Joint Mass Effect on Modal Analysis

[Bridge_Man]

Hi Guys,

I am working on an elevated water tank, and I wanted to add the Vertical mass of water as a lumped mass using a joint linked to a girder surrounding the water tank. when I add a joint mass I do not notice any effect on the fundamental period of the structure (I did make sure that am using the correct units, I did it using weight and using mass values, same answer). BUT when I try to distribute the exact same mass over the length of the girder mentioned above and apply that mass as a frame line mass it has a visible effect on the fundamental period.

I tried adding the same mass as a uniform load and then adding that load as a mass source and I got the same answer as when I add the frame line mass.

am going crazy here, why is this happening. knowing that when I try and add a Horizontal joint mass I can actually see its effect on the fundamental period.

Thank you and I appreciate the help

 

[JoshPlumSE]

You only added VERTICAL mass? When you add mass manually, it is important to realize that this mass is truly directional. Meaning that vertical mass will only affect vertical modes (for the most part).

If you did not add horizontal mass then this would not affect the horizontal modes of the structure.

 

[Bridge_Man]

JoshPlumSE , Thank you for your comment, maybe am missing something here. so please bear with me for a minute.

what would the modal fundamental period change drastically then when I change the type of mass applied (mass joint Vs frame liner mass) ? am still applying the same vertical mass, only using a different approach.

Thanks again, this would help me better understand what's happening.