structural CONNECTIONS Anatomy of a Mass Timber Bearing Intersection By D. Scott Nyseth, S.E., and Jason Smart, P.E.
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roper design of bearing intersections between mass timber members is critical to the overall success of a mass timber project. The details of these intersections have a significant effect on cost and schedule. This article focuses on the multi-story column condition, where loads from the column above need to be transferred down through the beam-column intersection, and the beams are supported using a bearing pocket instead of a bearing hanger (Figure 1).
Column Load Transfer
For maximum bearing capacity, wood columns should bear directly on wood columns end-to-end because wood is strongest when bearing parallel to the grain. For example, if a wood column were to bear directly on a wood beam below, the intersection would have about ⅓ the bearing capacity because compression design values perpendicular-to-grain are lower than compression design values parallel-to-grain. For efficient member sizes, consider a detail where Constructability an upper column bears directly on the lower column, When initially installed, cambered beam ends may need and a beam also bears on the lower column (Figure 2). special cuts to allow them to sit tight in the bearing Wood columns designed in accordance with the National pocket. Sealing/protecting ends of beams and columns Design Specification® (NDS®) for Wood Construction can is critical because the ends of these members have the have a maximum unbraced length-to-depth ratio of potential to absorb water if wetted during construc50 (up to 75 during construction). Shorter wood coltion, causing swelling and damage to the intersection. umns are controlled primarily by bearing area. Wood Computer Numerical Control (CNC) tolerances are columns used in typical structures must be upsized to small for most cuts, copes, and drillings (⁄16 to ⁄8 control column slenderness and buckling. This means inch); however, they are larger for longer slots that that there is typically more bearing area than required at need to be cut for items such as knife plates (⁄8 to ¼ Figure 1. End column with the ends of a longer wood column, allowing the design inch or more). CNC machine tooling for preparing beam bearing pocket. to incorporate beam pockets in the column without beam pockets and tenons are typically round cutting upsizing the columns. bits; therefore, the resulting surfaces will have rounded surfaces that For column-to-column compression load transfer or any other need to be addressed in the design. parallel-to-grain bearing, NDS 3.10.1 states that “bearing shall be on In the absence of a diaphragm, beam-column intersections should a metal plate or strap, or other equivalently durable, rigid, homogehave some lateral capacity for bracing and racking loads during erection neous material with sufficient stiffness to distribute the applied load” and for leveling and plumbing of the beam-column frame. Column to utilize the full compressive capacity of the wood for bearing. For stability is crucial for projects that use a concrete diaphragm instead end-to-end bearing where a rigid insert is required, NDS 3.10.3 allows of a Cross-Laminated Timber (CLT) diaphragm. In this scenario, the use of a 20 gauge or thicker metal plate placed between bearing multiple stories of columns, beams, and floors can be erected before surfaces to avoid a 25% reduction in bearing capacity. Without the the structural diaphragms are in place. This requires special design bearing plate, a 25% reduction in bearing capacity directly affects the attention, especially at exterior columns. In addition, the contractor amount of wood that can be removed for the beam bearing pocket. needs to provide additional shoring for these columns to stabilize the structure until the concrete diaphragm is in place.
Beam Bearing Pocket Intersections
Figure 2. Column-to-column bearing surface.
24 STRUCTURE magazine
The depth of a beam pocket is often limited by the tooling for a CNC machine. A common limitation is that the beam pocket must be less than or equal to 6 inches deep. This means that the pocket width is the only variable that can be changed by design to make the bearing area large enough for the beam reaction. Therefore, narrow beams with relatively large reactions are not a good combination for a beam bearing pocket intersection. Beam bearing pocket intersections do not necessarily have a positive connection to the column during installation. Therefore, consideration should be given to provide some type of connection to provide stability during construction. In addition, connections will need to allow the top of beams and floor to shrink down around the columns so that the
