Ok, first of all, I didn't find this problem easy and I'd like to get a second opinion before I claim to be correct in my calculations. I expect a civil engineer receives more training in this area than I have, but I think I figured it out. I found an equation online for wind pressure on a vertical wall and I rearranged it for your application. I came up with an equation for maximum stress (psi) in each post. The derivation is too complex to explain here, but here is the end result:

(I developed an equation for moment, integrated it over the surface, and divided by the section modulus to get stress)

Stress = 0.000234*(Height^3)*(Wind Speed^2)

This is assuming 8-foot post spacing, height in feet, and wind speed in mph

Stress = 0.000234*(8ft^2)*(75mph^2) = 675 psi

A chart indicates that the maximum stress for structural timber is approximately 5000 psi. Therefore, you shouldn't have any problems based on that calculation. If that equation is correct, the fence should be able to withstand winds in excess of 200 mph. (Which seems pretty unlikely to me, but I can't find my mistake)

Someone please check me on my calculation or offer a different solution.

This calculation is based on information from these websites:

http://instruct1.cit.cornell.edu/cou...wind/wind.html
http://jan.ucc.nau.edu/~dsl/egr437/c...n/bending.html
http://www.woodbin.com/ref/wood/strength_table.htm