luketreework

In the past, people used a combination of pulling and winching tests to research various tree biomechanics. The goal of those was to determine their resistance and how durable against rupture and uprooting they were. As a result of such tests, the tree was usually brought to ultimate failure and destruction. 

Nowadays arborists and arboricultural consultants employ a set of non-invasive tree risks assessment techniques, which preserve the trees they are conducted on. Static load tests are now the common method that experts utilise after it was developed at the University of Stuttgart in Germany. 

A static load test consists of three steps: 

• Pulling test – this methodology aims to assess the risk of failure (usually related to uprooting or stem breakage). It is achieved by pulling the tree to simulate wind loading in moderate power. Things like root plate inclination and changes in the length of fibres are then measured to determine the results. As for the tools used for the job, it is usually a rope winch or a grip hoist that apply quasi-static forces then measured by an in-line dynamometer. The idea behind the test is simple – as the tree experiences increased load, its stem will bend and the root plate will tilt. Arborists can detect these changes with very sensitive instruments. By then looking at the degree of compression and exerted extension within marginal fibres, the experts can better determine fracture resistance. 
  
  
• Wind load analysis – the goal of this test is to produce an estimate of the expected wind load of the given location. Any safe tree needs to be able to withstand the wind effects, as well as snow and ice. Wind loads depend on a few factors, such as expected wind speed and resistance of the tree to streaming air. The way to perform such analysis is to extract the crown in a digital picture and then the different levels are correlated with the wind speed. Experts are always mindful of the fact that wind speed increases with height above ground. There are national wind zone maps, which determine reference wind speeds and wind events. By using species-dependent parameters, experts are then able to assess the reaction of the tree to the wind. That is how they can streamline the crown or recommend a reduction in tree height. 
  
  
• Evaluation – once the required tests have been performed, arborists can begin the assessment and evaluation process by using the acquired data. The goal is to get enough information on the critical load of the tree and whether the conditions and environment it is in are potentially dangerous to it. The analysis results in factors of safety, with a value of 1 when the resistance against failure matches the expected wind load. However, according to most engineering standards, structures should have a factor of safety equal to 1.5, which is the aim of any static load test. 
  
  

It should be pointed out that trees often compensate for insufficient strength by utilising adaptive growth. Arborists will always take into consideration a tree’s ability to produce additional wood fibres in the areas of excess strain, often just by visually inspecting the tree. This aspect of the job is essential to deducing meaningful recommendations by arborists, who correlate results of technical inspections with their visual assessments. 

There are still certain limitations that any such project presents, namely the majority of factors at play. In the future, these tests will be better able to produce meaningful information. 

© Treework Environmental Practice