Behavior Of High And Intermediate Frequency Modes Of Structures Subjected To Harmonic Excitation

Dynamic analysis of structures subjected to harmonic and seismic loads shows that the contributions of high frequency modes are essentially static. The modal responses of the structures subjected to harmonic excitation are generally combined using square root of sum of squares (SRSS) rule for well separated modes. The combination of modal response s using SRSS will results in erroneous results for even well separated modes, if the high frequency modes having frequency greater than operating frequency are included. This paper makes an attempt to identify the behavior of the modes which have a frequency greater than the operating frequency. The rigid and periodic part of the response are assumed to be statically independent and the behavior of high frequency and intermediate frequency modes of structures subjected to harmonic loads are studied with the help of an example. The problems involved in using the avaiable modal combination rules in combining the modal responses, when high and intermediate frequency modes are included are discussed.