Estimation of extreme wind speeds for the development of wind zone map for the Philippines

by Nicetos E. Rosaria (2001). MSc Thesis, University of the Philippines, Diliman, Quezon City.

Adviser: Victor A. Pulmano, PhD. Co-Adviser: Benito M. Pacheco, PhD, PE.



Abstract: The thesis covers key concepts in the estimation of extreme wind speed for the development of a wind zone map. The present wind code provisions presented in NSCP-92 is modeled after UBC-91. Comparison of three different wind codes from the United States of America namely UBC-91, ASCE 7-88, and ASCE 7-95 showed that NSCP-92 do not have the same levels of safety as that of the more recent ASCE 7-95. A need to update NSCP wind provisions is therefore realized and ASCE 7-95 is eyed as the target model. However, adopting the ASCE 7-95 wind code requires that our present wind map be updated as well, in order to match the new definition of basic wind speed used in ASCE 7-95. Basic wind speed is defined in ASCE 7-95 as the 3-second gust speed at 10m above ground in Exposure C (or open terrain) associated with an annual probability of 0.02 of being equaled or exceeded (i.e. having a 50-year return period). It is estimated from an extreme value model or distribution that best describes the extreme wind speeds of a particular area.

Monthly maximum wind data of about 30 years from 50 weather stations nationwide, collected and compiled by the Philippine Atmospheric, Geophysical, and Astronomical Services Administration (PAGASA), were utilized in the study. These data were subjected to two methods of extreme value analysis, the classical approach and the Peak-Over-Threshold (POT) approach, to determine which type of extreme value distribution is appropriate. The method of moments and the chi-square test were used for the classical approach, while the De Haan method of parameter estimation was used for the POT approach. The results of the two approaches were generally consistent and most stations were found as best modeled by the Gumbel (Type I) distribution. Distributions of most stations under regions of different frequencies of typhoon passage were found with also same type.

Basic wind speeds for a 50-year return period for all stations were estimated. A procedure for estimating basic wind speeds at different return periods was also presented. Values of the estimated basic wind speed were utilized to develop a wind zone map. Similar to the present wind map, the developed map is divided into three zones. However, values of basic wind speeds assigned to each zone have significantly changed. Wind pressures on some selected structures resulting from the use of the proposed map in conjunction with the provisions of ASCE 7-95, which is eyed as the new model code for the forthcoming revision of NSCP, are then compared with those computed with the present wind code.