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Wind Speed Maps for the Caribbean for Application with the Wind Load Provisions of ASCE 7

The importance factor used in ASCE 7 for the computation of wind loads for the design of category III and IV structures is defined so that the nominal 50-year return period non-hurricane wind speed is increased to be representative of a 100-year return period value. This importance factor is not the hurricane importance factor, IH, but rather a factor used to increase the wind loads based on an occupancy classification. The importance factor is applied to the design of all category III and IV buildings whether or not they are located in a hurricane prone region. Following the approach used above to estimate the resulting ultimate load return period associated with the 100 year design

wind speed in the non-hurricane prone regions we find that:

T = 0.00228exp[10(V100 /V50 ) WLF ]

B-9

where for V 00 /V50 computed from B-4 and WLF = 1.6, we find that T=1,697 years. In the

development

of

Equation

B-9,

the

term

(V 00 /V50 )

WLF

replaces

the

WLF

used

in

Equation B-5, effectively resulting in a higher load factor for category III and IV structures equal toWLF (V100 /V50 )2 . Thus for Category III and IV structures, a design wind

speed of V1700 / 1.6 is appropriate.

In the versions of ASCE 7 since 1993 (i.e., ASCE 7-95 and beyond), the importance factor has been applied to the velocity pressure, not, the wind velocity as was

the case in prior editions. The design pressure in ASCE 7-95 and later is

qz = 0.00256Kz Kzt KdV 2 I

B-10

where the importance factor I is equal to 1.15 for category III and IV structures. For consistency in the hurricane prone regions, the importance factor should be defined as:

I = (V 700

/ V700 ) 2

B-11

Figure B-3 presents contour plots of I = (V 700

/V700 )2 where a large gradient of I

from north to south is evident, but over most of the region, I, is consistent with the 1.15 value given in ASCE 7. In the case of the Category II buildings where a 700 year return period wind speed represents an ultimate design wind speed for these Category II buildings, we find that for Category III and IV buildings a 1,700 year return period wind speed is representative of the ultimate wind load. Both approaches inherently include the variation in the hurricane importance factor in hurricane prone regions, but are tied back to a wind load factor equal to 1.6 as applied to the non-hurricane prone region of the United States.

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