(Based on observations and World Treehouse Conference 08)
Most of Our First Treehouses were built in open forests where we could pick those trees that were readily fitted with platforms. Our Next Treehouses were often built into trees that simply BEGged to be platformed – often within sight of a buddy’s backyard drinking deck. The recent surge in treehouse building, though, has been driven by folks wanting their treehouse to be built Right There, in THOSE trees – whether or not those trees are at all predisposed to platforming. The basic design issue, then, has become fashioning a flat spot among tree(s) that are not intrinsically candidates for this.
Unlike dead wood timber connection designing; you do NOT want multiple and light connectors – in order to minimize damage to the tree, we use minimal (necessarily heavier) connectors.
Unlike with most any other structural connection, connectors to trees CAN get stronger and stiffer with time. The trees DO envelope the connectors, so long as they continue to grow at all.
Wind driven deflections can cause fatigue failures in the steel components far sooner than most designers anticipate – treehouse design can feel a bit like designing airplanes, in fact.
Over framing (vs. flush framing with hangers) the platform is crucial, in permitting the main carrying members to be slid outward (or even replaced) as the trees grow. Generally, the headroom is available – the challenge is mostly in preventing joist rolling.
Struts vs cables (or struts AND cables). Struts are connected to the lower and therefore stiffer part of trunk. Cables indicate tension easily and (with some effort) precision. The combined use of struts and cables can (and has) prevented a collapse that could have happened with a single connector failure.
Revised Artificial Limb Design – nested heavy pipes with smaller lag at bottom/inner end. The first one installs as open ended can with a lag centered in closed/inner end holding it into trunk. Varied fittings for eventual extensions, cables, sliding seats are quite feasible.
Redundancy is - unusually for structural design - not always great in treehouse design. It IS possible for instance, for redundant supports to struggle against one another – specifically through trunk movement between supports, both in tall single-tree structures and any multiple-tree platforms.
Multiple connectors, aligned with the trunk, may not share load as evenly as expected, because of differential thermal movement between the trunk and any long, interconnecting steel fittings that run vertically, parallel to the trunk. This is true for the connectors and for framing that runs parallel vs. perpendicular.
Smoother trunk connectors seem as thought they would require less work for the enveloping tree. Saporrhoids seem a likely indicator cause of tree distress.
An adjustable and flexible compression strut might be sweet: could tune the length to carry desired load, internal shock absorbers could work with wind deflections to ease point loading to structure and tree. This component would be based in some combination/version of auto suspension technology and/or bicycle seatpost techniques.
I could go on, at some length, with a free body diagram analysis of the varying forces and connector designed in the classic/simple ladder/cleat we all nailed up the trunk.