ASCE 7-22 Gust Effect Factor

[Gforce1]

Hi,

I am looking for a couple of pointers regarding the use of ASCE7-22. I am more familiar with EN standards, but have some experience with ASCE for seismic design.

I am working out the wind load on a large container crane, which I am considering as an open truss structure. My method is:

1. Take basic wind speed for a risk category III from fig 26.5-1C
2. Set Kd at 0.85 (trussed tower)
3. For each member, or parts of members, work out qs based on their height above ground level
4. Calculate the natural frequency from my FE model (this is approximately 0.5Hz)
5. Work out the line load on each member from equation 29.4-1

The problem is that as the natural frequency is less than 1.0Hz the structure is classified as a flexible structure. I therefore am trying to calculate the gust factor from 26.11.5, but I am finding the code to be unclear.

- Do I need to calculate a difference gust factor for each member, by changing the Height parameter?
- In the calculation of Q I need to use the building width and height? Is this relevant to an open structure?
- In the calculation of the Rh/RB/RL factors what does is the significance of the power e is raised to; i.e. 2 subscript nh?

Many thanks

G

 

[JStephen]

Last item- I'm looking at ASCE 7-16.
Eq. 26.11-15a has a (1 - e^(-2*eta)) term. That is exponential function of negative two times eta, where eta is substituted in at the values shown to get the different R terms.

On the overall question- ASCE 7 is written for buildings and "other structures". Some "other structures" are covered very well, some just aren't. In ASCE 7-16, 26.11.6 allows determination of G by "rational analysis" but gives no clue on how that would be done (commentary might help, but I don't have it here with me.)

I am not familiar with the different crane codes, so I don't know if there would be any guidance in them.

That whole section is just irritating to use. It's unduly complicated, so you'd think you're figuring G to six or eight decimal places. Then you come to beta, the damping factor, and you're just going to toss some number in there for lack of anything better.

 

[Gforce1]

Is this a misprint; i.e. e^(-2*nh)?

The equations although awkward do allow G to be calculated. However they use the overall building width/height; can I use this method for an open structure?

 

[SWComposites]

Why would the wind load on a building and an open truss structure be the same?

 

[Gforce1]

I didn't say they were.

I have worked out the drag load on individual members. The structure is flexible and so this needs to be included in the calculation of the gust factor.

The equations for the gust factor make reference to the overall building dimensions. I am trying to understand how to calculate a gust factor using the guidance provided in the code or if I need to look elsewhere.

 

[bones206]

- Do I need to calculate a difference gust factor for each member, by changing the Height parameter?
I don't think so. The gust effect factor is based on global dynamic response to wind gustiness, so I would recommend a single factor based on the global natural frequency of the structure.

- In the calculation of Q I need to use the building width and height? Is this relevant to an open structure?
I don't think the original gust effect formulations considered open structures, but in the grand scheme this is just an approximation anyways. So I would recommend using the overall height and width, since that it closest to the original basis of the formulas. You could consider using a conservative factor of 1.5 or 1.6 and skip the equations altogether. That should envelope any possible gust effects.

 

[Gforce1]

Thank you bones206, that answers my question!

 

[Gforce1]

The only issue left is the printing of the equation for Rh etc. Is this a misprint and should it be:

e^(2 * nh)?

 

[bones206]

yes, I think it is e^(-2*nh)