About Us

SIM: Static Integrated Measurement MethodSIM: Static Integrated Measurement Method

The SIM Group is an association of international experts from 8 European countries, specializing in static/dynamic methods for the safety assessment of trees. DR. ING. LOTHAR WESSOLLY, the author and developer of these methods, is a member of this group. Landscape architect Günther Sinn initiated the static approach to tree safety in 1984 and became involved in the Special Research Area in 1986.

Tree statics means orienting to common practices in structural engineering. This is because a biological structure, like a tree, is also subject to the laws of physics.

Specifically, this means determining the failure loads that would lead to a fracture or tipping failure, and mathematically comparing them with the forces acting on the tree at wind strength 12.

From this, the exact, legally binding fracture and stability safety values can be calculated.

This enables a targeted approach to preserve valuable old trees, or those damaged by construction measures, in compliance with safety regulations according to § 823 BGB (German law) for as long as possible and with legal accountability.

The evidence-secure Elasto/Inclinometer method, due to its closed computational procedure, is clearly superior to other diagnostic methods such as resistance drilling or sonic testing, especially in a legal context.

This is all the more important as trees, unlike technical structures, have unknown baseline conditions regarding their fundamental safety. For many technical constructions, simple visual procedures are often sufficient due to the clear structures and the presence of building plans. However, for trees, drilling is not permissible, and sonic testing is far from sufficient for accurate safety determination.

Origin

Inspired by landscape architect Günther Sinn, who recognized that the critical element, statics, was missing in the safety analysis of trees practiced until then, the scientific foundation of this approach was established 38 years ago at the University of Stuttgart. DR. ING. L. WESSOLLY served as the responsible project leader within the Special Research Area "Natural Constructions" from 1986 to 1991. The first measured attempt to break a large tree was carried out in 1987 on a hollowed-out blood beech in the Villa Berg Park in Stuttgart. Several other trials followed to test our approach.

Since not all values needed to solve the static equations for trees were known, special measurement methods were developed with the support of the German Research Foundation to fill this gap. This includes the Stuttgart Strength Catalogue, as no usable material data for green wood existed worldwide. Established wood research was only interested in dry wood. The creation of the Generalized Tipping Curve was based on tipping experiments to failure on several trees in Baden-Baden and the analysis of tipping behaviour in several hundred trees that had been examined for safety assessments.

For trees, knowledge of essential aerodynamic values was also lacking, such as air resistance, dynamic tree response to gusty winds, and the terrain factors influencing near-ground storms. Fundamental measurements were conducted using the newly developed elastometer on trees during the Vivian and Wiebke hurricanes in 1990 and during Mistral storms in Corsica.

Safety assessments were carried out in the first three years in collaboration with Günther Sinn. After 18 years of engineering work in the fields of measurement and testing technology, statics, dynamics, and aerodynamics, a startup, possibly now "tech for nature," was established at the University of Stuttgart. It has since become an accredited expert office for tree statics under DR. ING. L. WESSOLLY. The office was accredited in the aforementioned Special Research Area until 1995.

The SIA method was subsequently developed, allowing an assessment of the fundamental safety of trees. This method enabled many practitioners and experts to determine the fundamental safety as an essential reference value for tree safety. The SIM Group provides this valuable tool for free use on the internet at simgruppe.de.

Measurement Methods

The method known as the Elasto/Inclinometer method determines the response of a tree to the application of a wind replacement load generated by a pulling device. This involves recording values in which all the influences contributing to the safety of the tree are integrated. To determine fracture safety, the strain on the representative outer fiber in the load direction just beneath the bark is measured. A base length of 20 cm between the tips seated in the bark has proven effective, as it allows for tracking the load capacity over the trunk height. Larger base lengths cause the weakest cross-section to disappear in the averaging process and are thus not permissible. The quality of the tree's anchorage in the soil is shown by the inclination of the root plate.

This measurement is carried out using specially developed strain gauges (elastometers) and inclination sensors (inclinometers). The elastometers, with a resolution of 1/2000 mm, measure the strain of the representative outer fiber, while a pulling device applies a wind replacement load to the tree crown. Simultaneously, the inclinometers, with a resolution of 1/1000 degrees, measure the inclination of the root plate without causing any injury to the tree.

In determining fracture safety, the elasticity limit is the reference value. It is found in the Stuttgart Strength Catalogue for 26 tree species. For stability safety, the Generalized Tipping Curve is used, which WESSOLLY discovered during the analysis of several hundred pulling tests.

During the measurements, all parameters that influence the failure forces in breaking or tipping are automatically included. This is true regardless of whether the trunk cross-section is hollow or has a structure like the Eiffel Tower, or how the roots are arranged in different ductile soil structures and types. The simulated wind load is applied in 5-8 measurement steps, and the associated strain and inclination values are documented. Possible nonlinearities, such as initial compliance in the measuring tips, are mathematically corrected.

Thus, the loads that lead to failure through breaking or tipping are known. To solve the static/dynamic equations, the occurring hurricane load at the location of the leafed tree is still needed.

Through automated continuous measurement of all parameters during the pulling test, thousands of individual data points are created, without achieving higher accuracy. Instead, this method and its representation lose clarity, as it also captures load application and dynamic tree reactions, along with any wind influences, which can distort the relevant measurements.

Plane Tree, Hermannshof Arboretum, Weinheim, Trunk Diameter 2.19 m. Sixth Control Measurement. Fundamental Safety 1111%, Fracture Safety 200%, Stability Safety 200%. Responsibility taken for the next 3 years.Plane Tree, Hermannshof Arboretum, Weinheim, Trunk Diameter 2.19 m. Sixth Control Measurement. Fundamental Safety 1111%, Fracture Safety 200%, Stability Safety 200%. Responsibility taken for the next 3 years.

Load Analysis

The load analysis in the near-ground boundary layer wind is based on the preliminary work of DAVENPORT for hurricane wind speeds: These occur much more frequently in our region in local thunderstorm winds than large-scale hurricanes, which are recorded in European regulations and differentiate between locations and directions. Thunderstorm hurricanes can occur from any direction at any location in our region. Therefore, the aforementioned regulations are incomplete for the safety diagnosis of trees and leave a significant safety gap.

The Elasto/Inclinometer method includes the dynamic components of the hurricane as well as the oscillation willingness of the trees. The cw-values for trees were first recorded by WESSOLLY through elastometer measurements during the Vivian and Wiebke hurricanes in 1990, and later during several Mistral storms in Corsica.

Suggestions are provided in the Stuttgart Strength Catalogue. The threshold for safety decisions is, as in aircraft construction, the safety factor of 1.5 = 150%. This approach has been proven legally binding for the SIM members in more than 25,000 tree assessments and has become the standard for the well-being of the trees.

A higher safety requirement, as is common in construction, with safety factors up to 6, would have led to completely unnecessary, massive interventions in the examined trees. With the elasticity limits of green wood in the Stuttgart Strength Catalog and the Generalized Tipping Curve for trees according to WESSOLLY, reliable reference values for safety assessment are available.

Recognized State of the Art

The experience of all SIM members is based on more than 25,000 liability-bearing tree statics assessments. Of these, 17,500 are currently available for viewing in a physical archive and an Access database at arboa Stuttgart (as of 1/2025).

With this number of assessments and a comprehensive 30-year field trial for the SIM approach, using cw-values and material data from the Stuttgart Strength Catalogue, the Generalized Tipping Curve, and the applied terrain values, and the 1.5 safety factor as a threshold, the state of the art has been reached. This is crucial because our activity is integrated into the legal category of traffic safety obligations.

Archive with over 17,500 Tree Safety Assessments using the Inclino/Elastometer Method. Location: arboa Tree Safety, Stuttgart (Germany).

Through regular training throughout the year, exceptional qualifications for the members are ensured.

In this regard, the communication center Meck in Frick (Switzerland), owned by our member M. ERB, plays a particularly important role. Our Institute for Tree Diagnosis e.V. is also based here in Frick (CH), Stuttgart (D), Vienna (A), and Varese (I). At this location, training to become a certified tree statics expert is offered, which allows graduates, after successful examination, to apply the Elasto/Inclinometer method with the corresponding measuring instruments.

The following major standard works in the field of trees have been published by members of the SIM Group:

• SIEWNIAK, M. / KUSCHE, D.: Baumpflege heute 1984, 1988, 1994, 2002, 2010, 2020, Patzer Verlag
• WESSOLLY, L. / ERB, M.: Baumstatik und Baumkontrolle 1998 and 2014, Patzer Verlag
• WESSOLLY, L. / ERB, M.: Manual of Tree Statics and Tree Inspection, 2016, Patzer Verlag
• SIEWNIAK, M. / WESSOLLY, L. / BOBEK, W. / SIEWNIAK, M.: Statika drzew. Analiza zawodnosci, Warsaw 2020
• BENK, J. et al.: Praxishandbuch Wurzelraumansprache, Arbeitskreis Baum und Boden 2020
• ZANZI, D. / CASTIGLIONI, M.: Fito-Consult & gli alberi, quarterly journal since 1989
• BADER, C.: Untersuchungen über den statisch wirksamen Wurzelraum, thesis FH Nürtingen 2000
• RAU, S.: Verankerungsbedarf von Solitärgehölzen, thesis FH Nürtingen 2001
• LESNINO, G.: Comparison between Sonic Tomography and Pull Tests, Baumtage Süd, Böblingen 2011

Further Involvement in the Field:

• WESSOLLY, L.: Member of the working committees for ZTV Tree Care since 1992 – 2013, FLL Working Groups on Tree Plantings 2015, Tree Inspection Guidelines, and Investigation Guidelines 2013
• BENK, J.: Working committee member for ZTV Tree Care 2006, Board Member ISA Germany
• BREHM, J.: Head of FLL Committee on Tree Valuation
• VENZKE, C.: Former senior lecturer at LVG Heidelberg and Justus-Liebig-Schule in Hannover, former board member of ISA Germany
• ZANZI, D.: Best friend of A. SHIGO and organizer of countless expert conferences in Italy
• ERB, M.: Lecturer at the Swiss Federal Tree Care Training
• RAU, S.: Training leader for the Certified Tree Inspector program, Institute for Tree Diagnosis
• SCHABEL, P., KLIMA, R.: Involvement in tree-related matters in the ÖNORM
• SIEWNIAK, M.: Former full professor at the Universities of Warsaw and Krakow, Poland
• STIEMER, S.: Former full professor at the University of Vancouver for Structural Engineering, Canada
• MORELL, R.: Lecturer for Arboriculture at FH Rottenburg and FH Weihenstephan
• SCHINDLER, R.: Trainer, e.a. Baumgenossen

A comprehensive list of technical publications can be viewed under Downloads, where interesting measurements are also depicted.

Through active involvement and membership in all important national and international associations, the SIM Group is excellently networked in the field of tree safety in Europe.

In 2009, the SIM Group was founded after L. WESSOLLY left the SAG Baumstatik e.V. which he co-founded. Two-thirds of the members from that time then joined the SIM Group.

So far, the SIM Group has formed an association of independent experts who focus on tree safety. According to recent FLL statutes, we, as an independent group, were not allowed to send representatives to the working committees. Therefore, a registered association was established in Vienna in November 2024 under German law to fulfil this formal requirement. SIMON RAU from Loßburg, DINO HIEPLER from Tübingen, and treasurer CHRISTIAN WAGNER from Vienna form the board.

The development of tree statics has raised the safety assessment of trees to a new level, achieving parity with engineering. The interdisciplinary exchange between biology and engineering has led to a quantum leap in the preservation of valuable trees.

 

The headquarters of our Institute for Tree Diagnosis e.V. are located at Geißgasse 17, 5070 Frick (Switzerland), with branches in Stuttgart (Germany), Vienna (Austria), and Varese (Italy).