Role of technical monitoring and computer modelling in protection of historical and architectural monuments
In the 21 century we have an opportunity to enjoy the sight of unique monuments of history and the architecture, which preserved and reached up to our time. It became possible due to the fact that during the life of the constructions they were maintained in operational status by our contemporaries. Scientific elaborations have been constantly conducted on such objects, which the results are very topical now.
Unfortunately, many of unique constructions are located in the mega cities with intensive going constructions in the surrounding area. That makes a negative effect on monuments of history and architecture, namely:
Increase of bad effect from constructions on the foundation of considered objects, that can cause additional settling of such objects from mutual influence of the foundations.
It changes the level of subsoil waters and the direction of their flow. There is a probability of soil wetting under the base of foundation, having soil settling properties.
It increases the probability of technogenic failures (break of engineering services ).
It increases additional dynamic influence at object, for example, from passing subway in the nearby territory, or increasing road traffic.
Besides there is also a probability of seismic effect on historical constructions, located in seismic activity area, or reachable seismic waves.
If the construction has got network of the developed power cracks, even little seismic effect can lead to the reduction, or loss of bearing ability of the constructive system.
To preserve historical buildings for future generation is also a mission of contemporaries. One of preservation options for the monuments can be the elaboration and creation of monitoring system, which the base shall be a computerised computation model.
Such work has been executed for the Saint-Sofia Cathedral of Sofia Kievskaya reserve. Elaboration of computation model allows to compile and generalize the information about the construction and about the physico-mechanical properties of material and the bases below the foundation. Performance of computation spatial settlement model together with the bases, will allow to define the mode of deformation condition of investigated construction, to define the most problematic parts, to estimate the probability of creating of new cracks at various additional influences.
Let's consider stages of information compiling for the elaboration of computation model. Considering that normally, monuments of history and architecture are investigated for a long period of time, first of all we need to study the references.
It's necessary to generate a set of construction drawings-plans, facades, cuts. For this purpose they use either the existing information (archives data) or carry out new measurements of the construction. During the elaboration of computation model of Cathedral both options were used. First, the existing package of drawings was not full, secondly the construction of Cathedral has a complicated irregular structure in plane and on height, and to verify the numerous data of drawings they had to make additional measurements in order to create the proper computation model. One of the options of construction measurement, can be surface scanning (from the internal and external side). Performance of scanning works should be accompanied and controlled by precision sensor measurements.
As experienced in the past, when using the construction scanning, essential discrepancies between the measurements (executed by means of measuring tools and by means of scanning were noticed.
Probably, it's expedient to use the scanner only in remote places (for example, for the definition of the sizes of vaulted overlaps).
For the creation of correct computation model, it is advisable to have the measures as much precise as possible and close to the original sizes of the construction.
Let's consider in details the information needed for the elaboration of computation model.
When researching the walls they consider the presence of niches (height, depth, width), changes of walls configuration, variation of thickness, presence of apertures (including arch type). Besides, it is advised to underline the following fact. In the wall constructions of the Sofia Cathedral (during early reconstruction) in order to increase the spatial rigidity, they have built in metalic items: tension bars, inhalings, cramps. It is necessary to define the element profile, their arrangement in plane and height, degree of joint inclusion in bearing constructions. Such constructive protection can appear at other monuments of history and architecture.
Picture 1. Fragment of the computation model - east wall of the Sofia Cathedral where you can see the complexity of the relief.
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| Picture 1 - Fragment of the computation model - east wall of the Sofia Cathedral |
Overlaps and coverings. Special attention needs to be drawn to overlaps in the form of vaulted systems (cylindrical, crossed sailing, in the form of sphere, etc.) - height, diameters, line of arch components crossing. It is important since that on the level of the rest point of such overlaps on walls (depending on the form of the arch) there are horizontal reactions to the walls (on the rest point), that creates additional pressure and deformations.
The coating is included in the computation model if it is a part of a bearing system and effects the rigidity of the construction in total. If it is, for example a wooden structure, it is supposed to consider to be a kind of loading on bearing constructions.
Foundation system. Normally, the foundation system at the active exploitation of the construction is located in the bases.
For the reconstruction of general view and creation of foundation system drawings, first of all, it is necessary to study references from literature and archival materials as, probably there you can find the necessary information. If it's not available, it is required to open the foundation in several accessible places by driling the holes, in order to define the bases' depth and to estimate properties and structure of their substance.
At the time of performed works with Sofia Cathedral computation model, drawings of the foundations did not exist. For the development of conceptual view of foundation system of the Sofia Cathedral they used data about width, depth batter in accordance with holes, which were made by the scientists of the previous generation during the reconstruction. On the basis of those information all data was generalized and the conceptual view of foundation system of cathedral was recreated. Picture 2. The view of the Sofia Cathedral foundation system computation model, elaborated and used in the computation model.
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| Picture 2 - Conceptual view of Sofia Cathedral foundation system |
As the Sofia Cathedral is acting museum, it was not possible to make the natural researches on the foundation system. During the monitoring system creation, the depth of the foundation along the contour of building was tried to define by the acoustic device. This work has been determined negatively, as the obtained data was not matching the foundation description found in the literature references and archival depository.
The results of the construction system research and the creation of the computation model brought up the amplified set of drawings made according to data, received during the work on the creation of the monitoring system.
The computation model is the interrelated copy of the existing construction. Thus, when elaborating the model there sometimes occur incompatible sites, which need arrangements for additional gaugings and measurements in order to eliminate the discrepancies.
Therefore such works (creation of the construction computation model, elaboration and editing of sets of drawings) are recommended to conduct simultaneously, as it allows practically authentically (with the minimum idealization) to develop the computation model in full conformity with the corrected drawings.
In the computation model of the construction there used physico-mechanical characteristics of construction material as basic data. For this purpose bearing structures are necessary to investigate as by destructive so by non-destructive methods. The destructive method considers the test of samples for the definition of strength at compression and bend. Samples of layings can be selected in museum depository, or in other accessible way.
Obtained data is used, as the standard sample, at definition of elements durability of constructions on different parts of walls by non destroying acoustic methods.
For the definition of physico-mechanical properties they make ground geological researches within the limits of the plane of investigated construction, or they use data, which is obtained earlier.
The strained condition. Now the construction of the Cathedral has a significant number of breaks and cracks, supervision for which is conducted by the museum staff. All fixed cracks and breaks, included in the computation model.
The computation model of Saint-Sophia Cathedral bearing system and photo of east side.
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| Picture 3 - Computation model of Sofia Cathedral bearing system |
Before they start jobs, developers must study the object visually in order to define the most acceptable way of structure modelling (application of flat, three-dimensional, shank, final elements, etc.)
It is necessary to note, that when elaborating the computation model they used three-dimensional final elements (FE), because the thickness of walls was differentially changing. They also used flat and rod final element FE.
The configuration of cathedral walls, arch systems of apertures, vaulted overlaps and also Northern and Southern towers - have a very complicated structure. Nevertheless, developers of the model tried their best to execute a model as much close as possible to the original forms.
On picture 4 there are fragments of the computation model of Northern and Southern towers.
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| Picture 4 - Fragments of the computation model of Northern and Southern towers |
The complexity of the construction is unique. Overlaps of towers look like torus.
The loads accepted in the computation model.
Calculation are done based on the dead loads, temporary snow weight loads and on dynamic influence from actual accelerograms, defined by Geophysics research Institute.
Compilation of dead loads on bearing system was an important stage in series of the spade-works performed. First of all it is a complicated structure of walls' laying.
The cathedral foundations are combined basically from rubblework. The percentage of use of natural stones was decreasing from the ground level of walls to the 2-nd floor and also in places of arch apertures, in the walls of the 1st floor and bearing walls. All those factors have been considered in the computation model. The weight of the plastering was considered as well. A considerable layer of plaster of the different thickness was put on the walls, adding weight on the walls of Cathedral. Overlappings on the first and second floors, as mentioned above, represent the vaulted system. In bosoms of the arches they made fillings on which floors of the second floor were laid. All this also was taken into consideration while determining and estimating the loads.
The computation also has been made subject to wetted subsidence of the bases to have a possible effect on soaking from the western part. The computation of the construction was carried out on the basis of its spatial computation model elaborated for the calculation of the foundation. For the computation they used "LIRA-WINDOWS" program, the basic method of which is the use of final elements in dynamics (elaboration of NIIAS-Kiev scientific research institute).
As a result of computation they got a mode of intense-deformed condition of Cathedral bearing system, they have analysed the walls durability, heel of the foundations, dynamics of the cracks development.
By having such a model in a monitoring system, in case if it's needed it's possible to add the elaborated constructive protection in it and to check how it works in the existing structure of the construction.
Also, in case of occurrence of new influences it is necessary to apply the elaborated model in order to estimate the degree of such influences.
To show how the model works thereby in picture 5 there are the isofields of movings on axis OX.
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| Picture 5 - Isofield of movings on axis OX |