ASCE/SEI 7-05 Section 6.5.8.2 gust effect factor

[djbse]

Has anyone had the opportunity to go through this design section successfully? If so, I am curious how two of the factors were determined:
1) building natural frequency
2) damping ratio, percent of critical
Any help, comments or suggestions would be most welcomed.
Thanks

 

[NewImhotep]

1) building natural frequency

you can use model analysis by software like ETABS, STAAD .or approximately by formulas like (50/h^0.75) where h the structure height in feet.

2) damping ratio, percent of critical

I am using 1% to 2% for concrete structures and half of that for steel structures I think most of the designer using the same values

Regards

Ahmed

 

[UcfSE]

As ahmed said, you can use software for the natural frequency, or approximate forumlae, which you may find in the seismic sections of the code you are using.

What you need to do is an eigenvalue analysis. For simple cases with fewer degrees of freedom, or singleDOF, this is not overly difficult by hand. The results will tell you the building natural frequency (reciprocal of the period) for the different modes. You are looking for the first mode for this calculation. If you solve by hand, make sure you aren't using the angular natural frequency; what you want is the frequency that is the reciprocal of the period.

For damping, I use 5% in general, and by general I mean very general. Check how sensitive the result is to the assumed damping. It may not be that critical.

 

[WillisV]

UcfSE, though 5% damping is generally assumed for seismic calcs, most of the literature recommends much lower values for wind gust calculations (in the range of 1 to 2%). Take a look in the ASCE 7-05 commentary for the wind section.

 

[djbse]

AhmedR, UcfSE, and WillisV.....Thank you for the information. This answers my question.

 

[UcfSE]

Thanks for pointing that out Willis. I was mistaken.