Non Technical Summary
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.
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Goals / Objectives
The GOAL of this Phase II SBIR-funded research is to make round timber structures a viable alternative to steel and concrete framing systems by developing cost effective selection, grading, and manufacturing techniques, and establishing the design capacity of round timber assemblies. Phase II results will allow for the scaling of round timber manufacturing. They will also respond to building officials concerned with the structural quality and performance of branched timbers, while introducing to the construction market a viable branched column-truss assembly using both branched and straight timbers. Phase II technical OBJECTIVES are to: 1) Continue axial mechanical testing of branched timbers in Ash and in White Oak. 2) Develop a prototype grading system focused on optical NDE data and branched timber geometry. Further develop and test novel sub-assembly connection methods for branched column-truss assemblies to best utilize the strength of branched timbers. 3) Test prototype column-truss assemblies for axial loading the field. 4) Use test data to establish design values for the connections, the Y-branched columns and trusses to assist in grading timbers and engineered framing systems for Phase III commercialization. EXPECTED OUTPUTS are: 1)Structural axial load capacity tables for Y-branched timbers: For round timber to penetrate mainstream structural systems markets, structural engineers in the industry will need key data on the axial load capacity of Y-branched timbers. 2) Structural load tables for branched column-truss assemblies. 3) Steel connections that leverage the powerful axial and lateral loading capacity of Y-branched timbers. 4) Cost effective and efficient selection and inventory of raw material from the by-product of healthy timber management.
Technical objectives will be addressed through the following major tasks: 1) Specimen Selection: Branched members will be selected, harvested and prepared by WT crew. Selection will be based on geometric criteria including length, branch diameter, and angle between branches. Each specimen will be completely stripped of bark and kiln dried in order to ensure consistent moisture characteristics. 2) Collect Physical and Optical NDE: Each timber will be measured, photographed and documented (data will include: found location, visual marks, slenderness ratio, branched angle, out-of-plane ratio, etc). Parametric optical data will be collected using calibrated high resolution digital cameras; parallel and perpendicular to the branch direction. Using image analysis edge detection routines in MATLAB, the branch edges, branched angle and diameters will be established for key points. 3) Compressive axial testing of individual specimens: Specimens will be oriented with branched ends to the floor and loaded with an hydraulic actuator capable of applying 300,000 lb. Branched ends will be held by supports that restrain lateral movement while allowing for limited end rotation. All testing will be conducted in deformation control, so that failure of the specimen will occur between five and ten minutes (ASTM D198). Out-of-plane displacements will be measured using three calibrated digital cameras. 3D digital image correlation (DIC) techniques will determine out-of-plane deformation and document the failure modes. 4) Design and testing of sub assembly connections: WT architects, structural engineer and machinist will collaborate with FPL engineers to develop two to three connection designs for round wood elements necessary for the construction of the branched column truss system. Once the initial designs have been fabricated, they will be evaluated using sub-assembly connection tests (five tests per connection). 5) Testing of Branched Truss assemblies: Three independently observed tests will be conducted on branched column-truss assemblies to assess the ability of branched columns to reduce effective truss spans and develop structural frame behavior under typical roof load conditions. 6) Frame analysis: WT structural engineer will perform structural analysis of the branched column-truss assembly to determine test loading, connection forces and expected deflections using geometry of the branched columns provided by NDE and optical analysis methods described above. The loads to be placed on the test frame and the forces in the connections will be determined along with estimated deflections of the frame timbers. 7) Correlation of destructive tests to NDE data: Optical measurements determined via image analysis will be compared to physical measurement. Image analysis measurements will be used to generate structural models of each Y-branched axial specimen to determine the regions and values of maximum stress. In Phase II, the structural analysis model will include the effect of geometric non-linearites. Setting the limits for out-of-plane specimens imperfections is critical to this development. 8) Development of a prototype grading system.