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

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

(OP)
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.

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

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.

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

(OP)
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.

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