what is difference between eigen frequency and rayliegh frequency? kindly tell me why the staad is showing difference in both the values of frequnecy for the loading?
Attached is PDF file which provides information on the two methods available in STAAD for calculating frequencies.
Of those, the eigenvalue method is a better choice since there is less likelihood of the user providing input that would be erroneous. For the Rayleigh method, since the quality of the result depends on how close a resemblance the static deflection has to one of the mode shapes of the structure, the possibility of the frequency being an unrealistic value is greater.
In the program's main screen, go to Help-Contents-Application Examples-American examples-Example 28. It demonstrates the procedure for calculating the frequencies per the eigenvalue method.