical Condition ... - Science Direct

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ScienceDirect Procedia Engineering 195 (2017) 32 – 39

18th International Conference on Rehabilitation and Reconstruction of Buildings 2016, CRRB 2016

Supporting historical structures technical condition assessment by monitoring of selected physical quantities Łukasz Bednarskia, Rafał Sieńkob,*, Tomasz Howiackib a

AGH University of Science and Technology, Mickiewicza Al. 30, 30-059, Poland Cracow University of Technology, Warszawska St. 24, 31-155 Kraków, Poland

b

Abstract Assessment of historical structures technical condition is difficult due to the lack of technical documentation and inability to clearly identify the strength-strain characteristics of materials, from which they were erected. Thus, creating numerical models of objects made many years ago, is very complicated and the results reliability is very limited. The article describes an example of structural health monitoring system application, which aim was to perform continuous, remote and automatic measurements of selected physical quantities, important for crack state estimation in the walls and other structural members in historic, parish St. John the Baptist Church in Jangrot (Poland). When designing the system, the attention was paid to minimize costs by selecting optimum number of measuring points and an appropriate measuring technology, as well as a small interference in historical substance. Measurement data collected during 12 months were the basis for assessment of structure technical condition. 2017The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © 2017 © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 18th International Conference on Rehabilitation and Peer-review underofresponsibility of the organizing committee of the 18th International Conference on Rehabilitation and Reconstruction Buildings 2016. Reconstruction of Buildings 2016

Keywords: crack monitoring, structural health monitoring system, historical structures

1. Introduction Assessment of historical buildings technical condition is difficult due to the lack of technical documentation and inability to clearly identify the strength-strain characteristics of materials, from which they were erected. Thus,

* Corresponding author. Tel.: +48-502-646-975 E-mail address: [email protected]

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 18th International Conference on Rehabilitation and Reconstruction of Buildings 2016

doi:10.1016/j.proeng.2017.04.520

Łukasz Bednarski et al. / Procedia Engineering 195 (2017) 32 – 39

creating numerical models of objects made many years ago, is very complicated and possibility of obtaining reliable results based on this models is very limited. This article describes an example of application of structural health monitoring system, which aim was to perform continuous, remote and automatic measurements of selected physical quantities, important for crack state estimation in the walls and other structural members in historic, parish St. John the Baptist Church in Jangrot (Poland). The structure was built with ceramic bricks in the years 1822 - 1824 by the Cracow architect Jan Beka, in the NeoByzantine style, founded by priest Jan Paweł Woronicz. Inside there are 5 altars (including the main altar dating back to years 1892-1893).

Fig. 1. The view of parish St. John the Baptist Church in Jangrot from the south-east side

During the church technical condition inspection in June 2015, on its walls numerous cracks and even ruptures have been found. When planning repair way of such kind of damage, it was important to acquire knowledge if the cracks are changing its width over the time and, if such changes take place, what is the reason of its appearance. In some cases, the crack may be the first sign of deteriorating structural condition, and determination of crack width changes over the time could be very helpful in assessing the progress of hazards affecting the building safety [1]. Taking into consideration the main aim of structural health monitoring system, during its design the particular attention was paid to two aspects: x economical: cost minimization through the selection of appropriate measurement techniques (which allow for reuse of sensors) and the application of optimal number of measuring points; x visual: minimum interference into historical substance of the Church.

Fig. 2. Interior of parish St. John the Baptist Church in Jangrot

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Fig. 3. The example of crack on interior wall of the Church

2. Description of structural health monitoring system In order to observe any changes in crack width on selected structural elements four vibrating wire (VW) displacement sensors Geokon model 4422 were installed. VW sensors are characterized by high accuracy, resistance to environmental impacts and long-term stability of measurements, hence they are widely used in structural health monitoring systems [2, 3, 4]. The measurement data were recorded remotely and automatically with a frequency of 4 times per hour and stored in non-volatile memory of the server, on which special measuring platform was installed, dedicated for monitoring systems. This application allows for managing a large number of data, including the possibility of creating of any computational models, presenting the results in real time, eg. in the form of charts, or archive the collected information. The main task of the software, however, is the current analysis of raw measurement data by converting it into the quantities important from engineering point of view. In this case, the change of natural frequency of string in displacement sensor was converted into the change of crack opening width (mm). The values of crack width change obtaining this way are characterized by accuracy of 0.005 mm. During the measuring of crack width changes, the air temperature was also measured in the vicinity of the walls, on which measuring devices were installed. As a result, it was possible to provide thermal correction for measurements obtained from vibrating wire sensors, and assessment of global construction work loaded by variable temperature was facilitated. Therefore, it was possible to answer the question whether the changes in crack width result only from thermal changes or mechanical loads, that could be potentially dangerous from Church technical condition point of view.


Fig. 4. Exemplary measuring point – vibrating wire displacement sensor Geokon model 4422

Automatic monitoring was supplemented with eight measuring points, in which manual SHM-X crackmeters were installed in basic configuration, ie. allowing for realization of measurements only on the direction perpendicular to the crack. During the measuring period this crack width indicators were read with variable frequency using a caliper applied to appropriate notches, located on measuring pivots, installed on both sides of the crack. Manual measurements were performed with accuracy of 0.05 mm.

Fig. 5. The scheme of SHM-X crackmeter [12] (a) and example of measuring point (b)

Installation of measuring devices was performed in June 2015. Then the readings of crackmeters were also made, which were treated as reference readings, as well as the zero values for automatic vibrating wire sensors were defined on measuring platform. In this article, all presented values of crack width changes shall be treated as relative values, referred to the initial state. Fig. 6 and 7 show a top view as well as longitudinal cross-section of the Church with location of measuring points: both manual crackmeters and automatic vibrating wire displacement sensors.

Fig. 6. Location of measuring devices – top view

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Fig. 7. Location of measuring devices – longitudinal cross-section

3. Interpretation of results In presentation of measurement data the principle was assumed, that plots of crack width changes (blue line) will be presented together with air temperature changes (green dashed line) in the immediate vicinity of the wall surface, on which adequate sensor is installed. Displacements corresponding to decrease in crack width was marked with "-" sign, while increase was marked with "+" sign. Collected measurements results were processed by removing electromagnetic interference. Against the background of measurement data, trend lines of measured physical quantities are presented by a sixth degree polynomial. As a result, it is possible to transparently observe a long-term structural work in the context of changing thermal conditions.

Fig. 8. Point D1: changes of crack width on the background of temperature changes





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Observation of changes in crack width including temperature measurements, indicate for clear impact of thermal changes on crack width over the time. This effect varies depending on the location of the damage and its statistical representation may be a correlation coefficient KΔlΔT. Its estimator is given by the following equation: n

K ΔlΔT =

∑ ( Δl

i

i=1

n

䌥( Δl i=1

i

Δla )( ΔTi Δla ) 2

ΔTa ) (1)

n

䌥( ΔT i=1

i

ΔTa ) 2

where: n – numer of samples in whole measuring period, i =1… n – index of i-th sample, Δl – measured change of crack width, ΔT – measured change of temperature, Δla – averaged value of crack width change, ΔTa – averaged value of temperature change. Thus the values of coefficients obtained this way are as follows: x x x x

Correlation coefficient in D1 point: -90,5 % Correlation coefficient in D2 point: -85,2 % Correlation coefficient in D3 point: 97,6 % Correlation coefficient in D4 point: -91,7 %

The calculated correlation coefficients exceeding the absolute value of 85%, indicate that there is a very strong correlation between changes in crack width and the temperature value. The largest dependence was observed at a point D3 (nearly 100%), located on the lower surface of inner wall arch keystone in south matroneum. At this point, crack width was changing in direct proportion to the temperature changes. In all other measuring points this relationship was inversely proportional, ie. with increasing temperature, crack width decreases. Different structural behavior at D3 point may be due to the actual boundary conditions and static scheme, forcing this kind of arch operation under the influence of temperature changes. The biggest change in absolute value was recorded in point D4, located on the lower surface of outer wall arch keystone in the same matroneum. For this sensor it was also the biggest change in crack width. In analysed period of time the maximum difference was approximately 0.8 mm. In other points, these values did not exceed 0.3 mm. Analysis of measurement data from points D1-D4 clearly indicates that the operation of cracks located within the church walls is mainly related to thermal impacts, and negligibly with mechanical. Because there is a slight difference between the outside temperature and the temperature inside the Church, it must be concluded that the crack behaviour is mainly connected with deformations derived from axial compression and tension of the walls, and less from the effects of bending moments. The results of measurements carried out with the application of manual crackmeters indicate an inverse relationship between changes in temperature and crack width, ie. with increasing temperature, the crack width decreases, and vice versa. Crack width changes in absolute value, however, were slight and in the considered period did not exceed 0.15 mm.


4. Conclusion Measurements of physical quantities important for crack state of historic church in Jangrot, conducted throughout the whole year, revealed that: x there is no significant effect of mechanical loads (related with eg. the uneven subsidence, overloading of structural members, etc.) on crack behaviour within the church walls, x there is a direct relationship between crack width changes and temperature changes. Analysis of data obtained from structural health monitoring system showed, therefore, that the existing cracks do not affect security state of the parish Church in Jangrot. During the church construction renovation design it was recommended to analyze the rightness of application of permanently elastic materials to perform fulfillment in existing cracks and ruptures. The material having a high elastic modulus can cause the appearance of crack or rupture at the location spaced few or over a dozen centimeters from the repaired damage. Continuous measurements of crack width changes using automated devices are increasingly applied for the realtime assessment of structures technical condition and as a basis for preparing expert elaborations. Today, scientists and engineers worldwide are working on the new measurement solutions, enabling even more complete analysis of crack state in masonry structures and made of concrete (reinforced concrete, prestressed concrete). Promising results in this regard, confirmed by numerous studies [5, 6, 7, 8], provides optical fiber measuring technique. Its advantage over traditional spot-measurement techniques, is the possibility of carrying out strain and temperature measurements continuously along the length of structural member [9]. This allows for the replacement by a single optical fiber a very large number of spot sensors [10] and, as a consequence, it also brings economic benefits. In the near future, optical fiber measuring technique has a chance for practical application in a number of engineering and expert issues, including structure crack state analysis, both to locate potential damage (cracks) as well as for quantitative evaluation [11].

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