Pulling tests
Pulling tests – or static load tests –have been used for around 25 years to determine whether a tree poses a risk to public safety. The test involves applying relatively low load to the tree by means of a cable that is secured in the tree crown. As the load increases, the trunk and the root system respond by deforming slightly. These reactions are not visible to the human eye but can be measured by highly sensitive instruments. The three values that are continually recorded are:
- the force applied to the tree
- the extension or compression of the trunk’s marginal fibers
- the tilt of the root base.
These values can be electronically stored and monitored on a computer. This ensures that the tree is not “overloaded” and that its load-bearing structure, bark and roots remain undamaged. Each pulling test is stopped once a relatively small reaction has occurred.
The test values are then evaluated in the ArboStat software that was specially developed for the task. The wind load exerted on the tree, the material properties of the green wood, and the geometry of the loaded trunk are all taken into account. (See our pdf-downloads on this topic.)
The instruments used for pulling tests are:
- elastometers (stem strength), accurate to 1/1000 mm
- inclinometers (anchoring strength of the root system), accurate to 1/ 500 °
- dynamometers (tensile force and cable angle), accurate to 30 daN (kp)
- laser altimeters