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Another Wind Load Question

Another Wind Load Question

Another Wind Load Question

(OP)
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??

RE: Another Wind Load Question

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

RE: Another Wind Load Question

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.

RE: Another Wind Load Question

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

RE: Another Wind Load Question

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

RE: Another Wind Load Question



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.

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