Adjustment Factors

The more building there is above the floor you are bracing, the more wind pressure you will need to brace against. This includes both the floor above and the height of the roof, which adds to the “sail” area. Steeper roofs increase this sail area and therefore require more bracing.

The more exposure, the more bracing required. In Exposure B, where terrain, trees, or other buildings provide shelter from wind, the adjustment multiplier is 1 — no change. But in Exposure C (open grasslands or flat plains with few trees) or Exposure D (next to unobstructed open land such as a salt flat or a lake that freezes in winter), for a two-story building the required bracing length must be multiplied by 1.30 or 1.60, respectively.

If you have 10-foot walls, your adjustment factor is 1 — no change. But for 11-foot walls, you have to multiply by 1.05, and for 12-foot walls, the multiplier is 1.10. The good news here is that you get to reduce the bracing if the walls are shorter than 10 feet: for 9-foot walls, you multiply by .95 (a reduction), and for 8-foot walls you multiply by .90.

If you’re resisting the wind force with just two outside walls, you stick to what came out of the table, plus the other adjustments already applied. But if you’re using three, four, or five braced wall lines to pick up the load, you need to multiply by the appropriate number found in the table at left.

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