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

Definition of Basic Wind Speeds Used in ASCE 7

The purpose of this appendix is to review the process used by ASCE 7 Wind Load Task Committee (WLTC) in the development of the wind speed map given in ASCE 7-98 and beyond that presents a design wind speed map that is defined by wind speed contours that represents the 500 year return period wind speed divided by the square root of the

load factor (i.e. 1.5 ). The goal of the WLTC was to develop a wind speed map that

yielded approximately risk consistent designs (for wind resistance) in hurricane and non- hurricane prone regions of the United States. To reach this objective the WLTC developed an approach that, while approximate, resulted in a design wind speed map that incorporated a hurricane importance factor into the specification of the design wind speeds. The approach essentially involved equating the return period associated with exceeding the ultimate wind load in both the non-hurricane and hurricane prone regions of the United States. The methodology allowed for the implied hurricane importance factor to vary with location rather than using a single value as had been used in prior editions of the standard. The approach taken by the WLTC is extended here for the case where the wind load factor is equal to 1.6 rather than 1.5, and is further extended to determine the effective return period associated with the ultimate design of Category III and IV structures (as defined in ASCE 7).

Prior to the introduction of ASCE 7-95, the design wind load equations in ASCE 7 included a multiplicative term in the form of a hurricane importance factor. This hurricane importance factor was introduced to take into account the fact that the tails of the wind speed exceedance probability distributions for hurricane winds are longer than those associated with non-hurricane winds. The hurricane importance factor varied from about 1.05 at the coast and decayed linearly to 1.0 at a distance of 100 miles inland. The hurricane importance factor in ASCE 7 and it predecessor (ANSI A58.1) was applied to the 50-year return period wind speed given in the standard, not the resulting velocity pressure. Thus, using the ASCE and ANSI provisions, buildings and structures located near the coast were designed using a wind speed that had a longer return period than those located 100 miles or more inland.

In the development of the wind speed map given in ASCE 7-95, the hurricane importance factor was incorporated directly into the wind speed map (i.e. wind speeds along the hurricane prone at the coast were increased by 5% and wind speeds 100 miles inland were left unchanged, and those in between were adjusted through linear interpolation of the hurricane importance factor).

In the development of the design wind speed map used in ASCE 7-98 the WLTC re-visited the hurricane importance factor that had been in use in the US standards since 1982. The primary reasons for re-visiting the hurricane factor was the recognition that the importance factor likely varied with location along the coast and using a constant value of 1.05 was not appropriate.

The approach taken to develop a varying importance factor began with the premise that the nominal wind load computed using the methods given in ASCE 7, when

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