Wind load simulation & tree statics:
Management for extreme climatic events
Report on the fourth Treeworks Environmental Practice/AA Seminar held on the 5th & 6th December 2005 at Ashton Court, Bristol
Seminar IV
Originally published in AA newsletter issue 132 by Mark Nankervis of the AA Western Branch
The recent seminar titled Wind Load Simulation & Tree Statics is the fourth in a series of seminars being hosted by Neville Fay and Treework Environmental Practice in association with the Arboricultural Association. The aim of the seminar series is to introduce and promote work which has significance to arboricultural theory and practice, but has not previously been widely considered due to barriers of language and the lack of exchange of information between disciplines.
The engineering concepts associated with statics are well known and have been in use for a considerable time. On the other hand statics is relatively new to arboriculture and is still in the process of being tested, understood, applied and assimilated.
Mechanics is a branch of physics primarily derived from Newton’s three laws of motion, and is involved with the study of the behavior of a body under the effects of forces that influence its state of rest or motion. This field of physical science covers the study of rigid and deformable-body mechanics, and fluid mechanics. It is rigid body mechanics that is specifically relevant to the field of tree statics.
Rigid-body mechanics is subdivided into dynamics (the study of accelerated motion) and statics (the study of bodies at rest or moving at a constant velocity). Statics is really a particular theoretical case of dynamics (where the acceleration of a body is equal to zero), though it has become a distinct field of theoretical mechanics. Statics uses standard concepts of force and moment and applies physical-mathematical modeling.
Tree statics therefore studies the tree in terms of its structural properties applying conventional engineering principles to calculate a safety factor; applying similar scientific principles to those used to assess the safe construction of buildings or bridges. When applying tree statics, the tree is considered as a structure in terms of its material properties, geometry and the forces likely to act upon it.
The Seminar was held on the 5th and 6th of December at Ashton Court Estate in Bristol. Those who are familiar with this estate will be aware of its stunning location and its setting with extensive historic grounds peppered with veteran and ancient trees.
Elastometer
The seminar was co-presented by Dr. Peter Horacek, an eminent European wood scientist, who is head of department at Mendel University, Brno in the Czech Republic. His partner for the seminar, Dr Jaroslav Kolarik, is a respected author, international speaker and arboricultural consultant. Dr Kolarik has been applying tree statics since 1996 to help inform consultancy advice.
The static Integrated Assessment (SIA) approach is based upon the concept of adapting the basic principles of physics and engineering standards developed for building design. As described above this requires knowledge of material properties, the size and from of a structure and the forces it will be subjected to (known as the triangle of statics).
These are adapted into a tree surveying system requiring only basic measurements and information about the individual tree to arrive at a calculation for the safe dimensions of that specimen.
We were also introduced to the Static Integrated Method (SIM). This principally involves what has come to be known as ‘the pulling test’; a device-based method used in specific circumstances in conjunction with the SIA where more information is required (in a similar fashion to using decay detection and mapping equipment to provide further information in cases where Visual Tree Assessment may be considered insufficient). The pulling test is used to assess breaking resistance of the trunk as well as root plate stability.
The methodology and research being presented was pioneered at Stuttgart University by Lothar Wessolly approximately twenty years ago. Although the system is used in many European countries it is puzzling that such a potentially useful system and diagnostic tool has had such limited impact in the English speaking countries over this time.
In the case of SIA and SIM, the systems have been researched, developed and applied in mainland Europe and the translation into English of relevant information has been very limited, obviously restricting its introduction and application for UK arboriculture. In Britain we have also adopted a series of different inspection tools and methodologies to inform our consultancy. Dr. Kolarik pointed out that in The Czech Republic the type of damage caused by the invasive tools that we regularly use in this country is not tolerated. The Pulling Test is non invasive. It was acknowledged that there has also been some conflict with the hugely influential Clasu Mattheck and that this has not helped with the recognition or promotion of the tree statics methodology.
This seminar involved highly technical information requiring us to polish up our numeracy skills (and deal with equations and formulae with which we were not accustomed). Most arboriculturists, (not having selected their career on the basis of proficiency in maths and physics) were struggling like myself to maintain a grip on the concepts. I make my confession here that although I once appeared on 15 -1 , I was not the ‘1’. I am not a physicist or a wood scientist so I am presenting the systems as accurately as my understanding of the seminar permits, and I thank Paul Muir of Treework Environmental Practice for his invaluable help in this. In the interests of understanding and restricted space I have not delved into deep technical detail or language where this has been avoidable, and provide an outline below of the principle concepts. Such technical details will be presented in the scientific Arboricultural Journal once peer reviewed.
STATIC INTERGRATED METHOD (SIM) – A SUMMARY THE PULLING TEST
This considers wind load simulation to assess the theoretical stability of the tree, and applies this assessment through a field trial approach known as the ‘pulling test’. The components of SIM are summarised below:
- WIND LOAD SIMULATION
Wind load is estimated through recording and factoring in the height, crown area (from a digitised photograph of the tree), together with the species and crown density of the tree. These details are then used in calculating the coefficient of drag of the tree.
Also factored into this estimate are site factors that may modify the wind speed of a Beaufort Force 12 gale; these include the context in which the tree is growing; whether it is located in a city, town or country. The tree’s elevation above sea level can have a very significant effect on the wind speeds in a locality.
- THEORETICAL STABILIY
Using the measured diameter at breast height (dbh) of the tree and knowledge of tree species material properties the theoretical stability of the specimen can be calculated.
If the tree was subjected to a Beaufort Force 12 wind then the material at the base would be subjected to bending moment calculated using the coefficient of drag of the tree and the length of the lever arm (height of the tree at the centre of gravity of the crown). The tree’s own weight and torsion calculation are also taken into account.
A safety factor is calculated which will inform whether there is sufficient cross sectional area at the base of the tree (assuming sound wood throughout), to support this bending moment/load.
This calculation is carried out before any pulling. The information is used to determine an appropriate force to be applied during the Pulling Test to avoid the risk of any damage to the tree.
The Pulling Test
- THE PULLING TEST
Practical field approach to simulation of wind loading.
Through applying a measurable load using a manual winch a bending moment can be calculated (the force applied multiplied by the height at which the cable is attached).
Wood fibre deformation is measured to a high degree of accuracy using an elastometer (above) – a small sensor device attached to the tree along the plane in which the tree is being pulled.
We know from laboratory tests that there is a fixed relationship between stress (force divided by area) and strain (the deformation of wood fibres) as described by the modulus of elasticity (Young’s Modulus) of the particular species of wood.
Consequently with a measured load, a known dbh and a measured strain response we can make comparisons between the tree being tested and the readings which would be expected if the tree were sound. The implications of a lower modulus of elasticity (lower material stiffness) are either that the tree is hollow (the load exerted is being spread over a smaller cross section) or that the wood quality is lower than expected for the species.
The Inclinometer is another small sensor device attached to the base of the tree along the plane in which the tree is being pulled. The degree of root plate tilt is measured in response to a known load. How far the tree would tilt if subjected to a force 12 wind load is read from a generalized tipping curve – plotted from destructive testing.
The Pulling Test takes two people approximately two hours to undertake. It is performed from two different directions at ideally 90 degrees to each other. The elastometer is positioned on areas of the stem along which the practitioner suspects (through using VTA) that a potential fault exists in the plane of the load being applied. The point of primary failure is known to be on the compression side of a stem (wood is approximately twice as strong in tension as compression).
Static Integrated Assessment (SIA)
The Static Integrated Assessment approach is based upon the system used to assess the wind load and the theoretical stability of the tree as described in 1 and 2 above. Rather than using a digitised photo of the individual tree crown being assessed, the generalized crown shape is fitted to the closest of four crown shapes provided.
SIA is theoretical. In terms of statics the system considers the maximum load the tree is expected to experience, the laboratory tests of sound wood for expected wood properties, and the diameter of the structure expected to support the crown.
Where the trunk diameter does not meet the minimum size required for static equilibrium, the methodology can be used to provide a calculated and measured recommendation as to how far the lever arm, (height of the tree), and sail area (shape and extent of the crown) need to be reduced by reference to a graph through various levels of crown reduction.
Issues
The SIA can be effectively employed during the surveying of large populations of trees. Experience has shown that approximately 97% of trees fall within the safe category and require no further action or investigation.
Collecting only basic tree information during a large scale tree risk survey can quickly identify which trees (with a low margin of safety) require the most urgent treatment or a more detailed assessment. In other words if the basic dimensions of the tree indicate that a sound stem would yield a 400% safety factor we can assume that such a tree can tolerate huge volumes of decay before posing a significant risk of failure. In this way the trees most likely to be hazardous can be identified and together with information about risk zoning of the site this can be used to target or sequence resources for further investigation.
The Pulling Test provides an excellent way of testing tree stability on development sites where root plates have been damaged. In the Czech Republic where root plates have been damaged on development sites, developers have been made to either pay for the amenity valuation of the tree, or to commission a Pulling Test in order to determine the options for safe retention of a structurally damaged tree.
When using the SIA recommendations, tree reductions are generally the only effective way of reducing the lever arm. Prior to implementing a reduction, however, it is of course necessary to consider the consequences of such work on factors other than tree stability:
- it could spoil the amenity of some trees
- it could stimulate prolific localized regrowth about large pruning cuts
- it could expose previously sheltered bark and foliage to sunscorch
- it will remove material capable of photosynthesis
Dr. Kolarik countered these issues by pointing out that because of the influence of the highest branches in terms of increasing the lever arm on the tree, even surprisingly modest crown reductions can often result in an acceptable safety factor for a tree previously assessed as ‘unsafe’. These small reductions, often in the order of one to two metres have been demonstrated to not induce rapid regrowth due to small diameter pruning cuts and the reduction of auxin production resulting in increased levels of lower crown growth. These phenomena were observed even in a two metre reduction on a Poplar, and Neville Fay had the experienced the same with one to two metre reductions of fully mature and veteran beech trees. Obviously this approach is not viable on many conifers and very unsound tree would require reduction of a scale such that considerations in the previous paragraph are likely to make reduction an inappropriate option.
The Pulling Test similarly requires the replacement of pulsing and gusting wind with a measured pull from a winch, and the root plate tests are compared against criteria which are not species, soil type, or soil moisture sensitive.
Interestingly this method of tree assessment can provide ‘safe’ residual sound wall thickness recommendations which are far smaller than the ratio of sound residual wall thickness (t) to stem radius (R) proposed by Mattheck and others, i.e. one third. A ‘safe’ radius of one tenth the diameter was shown to be possible in certain cases. This was the cause of much discussion and received much anecdotal support from consultants particularly involved with veteran tree management. The point was made by one fellow attendee over lunch that many stable trees are unnecessarily condemned because they have less than the one third sound wall ratio, but demonstrate their stability year after year. While the biomechanic ratio (t/R) has provided a credible and accepted measure through which to retain basically stable trees with cavities, and is in line with my experience, SIA could help us to take this possibility another step forward.
The use of the SIM system requires membership of the SAG Baumstatic group run by Wessolly, however SIA is freely available through their website http://win.aow.cz/sia/sia_en.asp .
Presumably acceptance to the procedure by insurance companies will be another critical factor in its future influence in this county. The twenty years of experience on the continent has provided a reported track record of reliable advice.
Limitations of the system are that it is not adapted for use on close grown or woodland trees, and it is not adapted to test individual parts of trees. Research is currently being undertaken on both of these situations.
A fundamental necessity of the system is that the SIA method requires simplification of data in order to make the process manageable. Such simplifications have been made in the description of the factors
- canopy shape
- type of wind exposure
- crown density.
The Pulling Test similarly requires the replacement of pulsing and gusting wind with a measured pull from a winch, and the root plate tests are compared against criteria which are not species, soil type, or soil moisture sensitive. However at each stage within the model margins of safety are introduced. Despite these safety margins the recommendations are generally far less drastic than those arrived at using our established hazard assessment and mitigation methodologies. This point is due to several factors. It could be argued in UK arboriculture that we overstating the magnitude of the load applied by the wind or that we are understimating the strength of a thin residual wall on a large diameter stem (the significance of the geometry of the structure), particularly as considering t/R does not take into account the magnitude of the load and lever arm.
There is a limit in terms of the numbers of tree species, which have had their wood properties laboratory tested. There may also be some differences in the wood strength characteristics of a Beech tree growing in central Europe, for example, and the same species growing in the South West of England.
The question of the different degradation characteristics of brown and white rots was also discussed by Dr. Horacek. The criticism that has been levelled at the Pulling Test is that the test measures the stiffness of wood at low loading and makes an estimate of strength at high loads at the point of primary failure. There was some discussion over this point, which needs further clarification if the theory is to be fully accepted. However, the practical application of the system has to a large extent been ‘reliability tested’ over a twenty year period.
Dr. Horacek stressed that the SIA and SIM systems are simply tools to be used along with our other systems, knowledge and equipment, including VTA.
Conclusion
The seminar demonstrated in great detail the principles and methodology upon which the systems are based, and how they are used in the field. I am not in a position to query the mechanical equations and mathematical proofs upon which this system is based, but the general premise of the interaction between materials, structures and forces is rational, and the safety margins used are sensible to compensate for the simplification of the input data.
Generally the principle of a calculated, objectively justifiable specification for pruning operations which are generally less sever than those recommended by competing systems of assessment is a positive step. The calculations are not open to interpretation influenced by the consultant’s background and extent of experience (or lack of it). If the results provide reliable information and undermine doubt, they also undermine the ‘if in doubt have it out’ philosophy of some hazard assessment proponents.
Many trees requiring only a light reduction under this system would otherwise be felled.
In terms of reliability, significant experience has trialled the accuracy of the system in consultancy over the past 20 years. For consultants generally this record of performance in the field is going to be persuasive: what greater endorsement is there?
Keep an eye out for tree statics. I would expect to see the systems growing greatly in influence and becoming part of our arsenal of consultancy systems and equipment over the next few years.
Mark Nankervis wishes to thank Paul Muir & Neville Fay of Treeworks Environmental Practice for technical assistance with this article.