TREE HOUSE ENGINEERING FOR CODE APPROVAL
An Overview by Charles S. Greenwood, P.E. © 2013/2016
STRATEGIES FOR CODE APPROVAL AND THE NEED FOR SPECIFIC CODE LANGUAGE
Building Standards, an ICBO publication, presented in the July/August 2000 edition an article authored by myself and David Bassett, P.E,. C.B.O, entitled “Habitable Treehouses: Not as Simple as Swiss Family Robinson”. This summarized nearly a decade of work in this field.
In the thirteen years since there has been an explosion in the construction of tree-mounted structures around the world. Much has been learned about what to do and what not to do. Many permits have been granted, even for large public-use wheelchair accessible projects by referencing IRC R202 (“….other independent systems”), R301.1.2 (“…..accepted engineering practice”), R 402.1 (“…rot-resistant wood foundations”); liberal use of the performance-based provisions as well as alternative testing procedures may also be required. However, typical Codes are NOT a good fit to treehouse engineering and construction methodologies.
SIGNIFICANT DIFFERENCES COMPARED TO “GROUND-MOUNTED” STUCTURES INCLUDE:
99.9% OF TREE HOUSES HAVE NOT GONE THROUGH AN ENGINEERING OR CODE REVIEW.
NON-COMPLIANCE BY TREE HOUSE OWNERS CAN BE TRACED TO (1) THE LACK OF A REASONABLE CODE PROCESS TO FOLLOW, AND (2) THEIR NON-CONFORMIST ETHIC.
CONSTANT CYCLIC LOADING AND METAL FATIGUE CAN LEAD TO CATASTROPHIC FAILURE. BUILDING PROFESSIONALS MAY NOT BE TRAINED IN THIS TYPE OF ANALYSIS.
FLEXING IN BEAMS, WALLS, JOISTS, ETC., REQUIRE THAT MEMBERS BE DESIGNED FOR MAXIMUM DEFLECTION WITHIN THEIR STRENGTH LIMIT. L/360 IS LIKELY TO CREATE UNSAFE STIFFNESS IN TREE HOUSE STRUCTURAL SYSTEMS.
VARIATIONS IN LOAD COLLECTION AND DISTRIBUTION MAY REQUIRE EQUALIZATION BEAMS AND OTHER UNUSUAL TECHNIQUES.
ALL BUT THE SIMPLEST TREE HOUSES MUST BE ENGINEERED USING FINITE ELEMENT ANALYIS PERFORMED BY AN EXPERIENCED PRACTITIONER TO DETERMINE DYNAMIC FLEXIBILITY AND STRESS CONCENTRATIONS. AGAIN, BUILDING OFFICIALS MAY NOT BE FAMILIAR WITH FEA.
Code language may create problems rather than solve them. For example, NFPA rules regarding emergency power systems used in hospitals and other critical facilities unfortunately contributed to unnecessary injury and death during Super Storm Sandy.
What should be the intent of Building Codes? Generally accepted as the first building code (3,700 year ago!), the Code of Hammurabi provided punishment to be made proportional to the injury, and ultimately Rule 229 declares that “If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death”. The intent of modern codes should be to avoid that outcome.
DEVELOPMENT OF TREE HOUSE SPECIFIC CODE LANGUAGE SHOULD FOLLOW INTERNATIONALLY RECOGNIZED METHODOGIES FOR PERFORMANCE-BASED DESIGN.
The performance-based BCA (Building Code of Australia) was drafted following consideration of numerous overseas models (including the New Zealand, British, Swedish and Dutch examples) to suit the Australian building regulatory environment. This has meant that the performance-based BCA substantially includes the technical requirements from the previous BCA90, with a ‘performance hierarchy’ built around them. The hierarchy of the BCA is shown below.
INCLUDEPICTURE “http://www.abcb.gov.au/en/about-the-national-construction-code/the-building-code-of-australia/~/media/Images/About%20the%20NCC/BCA%20Hierarchy.ashx” \* MERGEFORMATINET
See also ASCE/SEI 7-10, Chapter 1, where the concept of Performance Based Design Engineering is introduced, and the terminology of Occupancy Category is replaced by Risk Category.
Please provide your contact information if you would like to work on Tree House Code language. This will best be accomplished by the P.E. and C.B.O. community.
STRUCTURAL PROPERTIES OF LIVING TREES
Tree stems and limbs are highly flexible vertical and horizontal cantilevers; they provide the biological mass transport system for nutrients between the solar-powered “engine” found in the leaves and the complex biochemical “factory” found in the root system. Roots also act as a diffuse soil anchoring system, capable in almost all cases of resisting enormous repetitive loads without failure. Please see “Improving Tree Health: Myths & Elixers” by Olaf K. Ribeiro, Ph.D. for an introduction to root physiology.
Trees grow upward from the top and outward from their centerline.
The tree diameter above the root buttress, typically expressed as “DBH” (Diameter at Breast Height), may be taken as the most accurate measure of the actual strength of that particular tree; given the specie properties you may calculate the probable maximum strength and therefore the load the tree is designed by nature to resist.
Trees respond to imposed loads, adding both quality and quantity growth at stress points. This occurs quite rapidly, even in one season. Try that with your typical engineering materials.
Young trees grow upward without any safety factor as nature deems it more important to established solar contact at its crown. Stem failure will simply place the nutrients back into the carbon cycle and a new tree will try again to reach the sunlight.
The safety factor for an established tree is approximately 1.5. This is inferred from the ability of the tree to successfully resist loads that it has never previously experienced.
Hyper-elastic behavior is defined as the ability to temporarily withstand loads that are well above the laboratory derived rupture strength. This is similar to the behavior of injection molded plastic parts where the addition of water increases the short-term load resistance, albeit at lower total strength.
The MOE of living trees is about 70% or less of the same tree cut and dried for lumber.
Load Bearing Biological Structures always try to grow into a state of constant mechanical stress. The corollary to this is a marked reduction in strength after cutting the living tree into convenient shapes – two thirds of the living strength may be lost.
Trees do not necessarily exhibit similar deflections and natural frequencies in response to wind loads. They use torsional oscillation (much like an egg beater) to dump energy by friction into the comparatively thick viscous air, and if necessary, they shed limbs preferentially to preserve the main stem. This makes behavior unpredictable, even for the same species and size.
The exact mechanisms controlling living tree behavior are yet to be fully understood, but this does not interfere with using what we have learned about placing significant loads into living trees over the last twenty years.
Let’s look at the engineering properties of the tree itself:
Thus a mounting system placed 36’ above grade in two trees with characteristics defined above must achieve eight feet of total translation capability. Reductions in tree diameter will of course increase deflection but this is limited by the ultimate rupture strength of the stem.
Limbs can be analyzed in a similar manner. We must accept the root foundation strength on the basis of previous survival of the specific tree in question and of similar trees at the site.
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 (Also refer to more recent information)
Species Fiber Stress (psi)
At elastic limit At Rupture MOE (1,000’s of psi)
Comp. Perp. Grain, psi
Cedar, Incense (green) 3350 6200 790 3150
Cedar, Red (green) 3150 5200 940 2770
Douglas Fir (green) 3570 7700 1510 3870
Spruce, Sitka (green) 2740 5700 1230 2670
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. He states that .25 reduction for “variability”, .25 for maximum defect allowable in grade, and .432 for long term dead load. This creates an allowable working stress for dead loads applied to shapes cut from whole logs at 31.2% of test values.
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 design value of 1,978 psi.
Living (or cut whole) trees have by definition grown their fibers to achieve equal stress distribution. Nature provides many dozens (hundreds in some cases) of tightly spaced laminations naturally, each grown in response to the actual stress found at that point in the tree.
We can de-rate the tree itself, based upon its ultimate rupture strength x 0.31.
Using this value as the “Tree Design Strength” we can calculate the acceptable maximum moment at ground level.
Nature establishes an approximate safety factor of 1.5 for an established tree; using the same de-rating logic suggests that any loads added to the living tree should not exceed 0.31 x 50% (the “extra” strength). This is the basis of the
“15% Rule” that has been respected as the Maximum Load in most engineered designs. This can also be applied to the vertical strength of the stem.
ENGINEERED SUPPORT SYSTEMS AND MOUNTING IN TREES
THE 15% RULE ALLOWS THE MAXIMUM VERTICAL AND HORIZONTAL LOADS TO BE DETERMINED BEFORE YOU BEGIN TO DESIGN THE TREE HOUSE.
LIVE DESIGN LOADS DEPEND UPON THE LOCATION, SIZE AND PURPOSE
TOTAL LOAD MUST INCLUDE THE TYPICAL 10 PSF FLOOR AND ROOF DEAD LOAD, AT LEAST 8 PSF WALL LOADS, AND A VALUE FOR THE FOUNDATION SYSTEM THAT IS AT LEAST 10% OF THE L+D FOUND ABOVE. OTHER LIVE LOADS INCLUDING WIND, SNOW, ICE, WATER PENATRATION, ETC. MUST ALSO BE ACCOUNTED FOR. GAZEBOS CAN HAVE A FULL SNOW LOAD ON THE ROOF AND ANOTHER WIND-BLOWN FULL SNOW LOAD ON THE DECK.
ACCOUNT FOR UNUSUAL ADDITIONAL LOADS INCLUDING TIMBER FRAMING, ART OBJECTS, HEAVY FURNITURE, BRIDGE/ZIP LINE CONNECTIONS, ETC.
OCCASIONALLY THE SEISMIC LOAD WILL RULE, BUT TREES WITH THEIR ROOT SYSTEMS ARE VERY GOOD AT DECOUPLING THE SHORT TERM GROUND MOTION; THEREFORE USING THE 1-SECOND VALUE IS ACCEPTABLE.
WIND LOADS CREATE MOVEMENT INCLUDING TRANSLATION AND TORSIONAL OSCILLATION, AND THESE MOTIONS ARE SUBJECT TO RANDOM REVERSALS.
“TAB” = TREEHOUSE ATTACHMENT BOLTS
For a small tree house we can assume an effective L+D (including walls and supporting beams, etc.) of 100 PSF in most cases. Above 150 S.F. a more detailed tally will be required. This means the TABs will need to support 15 Kips, and with true ratings in the 3,000 pound range or less, at least 4 or 5 of the most common type will be required.
The design is effectively the combination of a 3” O.D. by 1” long TECO-like shear ring mounted on a threaded stud, usually 1.25” in diameter; lengths and collar details may vary widely.
A one inch diameter lag bolt with a 6” long thick walled tube (holobar) w/ 2” O.D. x 1” I.D., with at least 3” imbedded into competent sapwood and heartwood, will support 5 or 6 Kips and allow for some future tree diameter growth. It is also easier to install and less expensive. This TAB design takes advantage of the much greater pull-out resistance of the lag thread, and therefore makes a great upper mounting point for a turnbuckle attached to a long shaft lower TAB.
Since it is preferable to keep the load centered upon the vertical axis of the tree stem, it is good practice to mount support beams on both sides of the host tree. On small trees the only option is to use a through fastener style TAB, which can simply be a solid shaft placed through a suitable hole bored through the tree with a ship auger. 3M DP 190 epoxy is frequently used to fix the through fastener to the tree, as well as to seal the wound. This method is gaining ground when used with larger trees, and is essentially mandatory when lateral tension loads are attached to the tree such as zip lines, cable and chain bridges, etc.
Large competent hardwood trees can utilize a pair of “side mount” TABs rather than a through fastener, but the maximum available compression strength perpendicular to the grain must be watched closely.
The total L+D load is the best determinant of the TAB shaft diameter. Strength increases by the ratio of the square of the diameter, so a 1.75” diameter shaft greatly exceeds the load capability of a 1.25” diameter shaft, and so on.
Since the shaft itself is supported by its own bending strength plus the support provided by the collar bearing upon the crushing strength of the grain (both for vertical and horizontal loads), it is obvious that large and long collars provide substantial increase in TAB load rating.
For a critical non-redundant primary support TAB, it must be assumed that load reversals during the life of the structure will exceed many tens of thousands, if not more. This means that spring steel is mandatory, and that is accomplished by specifying the proper alloy followed by proper heat treatment. Such a part should NOT be installed with a pipe wrench or similar tool since scarring will allow stress cracks to form, potentially leading to catastrophic failure. It is not difficult to provide a mounting scheme on the end of a TAB that eliminates this dangerous practice.
MINIMIZING THE TOTAL NUMBER OF TABS IS IMPORTANT TO TREE HEALTH
For each 3” collar 7 in. sq. of the cambium layer is sacrificed for the TAB. For an eighteen inch diameter tree with two similar TAB penetrations, 6” of the 56.5” total circumference, or about 10%, has been sacrificed. If the 1.75 inch diameter “HL” style TAB is used in a competent tree, as much as 30 Kips can be supported rather than about 6 Kips with the short-collared 1.25 inch diameter TAB.
The practice of wrapping the tree with cable cinched around wood blocks, as is done in some areas, is a particularly egregious form of mounting since little vertical support is developed while at the same time causing a massive amount of injury to the cambium layer by squeezing it enough to stop biological transport flow. Never do this.
A better approach is possible when used only for horizontal load support; this approach places a formed steel ring around the tree some distance away from the bark, with at least three threaded pins tightened into the sapwood to position the device. It offers little vertical support for tree house and platform loads, but achieves better tree health than the cinched block method.
GENERAL DISCUSSION OF TREE HOUSE ENGINEERING ISSUES
Charles S. Greenwood, P.E., LLC, makes no claim or warranty that the information presented herein is directly applicable to any other situation or circumstance save for the educational exercise of the attending engineers and builders. If you are considering building a tree house that may need Building Code approval, it is necessary that you employ a Professional Engineer who has demonstrated their skills in this area of practice. It is far more expensive to engineer an as-built structure than to properly engineer it prior to construction. If you need these services, my contacts are 541 592 4100 and email@example.com.
LOAD COLLECTION: How big is it? What do the components weigh? What does it all weigh? How is the weight distributed? What’s the seismic load? What’s the “sail area” (wind load)? How about the live load? Snow load?
LOAD TRANSFER: How do you take that entire load you’ve collected and put into a few rather small fasteners and have it work properly? We are seeing a variety of problems with inadequate mounting methods. What can be done about it?
LOAD SUPPORT: Now your loads are bearing on the trees themselves. How do trees react to external loads? How much is too much? When is a tree too small to act as part of the support system? What happens when we mix “crutches/helpers” and trees together?
PERMITS: Do you need one? How do you get a permit if you need it? Why is the International Residential Code not very useful, but International Building Code is?
SINGLE TREE SUPPORT SYSTEMS
Tree Houses mounted in a single tree will use the classic “knee brace” or “hanging cable” solution; some of the vertical loads and typically all of the horizontal loads may be transferred directly into the stem of the host tree. The major disadvantage is that at some point the tree must penetrate the plan view, and probably (except in the case of very large support trees) the roof as well.
Deflection of the host tree over the vertical distance of the triangle created by a knee brace system will attempt to bend the building frame and/or the knee brace itself as the tree is likely to be a great deal stronger than the knee brace/frame system. Something has to give. Cables of course are laterally non-supporting but all of the horizontal load must be resisted by the interface between the house frame and the tree itself; some kind of fastening system will at least prevent destructive “bark banging”.
Very small backyard tree houses are good candidates for an adequate single tree.
TWO TREE SUPPORT SYSTEMS
A second support tree offers major advantages, along with a few added difficulties. Simple twin-beam support systems will often suffice, closely mounted at each side of larger diameter trees, or with the additional complexity of a moment frame (probably steel) with multiple fastening points. Robust joists or cross beams can cantilever outward in a balanced configuration on each side of the parallel beam lines, providing floor area quickly.
Once again, something has to give, so it is typical practice to mount the beams solidly to one tree and with UHMW (Ultra High Molecular Weight Polyethylene) wear plates at the other. For commercial and public use projects it is recommended to completely enclose both the fixed and sliding connections with steel “clasp brackets” that includes UHMW on all surfaces; this will allow movement AND prevent moisture induced beam rot as will be the case in a steel-to-wood interface.
While this arrangement favors a rectangular plan view, one may easily get creative. Adding knee braces to this type of support system can provide for greater spans, but remember that something has to give and it will probably be the frame itself – some amount of twisting and deflection is inevitable as the trees will rarely act in concert during wind events. Torsional loads also come in play with multiple support tree designs and this inevitability must be provided for. Fortunately, if the support beams are robust and not too rigid in plan view, and well-mounted with the beam mounts described above, there will likely be sufficient flexibility to allow for normal tree movements.
THREE TREE SUPPORT SYSTEMS
Now we’re getting creative, as the building and structural design can become much more interesting. Usually one tree will act as the “master” and the others will act as “slaves” as far as differential motions are concerned. Beam connections must be free to move with respect to each other.
This is where the analogy to early American railway locomotive design becomes significant. The European practice was to build very precise rail alignment on very stable roadbed, while the American practice was just the opposite – there was a lot of distance to cover, and the investors wanted to make the highest rate of return as soon as possible, so they made the crappiest track possible. Unfortunately, the engines in particular left the rails often. Rather than improve the track, designers realized that clever floating support linkages amongst the wheels and engine frame – suspension with load distribution and equalization – would keep the locomotives on the track most of the time. For tree houses, multiple sliding connections (with 4-bar knee brace links when required) provide load equalization during translation.
FOUR (OR MORE) TREE SUPPORT SYSTEMS
All of the above applies when you are blessed (or cursed) with numerous support trees. Remember, the translational complexity goes up geometrically with the number of support trees involved.
ARTIFICIAL TREES – GROUND MOUNTED SUPPORT POSTS
This can be a handy way out when you are unwilling to make your design fit what nature has provided, or your construction is too heavy to be supported by the available trees.
A “tether ball pole” is a good starting point for this discussion, as most of us have put concrete around a car wheel and mounted a pipe to it and then rocked its world. The idea here is to keep the system from acting like a bowling pin; these pins are also a gravity stabilized system, but one that is likely to fall over if the horizontal force exceeds the vertical stability moment. Wind machine and utility towers, and for that matter, sky scrapers, are large examples of a ground supported and gravity stabilized vertical cantilevers. Tree house posts can be rigid with slider tops, or flexible if your design is elegant.
TREE HOUSE ATTACHMENT BOLTS AND OTHER FASTENERS
Trees support vertical and horizontal loads by resisting the crushing forces acting on the grain of the formerly living wood – very little load is resisted by the living and “recently living” tissue. Wood is much stronger parallel to the direction of the grain, and this presents a significant problem when attempting to support horizontal loads such as zip lines, bridges, wind and seismic forces from structures, as the horizontal crushing strength drops typically to 10% of the above. Therefore, this engineer recommends eliminating as much as possible the support of externally imposed horizontal loads; obviously, a significant amount of horizontal load will transfer in any case from the tree house itself into the tree.
Compression against the grain (loads are parallel to the grain) supports gravity loads in tree houses. The most typical case is an inserted pin of some diameter, adequate for the loads involved; this can be a nail, a lag screw, an arborist shackle mount, a special lag screw or stud with an integral shear collar, or a pipe or tube going part way or all the way through the tree. All have been used, and all work when applied correctly. All penetrations can involve risk to the health of the tree, so proper practice is to minimize the total area of insult to the living cambium layer, and to treat and/or seal the injury. Cinch rings damage a much larger area of the cambium and therefore should not be used.
It is recommended practice to use doubled fasteners – one over the other – either in a simple pin with turnbuckle, or a double TAB with integral moment bearing beam support frame. Remember that when a cable support is used, the upper fastener now has a pull-out load added by the triangulation inherent in that design. Also, the cable mount can end up supporting a large portion of the load and therefore needs to be as strong as the lower TAB. Finally, all loads in trees create a biological response that induces the tree to add significant “bolstering” at the point of stress; this can occur in as little as one year and may become the dominant consumption mechanism.
Speaking of consumption (where the tree literally eats the fastener), one must decide how many years of service your tree house is going to provide before major re-mounting is required. Experience is showing that this becomes an issue much sooner than most builders expect. Extendable TABs are now coming into use to provide for easier “planned” re-mounting; having one’s beam lines and joists laid out in an easily movable pattern helps to facilitate this. Of course, many older tree houses have simply been partially consumed by the host tree(s) but the jury is still out on the effect of this type of damage. One school of thought is if you make the beams out of the same material as the tree they may not reject the tissue as they are consumed. Good luck with that.
Crushing support from the end grain into the TAB is analogous to a system of coil springs (literally each ring) supporting a horizontal leaf spring that is almost rigidly attached at its inner end. Thus at the inner end some of the compression force must be resisted at the top of the fastener to counter balance the upward support along the shank (or collar) as it nears the outside surface. There is almost no bearing strength over the outside inch of the fastener pilot hole, so it is up to the crushing strength of the wood several inches in to the tree to support your tree house. This why it is common practice to increase the bearing area by using large diameter collars of sufficient length – indeed, the same 3” diameter penetration through the cambium layer may provide three or four times as much support by using a long collar.
We should discuss the proper metallurgical properties for TABs. Specifications advocated by this engineer are to anneal after machining followed by quench and tempering to produce a Rockwell “C” hardness of approximately Rc = 35. With 4140 alloy this will achieve yield strengths above 100,000 psi. Since stress reversals often occur many times per day, it is predictable that without proper alloying and heat treatment, the steel will fail – just like putting a piece of metal in a vice and bending it back and forth until it fractures. Since the fracture will begin at a discontinuity on the surface, it is (or should be) absolutely forbidden to install TABs with pipe wrenches or other devices that can injure the highly-stressed fastener surface.
Relative compression strength parallel to the grain is shown in the following chart. A juvenile White Oak was used for load-to-failure tests.
A special tool was developed to take the above data (Provisional Patent Application filed). With this tool an installer can check and verify the vertical grain compressive strength of the specific hole being used to support a TAB. Rotating the tool 90 degrees offers a similar set of readings for the horizontal crushing properties.
Precise load rating is then made possible by mathematically analyzing the above data; this produces results that are specific to the particular fastener design and to the actual location into the tree in question. If, for example, the design load is greater than the load calculated from the above data, then a second fastener can be added some distance above or below.
FINITE ELEMENT ANALYSIS OF TREES AND TREE HOUSES
For any but the most basic tree house designs, simple 2-dimensional statics-based analysis, typically done by hand, is inadequate. Due to dynamic loads, trees and tree houses are best analyzed as 3-dimensional flexible objects akin to wooden boats or even space craft. This engineer has developed specific FEA techniques appropriate to tree house engineering that allows all manner of load combinations to be explored and demonstrated.
THE FUTURE OF TREE HOUSE LIVING: How do you plan for the long term use of your tree house? Is this a trend that will continue? What must you do when it’s time to re-mount your tree house? The picture shows the initial stage of re-mounting a large tree house.
The fasteners used in the installation of the above tree house have begun to be aggressively consumed by the support trees. Workmen shown above are evaluating methods that might be used to re-mount this structure in a manner that will not require additional major work in the future. Note the “temporary” cable bracing added during construction to support the very heavy loads encountered in this three-story structure. This “band-aid” can also be permanently removed by employing suitable beams in the re-mount.
The “XL SYSTEM” was employed to re-mount that tree house and will be available for similar re-mounting projects and new construction. It represents a break from previous methodologies and provides numerous engineering improvements including load capability, speed of installation and cost per ton of load supported. See treehouseengineering.com for details or contact Greenwood Engineering.
GENERAL RULES FOR TREE HOUSE ENGINEERING
The “15% Rule” – Trees are configured by nature to support all of their probable loads while maintaining some factor of safety for improbable loads, or for circumstances where the tree itself is compromised through injury or disease. There is general agreement that about 50% safety factor is achieved with trees of hardy species – but only after the tree has established itself amongst the competing adjacent trees. This engineer has advocated consuming no more than 1/3rd of that safety factor when adding a tree house load (V and/or H).
”Tree stem diameters allow load calculation through reverse engineering” – given the species and diameter it is possible to calculate the wind drag and weight load of the tree under study. This is actually much more accurate than estimating the wind drag and weight. The bending moment about this diameter is the starting point for the above “15% Rule”.
PERMITS: There is no prescriptive path for tree house construction, nor is there likely to be any time soon, if ever. There are simply too many variables and too many creative solutions to condense this subject into a procedure chart. On the other hand, the International Building Code may allow professionals to propose different methodologies if they are able to support them with mathematical analysis and testing. This is why the IBC is used to support your Code tree house, while the IRC may be used to define features and components. It would be very helpful to have a functional and separate document specifically written to meet the specific and unique challenges of tree house construction.
Load reductions commonly employed in Code analysis are not appropriate in tree house design. Methods that reduce for example the imposed wind load to some artificially low value are simply asking for trouble. The liability issues of tree house construction and use are not to be trivialized. On the other hand, the National Design Standards and timber engineering practice in general are a good design reference, as is “Wood Properties” (J. Winandy, 1994), U.S.F.S. Forest Products Laboratory, Wisconsin, USA.
Copyright, 2013, Charles S. Greenwood, P.E., LLC
Copyright 2013, Charles S. Greenwood, P.E., LLC Do Not Copy without Written Permission