Thursday, September 29, 2011

Storm Damage

This year has brought a lot of storms which are usually accompanied by strong winds. These strong winds and downdrafts are the culperate for many of the damaged and up rooted trees although ground saturation plays a big role as well. Always remember that storm damaged trees are some of the most dangerous. The reason for this is because after the tree is damaged there can be anything from pressure loaded branches to stress fractures that typically can't be seen so it is always better to give us a call at 262 646 3735, and we will take care of it for you. Here is a study done by the unversity of georgia on wind loads on trees.

Storm Wind Loads
On Trees
by Dr. Kim D. Coder, Warnell School, University of Georgia Feb. 2007

Trees must withstand wind to survive. Wind and gravity both impact trees in storms, but the dominant load is from wind. Storm winds push on tree crowns and stems causing root plates to rock. The result of storm winds are forces twisting and bending tree parts causing either the part to fail or the supporting soil to fail. Treessense structural stress and attempts to minimize failures through reactive growth. Trees modify their structure over time as they are challenged by wind. Trees are biologically designed to sustain average wind loads. What are the loads applied by storms to trees?

Sailing Along
A tree has a large sail area (leaves and twigs) held upright high into the air by a tapered mast (stem). The stem is woven onto a thick horizontal mat of large structural roots at the stem base which forms a root plate. Rapidly tapering roots grow away from the stem base to provide the tree with an absorbing surface. The center of gravity (effective weight center) of a tree is usually somewhere in the middle of the stem above the root plate. Gravity is pulling down on the tree all over but has a total effect focused at the center of gravity. Wind is pushing the tree around its center of gravity. The weight of the tree is pulled down onto the soil surface. As long as its center of gravity remains positioned above its supporting base, the stem is stiff enough to keep the tree upright. If the center of gravity for a tree is not positioned over its supporting base, gravity may topple the tree if the tensile and compressive strength of the stem and root base is inadequate. Storm winds can push the center of gravity in the stem out from over its supporting base. The combined result is wind moving the tree laterally and gravity pulling the tree downward, providing a rotation force rolling the tree out of the ground -- a load wheel.

Trees seldom fail under their own weight -- external loads stress and strain the tree until failure. Gravity loads trees in tandem with wind to initiate these failures, but wind loads greatly exceed gravity loads in most
situations. Wind is an acute mechanical variable for which trees must constantly adjust growth processes. Gravity is a chronic stress and strain with which trees must cope. Adding mass to a tree in the form of growth
(size, extent, and reach), or in applied water, ice or snow loads, can magnify gravity’s impact on the tree, causing fiber creep (permanent tissue changes) and failures. But, it is storm winds which push tree structures to their limits.

Impact Trinity
In order to more clearly understand storm initiated failures in trees, wind loads must be fully appreciated.
Wind has three primary components which impact trees: 1) velocity or speed (mile per hour or feet per second); 2) acceleration (velocity changes over time or velocity squared); and, 3) throw weight (mass of air)and its contents).