Since past masonry structures were built by time-honored method of trial and error. Medieval time masonry unions use time approved methods of masonry structure cross sections, shell geometry, proportions and forms. An objective of investigation is deformed structure of Riga cathedral in Latvia capital city. 13th century built masonry Gothic arch shells, which are built from solid clay bricks, are one of the first masonry shell covered structures in Latvia. Cross arch and star arch shell where commonly used as type of heritage building shell. Long building process and changes in finale view of cathedral was usual by-effect in building process centuries ago. Riga Dome Cathedral had long erection process with stop breaks and design changes. Few steps of erection and enlargement by changing city demands took place also. Tower and nearest roof parts were lost due to fire happened in 1547. The supporting piles and subsoil situation in Riga Dome cathedral are survived today and found in the unsatisfied conditions. For lot of buildings wooden piled foundations are used to transfer loads to dense sandy layer through loose sand located under footings. Wooden pile lifetime practically is not limited underground water minimum level but the piles state after eight centuries of building exploitation has been evaluated as unsatisfied.
Structural weakness or overloading, dynamic vibrations, settlement, and in-plane and out-of-plane deformations can cause failure of masonry structures. To prevent the accidental situation in heritage buildings safety criteria must be specified by determining deformation limit between interacting parts of masonry arch shell.
To ensure measurement of cracks displacement in Riga Dome Cathedral an automated cracks monitoring program has been developed, which allows for assessing the deformation tendency in the real-time mode and focuses on the zones with the highest deformation dynamics. The new method allows enhancing the accuracy of the existing monitoring program measurements through the analysis of the obtained results as well as substantiates the necessity of further research.
The cracks monitoring system installed in 2007 in Riga Dome Cathedral performs consecutive cracks measurement at 22 points. For assessment of cracks dynamics measurement of crack openings is performed automatically at two-hour intervals. By analyzing the load on Riga Dome Cathedral masonry vaults and cracks displacements further service safety of the building is forecasted.
SOFO (Surveillance des Ouvrages par Fibres Optiques) long base deformation sensor is a fiber optic sensor system made by Swiss Company Smartec. Long Base Deformation Sensors have been installed in 2007 to perform long term crack monitoring. The main reason of long base fiber optic deformation sensor usage is computational control of measurement data, long lasting life period and minimized casual factor’s influence on measurement’s quality.
The SOFO measuring system has resolution of 0.002mm and precision of 0.2% of the measured deformation. Smartec SOFO Lite signal conditioner allows control of a group of 12 fiber optic sensors, two conditioners have been installed in an enclosure to monitor 24 sensors, and remote communication is performed via an industrial PC and modem. Additional thermocouple sensors for external and internal temperature control are included in measurement program. Supports deformation can be evaluated from changing of cracks opening and as result defined criteria of safe exploitation building.
As example, SOFO long base deformation sensors readings in Figs 1 and 2 show seasonal crack oscillations by deformation sensors 5735 and 5740 in four-year period.
The sensors highlight hinge rotation mechanism of shell rib. Support deforming leads to crack opening at position of sensor 5739. Deformation measured by sensor 5735 is mainly due to seasonal temperature and humidity changes. Deformation progress highlighted by sensor 5739 is 0.06 mm/year. A total of 21 deformation sensors measure crack oscillations every two hours. Structure deformations as an exploitation condition and loading effects are recorded for future numerical model validation.
Additional 18 deformation sensors were additionally installed at the vaults of the Dome Cathedral in 2019. This system envisaged for expanding monitoring on second part of the Dome Cathedral building. The sensors were installed at the high 13-18 m from the floor level and are collecting information about opening and closing of the cracks in the masonry vaults.