Please refer to the following wiki:
communities.bentley.com/.../29023.calculating-modes-and-frequencies
You need to consider the gravity loads only. The limitation of Rayleigh has been explained in the wiki. If you want to use this method, you need to apply loading (gravity loads- DL+ 25% or 50% imposed load) in the direction in which you want to find out the fundamental time period in such a fashion so that the deflected shape of the structure in proportion with the fundamental mode shape of the structure.
If you are interested in EIGEN SOLUTION (this can be instructed by the program using MODAL CALCULATION REQUESTED command), you need to define a mass matrix which will be formed for gravity loads- DL+ 25% or 50% imposed load. For this process you need to define the loading in three orthogonal direction in the load case where you use MODAL CALCULATION REQUESTED command.
communities.bentley.com/.../29023.calculating-modes-and-frequencies
You need to consider the gravity loads only. The limitation of Rayleigh has been explained in the wiki. If you want to use this method, you need to apply loading (gravity loads- DL+ 25% or 50% imposed load) in the direction in which you want to find out the fundamental time period in such a fashion so that the deflected shape of the structure in proportion with the fundamental mode shape of the structure.
If you are interested in EIGEN SOLUTION (this can be instructed by the program using MODAL CALCULATION REQUESTED command), you need to define a mass matrix which will be formed for gravity loads- DL+ 25% or 50% imposed load. For this process you need to define the loading in three orthogonal direction in the load case where you use MODAL CALCULATION REQUESTED command.