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

Using a wind speed defined as in Equation B-8 eliminates the need to develop a map for both the 50-year return period wind speed and the importance factor. If a basic (design) wind speed associated with a 50 year return period was used in the Caribbean, in order to be consistent with the intent of the ASCE 7 standards, a load factor defined as

(V700 /V50 )2 would be used in place of a constant value of 1.6. Figure B-2 presents contours of (V700 /V50 )2 showing the variation of the effective wind load factor over the

Caribbean basin, varying from about 1.75 around Puerto Rico to in excess of 6 near Trinidad and Tobago. The very large ratios in the southern portion of the Caribbean occur because of the large number of years where the locations do not experience any hurricanes, and as a result the 50 year return period wind speeds are very low, but these locations experience strong winds from hurricanes associated with rare events.

Note that when the wind speed maps were being developed for ASCE 7-98, the wind load factor at the time was equal to 1.53, which the wind load task committee rounded down to 1.5 and computed an ultimate load return period of 475 years, which subsequently rounded up to 500 years. The final wind speed map used in ASCE 7-98 was

developed using Vdesign = V500 / 1.5 . During the same time period when the wind load

map was being developed, the ASCE 7 committee examining load factors increased the load factor from a value of 1.53 to 1.6. Thus when ASCE 7-98 was published there was a disconnection between the load factor used to develop the map and the load factor used in the wind loading equations.

1.75

1.75

2

2

2

2.5

2

3.25

2.75

1.75

3.25

2

2.25

2.25

2.75

2

4

3

1.75

4.5

6

5.25

4.25

2.75

2.25

2. 5

3.25

4 75 . 5

6

6.25

5. 5

3.75

3. 5

5.5

5.25

6

4

6.5

5.75

4. 5

5

4.75

6

5.75

6.5

3.75

9

8

4.25

7

5

3.5

5.25

Figure B-2 Contour plots of (V700/V50)2 As indicated above, when the correct load factor of 1.6 is used, a design wind

speed defined as Vdesign = V700 / 1.6 is appropriate. This design wind speed is equivalent

to designing a structure using the 700 year return period wind speed and a load factor of unity.

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