damping ratio (beta) for ASCE 7 gust effect factor calculation

[psaphin]

I am looking for clarification regarding the damping ratio (beta) as used in the ASCE 7-05 section 6.5.8. Accordingly, beta is the "damping ratio, percent of critical". So, the question is, if your damping ratio is 0.02*[critical damping], or 2% of critical damping, is beta=0.02 or is beta=2.0?

My interpretation is beta=2.0. My calculated Gf is 0.91. However, I noticed that a proprietary software package (won't name names) uses beta=0.02. Hence, their Gf is over 1.90. Otherwise, our calculations match perfectly.

Curious, does anyone know what ranges to expect for flexible gust effect factors? Thanks in advance.

 

[WillisV]

See the linked article below, in particular Figure 3. The Beta to use in calculations is the decimal value (0.02). 1.9 does seem very high though.

 

[psaphin]

thank you Willis!

I have found other sources that also indicate using the decimal value (0.02). The Gf=1.9 is a direct result of using a very low naturual frequency (default value) of 0.200. I utilized the equations in the commentary of ASCE7-05 (C6.5.8) and determined a frequency of 0.6. Resulting Gf is closer to 1.14... MUCH better than 1.9. Should probably verify with STAAD.

By the way, this was for a tall sign like those found at freeway intersections. Single steel pole supporting a billboard.

Thanks again.