The period of the structure based on UBC97 method A

[nour75]

I am designing an industrial framed RC structure that has a vertical steel platform towers attached to the roof (the mass of the steel structure is neglected and not contributing in the main lateral resisting system, just for accessing some steel silo structures for maintenance). The total height of the structure including the vertical steel platform towers is 80.0m, but the height of the RC frames (the main lateral resisting system) is 23.0m. the total height to the center of mass of the steel silos, considering they are full of material, is 34.0m. The seismic zone is 1 according to UBC97 code. In order to determine the period of the structure based on method A, one of my colleagues advised me to use the 80.0m for the hn, and another colleague told me to use the 34.0m, but I am not convinced, and I suggest to use 23.0m instead.
Are there any one suggesting another point of view?

M.Nour

 

[wadakis]

I do not fully understand the structure you are designing, however, if there is no mass at 80m, then this would most likely not be the height. Your system might meet the requirements of the building on a building section. This would allow you to deign the upper structure as an isolated building. Then up it's base shear by the ratio of the R factors for the top structure and bottom structure and apply it to the top of the RC strcuture. The upper structure seems simply enough to calc it's period by hand (sqrt(k/m) i believe). Or, just call the upper structure a "component" and use the Fp force.

 

[nour75]

Thank you for your quick reply. But I want to clarify my question, is the hn in Eq. (30-8) in UBC-97 is the vertical distance between the highest point in the main lateral resisting system and the foundation (23.0m) or between the center of mass of the silos and the foundation (34.0m). The silos are connected by steel columns to the roof of the RC building, these steel columns not considered in the main lateral resisting system of the building. The main lateral resisting system of the building is RC ordinary moment resisting frames (OMRF). Knowing that, the mass of the silos are significant compared to the mass of the RC building. (in other words, if we simulate this structure by a SDOF system, what will be the height of the total mass from the fixed base)

 

[prsconsultant]

Based on your description it seems like using a height of 34 m for period calculations would be appropriate. Although your RC frames stop at 23 m, there has to be a load path to transfer lateral forces to this level from the structure above. If you want to be on the conservative side, you can use 23 m. How much of a difference it makes in your base shear values by using 23m vs 34m?

 

[nour75]

Thanks “prsconsultant”. The base shear is increased approximately 30 % if I use the 23.0 m for the hn in Eq. (30-8) in UBC-97. The load path will be through the steel columns supporting the silos, but they are not considered in the main lateral resisting system of the building.