Another Wind Load Question

[jmendler]

I am reviewing the wind loads used in the design of several buildings at our plant. The wind speed used in the design is 120mph (ASCE-7 02). My question is, this is the 3sec gust, so how can I relate this back to the Saffir-Simpson scale which is based on a 1min average. The NWS says the 3sec. gust is generally 20-25% higher meaning my building is only good for 95mph sustained winds?? This brings me down to a Cat. I hurricane not a Cat. III? Anybody??

 

[mrMikee]

Table 1609.3.1 in the IBC shows the corresponding fastest mile wind speed to the 3-second gust wind speed. The V3s is in the range of 15 to 20 mph more than the Vfm.

-Mike

 

[Bagman2524]

The 3 sec gust wind is the basic wind speed V

Fig 6.1 in ASCE-7 is also at 33 ft above ground for Category C Exposure (Open terrain, with scattered obstructions having heights generally less than 30 ft.).

Velocity pressure coeffient (Kz and/or Kh) is used to adjust to various heights.

Gust Effect Factor G is used to modify the applied wind pressure based on gust intensity. (Doesn't apply in this case).

There is a mean hourly wind speed in ASCE-7 in Eq 6-12
Vz = b(z/33)**? x V(88/60) that converts the 3 sec gust to a sustained wind.

 

[warrenw]

jmendler:
I have a table from Florida 2004 Statewide Emergency Shelter Plan - Table G-1. Equivalent Basic Wind Speeds.

Saffir-Simpson Hurricane Intensity Scale:
Hurricane Category 1 =
ASCE-7 90mph (3-second Gust) = 75mph Fastest-Mile = 74mph 1-minute Sustained

Hurricane Category 2 =
ASCE-7 120mph (3-second Gust) = 100mph Fastest-Mile = 96mph 1-minute Sustained

Hurricane Category 3 =
ASCE-7 140mph (3-second Gust) = 120mph Fastest-Mile = 111mph 1-minute Sustained

 

[chichuck]

jmendler,

look in the commentary of ASCE 7-02; figure C6-2. This is the Durst curve, and you can use it to get a ratio of V1min to V3sec.

from that graph, V3-sec/V3600sec = 1.525
V1-min/V3600sec = 1.24

V1-min/V3600 = V1-min/V3600sec / V3-sec/V3600sec = .813

I'm not sure if you can use the IBC table 1609.3.1 to do this, it only relates Vfm to V3-sec, and does not involve V 1-hour at all.


regards,

chichuck

 

[STR04]

This link explains it...

http://floridadisaster.org/bpr/Resp...4SESP/Individual Elements/2004-SESP-AppxG.pdf

 

[structural01]

Catagory 5 huricane equals to ASCE 07-02 195mph wind, I am not sure if the figures and equations for Cp, Kz, qz, etc. still applies. ASCE 7-02 wind map only goes up to 150mph.