Wood’s strength varies depending on the direction that load is applied relative to the grain orientation in the structural member. Typically in wood design, the allowable stress that a member is capable of resisting is greater when induced parallel to grain rather than perpendicular to grain. When wood is loaded in a manner that creates tension perpendicular to the grain, its ability to resist this stress is minimal. The National Design Specification® (NDS) for Wood Construction does not list allowable design values for tension perpendicular to the grain and states “Designs that induce tension stress perpendicular to grain should be avoided whenever possible”. Figure 1 illustrates a conceptual drawing of a wood member loaded in tension parallel to grain on the left, and tension perpendicular to grain on the right.
Figure 1: Tension parallel to grain (a) versus tension perpendicular to grain (b)
Tension perpendicular to grain can lead to non-ductile failures characterized by a sudden and unexpected loss of strength. Such failures can occur without warning, potentially leaving occupants unaware of impending structural issues. Proper design and detailing can prevent tension perpendicular to grain loading situations.
When using engineered wood products such as Trus Joist® Parallam® PSL, Microllam® LVL, and Timberstrand® LSL, it is important to understand that these products are manufactured with their wood grain aligned parallel to the length of the member and do not have any inherent advantage for resisting tension perpendicular to grain.
Figure 2 illustrates a framing situation that should be avoided due to tension perpendicular to grain. On a simple span beam where gravity loading is applied below the neutral axis of the supporting member, tension perpendicular to the grain develops where the supported member connects to the carrying beam. It is recommended that the majority of fasteners be placed above the neutral axis by using a taller hanger to limit tension perpendicular to grain stress forming at the connection, or working with an engineer to reinforce the supporting beam. Figure 3 includes a connection that would avoid tension perpendicular to grain while allowing for bottom flush framing.
Figure 2: Improperly loaded beam-to-beam connection resulting in tension perpendicular to grain
Figure 3: I-joist loading below the neutral axis (a) causes tension perpendicular to grain. Strap type hangers shown (b) bring the hanger load above the neutral axis of the supporting member
Notched or tapered cuts on the tension side of beams will induce tension perpendicular to grain stress within the member and should be carefully detailed if unavoidable. Refer to chapter 3 of the NDS® for further guidance when analyzing tension notches at bearing.
Figure 4: Notched beam at support may induce tension perpendicular to grain stress and cause beam failure
Figure 5: Seat cuts that extend over the face of bearing (a) may induce tension perpendicular to grain, flush seat cuts with bearing below are preferred when framing a sloped member (b)
Cross grain bending is a type of stress that causes the wood fiber of a structural member to pull away from itself, inducing tension perpendicular to grain. This can be the result of connections such as nailer plates that overhang foundation walls (refer to Figure 6), or other situations where a loading causes the grain to pull itself apart. During design, conditions that would allow for cross grain bending stresses should be avoided.
Figure 6: Load applied to an overhung nailer plate induces cross grain bending
Finally, tension perpendicular to grain can arise from mechanical connections that restrict wood's natural shrinkage. When wood is installed with a moisture content higher than its equilibrium moisture content, or becomes wet during construction, it will shrink as it dries, with the most significant shrinkage occurring perpendicular to the grain. If a connection impedes this shrinkage, it can induce tension perpendicular to the grain at the connection points, see Figure 7. It is the responsibility of the Designer of Record (DOR) to ensure that connection details are appropriate for the application to limit crack development.
Figure 7: Avoid connections that restrict vertical shrinkage of the beam (a). Detail connections that allow vertical movement of wood fiber (b)
Avoiding tension perpendicular to the grain is critical to the structural performance with wood framing members. Engineers, designers, and drafters must understand how this stress develops and avoid framing designs that induce tension perpendicular to grain. Proper detailing of connections is crucial to providing a load path that avoids perpendicular to grain stresses. Design software tools may allow the user to model conditions that induce stress perpendicular to grain. It is the responsibility of the user to recognize and avoid these conditions.
If you have any questions regarding Trus Joist® products, please don’t hesitate to contact our technical support team at 888-453-8358 or email techsupport@wy.com.
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