Section 3.10.1/Compression Perpendicular to Grain (Bearing)
NDS 2018 Section 3.10 covers bearing design for wood members loaded perpendicular to grain including the Cb bearing area factor.
Wood members loaded perpendicular to grain (bearing at supports, under concentrated loads) must be checked against the adjusted compression perpendicular design value Fc-perp'. The reference design value Fc-perp from the NDS Supplement is adjusted by applicable factors including wet service, temperature, and the bearing area factor Cb. The bearing area factor Cb from Section 3.10.4 applies when the bearing length along the grain is less than 6 inches and the bearing is at least 3 inches from the end of the member. For a 1.5-inch bearing length (typical for a stud on a plate), Cb = 1.75, significantly increasing the allowable bearing stress.
Why this section exists
Wood is weaker perpendicular to grain than parallel to grain because loading across the fibers crushes the cell walls rather than loading them axially. Bearing failures appear as crushing and indentation at supports, under point loads, and at connections. The Cb factor accounts for the fact that short bearing lengths distribute load to adjacent uncrushed fibers, increasing the effective bearing capacity. Without Cb, many conventional wood framing connections (studs bearing on plates, joists on beams) would appear to fail bearing checks.
What plan reviewers look for
Plan reviewers check bearing stress at supports and under concentrated loads. They verify the bearing length and the applied Cb factor. They check that the bearing area is adequate for the reaction force. For beams bearing on posts, they verify the contact area and the compression perpendicular stress. They check for bearing plates or other reinforcement where the bearing stress exceeds the adjusted capacity.