PART VI 



FAT {QUE. EMBRITTLEMENT AND VIBRATIONS 



Any structural member subject to tensile stresses 

 caused by loading conditions which repeat them- 

 selves can fail due to fatigue. the type of 

 structures herein recommended for the texas towers 

 include a bracing system which is subjected to 

 such conditions. therefore, design criteria must 

 be established which preclude the possibility of 

 fatigue failure. the primary loading conditions 

 which cause repeated tensile stresses in the brac- 

 ing system are wind and wave forces. as the stress 

 which a structural member can endure is a function 

 of both the number of repetitions and the magnitude 

 of the stress, the approximate number of repetitions 

 of the larger stress magnitudes during the entire 

 life expectancy of the texas towers must be evalu- 

 ATED. For THIS PURPOSE the life expectancy can BE 

 assumed to be ^0 years . the number of the stress 

 repetitions within 5^ years can be approximated by 

 an examination of the daily north atlantic weather 

 maps of the past ten years and listing the total 

 duration of all storms with wind velocities over 

 40 miles per hour . 



Assuming that all storms in the future 5^ years will 

 be similar to the storms of the last |0 years, each 

 wave height in the stairway approximation can be 

 associated with particular wave and wind forces on 

 the structure and in turn cause certain tenstonal 



STRESSES IN THE BRACING SYSTEM. |N THIS WAY THE 

 NUMBER AND MAGNITUDES OF STRESS REPETITIONS DUE TO 

 WIND AND WAVE FORCES THROUGHOUT THE TOTAL EXPECTED 

 LIFE OF THE STRUCTURE CAN BE COMPARED WITH THE FATIGUE 

 STRESS CHARACTERISTICS FOR THE STRUCTURAL MATERIAL OF 

 WHICH THE BRACING SYSTEM IS FABRICATED, 



DESIGN AND BRACING SYSTEM VARIABLES 



The DESIGN OF THE BRACING SYSTEM TAKES INTO ACCOUNT 

 THE AFFECT OF THE FOLLOWING VARIABLES! 



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