ASCE 7-10 - Wind Load on Box Truss - Fig 29.5-2 or 29.5-3

[ARS97]

Just as the title of the thread suggests. I've always used Fig 29.5-3 since I figured that a "trussed tower" sounded more appropriate for a box truss. However, I suppose "lattice framework" sounds similar too (Fig 29.5-2), plus it gives lower Cf values. Unfortunately, ASCE doesn't give much of an explanation of the intent of these figures, so I'm in debate. I'm just looking for what is most appropriate......any opinions?

 

[transmissiontowers]

ASCE 10 for lattice transmission towers and the loading guide ASCE 74 might shed some light. This is a very narrowly focused field. I have not read it but AASHTO has some publications for the lattice traffic structures.

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I have been called "A storehouse of worthless information" many times.

 

[chichuck]

andysines,

I favor using Figure 25.9-3 for a trussed tower. To use this correctly, the Cf is calculated using the solidity ratio on the windward face. The exposed area should be I think, the exposed area of the windard face. The coefficient in the Figure gives the total wind force on both the windward and leeward face. Refer to the following from Guide to te use of the Wind Load Provisions of ASCE 7-98: (Figure 6.23-3 in ASCE 7-05 is the same as table 6-13 in ASCE 7-98.)

In the Frequently Asked Questions:

21. In calculating the wind-forces acting on a trussed tower of square cross-section, see Table 6-13, shoiuld the force coefficient, Cf be applied to both the front and the back (windward and leeward) faces of the tower?

No, the calculated wind-forces are the total forces acting on the tower. The force coefficients given in Table 6-13 include the contribution of both front and back faces of the tower as well as the shielding effect of the front face on the back face.



That explains why the coefficients you figure from Figure 29.5-2 are lower than the ones from Figure 29.5-3. If you wanted to use Figure 29.5-2, you would have to include the effects of the leeward face, and also account for the shielding. The two faces can be accounted for, refer to Wind Loading of Structures by John D. Holmes, or to Wind Effects on Structures by Simiu and Scanlan. I"m not sure if those referenes account for the shielding.

chichuck

 

[ARS97]

This falls in line with what I had already believed......thanks for the input fellows....