Wind load on an open frame.

[WARose]

Looking at ASCE7-05, you have procedures for calculating wind loads if the structure could be considered a “trussed tower” or another type of frame. But how about this: what about just figuring a load on the individual members? The catch would be *how*? You could use equations 6-27 and 6-28 [in ASCE7-05]……i.e. F=qG(Cf)A. But then the problem becomes: what to use for Cf? You are trying to figure the load on a member on a per linear foot basis…..and I’m not sure that any of the coefficients in Figures 6-20 through 6-23 are applicable.

Any ideas?

 

[delagina]

wind load for petrochemical facilities is a good guide for open structures. we usually compute the solidity ratio to come up with Cf values.

 

[WARose]

The thing about that though: it’s looking at the frame as a whole and applying the loads on one face. The approach I am talking about [admittedly a bit more tedious] involves figuring the load on each member. But I am a bit confused as to the "how" for the reasons I stated above.

 

[delagina]

Cf would be 2 for components like flat plate/ladder/etc.. based on the table

 

[SAIL3]

I agree with delagina....use Cf of 2..no shielding.This makes it
more convenient if you are using a computer program to do the analysis.If by hand, the solidity ratio method may be more convenient.
The following is an excellent ref. on wind on open frame structures, pipe racks, access, vessels etc. It is very very practical:
"Wind Loads and Anchor Bolt Design for Petrochemical Facilities"
Published by ASCE in 1997...ISBN 0-7844-0262-0

 

[WARose]

Using a Cf=2 seems sort of like overkill. On figure 6-22 [of ASCE7] the only time Cf is equal to 2 is when the ratio of solid area to gross area is less than 0.1……my ratio is 1 [which is off the chart]. And I’m not sure this chart is really applicable anyway (based on how I am treating it). [I appreciate the responses by the way.]

 

[SAIL3]

Use Cf of 2 for loading(#/ft) on an individual member which, I believe, was your original question.

 

[WARose]

It was my original question….but I have to wonder about using a Cf=2 for the reasons I stated. (Not that there is anything wrong with being conservative.)

 

[WillisV]

Agree with all others - the Cf would generally be 2.0 - even though you are calculating the load on an individual member, you use the solid to gross area of the structure as a whole (not just the piece you are looking at), which is generally less than 0.1 for typical open structures thus a Cf of 2.0 from Fig 6-22.

 

[delagina]

you can compute solidity ratio of the frame then whatever Cf it comes up you can use that on individual members as uniform load.

i actually do this all the time coz i let Staad apply the wind load on each members as uniform load instead of me putting it as concentrated load. but the Cf i use is based on frame solidity ratio.

the Cf 2 i said is for components that is not part of the frame solidity ratio calculation like ladders, cantilever beams, etc..

 

[transmissiontowers]

ASCE 74 has some tables for structural shapes. We use it for my transmission towers which are mostly angles and for substation structures which can be structural tubes and tapered multi-sided tubes along with WF and many other shapes. A Cf of 1.6 is used for most structural shapes. A 2.0 is used for flat plates with sharp edges.

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