Wind Coefficient for Open Frame Structure Question

[SPS110615]

I have an open frame structure with Sf/B=0.42 and N=15. The ASCE book "Wind Loads for Petrochemical and Other Industrial Facilities" is being used to determine Cf for an Open Frame Structure. Figure 5.1 for Sf/B=0.5 limits the number of frames, N, to 10. Is the reason for the maximum of N=10 in this figure? Does the percentage of the total wind force diminish to a point past 10 frames that it is negligible? Has anyone dealt with this before or have any insight as to what to do to determine Cf if N>10?

Thanks in advance!

 

[ajh1]

The ASCE 7-16 public draft includes a provision (28.3.5) for determining longitudinal wind forces (perpendicular to the frames in question) for open structures. The basic equation is:
p = qh * GCpf * Kb * Ks
Kb = frame width factor = 1.8 - 0.01B for B < 100' or 0.8 for B > 100' B is the width of the building in the frame direction.
Ks = shielding factor = 0.6 + 0.073(N-3) + 1.25 * phi^1.8
phi - solidity ratio = As/Ae As = effective solid area of the endwall, Ae = total endwall area of an equivalent enclosed building

The MBMA 2012 Manual includes a similar equation and was the originating basis for the ASCE formula.
The Ks term is different there and is as follows:
Ks = 0.20 + 0.073(N-3) +0.4 * e^(1.5*phi)

I'm not aware of why the two equations are different, nor have I done any investigation to see how closely they compare.
In both cases there is no upper limit on N, but N is restricted to not be less than 3 for somewhat obvious reasons.