Why temperature changes every reading

Building materials expand when warm and contract when cold. Steel and concrete do so to a similar extent, around one hundredth of a millimetre per metre and degree. That sounds small, but it adds up: a 100-metre steel superstructure becomes about 36 millimetres longer or shorter over a temperature difference of 30 degrees. This movement is intended, and bearings and joints are built for it.

For monitoring, this means that almost every measurand on a structure carries a temperature component. Cracks open when the component cools and close when it warms up. Girders bend when the sun heats the top more than the bottom. Bearings move over the day and over the year. This component is often larger than the change that actually matters.

Component temperature is not air temperature

A component follows the air temperature, but not immediately and not evenly. Massive components respond with a lag because they store heat. Components in the sun become considerably warmer than the air, while components in the shade stay cooler. On a steel girder, the top flange may be in the sun while the bottom flange stays in the shade.

Design therefore distinguishes between a uniform temperature component, which makes the member longer or shorter, and a temperature difference across the depth of the cross-section, which makes it curve. This is also how the standard for thermal actions, EN 1991-1-5, describes it. For monitoring, it follows that anyone wanting to understand the effect of temperature on a component may need the component temperature rather than just the air temperature.

Three ways to the temperature

Which temperature is needed depends on the question. There are three options, which can also be combined.

  • Sensors on the component. Temperature sensors sit directly on the component surface and measure the component temperature. Several sensors per cross-section show the differences across height and width. Air temperature is measured by a sensor in a ventilated radiation shield.
  • Weather station on the structure. It records air temperature, humidity, precipitation, wind, air pressure and global radiation exactly where measurements are taken. Solar radiation and weather can then also be read as causes of a movement.
  • German Weather Service data. The German Weather Service (DWD) publishes the readings of its stations as open data. To correct an existing monitoring system, the nearest station is used, with no additional device on the structure and also retrospectively.

How temperature correction works

The correction determines from the data series itself how strongly a measurand responds to temperature and with what lag. To do this, measurand and temperature are analysed together over many temperature changes. The relationship gives the share that comes from temperature alone. Removing it from the data series leaves the trend: the change the structure itself shows.

In a project on a motorway bridge, the crack width followed the air temperature with a lag of four to five hours, by 3.9 to 5.9 micrometres per degree. The median daily fluctuation was 83 micrometres. Only after removing this share did it become visible how the cracks actually changed over the weeks. The motorway bridge reference describes how the project was set up.

Correction is also the basis for sensible thresholds. If a warning threshold lies in the middle of the normal temperature cycle, it triggers on every summer day. Set on the corrected value, it triggers when the structure really changes.

Temperature during construction of steel bridges

In steel bridge construction, temperature is not just a disturbance but a reading in its own right. The length, level and bearing position of a superstructure depend on how warm it is at the moment of surveying and how the heat is distributed across the cross-section. The component temperature at all measuring points at the same time is therefore needed.

A proven layout uses four sensors per measuring point: on the top flange, the bottom flange and the outer and inner webs. They give the mean component temperature and the temperature differences across height and width. The sensors sit on the outside of the coating, a wireless measuring node per measuring point transmits the readings, and no cables are run across the structure.

What matters in the measurement concept

  • Sensor position. On the component surface, with thermal paste and a cover cap against direct sun and precipitation. Air temperature belongs in a ventilated radiation shield.
  • One time axis. Temperature and measurand must be available at the same time, otherwise the relationship is wrong.
  • Measuring interval. The daily cycle needs several readings per hour or at least per day, depending on the measurand. In the motorway bridge project, four readings per day captured a median of 96 per cent of the daily fluctuation; hourly readings captured it completely.
  • Sensor checks. Comparison with a calibrated reference thermometer at commissioning and at every maintenance visit. In quiet night hours, the sensors of a measuring point must read the same.

How Infrasolute measures temperature on bridges and structures is shown on the temperature monitoring page. How the platform analyses temperature and measurands together is described under AI analytics.