Calculating ASCE Wind Loads on a Pratt space truss 4

[jochav5280]

Ladies and Gents,

I am a new EIT and have not been able to find any good examples of using the ASCE wind load spec., specifically on space trusses and open industrial structures. The spec. just shows a bunch of equations, but no examples, which has been challenging for a new engineer to understand. My company uses section 6.5.13 of ASCE 7-02 "Design Wind Loads on Open Buildings and other Structures" to determine the wind loads we apply to our space trusses, which consist of two parallel connected planar trusses. I am not confident that we have been applying these loads correctly and would greatly appreciate your opinions. The equation for this section is as follows:

(Eq. 6-25) F=qzGCfAf (lb)(N)

where
qz= velocity pressure
G= gust effect factor
Cf= net force coefficients from Figure 6-18 through 6-22
Af= projected area normal to the wind except where Cf is specified for the actual surface area, ft^2 (m^2)

We calculate the Cf-factor using Figure 6-22 "Trussed Towers". We consider our cross-section to be a "Square" cross-section and in general assume the epsilon value to be equal to 0.5. This produces a Cf value of 2.05. First question I have is whether or not we are calculating the Cf value correctly or if we should be using another equation to calculate this value? Any recommendations would be greatly appreciated!

Next we need to determine the Af-value. We in general assume that the Af-value is equal to our panel depth multiplied by the epsilon factor, which will give us an area per unit length. We then multiply this value by all the other factors in (Eq. 6-25), which gives us a force per unit length. Should this load be applied just to the windward truss, or should the same load magnitude also be applied to the leeward truss? The wind will hit the front and rear trusses, but I'm not sure if this same load should be applied to each indivdual truss.

Any recommenations or help would be greatly appreciated!

Thank you,

NewEIT

 

[JStephen]

If the geometry in question is not actually included in ASCE 7, I don't know that there is a "right" way to go about determining loads based on the standard. You can make reasonable approximations and assumptions, but those could vary considerably from engineer to engineer.

Where one member is theoretically shielded by another, a wind blowing slightly off-axis would eliminate that shielding, so I would figure wind loading on both.

 

[csd72]

What you do may sound okay on the surface, but what if your wind blows at, say, 5 degrees off the perpendicular? I guarantee that what you will see from that angle will be more like a solid than a 50% area.

I would tend to design it as a solid area for full depth of truss, or as you have done, whichever is worse.

The only real accurate way would be to get a wind tunnel test or find a paper that discusses this such as

http://cat.inist.fr/?aModele=afficheN&cpsidt=2134928

 

[GPekmezi]

Hello,

Check out the wind tunnel results in the link below with particular attention to pages 101, 105, 118, and 164. It is important to keep in mind that this is a dissertation rather than a peer-reviewed article, so you shouldn't use it for anything other than comparison.

http://etd.lsu.edu/docs/available/etd-11082007-194406/unrestricted/Amoroso_dis.pdf