Since past masonry structures were built by the 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 the investigation is the deformed structure of Riga Cathedral in Latvia’s 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 shells were commonly used as types of heritage building shells. The long building process and changes in the final view of the cathedral were usual by-effect of the building process centuries ago.
Riga Dome Cathedral had long erection processes with stop breaks and design changes, few steps of erection and enlargement by changing city demands also took place. The tower and nearest roof parts were lost due to a fire in 1547. The supporting piles and subsoils in the Riga Dome cathedral survived today and are found in unsatisfactory condition. For many buildings, wooden piled foundations are used to transfer loads to dense sandy layers through loose sand located under the footings.
Structural weakness or overloading, dynamic vibrations, settlement, and in-plane and out-of-plane deformations can cause the failure of masonry structures. To prevent the accidental situation in heritage buildings safety criteria must be specified by determining deformation limits between interacting parts of masonry arch shells.
To ensure measurement of cracks displacement in the 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 for 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 monitoring system installed in the Riga Dome Cathedral performs consecutive cracks measurement at 22 points. For the assessment of crack dynamics, the measurement of crack openings is performed automatically at two-hour intervals. By analyzing the load on the Riga Dome Cathedral masonry vaults and cracks displacements, further safety service of the building is forecasted.
SOFO (Surveillance des Ouvrages par Fibres Optiques) long base deformation sensor is a fiber optic sensor system made by Smartec. Long Base Deformation Sensors have been installed in 2007 to perform long-term crack monitoring. The main reason for long base fiber optic deformation sensor usage is computational control of measurement data, long-lasting life period, and minimized casual factors’ influence on measurement quality.
The SOFO measuring system has a resolution of 0.002mm and a 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 the measurement program. Supports deformation can be evaluated by changing cracks opening and as a result defined criteria for safe exploitation building.
For example, SOFO long base deformation sensor readings in Figs 1 and 2 show seasonal crack oscillations by deformation sensors 5735 and 5740 in four years.
The sensors highlight the hinge rotation mechanism of the shell rib. Support deforming leads to the crack opening at the 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 expanding monitoring on the 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 the opening and closing of the cracks in the masonry vaults.
