Wind Speed Maps for the Caribbean for Application with the Wind Load Provisions of ASCE 7
multiplied by the wind load factor, was representative of an “ultimate” load. Furthermore, it was assumed that the variability of the wind speed dominates the calculation of the wind load factor. The ultimate wind load, W, is given as
W = CF (VIH )2WLF
where CF is a building/component specific coefficient that includes the effects building height, building geometry, terrain, gust factors, etc., as computed using the procedures outlined in ASCE 7, V is the design wind speed, WLF is the wind load factor, and IH is the hurricane importance factor.
In order to estimate the value of the hurricane importance factor, IH, the committee required that the annual probability of exceeding the ultimate wind load in the hurricane and non-hurricane regions of the US should be the same. Note that requiring the annual probability of exceeding the ultimate load in the two areas (hurricane vs. non- hurricane) to be the same does not mean that the annual probabilities of failure are the same. Recalling that the nominal design wind speed in the non-hurricane regions of the United States is associated with a return period of 50 years, the WLTC sought to determine the return period associated with the wind speed producing the “ultimate” load in a representative non-hurricane prone region. As defined in ASCE 7-98, over most of the non-hurricane prone coastline of the United States, the wind speed for any return
period can be computed from:
) ] 1 2 l n ( 1 . 0 3 6 . 0 [ 5 0 T V V T + =
where T is the return period in years and, VT is the T year return period wind speed. In the non-hurricane prone regions of the United States, the ultimate wind load occurs when:
T F V C 2 =
F V C 5 2 0 =
L F T W T V V = + = ) ] 1 2 l n ( 1 . 0 3 6 . 0 [ 5 0
and from B-4, the return period T associated with the ultimate wind speed in the non-
hurricane prone portion of the United States is:
T = 0.00228exp(10 WLF )
Using the wind load factor of 1.6 as is currently specified in ASCE 7-05, from (B- 5) we get T = 709 years.
F i g u r e B - 1 p r e s e n t s a c o m p a r i s o n o f ( V T / V 5 0 ) 2 ( i . e . a s u r r o g a t e f o r t h e w i n d l o a factor) plotted vs. return period for a hurricane (in this case Grand Cayman) and a non- hurricane region. The comparison shows that for T=709 years, the wind loads for a d