Gust effect factor in ASCE-7 1

[capablanca]

For rigid structures the ASCE 7,chapter 6 Wind Loads,
indicates a gust effect factor of 0.85.My question
is how a factor to account for a dynamic effect can
be less than 1.0? Thank for any info on this!

 

[meca]

When you run into flexible structures (NF < 1 Hz), you will calculate Gust Factors in excess of 1.0 .

 

[capablanca]

Thanks for your comment.I agree whith it but my concern is
for rigid structures where the gust factor is 0.85.Perhaps
this number is due to some spatially pressure averaging.

 

[chichuck]

I too found this to be confusing. I researched it, and found the following article. &quot;On the formulation f ASCE 7-95 gust effect factor&quot; by Giovanni Solari and Ahsan Kareem. It is found in the Journal of Wind Engineering and Industrial Aerodynamics 77 & 78 (1998) pp. 663-672.

Before going on, understand that in ASCE 7-95 and 7-98, the gust factor is a combination of a gust load effect and a dynamic structural response. For rigid structures, the dynamic response is insignificant. For flexible structures, it is significant, and this results in a higher overall gust factor than for a rigid structure.

This article is about the 7-95 gust factor, but on reading it, you will see that it also matches the gust factor in ASCE 7-98. It is a bit unclear on some of the details, but the gist of the main point is in section 2 of the article. &quot;... in ASCD 7-95, the use of 3s gusts introduces a new twist, as the design wind already included the effects of gustiness. Accordingly, for a very small size structure, the new gust factor reduces to nearly unity and further decreases due to the spatial averageing resulting from a lack of contemporaneous gust action.&quot;

This second sentence is a bit confusing, and is clarified by another paragraph, earlier in section 2:
&quot;Gustiness in wind introduces dynamic loading effects on the system, which can be examined in terms of a gust loading factor. In order to evaluate the peak response of the system, the peak wind load must be considered. Maximum load effects are due to correlated high pressures over the entire structure in the form o feddies a least of size of the structure. Eddies of a small size compared to the structure impinge successively rather than instantaneously and hence are unable to correlate significant pressures over the whole structure. Eddies which are larger envelop the entire structure and hence are able to cause well-correlated pressures.&quot;

I think this explains why the gust factor can be less than one.

These words describe the dynamic loading on a structure. For a rigid structure, the dynamic response is insignificant. For a flexible structure, the dynamic response portion of the gust factor becomes significant, and increases the overall gust factor.

I hope this makes it a little clearer.


Regards,


Chichuck

 

[agama]

&quot;Gustiness in wind introduces dynamic loading effects on the system, which can be examined in terms of a gust loading factor&quot; is the key on this issue. It will always be confusing trying to compare gust factors among codes. This is due to the period of time used to estimate wind speeds. The period associated to the lull zones on the energy spectrum of wind (wich are useful for wind engineering purpuses), have a relatively broad range. Hence, the averaging time for the reference wind speed is diferent for diferent codes, British, Australian and Canadian codes use 1 hour, American ASCE-7, Mexican CFE, Brazilian code, 3 sec. Japaniese, Eurocode and China National Standard 10 minutes. In any case, the wind speed considered for design purposes is a mean wind speed. Evidently in order to take into consideration the máximun response of a structure, we will have to apply a gust factor. Nonetheless, the gust factor will be greater for averaging times colser to 1 hour than for 3 seconds. For 3 second gusts, factor will be closer to unity. Now regarding to gust loading factors being less than 1.0 for rigid structures, consider again that the reference wind speed is a mean wind speed. At first instance codes are general and general are the preaussures we get. Next, codes atend to structural response and shapes. If the mean wind speeds are refered to a particular averaging time in a particular code, then it is allways possible for the average presassure to be too high for a rigid structures and might be considered.

 

[cap4000]

There seems to be a conflict with the Gust Factor and a new term called "Turbulence Intensity Profile" proposed by Yin Zhou and Ahasan Kareem in the Journal of Structural Engineering in August of 2002. Perhaps,this is where the 0.85 reduction just might be coming from?? C.D.Durst is also a leading researcher regarding wind pressures.(Durst ASCE Model was done in 1960). Flexible Structures are penalized as usual, whereas Rigid Structures get cut a break. Good Luck.