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

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

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

(OP)
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?

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

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.

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

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.

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