Structural Testing of Branched Timber and Truss Assemblies

USDA secretary, Tom Vilsack, has urged the Forest to prioritize "protecting and maintaining all American Forests, including state and private lands. ... The Forest Service must play a significant role in the development of new markets and ensure their integrity." Small-diameter round timber is a vast and under-used waste product of well-managed forests. At the same time, the structural building systems market is currently dominated by steel and concrete structural systems. Round timber can substitute for steel and concrete in medium and large scale construction under Type IV: "Heavy Timber Framing." Round timber can qualify for up to eight US Green Building Council Leadership in Energy and Environmental Design (LEED) credits, more than triple any other "green" structural material and almost a third of the credits necessary to achieve LEED Silver rating, now the standard for new federal buildings. This convergence of factors will benefit WT's efforts to build an industry. Research conducted at the FPL indicates the superior strength of round wood timbers. Small-diameter round timbers are 50% stronger in bending than an equivalent square section of milled timber. WHOLETREES ARCHITECTURE AND STRUCTURES (WT), a leader in round timber design and construction, has conducted Phase I research into the strength of the branched connection between tree limbs and trunk. Structural tests, conducted at the Forest Products Laboratory in Madison, WI, applied lateral or axial force to a variety of branching Ash tree specimens and found the strength of the branched connection to be significant. Phase I testing demonstrated that out-of-plane bending of tributary branches or stem is critical to overall timber load capacity. Optical metrics of branched timbers along with structural analysis can be effective predictors of load capacities. A combination of slenderness ratio, branched geometry, and out-of-plane curvature seem to be a strong estimator of the load capacities. WT will, in Phase II, conduct focused testing to refine the selection criteria for specimens - a combination of slenderness ratio, branched angle and out-of-plane curvature - and develop the best visual parameters for selection. These selection criteria will be used for in-field sorting, and to develop proprietary grading software. Designing and testing original connection subassemblies will be crucial to bringing the strength of the branched timber to market. WT will build successful connection designs into full-scale branched column-truss assemblies.