问题求教：美国土木工程师协会ASCE7-05第6章风载荷

在ASCE7-05 Chapter 6中有这么一条定义：
Building,OPEN: A building having each wall at least 80 percent open. This condition is expressed for each wall by the equation A0>=0.8Ag, where
A0= total area of openings in a wall that receives positive external pressure, in ft^2.
Ag= the gross area of that wall in which A0 is identified, in ft^2.

A0的定义应该就是敞开的面积，为什么说是被施加正压？

请大家给予指点，谢谢。