Precision digital instruments to measure dynamic wind loads on trees during storms

Precision digital instruments that measure the dynamic wind loading on trees during wind storms are described. The instruments are 500 mm in length and attach to the trunk of the tree near the base. Two instruments are used on a tree and are oriented in orthogonal directions, usually North/South and East/West so that resultant wind forces are recorded, no matter from which direction the wind blows. The principle used is to detect linear strain in the outer fibers of the tree trunk, as the wind induces bending. The sensor used is a digital probe with a precision of 1 μm, which gives a strain resolution of 2 × 10−6 microstrain. Output is connected to a computer or data logger for recording with sampling rates of 20 Hz. The instruments measure strain directly and are thus called strainmeters. The accuracy of the instruments was tested under laboratory conditions and calibrated on a tree with a static pull test to relate strain readings to bending moments of the trunk. The strain meters were used under field conditions over a 12-month period and recorded dynamic movement of trees under storm conditions. The method of using these instruments to measure wind forces and dynamic tree response is described. Results from field tests of trees under high wind conditions are presented, including displacement data, wind loading and spectral analysis.