Pulling Test
14Pages

The FAKOPP pulling test 1. Introduction The stability of urban trees is a key question that affects everyone. Diseased and unstable urban trees pose much risk for everyone, and are a serious liability for municipalities in case of an accident. Tree stability and safety assessment is therefore of the utmost importance. In the meantime it tends to be much neglected in many areas. At present, the most accepted method for evaluating the safety and stability of trees is the pulling test. It involves applying a bending load on the trunk via a cable attached to the tree. The method can be used either to assess the uprooting stability of the tree (by measuring the inclination at the bottom of the trunk), or to establish the risk of trunk breakage (through measuring the bending stresses using extensiometers attached to the trunk). Both of these methods are introduced here briefly. 2. Pulling test for uprooting safety evaluation The pulling test is based on affixing a cable at approximately mid-height to the tree to be evaluated, and applying a moderate load, while measuring the inclination at the base of the trunk. The induced inclination is slight (less than .25 degrees), to make sure that the test itself does not damage or start uprooting the tree. The cable is attached to the tree at approximately mid-height in the crown. This typically requires a ladder, or climbing the tree to the appropriate height. A metal cable of appropriate loading capacity is attached to the trunk. A soft belt is typically used for this to avoid damaging the tree. The other end of the cable enters a winch, which is affixed to an anchor point. The anchor point can be any object that is safely secured to the ground, most often a stump or the bottom of another tree. If another tree is used, care should be taken that the bark is not damaged (typically using a soft rope or belt).

The winch applies tension to the cable. A load cell attached to the cable measures the tensile load. Since the cable is at an angle, the horizontal component of the load is calculated and used for the evaluation. A relatively moderate load is applied in order to avoid causing damage to the trunk or uprooting the tree. Load is continually measured and sent to a computer for recording and evaluation. Uprooting safety evaluation requires inclination, as well as load data. Inclination is measured at the tree collar. The inclinometer provides data of sufficient precision and frequency. This data...

- crown surface area, - air density, - wind velocity - aerodynamic drag factor - the height of the crown centerpoint The drag factor is a constant that is different for each wood species. The drag factor values are provided in the table at the bottom of this page (based on Wessoly and Erb 1998). Comparing Mwind to Mmax, we can calculate the so-called Safety Factor (SF) that indicates the probability that the tree will be uprooted at the given wind velocity: SF= If this value is above 1.5, the tree is safe, while a SF below 1 signals high risk. In-between these two values, there is a...

The testing procedure is much the same as in the case of uprooting safety determination, but in this case an extensiometer is used on the compression or the tension side (or possibly both sides) of the trunk, instead of an inclinometer. The load and deformation data are collected analyzed by a computer software. Trunk safety is determined based on the so-called linear elastic limit. When trees bend, up to a point, their deformation is linear. More importantly, this deformation is not permanent, and, up to this point, there is no permanent damage to the trunk. This safe limit of relative...

4. The FAKOPP pulling test apparatus The FAKOPP pulling test apparatus is capable of both tests described above. The components, test procedure and some important safety advice are described below: 4.1. Components Cable and winch The system contains a 20-meter (65 feet) length of high capacity metal cable with a 1.6 metric ton (optionally 3.2 ton) manually operated winch. The winch has a ratchet mechanism that multiplies the force of the operator to exert sufficient tension on the cable. The cable and the winch are equipped with safety hooks and two soft belts for fitting it around the tree...

Inclinometer - Inclinometer sensor, ST-015 - Ratchet lashing - Sensor mounting plate - External battery unit with bluetooth signal source - biaxial leveling device - 9V rechargeable battery + charger - Measurement range ± 2 degrees - Resolution: 0.001 degree - Temperature compensated - Sampling rate: 10 Hz - Mounted by a single screw - Operating voltage and current: 12V, 20 mA - Weather proof, IP65 Extensiometers The system includes two LVDT extensiometers that are mountable on the tree trunk. Signal is sent to the central unit via a common interface box. Extensiometer parameters: - LVDT...

Sampling rate: Power source Note: calibration constants are indicated in the supplied calibration sheet and are to be entered in the software upon the first time the device is used. The FAKOPP Pulling test software - PC software, runs under Windows7 or higher, - Simultaneous load, inclination and extension measurements, - Load-inclination and load-deformation curves generated - Automatic Safety Factor calculation for uprooting and trunk safety - Can handle one inclinometer and two extensiometers simultaneously - Drag Factor and Elastic Limit entered manually - Continuous software...

Connect the load cell to the external display unit and the display unit to the central unit. Install the first inclinometer sensor mounting plate on the tree trunk, as close to the ground as possible, using the ratchet lashing. In case of decay in the tree trunk, the position of the inclinometer may affect the test result. We recommend that you avoid decayed areas when mounting the device. Attach the inclinometer sensor and level it using the biaxial leveling device. Attach the battery back and turn the inclinometer on. Connect the battery pack to the central unit. Optionally, repeat the...