Monitoring systems for cubature constructions and engineering constructions are used to increase their safety of use and reduce the costs of snow clearance (snow clearance, including roof repairs after snow clearance).In 2009, this kind of monitoring was recommended to be applied in public use buildings in Poland. Automatic measurements allow collecting a lot of information about loads and how construction reacts to them. On this basis, safety parameters that define stress of the construction are evaluated. The measured variables are as follows: loads (e.g. snow loads), deformations (stress) and displacements (bending). Measurements are taken from representative points and construction areas.
The direct measurement of snow load is the simplest form of monitoring. It is reliable only if the weight of snow is considered – weight that differs depending on climatic conditions and the conditions of the area where snow is lying (shape of the roof, height above the ground, temperature on roof surface, vicinity of other buildings etc.). There are automatic methods of measuring the weight of snow, nevertheless in the majority of cases, it is measured manually, which requires taking a series of measurements and is ineffective. Automatic systems, which measure other variables and provide continual information about how construction reacts to loads, solve the problem. In fact, a user is interested in the information about the safety of construction object and load is an element (not the only one) of it.
Snow loads are only one of many loads that influence construction and in many cases, constitute only a small fraction of total loads. In the majority of cases, bending, due to these loads, increases by a few or a dozen or so millimetres, thus requires high accuracy of measurements to be noticed. Measurement of displacements informs about the bending of, for example, roof girder, nevertheless to get such information additionally the displacement of all the supports needs to be measured. The assumption that the displacements of supports are the same, thus reducing the number of points of measurements may result in mistakes. In the majority of cases, displacements are measured by means of geodetic methods (total stations, rangefinders) or by means of dumpy levels that are not very accurate, as regards the expected displacements. Total stations and rangefinders require a stable point of reference which in case of total stations, in cubature construction objects is the floor. This makes it difficult to change the arrangement of rooms and the optical elements need to be cleaned occasionally. Access to sensors cannot be restricted even in case of battery-powered sensors. Displacements can be measured accurately by means of inductive sensors, nevertheless it is problematic to have them mounted in a big construction object, especially that they constantly need power supply to provide continual measurements.
The measurements of deformations are much more accurate. These measurements can be made by means of strain gauges, nevertheless they are not very durable and similarly to inductive sensors, require a constant power supply. String sensors do not have such flaws (deformation sensors are mounted in two points where deformations are determined on the basis of the frequency of vibration of the inside string) similarly to optical fibre sensors. The oldest working string sensors are 30 years old now.
“SnowMonitor” by Neostrain that aims to determine safety level connected with snow loads makes use of string sensors. The system comprises deformation sensors (additionally equipped with temperature sensors), a transmitter (transforms signal from extensometers into a computer-readable signal) and a computer. All the elements are connected with a wire that aims to power the system and send signals from the sensors to the transmitter and to the computer. This technology allows avoiding signal interferences, measurements can be taken with any frequency, not being restricted by the life of power supply. For this reason, it is also possible to take measurements in the summer. In addition, the system is equipped with its own emergency power supply even though information is not lost in case of power cut.
Deformation sensors are grouped and located in measurement points and monitoring areas. Monitoring areas, on the other hand, are located in representative roof places, e.g. where snow keeps laying. Well-planned sensor distribution allows detecting gravity loadings (e.g. snow, water) and eliminating temperature and wind influence, which is not possible in case of, for example, displacement measurements. What distinguishes the system is the calibration divided into two stages: first happens on the basis of project measurements or trial loads, the second stage takes place in winter on the basis of snow loads, which assures the system works appropriately and additionally allows determining alert thresholds that inform the user about safety, as regards snow loads. As a rule, three thresholds are determined: the first one informs that snow clearance needs to be prepared, the second threshold informs that snow clearance needs to be started and the third suggests that people leave the building. All information is saved on computer disk and is available to authorised people via Internet.
To conclude, it needs to be admitted that currently, “SnowMonitor” is the most advanced system of monitoring roofs under snow load.
Roman Wróblewski PhD Eng.