ASCE 7-05 wind loading Case C for freestanding signs??

[Engineerataltitude]

First time I've done a freestanding sign design since CA formally adopted ASCE7 for wind in '07...
Fig 6-20 presribes 3 load cases to analyze. Case A & B are pretty straitforward. Load Case C not so much. The figure shows a graduated, stepped down uniform load resolved into progressively reduced single forces. Only problem is that I can't find any where which specifies how much each "step down" is. Just eyeballing it looks like the loading steps down by 25% each time. Is that it? Anyone have any idea?

 

[nutte]

California, Canada, or Construction Administration...???

That table gives you a Cf for each region. That's what you'd use to calculate the force on each region.

 

[Engineerataltitude]

I do get that part. Different Cf's for the 3 different load cases. What's not clear is when you look at the plan view of loading for Case C (in the figure) it shows the F (calculated from the Cf for Case C) loading the sign in some kind of progressive reduction from the edge. How much does each load drop for each step? Is that defined somewhere that I am missing?

 

[nutte]

Each region has its own Cf. For a sign with 4 regions, the first one has a Cf of 2.90, second 1.90, third 1.30, fourth 1.10. Each F is a function of those factors.

 

[Engineerataltitude]

Got it! Missed that aspect of the Cf chart. Appreciate the clarification.