Treehouse Design and Research
Discussion of the Derivation of Allowable Working Stresses:
3.1. U.S.F.S Circular #213, presented in Mechanical Engineers' Handbook, Lionel S. Marks, Editor in Chief, First Edition, Seventh Impression, 1916
Species Fiber Stress (psi)
At elastic limit At Rupture MOE (1,000's of psi)
Longleaf Pine 5090 8630 1662
Hemlock 3410 5770 917
Douglas Fir, grn 3570 6340 1242
Redwood (81% wet) 4530 6560 1024
Redwood (69% wet) 5020 7460 1101
3.2. Unit Stresses in Structural Materials, A Symposium, Transactions of the American Society of Civil Engineers, Vol. 91, 1927 containing "Unit Stresses in Timber", J. A. Newlin, U.S.F.S. Forest Products Laboratory
Newlin had at that time conducted 700,000 strength tests of wood at the U.S.F.S. Forest Products Laboratory. States that .25 reduction for "variability", .25 for maximum defect allowable in grade, .432 for long term dead load, or working stress for dead load at or below 31.2% of test values. Additional factor of safety added as appropriate.
3.3 Values for fiber stress in bending at rupture were obtained for many species in green and dry condition from tests conducted over a forty year period by the U.S.F.S. Forest Products Laboratory. Their research lead to the widely adopted de-rated numbers we see in today's codes. For example, if rupture occurs at 6,340 psi and we use 31.2% of that as the design limit, we get a value of 1,978 psi. This limit is also applied to the tree itself when considering the increase in stress due to the installation of the tree house (31.2% X 50% inherent tree safety factor = 15.6% allowable nominal stress increase). Similarly, Red Oak in green condition would be rated at (.312 X 3,930 psi) 1,226 psi.
This engineer has conducted tests on fresh cut Douglas Fir poles (8" and 10" dia.) which showed that the elastic limit (defined as a value, which would allow the initial shape to quickly return after deflection) exceeded 8,000 psi with failure occurring at roughly twice that value. In other words, Doug Fir grown under suppressed canopy conditions is likely to greatly exceed accepted code limits prior to being sawn into pieces. It is relevant to note that the above cited U.S.F.S. work was conducted up to 100 years ago, a time when the subject tree of this analysis was well established. The recent derating of in-place wood strength values and the plethora of new engineered glued wood products reflects the commercial use of newer trees grown in ways that do not reflect the properties of older trees. It is thus not only appropriate but necessary to cite the earlier studies as the basis of this analysis.
Discussion of Tree Attachment Technology:Many a child's tree house has been successfully attached to its tree using spikes or lag screws. The recent interest in adult tree houses has seen a growth of various schemes including lagged steel brackets, arborists forged steel cable attachments, and several variations on the well tested timber washer scheme. The World Tree House Association has conducted tests at its yearly meetings in which this engineer has witnessed and participated. This effort lead to the adoption of a somewhat standardized "artificial limb" which consists of an alloy steel lag screw with integral shear washer, given the name "GL" (or Garnier Limb) in honor of modern tree house pioneer Michael Garnier.
Failure of tree attachment hardware is defined as the load which causes irrecoverable crushing of the cambium layer that the fastener bears at least partially against. This engineer's research in this area involved the production and testing of a number of special fasteners that might themselves fail against the cambium layer, successively increasing the fastener strength until the cambium layer itself failed. This process allowed the live tree compressive strength against the grain for typical Doug Fir to be identified at 450 to 500 psi rather than the 1,800 psi allowed by codes for dense select structural material. Although not used in this project, design of improved fasteners allowing 1,000 psi grain compression have been verified recently by substantial testing.
Red Oak is rated at 254% of the hardness of Douglas Fir which has been extensively tested for fastener behavior. Thus it is safe to assume that a fastener design tested in Douglas Fir would have at least the same rated value in Red Oak, and likely a much higher rating.
Special Tree Fasteners are the result of on-going testing and development and analysis. The "standard Garnier Limb" is a collared stud with 1.25" diameter shaft, with lag type threads on the tree end. High performance versions of this type of fastener have resisted 12,000 pounds installed in living Oak without plastic yielding of the fastener. Crushing of the growth rings did occur but it is likely that the tree could rebuild against the load bearing face. This type of fastener is rated at 6,000 pounds placed vertically 2" out from the collar surface, and many are in service around the world.
Special cantilever type tree fasteners were designed and installed for the Tree House at the Island Wood Learning Center on Bainbridge Island near Seattle, Washington. These use 1.75" diameter 4140 shanks and very generous bearing area to allow up to 5" of growth in the host tree which is young, healthy, and actively growing. A design life of fifty years for the tree house structure was thereby provided.
Arborist type tree fasteners are assumed to provide approximately their rated value, and similarly for the chains and cables and related hardware used in many places.
©2007 TreehouseEngineering.com Charles Greenwood PE/Greenwood Engineering- All rights reserved.
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