When is a crack stable? Gathering data over time allows an engineer to make a judgement about when a crack is worsening, or if it is stable. Believe it or not, the attached output from an electronic crack gage are readings of a stable crack. The gages were installed in a Federal period building in SoHo. The crack opened up suddenly during adjacent construction; Old Structures asked Tectonic Engineering to install several gages so that we could keep a close eye on any further movement (“real time” versus manual readings).
(Click to enlarge.)
The scale of this data is such that the temperature (degrees Celsius) and the crack width (millimeters) appear to be large swings. In reality, the movement is on the order of 1/64th to 3/8 of an inch, with temperature varying between 67 and 74 degrees Fahrenheit. As the temperature increases, the crack width decreases; as the temperature drops, the crack widens. Once a crack has formed, it will continue to fluctuate slightly due to temperature changes even though it is classified as stable (think of a superficially patched crack that has re-opened repeatedly). The key to reading monitoring data of this type is to not focus on the daily fluctuations, but the straight-line trend of the cumulative data. If the average of all the readings produces a horizontal line, the crack can be categorized as stable. If there is a trend, e.g. opening of crack, the line will be sloped upward. This tool can be used “real time” to determine if adjacent work needs to be halted until the cause of the movement is ascertained. For a fuller picture, correlate the crack gage readings with the seismograph and optical survey readings.
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