The Static Integrated Assessment (
SIA) and Static Integrated Method (SIM) methods were first published by Sinn and Wessolly (1989) based on research carried out at Stuttgart University. The assessment of tree statics has been selectively explored and applied with varying levels of understanding. It is acknowledged that there is no perfect method that covers all possible cases.
The approach published by Erb and Wessolly (1998), is based on a common understanding of the behaviour of mechanical
testing in the construction industry. This approach may be simplified by representing the components of static forces, and is known as a model termed the "Triangle of Statics".
Fig. 1: The Triangle of Statics after Wessolly (Erb, Wessolly, 1998).
The diagram shows that the requirements for the stability of any construction involves the following components a load, its material properties and the geometry of the load-bearing structure. When considering the strength and stability of trees, these components are represented by wind force, quality of wood of the species, and stem size and shape, respectively.
Using the common approach to tree assessment, including special methods such as the Visual Tree Assessment method (Mattheck and Breloer, 1995) the tendency is to focus on two of the above components (viz. material and geometry). The consideration of load analysis is crucial information for assessing tree safety; for example, a defect (such as a cavity) in a tree with height of 25m is far more likely contribute to structural failure, compared to the same defect in a tree only 10m high.
The Static Integrated Approach incorporates two techniques; the first being a device-supported approach and the second being a visual assessment method. Both systems incorporate the three components of tree statics and are outlined below as the Static Integrated Method and the Static Integrated Assessment.
Static Integrated Method
The Static Integrated Method (SIM) is a device-supported approach, which measures both the breaking resistance and the tipping resistance of trees. It was designed by Sinn and Wessolly (Sinn, Wessolly, 1989) to produce exact data about the reaction of a tree to an artificially imposed load. In practice this method has been used since 1995 by arboriculturists who are affiliated through a group called
SAG-Baumstatik. This method applies a pulling test to a tree (involving the use of a hand-winch) to measure the response of the structure as a whole according to the following two parameters:
-
The change of length of wood fibres in the outer parts of stem in the pulling direction (measured by an elastometer to a resolution of 0.001 mm over a 25 cm distance).
-
The change of angle of the stem base. (The tipping procedure is measured by an inclinometer to a resolution of 0.01 degrees.)
The first step applies a recorded and measured force to the tested tree by means of the winch and a
dynamometer, linked "inline" via the winching cable. The measuring procedure lasts for approximately five minutes. The pulling test is non-destructive for the tested tree and may be repeated by placing the elastometer at various positions on the stem.
The second step involves carrying out a load analysis. A digital camera image of the tested tree is drawn. Its crown shape is digitally outlined and the crown spatial distribution is analysed in relation to a wind-distribution curve (Davenport, 1965).
On the basis of these inputs it is possible to calculate the following:
- the centre of gravity of the tree
- the eccentricity of the crown
- the crown sail area
- the moment of wind load (due to a wind speed of 32 m/s; i.e. force 12 wind on the Beaufort Scale)
The third step applies knowledge of the material properties of the tested tree species, according to the measured dimensions (DBH, height) of the tree and its reaction by the pulling test. The breaking and tipping resistance of the tree are calculated using experimental data published by Erb and Wessolly (1998).
Fig. 2: After (Erb & Wessolly, 1998).
The advantages of this approach are:
- the system uses non-destructive testing
- the test is repeatable as a check in the present evaluation or on future occasions
- if used appropriately, the result of the assessment is independent of the practitioner
- the method provides a basis for testing the uprooting resistance of trees. This may be of particular benefit in assessing effects of damage by developers on root anchorage.
Restrictions of this approach are:
- it is costly and time consuming. It takes about two hours for two people to test one tree.
- the method was designed only for testing the whole tree and not its parts (branches, multiple stems etc.).
Static Integrated Assessment
This method (SIA) depends on visual data but is based on a history of experimentation involving pulling tests applied to several thousand trees. The use of the SIA method in arboricultural practice pre-dates its first publication (Erb & Wessolly 1998) by a number of years.
The underlying principles of this method are derived from SIM, but it involves only a mathematical simulation of the inspected tree, as if it were loaded by a very strong wind (
hurricane) equivalent to Beaufort Storm Force 12 (32 m/s). This simulation integrates both information derived from visual assessment and pre-existing experimental data.
The visual and site assessment needs to incorporate the following data:
- tree species
- measured diameter and height
- thickness of bark
- shape of crown
- position of the tree (countryside, village, city).
- if the inspected trees are growing in avenues or are in rows on roadsides the distance between the trees is also recorded.
The SIA method applies information from the above observations and data to determine whether the breaking resistance of the tree is within safe limits.
In practice this is carried out by reference to a series of diagrams, which focus on four key indicators:
- Consideration of the relationship between tree height, diameter and crown shape.
- The underbark diameter of the measured tree is compared with that which would be required for a model tree sufficient to withstand a (hurricane) force wind.
- In circumstances where cavities may be present, observational data are used to calculate the minimum residual wall thickness of the main stem required for adequate strength (Mattheck & Breloer 1995).
- In circumstances where the Breaking Resistance Value is less than 100% (where the value of 100% represents an adequate safety limit) a graphical image of the minimum level of crown reduction is generated. This diagrammatic presentation provides guidance for the minimum recommended level of height reduction necessary to re-model the tree as an acceptably strong and stable structure (in terms of static forces).
The advantages of SIA method:
- this is a very quick and user-friendly method to apply, once the necessary training has been obtained
- the method provides the means to take account of hidden defects including any imbalance between the height and diameter of the tree
- due to its ease of application it provides the means of cost-effectively providing information regarding tree stability on large populations of trees, and may be a valuable asset for carrying out street tree inventories etc., which may then be used to inform risk management procedures.
- the system offers practical guidance on the minimum size of crown necessary to achieve stability in the face of strong winds
The restrictions of the SIA method:
- the system can be used only for open grown trees or trees in avenues (including highways, parkland and formal garden situations), but cannot be applied to trees growing in close-canopy conditions.
- the list of tree species is currently restricted to nineteen of the most commonly planted trees
-
- only a limited range of factors that may influence the static functioning of the tree structure can be taken into account (including height-diameter imbalance, cavity/hollowing, weak fork formation)
The SIA method may be applied when carrying out individual tree inspections, as one of the tools used to diagnose the structural strength and stability of trees and to inform potential remedial action. It may be combined with other device-based techniques for detailed tree assessment. Alternatively, the SIA method may be used for large-scale tree inspections to inform decisions on tree management according to treatment priorities to meet duty of care.
References
Davenport, A.G. (1965):
The relationship of Wind Structure to Wind loading Effects of Buildings and Structures, HMSO, London
Erb M., L. Wessolly (1998).
Handbuch der Baumstatik und Baumkontrolle. Patzer Verlag
Horáček, P. (2003): Introduction to the Tree Statics & Static Assessment, Tree statics & dynamics seminar, Westonbirt
Lonsdale, D. (1999).
Principles of tree hazard assessment and management. Research for Amenity Trees No. 7, The Stationery Office, London
Mattheck. M. & Breloer, H. (1995).
The Body Language of Trees: A handbook for failure analysis, (Research for Amenity Trees 4), HMSO, London
Sinn, G. & Wessolly, L. (1989).
A Contribution to the Proper Assessment of the Strength and Stability of Trees. Arboricultural Journal. 13, 45-65.
The original paper from Jarek Kolařik this is based on can be found
here.