Q: I have come across something in the new simplified wind design tables in ASCE 7-10 Chapter 27 that doesn’t make sense to me. In Table 27.6-1, it has different values for the wind pressure at the base of the building based on the height of the building. It is our understanding that the windward pressure at the base should not vary due to the height. When we remove the leeward pressure, which does vary based on the height, from the tabulated values, we do not get the same windward pressure for all building heights. Please explain why this is the case.

A: The ASCE 7-10 simplified method of wind analysis was calibrated so that the final story shears and story moments match closely with those determined from the “exact” method (ASCE 7-10 Directional Procedure, Chapter 27, Part 1). This was necessary because the wind pressure distribution along the height of a structure in the simplified method is different from that in the exact method (as shown in the figure), and the tabulated pressure values, p₀ and p_h, had to be adjusted to account for that. This, coupled with the fact that a simple method had to be suggested for splitting the total pressure into windward pressure and leeward suction, resulted in some increment in the windward pressure at the base as the building height increases. This may seem unscientific, but it was determined to be a small enough price to pay for the sake of simplicity while keeping the important design parameters (story shears and story moments) correct.

Mr. Larry Griffis (the primary developer of the simplified method) explained this in a SKGA web seminar. More information about this web seminar can be found here.