trussed tower wind loads - epsilon (ratio of solid to gross area)

[stuctural101]

This is in regards to the calculation of Cf (see Fig. 6-22 in ASCE 7-02) for trussed towers using the following formula:

Cf = 4.0e^2 - 5.9e + 4.0, where e is epsilon

Max Cf = 2.1 when e=1
~Min Cf = 3.94 when e=0.01

As epsilon (ratio of solid to gross area) increases, Cf descreases. According to this, as the element in question becomes more "solid", Cf, and therefore the wind force F in Eq. 6-25 decreases. This seems counterintuitive to me, since the more "solid" something is, the more surface area it has for wind pressure to act upon. What am I missing here?

 

[nutte]

As the tower becomes more solid, Cf decreases, so your wind pressure decreases. The area increases, however. I would expect that the net effect would be that your wind force F increases.

 

[Bagman2524]

Fig 6-22 shows Cf increasing for more solid round members but decreasing for more solid Flat-sided members. I'm not sure I understand this logic either. Try the Wind Loads and Anchor Bolt Design for Petrochemical Facilities Standard. It seems to have a more reasonable approach.