Frank Rinn - Technical Tree Inspection

© DIPL.-PHYS. FRANK RINN HARDTSTR. 20-22 D - 69126 HEIDELBERG GERMANY TEL +49 6221 3143 -87 FAX -88 WWW.RINNTECH.COM

Technical tree inspection

Dipl.-Phys. Frank Rinn, Heidelberg, Germany

2008 TECHNICAL INSPECTION OF TREES 2 / 148


Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Urban green. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Inspection, assessment, examination, evaluation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Successful trees. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Pre-requisites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Load to trees and other stress factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Wood anatomy, tree biology and mechanics.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Wood anatomy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Decay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Some typical tree growth symptoms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Tree failure.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Technical methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Height over diameter ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Drill resistance measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Stress wave tomography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Wind load analysis.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

Error calculation in tree assessment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Pulling tests and safety calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

Application examples: expert reports on trees. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

Final conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147



Introduction

Urban green

Urban green has more positive effects than just making cities looking nice. Urban trees

positively influence micro-climate, filter dust and air pollution, provide sun and wind

shadow. And they have a positive influence on economic value of shopping and housing

areas too.

Yet, urban trees have to stand a lot of stresses. Nature does not like air pollution and does

not need crown cutting. But urban green has to be adapted to boundary conditions of

urban life style, including the needs of buildings and traffic. However, the most dangerous

stresses to urban trees are affecting their roots.

Urban stress factors accelerate typical aging effects of trees, such as decay in stem and

roots. Stems can break, branches can be lost or trees fall over. Therefore urban trees have

to be inspected regularly in order to identify hazardous trees and to minimize risks.



Inspection, assessment, examination, evaluation

Urban tree inspection is normally done visual - mainly based on knowledge from more than

100 years of forestry and arboricultural research, enriched by modern approaches better

understand the

body language of trees.

Depending on tradition, laws, and standards, in some countries there is a clear line

between visual inspection and technical examination of trees. In Germany, for example, all

urban trees should be inspected visually (old trees at least one time a year). If there is any

doubt about safety, these potentially dangerous trees should be tested technically. To

distinguish the different procedures, technical inspection is called ‘examination’.

Because evaluation of the same tree can be very different by different experts, insurance

companies and judges more and more ask for neutral technical evidence rather than

subjective opinions.



Successful trees

Structure, size and functional capabilities of trees have overcome various difficult climatic

periods and diverse local conditions throughout the centennials. Thus trees can be called

biologically and evolutionary successful creatures of nature.

They obviously have developed successful concepts of how to deal with difficult

surrounding conditions. They seem to know how to react to external stress factors limiting

their possibilities to develop and grow. Otherwise they would have been diminished in the

evolutionary process by other plants.

And in fact, we find urban trees more than 100 years old even in cities strongly destroyed

several times by war. Trees from high altitude mountain areas are other examples for

being able to survive unfriendly climate and even severe mechanical damages.

The first step of learning how to inspect trees therefore is to understand their concept of

growth and of repair mechanisms to compensate internal or external stress factors.



Pre-requisites

Proper technical tree inspection requires deep knowledge about

! wood anatomy and tree biology

! wood mechanics and wood physics

! wind load and other stress factors

! limitations and possibilities of technical methods

! laws and standards.

The formal price of an inspection device is mostly less than the correspondingly required

investment into education, to learn how to apply the system, how to interpret and evaluate

the results.


Assuming compression strength of grren wood of 20N/mm², cross section area of 3.14*0.5*0.5m² (. 0,8m²)1


Load to trees and other stress factors

Loads

First of all the tree crown is a load to the stem. Big crowns can weight 20 tons or even

more. As long as the stem is vertically upright, it can carry big loads. Lengthwise

compression strength of wood is quite high. Theoretically, an oak stem without any defects

and 1 meter diameter can carry more than 2.000 tons of vertical load .1

However, usually the main load a tree has to carry, is wind load.



Wind load

Static load to a single tree F = 0.5 * A * Cw * v² * ñ

Example: F = 0.5 * 40m² * 0.5 * (30m/s)² * 1.25kg/m³ . 11.000 kgm/s² = 11kN � 1.1to

Variable Measure Depending on

v wind speed Time, height above ground, local stand topology, ...

A crown area Wind direction, foliage, wind speed, temperature, ...

Cw drag coefficient Tree species, foliage, wind speed, temperature, ...

ñ air density Temperature, altitude, ...

Porosity? + Dynamics? + Precision? + Swaying? + Torsion? + Damping? + Errors?

=> wind load calculation of urban trees contains many uncertainties to be specified!



Other urban stress factors

Although urban climate is often influenced by air pollution, this is not the main stress factor

affecting trees. Most urban tree problems are coming from other origins:

! wrong tree architecture from nursery(co-dominant stems and other potential defects)

! wrong treatment / missing preparation in nurseries for urban tree requirements

! wrong planting procedure (small place, un-adequate soil, ...)

! wrong pruning

! root cutting by ground / soil works

! stem damage (by car or work accidents, for example)

Most of these stress factors can be re-constructed by tree ring analysis (except for tropics)



Wood anatomy, tree biology and mechanics

! We distinguish between three major species groups of similar wood anatomical

properties

" conifers

" ring porous

" diffuse porous

! Trees within these groups often react quite similar to external load and stress factors.



Wood anatomy



Age trend

Decreasing ring width with tree

age is not automatically

correlated to decreasing vitality

of the tree.



Density trends

Conifers, ring porous, diffuse porous



Tree growth

genetics, biology,

soil, stand,

competition, climate,

weather

determine the amount of possible wood

growth

determines local distribution of growth:

where the tree builds up his new wood

mechanical stress on

the tree surface



Compression wood

Conifer trees react against static mechanical stress.



Typical ‘reaction wood’

(General overview, may be different in individual cases depending on conditions)

Load Static Dynamic

Tree group

Conifer treesCompression wood Bigger tree rings (low wood quality)

In extreme cases: spiral growth.

Broad leave trees

Tension wood

mature trees: broad rings on

compression side

? Bigger tree rings ?

(still subject to research)



Natural tree design?

! Each structure first breaks on its weakest point (‘notches’).

! Trees try to avoid notches (= points of maximum surface stresses).

! Trees accept higher stresses in roots and branches but not on the trunk.

! Safety factor of tree design is about ~4.5 (Mattheck)



! Because the stem is the most important part

of the connection between roots and crown,

the repair growth speed is maximum there.

! The repair growth can be simulated

mathematically: it follows the stress line.

The higher the mechanical stress

(compression or tension), the faster the

growth [as long as the tree is able to do so].



‘Adaptive growth’

! The cambium growth request depends on local stress.

! Contact to stiff materials cause compression stress.



Decay

! Fungal decay is a natural process,

necessary for re-cycling wood for new

growth!

! Reasons / origins of decay in urban trees:

" roots (soil works)

" stem base (direct damages)

" branches (wind or cutting)

! Different tree species react different to

decay due to their possibilities in

compartimentalization (Shigo).

(Shigo 1985)



! Root based fungi often do not affect the stem but

totally destroy the mechanic root system, without

weakening vitality correspondingly.

! Points to be inspected technically depend on

" tree species

" fungus

" origin of injury

" local stand conditions

(Shigo 1985)



Some typical tree growth symptoms

! Reactions to decay are mostly reactions to stress.

! The natural shape of tree stems is mostly round or

regular (there are some exceptions especially in the

tropics).

! Irregular forms mostly are a sign for decay.



Elliptical growth deviations from a

circle mostly indicate internal decay.



! The cambium reacts to

(internal) decay, as soon as the

mechanical stress is

significantly higher and if the

tree is vital enough to do so.

! As long as the decay only

affects parts of the tree, that

are not required for stability,

there is no visible outside

symptom on the tree’s shape.

! Thus trees can be decayed

without visual symptoms

because of different reasons!



! If you see a tree with obvious decay but no

reaction wood, the reason can be

" the tree is not any more vital enough to

react and to build compensatory wood (you

can check this by looking into the crown and

measuring shoot length)

or

" the decay area is small and not dangerous

for stability

or

" the decay is brown or wet rot in early

stages, only affecting bending strength but

not density and stiffness (dangerous!). =>

Technical inspection required!



! Such an early stage of brown rot can reduce

density and stiffness by 10% and at the same

time bending strength by about 90% (Wilcox

1978).

! The cambium can not recognize this defect

because it ‘observes’ stiffness. Bending strength

can only be observed by breaking the tree or

branch until failure.

! Extension of early stages of such decay patterns

can only be detected technically.

! If you find brown rot fungi at a tree, be careful

evaluating the tree without technical assistance!



Tree failure

Why and when do trees break or fall over?

(Mattheck & Bethge 2004)



Trees are designed to withstand wind until approx. 100km/h:



(Eucalyptus paniculata, Lavers 1983)

Vogel 1996



Skatter & Kucera 2000:

Data that were originally collected to study systematic asymmetries in the canopies of

Scotch pines (Pinus sylvestris) were used as input in previously developed models. These

models predict whether a tree will break due to bending loads or torsional loads during

critical wind exposure. Data from four pine stands were used in the study—two lowland

stands and two mountainous stands. For each of the stands there were large amounts of

both categories of trees: those predicted to break due to bending and those predicted to

undergo torsional failure. Moreover, there was no significant difference between any of the

stands when it came to the distribution of predicted failure modes. These two facts suggest

that the risk of bending failure and torsion failure is balanced so that neither is more likely

than the other.

The fact that torsion may be as critical as bending is a new finding.

(For. Ecol. Manage. 2000. 135:97–103)



Numerical failure criteria?

! How hollow is a tree allowed to be?

! How many roots can be missing until the tree in un-safe?

! There are some numerical approaches helping us to understand

" tree stability

" uprooting safety



The more hollow a

tree is, the higher

the risk of bending

breakage:



Statistical approach: natural failures in forests (Mattheck 1993):



Conclusion

! Trees react to decay

" by compensatory growth (adaptive growth)

" as long as they are vital enough

! As soon as the residual intact wall is less then 1/3 of the radius

" the tree is not automatically hazardous, but

" breakage safety of the fully crowned tree drops down

" risk should be assessed

" and probably load reduced

! Determination of residual intact wall thickness can be complicated, often technical

methods are required.



Technical methods

There are many different physical principles that can be used to inspect trees:

! mechanical (most commonly used)

" sound / stress waves: knocking, stress wave timing, ultra-sound

" drilling / pin-pushing, increment coring, resistance drilling

" inclination and elongation (observing under wind-load and/or pulling)

! electrical: 2-point or multipoint impedance (electrical resistance)

! electromagnetic: radar, x-ray, nuclear magnetic resonance, thermography

Most popular methods in urban tree care: drill resistance and stress-wave timing.



Measuring the tree?

! Each measurement is related to errors

" Using even simple technical equipment is always related to encounter errors

" When giving numerical results, an error evaluation should be included

! The more complex the numerical approach,

" the more errors add up

" the more difficult to explain the result to the customer



Height over diameter ratio

! From forestry, it is well known and accepted, that slenderness is a valid criteria for

characterizing storm resistance of dens forest stands.

! It is assumed that trees within stands are of higher risk, as soon as

tree height / trunc diameter > 70



For solitary trees, this ratio shall be about 50. Trees with h/d>50 are assumed to be of

higher risk for storm breakage of the stem.

(Mattheck 2001)



(Mattheck 2001)



Increment coring

Often done for tree ring analysis.



Climate reconstruction is often based on tree ring density analysis by of increment cores



using x-ray, optical or other scanning techniques.



Drill resistance measurements

Continuous measurement of penetration resistance of thin

needles entering the wood until ~1.5m depth.

Kipp 1990Rinn & FEIN 1987

Kamm & Voß 1984



History and origin of the RESISTOGRAPH® drill-resistance method

1970 Pin-penetration depth ~ wood density (Liechtenstein)

1972 In-sizing of wooden utility poles => needle penetration resistance (Germany)

1973 Resistance drilling by torque-measurement of particle boards (Germany)

1978 Recording soil penetrometer (USA)

1983 Recording motor power as measure of mechanical resistance (Japan)

1984 Spring-loaded recording of needle drill resistance with scratch pin on wax paper

1985 Needle drill resistance by motor power (Germany)

1986 Diploma thesis: tree ring analysis by resistance drilling (RINN, Heidelberg, Germany)

1987 First portable series of drills to inspect trees and timber (Rinn & FEIN, Germany)

1990 Patent on electronic drill resistance measurements (Rinn) and needle geometry

1993 RESISTOGRAPH® as registered trademark (mw. in more than 30 countries, Rinn)



The ‘real story’

! To make wooden utility poles more resistant against fungi, a German engineering

company started 1972 pushing thin needles into the wood in order to get the chemical

preservatives deeper in. Two retiring engineers of that company got the idea as a gift

for being applied as a patent (although it was not their personal idea). The patent was

later rejected by the German Patent Supreme Court due to older publications describing

similar devices (from Japan). In addition, several other experts contributed ideas how

to encounter the penetration resistance of needles, pins or borers into wood.

! 1986-88, in my physics diploma thesis at Heidelberg University, I checked if it is

possible to obtain information about tree-ring density variations based on the patent

application. It did not work in oak due to specific wood anatomical features but in

conifers. The profiles clearly revealed tree rings in conifers and showed typical

differences between intact and decayed wood in forth and backward drillings.

! All subsequently shown graphs, correlations and statements are valid only for these

electronically regulated, electronic and linear recording drill resistance devices!



Technical basics

An electric motor drives a fast rotating needle into wood. The mechanical torque at the

needle’s tip is proportional to wood density if the needle’s tip has a specific geometry. The

power consumption is proportional to the torque, if recorded electronically (linear axis!).

If these conditions are fulfilled,

the measured value can reliably

be correlated to material

properties (such as wood

density)!



Typical profiles in intact conifer and oak wood:



In conifers, compression wood can be identified by a broader latewood band:



Tree rings of diffuse porous species can be clearly visible (Fagus)

but can be invisible as well:



Poplar:



Platanus:



Tilia:



Tree-ring structure and ring width age-trend determine stem trends of density:



Typical profile

from oak with

high density in

the center:



Complete drilling through beech (Fagus) with a nearly constant density profile:



Tree rings can only be identified in the resistance profile if the needle penetrates the

borders perpendicular!



Defect and decay detection and evaluation in resistance profiles

! Defects are not always related to decay:

" included bark between co-dominant stems

! Depending on drilling point, the profile may appear as showing defect but

" it can be soil

" empty space between roots

! Thus it is important to

" first visually check the tree

" then evaluate the best position to drill

" then drill and cross-check the result with visual symptoms!



Included bark



Finding strong decay is mostly easy:



Decay my be small

and missed by

drilling.



Ring shake (fungal decay in one early wood band only), only visible by small depressions:



Strong decay is easy to identify but a low profile does not automatically mean ‘hollow’:



Compensatory growth is not always easy to evaluate (even in conifers):



Decay is not always concentric



The transition from intact to decay tells us something about compartimentalization:

Trends of decay expansion can be

estimated but, here we found two different

types of transition in the same stem disk!

Compartimentalization may work on one spot

but not on another one!



‘Difficult’

transition

from intact to

decay in early

stages of

brown rot

and soft rot.



! A tree without defined residual wall thickness.

! Drilling does not help

evaluating safety of such a

cross section.

! Many drillings would be

required to obtain the real

situation.



Residual wall thickness?



Drilling provides information valid for one spot!

Where and how often should we drill?



Decay by drilling?

! Since 1987 I am drilling standing trees.

! Until now we did not find any case where

decay started by drilling or extended

significantly.

! In most cases, there is a dis-colouration

around the drilling hole.

=>

! Resistance drilling is not non-destructive! Therefore, we should drill

" only if necessary and only on the optimum spot

" and we should extract as much information from the profiles as possible.

! Therefore many people now use stress wave tomography before drilling.

(Mattheck&Weber 2000)



Stress wave tomography

! ... using chains of electronically independent sensors was developed in the 1990ies in

order to make

" tree and timber inspection less destructive and

" deliver a more descriptive presentation of the internal state of the tree.

! The first publication describing combined stress wave tomography in wood came out in

May 1999 (Rinn: international Patent in EU and USA on stress wave and electrical

impedance tomography).

! Application is nearly non-destructive but due to physical properties of the method(s) it

does primarily not show the internal state of a tree!

! => Understanding wood anatomical and physical properties is one of the basic pre-

requisites for a proper application of these technical tree inspection methods and for a

reliable interpretation and evaluation of the results.



Basics of stress wave tomography

! First put the sensors around the tree

" The distance between and the total number of

sensors is free but should reflect the geometry

of the cross section.

! Then note their position in a table.

! Select height, location and tree species.

! Connect all the sensors by a cable.

! Knock on each sensor several times.

! That’s it.



Measurement principle

Speed of impulse = length of path / time of flight

Stress wave tomographs only measure time of flight.

Travel path and speed of the impulse are unknown!

There is no solution for the equation!



! The computer program re-constructs the inner situation of the measured cross-section

based on

" scientific knowledge and

" experience (of the manufacturer).

! Thus there are sometimes big differences between the different products!



Stress wave tomography is

! using only short impact pulses

! no ultrasound!

! only encounters the travel time of the beginning of the first incoming wave!

! not distinguishing between longitudinal and transverse waves

! strongly depending on wood anatomical influences by different species, such as:

" age trend

" species specific properties

" anisotropy

! thus depending on climate, stand,

wheather, season, ...



Typical travel paths in wood with defects



Example tomograms

Intact oak



Intact spruce



Extended decay in horse chestnut



Compartimentalized decay



Spatial resolution and precision?



No resolution defined in decay parts!



Colour games?

Subjective impression of colour scale influences evaluation!



Do stress wave tomograms of trees reveal wood condition?

! Generally NO !. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (luckily!)

! Sometimes YES!. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (helpfully!)



Example



Residual wall thickness from stress wave tomograms?

Sometimes YES, often NO!



Comparison with RESISTOGRAPH® profile

Stress-wave tomography of Kapok-tree at HortPark,

Singapur

Resistograph® drilling at sensor #7.



Conclusion

Stress wave tomography mainly

reveals mechanical compactness

of the wood in the cross-section.

This information is more

important for stability evaluation

than wood condition because it

correlates to moment of inertia

(and subsequently strength of

the cross section)!

Therefore we can calculate the

most dangerous wind direction

from relative determination of

moment of inertia of the cross

section!



Root analysis by stress waves (in use in expert reports since 2004)

! Finding roots

" to prevent damages by

constructions

" to check for damages by

construction works

afterwards

! Identifying the tree who is

" responsible for damages to

footpaths or walls or tubes

! Estimating anchorage weight

for stability evaluation.



Simple application



Typical results



3 of 16 trees (similar stand conditions but different root distribution)



Here we had to check to which tree the roots

belong that destroy the footpath.



Building permit? (red=planned building size, green = canopy area of trees)



Wind load analysis

! During the last years some experts are trying

" to calculate real wind load to the tree and

" to combine pulling tests in order

" to calculate tree safety.

! A closer look to the formulas and procedures inhibits

" systematic errors

" basic restrictions

" and potential reliability.

! First we have to evaluate real wind load, then material properties.



Vertical wind profile



Drag coefficient of trees

Varies with wind speed.

(Ruck 2008)



Drag coefficient of trees

Varies between species

(Ruck 2008)



Turbulence around a tree

(Ruck 2008)



(Ruck 2008)



Stand effect on wind-load

(Ruck 2008)



Structured

thinning at the

forest edge

helps prohibiting wind

breakage.



Dynamic swaying of crown parts

dissipate energy and helps prohibiting failure.

Cutting branches can even

increase total wind load

although crown mass and wind

sail area are reduced!

James 2003



Error calculation in tree assessment

Errors affect all measurements

Tree diameter



Tree height



Residual intact wall

Ît + Îr $ 30 - 50%

=> Î (t/r) >> 50%

=> be careful evaluating a tree by

one measurement only!



Example measurement of one tree by 15 experts:



Error calculation and cumulation



Each measurement is related to stochastic and systematic errors, an assessment of tree

height and diameter too:

ÎS = ÎH + ÎD

=> ÎS = (~±10%) + (~±5%) = ±15%

±15% is the mean error and does not necessarily cover the real value.

Mostly, if you regard one measurement, the real value is covered within the interval of the

doubled mean percentage error

=> ÎS . ±30%



Concepts of error description



Mean precent error (MPE) describes mean

percentage deviation of the actual value

from the mean value.

The corresponding error beams do not

necessarily cover the mean for all individual

values.

=> x ±MPE may not contain mean



x ±2*MPE mostly covers real value

=> if you make one measurement and you

do not know about the expected error, take

x ±2*MPE

as an easy and correct span!

Example for height/diameter

h=25m, d=50cm

=> h/d = 50 ±30% = 50 ±15

. 35..50.. 65



Stress wave tomography of the same cross section: different machines => different results



Drill resistance profiles from the same sample: different device versions, different profiles

(Mattheck&Bethge 1997)



Density profiles from x-ray and high-frequency measurements



Moment of inertia of cross section: critical influence of shape measurement!



Pulling tests and safety calculations

Wind force

Simple F = 0.5 * A * Cw * v² * ñ

a bit more precise ...

v(x,y) wind speed

ñ(x,y,T,p) air density

n(x,y) canopy porosity

wc drag coefficient

tu, gf turbulence, stand and roughness factors

Wind load at the stem base: Windforce * length of lever arm = 0.5 * A * Cw * v² * ñ * L



Precision of tree safety calculation by pulling tests?

Variance of bending moment B at stem base

ªB = ªA + ªCw + 2* ªv + ªñ + ªL

ªB $ (±20%) + (±30%) + (±2*30%) + (±5%) + (±10%)

=> ªB >> ±100%



Tree strength?

Pulling tests determine MOE (modulus of elasticity) by measuring stress and elongation,

but we need MOR (modulus of rupture) to assess tree safety.

MOR of wood can only be measured directly by bending until failure!

Thus many people are looking for non-destructive methods for MOE => MOR correlation.

The best wood grading machines achieve r²~0.6 for correlation from MOE to MOR for

small size timber without knots and decay.

Non-laminar fibres, knots and decay bring r² down to 0.1 or even less (Wilcox 1978).

=> More than 90% of the variance of MOR can not be explained by MOE

measurements in real trees with decay and irregular inside and outside

shape.

=> MOE by pulling tests do not allow to evaluate MOR of stems.



(Sander 2005)



(Niemz 1993)



Assuming the simple equation:

Tree safety = Tree strength / Wind load

=> ª Tree safety = ª Tree strength + ª Wind load

=> ª Calculated static tree safety $ ±200%

For static wind load without taking into account dynamic behaviour and torsion effects!

=> Estimation error for real dynamic safety will even be much higher!

Every expert claiming more precise values for tree safety has to clearly prove how he

achieves this - independent from methods and persuasions!



I think, tree experts should be honest and clearly point out in reports

that the evaluation may be based and/or guided by precise measurements or calculations

but at the end can not be precise at all and is influenced by individual subjective decisions

- yet still much better than doing nothing and evaluating without objective results!

! Conclusions:

" relative assessments of similar trees on similar stands under similar conditions work

" absolute determination of wind load, stem strength and safety is nearly impossible.



Application examples: expert reports on trees



Final conclusions

! A well trained visual tree inspection

" is the most important base for proper risk evaluation.

! Technical equipment helps in many cases,

" but deep knowledge is required about trees, wood and techniques

- before selecting the appropriate method and device

- for application at the tree

- for interpretation of the results

- and for evaluation of the potential risk.

! As Shigo already said: read, read, read, get educated and trained, and hurry up,

because who comes to late ...



! will face a problem catching the arboricultural train in time ...

Thank you for your attention!

Frank Rinn - Technical Tree Inspection

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Transcript of Frank Rinn - Technical Tree Inspection