ASCE 7-10 - Chapter29, Eq 29.5-1

[GJM62]

Hello, I am new to the forum.

I would like to ask for assistance in understanding how to apply Af in Equation 29.5-1. I am attempting to analyse an existing trussed tower. It is an open tower comprised of pipe columns and angle bracing on all four faces. I feel comfortable calculating force coefficient Cf per Figure 29.5-3. I understand that Cf includes contributions of both front and back faces of the tower so that wind does not need be applied to both windward and leeward faces.

My concern is calculating Af properly. ASCE 7-10 states Af = projected area normal to the wind except where Cf is specified for the actual surface area. What does the ASCE 7 mean by the term "except"? If I calculate Cf, what is the the proper way to determine Af to apply wind load at the top of each panel? I found in a FAQ regarding ASCE 7-02 that the area Af used to calculate wind force per foot of tower height is the solid area of one face divided by the panel height. Does it make sence to calculate Af in that manner and multiply by the panel height to obtain a force to be applied at the top of each panel?

Thank you in advance.

 

[RobertHale]

I think the "except" that you reference is indicated by Figure 29.5-2: Note 2 and Figure 29.5-3: Note 1. To me it seems to imply you are not allowed to take a reduction in the surface area for "Solid" walls and Signs with openings less than 30% of the gross area. The way you (and the FAQ) suggest seems to appropriately account for the solid area provided that is your load path.