damping ratio (beta) for use in calculating gust effect factor (G)

[mpdst11]

I am referring to ASCE 7-02, Section 6.5.8.2, Gust Effect Factor for Flexible or Dynamically Sensitive Structures.

The equation for the gust effect factor (G) includes the variable R, or resonant response factor.

The equation for R includes the variable beta which is a damping ratio.

As far as I can tell, there is nowhere in the code where it gives any equation or values for this damping ratio, beta.

Does anyone know how this value is calculated? Thanks for your assistance.

 

[CFSEng]

Look in the commentary in the back of ASCE 7-02. Under the section C6.5.8 there is a section titled Structural Damping. This gives you some ideas of possible betas to use as well as references. Looks like ratios of 1% and 2% are typical for the US for buildings.

Interesting to note is this states ratios of 0.15% to 0.5% for signs are typical but the guide to the use of ASCE 7-02 sample problem for a sign uses 1%.

With that said I may have missed something

 

[mpdst11]

I might be crazy, but I don't have a section about Structural Damping in my ASCE 7-02. I do have the example you referred to that uses a damping ratio of 0.1. But there is no section in my text that gives any suggestion where that number might have come from.

 

[CFSEng]

Oops... my mistake. The wind guide I have is for 7-02 but the commentary I was reading is 7-05. These minor details always getting in the way.

You know 7-02... 7-05... whatever it takes

 

[meca]

mpdst11, What type of structure are you analyzing? Are you sure it classifies as a flexible structure (NF < 1 Hz)?

 

[mpdst11]

Thanks CFS, I will have to take a look at the 7-05.

Meca, the structure I am analyzing is a 7 story concrete office building. I have it modeled in RAM and it has a natural frequency of 0.7 Hz.

I am trying to calculate the wind pressure on the penthouse and screen walls on top of the structure.

 

[WillisV]

See

http://www.structuremag.org/article.aspx?articleID=686

for discussion of frequency and damping values to use for wind calculations.

 

[Stazz]

On this subject...

I calculated the gust effect factor for rigid buildings across a range of heights for each exposure (B,C,D) and I am getting a factor that is above 0.85 mostly for heights under 400 ft. Is this true? Because if it is then the 0.85 default value isn't conservative. I've double checked my math and it looks ok but it doesn't make sense when ASCE-7 05 says that 0.85 is a conservative value.