A wall running perpendicular to the wind direction acts like a big sail (blue area), while the walls running parallel provide resistance. Short walls need enough bracing to resist the forces against the large “sail” area created by the longer wall (left), and vice versa.
A braced wall line (BWL) does not have to align with a wall on the building. The code allows a BWL within 4 feet on either side of a wall. In the floor plan shown here, the gray BWL would not work for the entire side of the building, because it would be too far from the garage-opening wall. But together, the two red BWLs would work.
This excerpt from IRC Table R602.10.3(1) enables us to find required wall bracing lengths. For a given wind speed and building story, locate spacing of each BWL in the third column, then follow across to the column for the bracing method you are using. In the examples on pages 32–34, we are using the middle set of values (highlighted above), which apply to the first floor of a two-story house or the second floor of a three-story house, and pulling our bracing lengths from the last column, which covers continuous sheathing-wood structural panels (CS-WSP).
To qualify as a braced wall panel (BWP), a section of sheathing cannot have any openings in it and must be fastened following the minimum specs shown above. BWPs must be spaced no more than 20 feet apart, and begin within 10 feet of each end of the intersecting braced wall lines (BWLs). In this example, the intersecting BWL on the left aligns with the corner, so the 10-foot maximum dimension coincides with the corner. On the right, however, the BWL is offset to the house interior, creating an allowable unbraced length of wall that is greater than 10 feet from the corner. If an intersecting BWL is offset toward the exterior of the house wall, the distance to the corner would be less than 10 feet.