Tham khảo tài liệu 'marine_structural_design episode 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 58 Part I Structural Design Principles mo and m2 need to be calculated properly when applying Eqs. and . Table compares the short-term extreme values obtained by two different methods. Method I uses the weighting factors listed in Table to calculate the mean values of mo and m2 while method n uses each member of the spectral family in Table and takes the maximum . xPEy max xpEy Hs Tj j The extreme values provided by the latter are up to 16 larger than those obtained using the former method. This is understandable because the sample size or exposure time for the latter is relatively larger. In this example exheme values for Hs with risk parameter a 1 are directly applied. Obviously the final extreme values of responses are dependent on the designer s discretion and choice of Hs- Table Short-term Extreme Values of Dynamic Stresses for Deck Plates Zhao bai Shin 2001 Method Wave Spectrum Return period years 20 50 100 W156 JONSWAP W156 Bretsch. I W391 JONSWAP W391 Bretsch. W156 JONSWAP W156 Bretsch. n W391 JONSWAP W391 Bretsch. stress in unit Kgf cm2 Long-Term Extreme Approach A long-term initial cumulative probability distribution function P x of responses is required when predicting a long-term extreme value. Although function P x cannot be predicted explicitly due to the complications of the responses in various sea-states it can be built up approximately through accumulations of various short-term statistical analysis. Generally P x can be of the form P x 1 - exp - x WỈO Weibull distributions or log-normal distributions are commonly used for P x . The Weibull cumulative probability distribution function can be represented as P x l-exp - I m p m 0 where parameters m p and y can be determined from the observed data by the least-squares fitting method. .
