Combined Period

[haynewp]

When you have two different seismic resisting systems in the same direction (say concrete shear walls at the first floor and steel braces at the second), so that there are different Ct values at each height, how do you get an equivalent period for the whole building (Tcombined)?
Ref. IBC 1617.6.3.1

 

[JAE]

Under the IBC I don't think you can combine the approximate T values (Ta) in any way. By default, it appears that you must simply take the smallest T that applies to any part of your lateral resisting system. The approximate Ta just doesn't have the accuracy to do anything else.

If you think you need to take advantage of a higher T value then you must calculate T per 1617.4.2. (first sentence). With a calculated T, you would still have to meet the requirements of the second sentence of 1617.4.2 where:

Your calculated T < Cu x Ta. This still puts you back to assuming a Ta based on a Ct. But it would give you a higher applicable T than just using Ta alone.

 

[haynewp]

I was trying to use the 2 stage analysis in exception 3(If I remember correctly) to 1617.6.3.1 I tried for a while to find some combination of the Ta of each story and it just didn't make sense. They some how did it in the IBC seismic design guide book but didn't say how. I thought it was going through the stiffness calc's of each story in some way. I don't have the IBC with me now I don't remember what 1617.4.2 says. Maybe that is the method they used.

 

[JAE]

Exception 3.4 talks about the period, T.

It says: &quot;The period of the entire structure shall not be greater than 1.1 times the period of the upper portion considered as a separate structure fixed at the base.&quot;

I had not seen that before as we've never performed a two stage approach (the 10:1 ratio on stiffness required to use this approach per exception 3.3 limits how often you can use it). So you would analyze the upper portion of the structure using the period for the upper. The base shears at the base of the upper would then be applied to the top of the lower times the factors designated in exception 3.2.

The lower portion analysis: For the period, would you use 1.1 times the period of the upper portion or would you use the period calculated for the lower portion system with the full building height as h??

Personally, I would model the building and use RISA to direct calculate the whole-building period, T. Then I would apply the value Cu (from table 1617.4.2) to a Ta value determined from Equation 16-39 or 16-40 to get the upper limit.

 

[haynewp]

I may just ignore the exceptions and be conservative. Thanks for looking at it JAE.