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http://www.thetreehouseguide.com/press/latimesaug052004.htm

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http://tfguild.org/confs/cfwest2000.html

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New Inovations
The superposition of building loads on a living structural system creates a difficult problem for analysis. Living structural systems attempt to achieve a state of uniform stress by growing additional solid material to support additional loads (Ref #12 below). This implies that after an appropriate length of time, the tree itself will compensate for applied external loads.
This has in fact been observed by this engineer and other researchers; it also means that during the first several years there will be maximum stress on the tree connection system. It is therefore common practice to remove some of the limbs of the tree to reduce both weight and wind drag during this compensatory growth period.
During initial research into engineering solutions for tree house structures in the early 1990's little definitive information was found regarding the development and distribution of stresses in living trees although the relevant technical literature was well searched. Nevertheless, some important information was identified regarding the actual value of wind loads transferred from the leaf mass into the relatively flexible tree structure. Speck, Spatz, Vogellehner ( Ref #1,) stated that 50% of the projected area acting as a flat plate will approximate the wind load.
Dr. Claus Mattheck has since published his very good book entitled "Tree Mechanics" (see also www.vtaseminare.de) that discusses stresses in living trees in an informative and entertaining way. His work corroborates the assumptions presented herein but does not address codes.
It is certain that increasing wind velocity causes leaves and stems to shed load since they are no longer able to structurally resist the wind loading. The projected area ultimately reduces to that surface area described by the trunk, limb, and stem outline. Thus the wind load caused by the leaves themselves will reach a maximum value and then remains nearly constant or actually drops (leaf stem and branch failure) with increasing wind velocity. The wind loads on the remaining projected area continue to climb with wind speed, but there is considerable non-linearity since the highly flexible portions of the tree structure are limited in their ability to transfer increasing bending moments into the more ridged portions. This results in a combination of bending about the base and upward tension.
As the wind speed continues to increase, the trunk either de-laminates or the entire root system is ripped from the ground. Examination of nearby trees at this site shows that some have continued to grow and thrive for extended periods of time, suggesting that at this particular site de-lamination and uprooting are relatively rare. Technical articles stated that living systems approach a safety factor of 50%, which may explain this robust performance. Some safety factor is initially consumed by the addition of the tree house loads, but, as noted previously, trees will reconfigure to achieve the original safety factor.
The engineering solution consists of calculating the moment about the base due to forces acting on the tree itself and the total moment including tree house. The estimated wind force acting at the center of drag (at the center of mass for seismic loads) multiplied by the distance above the ground to the center of force provides the tree moment. The moment due to the tree house is found in a similar manner.

 

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