Modeled frame seismic period determination

[port125]

We are designing a two story retail facility and would like to utilize the ASCE section 12.2.3.1 for two stage equivalent lateral force. The controlling factor for our design is meeting the requirement to have the period of the entire structure be < 1.1X the period of the upper portion.

I have the period for the upper bay from a RISA model of the brace frame.

When I model the full bay (lower and upper portion) I get about 50% mass participation at two very different periods.

What period should be used to determine if it is < 1.1X the upper frequency?
Is it common practice to use the sum of the squares proportioned to mass participation? or to use the period with 51% mass participation? or use the higher/lower values?

Thanks

Ryan

 

[weab]

I believe that RISA says that to determine the natural frequency, you should have >90% mass participation. Check me on this by looking at the RISA help section. The point is that you may have a problem with your model if all you get is around 50% participation.

Anyway, I am not thoroughly knowledgeable about the section in question. However, I'd think that the periods that you need to compare are the ones with the highest mass participation. Compare the frequencies for a 1.1 ratio.

 

[port125]

Thanks for the reply.

I believe what is happening is that the main floor level is getting about 50% mass participation and the roof level is getting about 50 % as well with two different periods.

I have close to 100% mass participation over two different frequencies. I think the cause of this is the first item in the code that states that the stiffness of the 1st needs to be 10X the upper level. Causing the building to vibrate with two different periods.

RISA is calling the 1st mode the fundamental period, however it shows less than 50% participation the 2nd mode shows the remainder. (And eureka if we can use the 2nd mode as the frequency we fall into this particular section of the code and we save a bundle on our roof diaphragm fastenings.)

 

[weab]

I have to stress that I'm not familiar with that part of the code. It would be nice if someone else can confirm the code intent.

 

[Qshake]

The code is clear that to use a simplifying analysis you must have a stiffer lower bay such that the overall building period is less than that of the upper and generally more flexible bay. When this condition is met, you can then presume to have a single level structure so to speak. You could probably just as well check this by using the stiffness of the columns for the upper section and compare that to the combined or coupled stiffness of the columns for each level.

As to the mass participation - this is really another matter altogether and is an indication of the dynamic model performance. When we analyze structures using dynamic modeling we strive to achieve equal to or greater than 90% mass participation over several modes anywhere from 10 to 100 or more pending the size of the model.

I can think of a variety of reasons you would not or should not expect to have 90% mass participation in each mode when modeling a two story structure. Certainly one reason is that the lower bay is sufficiently braced or with shear walls to prevent modal displacements. Another could be that a majority of the mass is on the second level.

If you focus on the stiffness of the building as a whole and the upper level and then compare you should be able to move forward. I would not tie the model dynamic performance to a lateral force method requirement. This, to me is like apples and oranges.

Good luck.

Regards,
Qshake

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