Hurricane Basic Wind Speed

[ibro]

Customer specified category 5 hurricane level for wind loading. What basic wind speed should be used for wind analysis based on IBC/ASCE 7?
Thanks, Ibro V.

 

[boo1]

Category Five Hurricane: Winds greater than 155 mph link:

http://www.nhc.noaa.gov/aboutsshs.shtml

The 2003 IBC wind loads come from ASCE 7-02. Typical 3-second gust basic wind speed criterial are specified and used for design. ASCE 7-98, figure C6.11, "Durst curve" can be used to convert to the fastest-mile basic wind speed.

Vfm=(V3s-10.5)/1.05 ASCE Figure 6-1

Cheers
Boo1

 

[JAE]

boo1 - for a mechanical engineer, you're amazing!

 

[Lutfi]

Well-done Boo1.

Ibro, what are you designing? Three second gust near over 173 MPH seems excessive unless you aredesingin a shelter. Let me caution you about using the ASCE-7 importance factor of 1.15 is you are designing a shelter or critical facility. The text below is my explanation:

FEMA Standard 361 requires the use of importance factor of 1.0, for shelter designs, in accordance to section 3.5.2. The following quote is FEMA’s rational for using importance factor of 1.0 “The importance factor (I) is set equal to 1.0. The importance factor for wind loads in ASCE 7-98 is designed to adjust the velocity pressure to different annual probabilities of being exceeded (different mean recurrence intervals [MRIs]). Since the wind speeds in Figure 2-2 are already based on very great MRIs (i.e., low exceedance probabilities), they do not need to be adjusted with the importance factor.”

I got your e-mail on the crane runway. I have been out of town. Do you still need my input?


Regards,
Lutfi

 

[chichuck]

boo1,

where did you get your equation? A few years back, I attended the ASCE Wind Loads Seminar. They told us that to use the Durst curve, you must take your fastest mile speed, convert it to a 3600-sec "gust" and convert that back down to a 3-sec gust. The reason is, each fastest mile speed corresponds to a different "gust duration" and you need to convert to a common base, they used 3600 secs. This way, there is a different multiplier for each and every value of fastest mile speed.

regards,

chichuck

 

[chichuck]

boo1,

I looked at the link you provided. It says those winds are based on a 1-min duration. That means there is a unique conversion factor for changing over to 3-sec gust, just as your formula indicates. ASCE 7-98 doesn't refer to fastest-mile winds, it uses the 3-sec gust. I think what you meant was use the Durst Curve to change from 60-sec duration to 3-sec duration.

regards,

chichuck

 

[JStephen]

It might be simpler to kick it back to the customer and ask for a specific wind speed for 3-second gust.

The problem you have is a Category V hurricane is not a 155mph wind speed- it is that "or greater". IE, the design range you're looking at begins at the 155mph mark, rather than ending there.

 

[samdamon]

Unusual. Never had a client ask for a particular storm level to be used in design, have always used code minimums. Ibro you may want to ask your client to confirm the return period of the storm also, default is 50 yrs I believe.

 

[boo1]

Chichuck, My point was the different criteria for wind speeds. It can be confusing what wind load provisions apply. The wind speeds associated with each category of storm are considered to be 1-minute sustained wind speeds (Powell et al. 1994). These wind speeds are converted to equivalent 3-second gust speeds using Figure C6-1 in the Commentary of ASCE 7-98 (Durst 1960). The 155 mph wind speed is also list in ASCE 7-05 Table C6-2. ASCE 7-02 Figure C6-2 was revised in ASCE 7-05 Figure C6-1, which is also in 2006 IBC 1609.3.1, Table 1 and R301.2.1.3.

These two links explain it well:
<

http://floridadisaster.org/bpr/Resp...SESP/Individual Elements/2004-SESP-AppxG.pdf>

see Table G-1. Equivalent Basic Wind Speeds

For the equation reference see:

http://www.iccsafe.org/cs/codes/2004-05cycle/2004-05ROH.html

Code Development: 2004-2005 Code Development Cycle / structural: Errata S32-04/05, R301.2.1.3 page 97-99

Ibro, consider what importance factor apply, I have seen FEMA referencing the use of 1.0 (improtance factor) vs 1.15 when the Cat5 wind loads were imposed.

Cheers
Boo1

 

[SteveFehr]

South Florida has 140 and 150mph wind zones which have considerable lessons learned cranked into them; if the customer wants to withstand a category 5 then you'd probably be well off just picking up a copy of the south florida codes and building to that rather than trying to calculate out straight stresses from IBC. (Or are they all in IBC now? They might be...) When I was designing my house (only 110mph here, thankfully), I was chatting a good deal with the city engineer who recently came from south florida- there are a lot of things at 150mph that you don't even have to worry about at slower speeds, things that I certainly wouldn't heve even thought of but are just as important as simple uplift and shear.