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RE: Fundamental Period Question ASCE 7-05 12.8.2

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I understand the coordination of the Seismic Design Category requirements of Section 11.6. This still does not answer my question since Section 11.6 is followed regardless of all other requirements. The trigger in section 11.6 is the 1-second period S1 which would push the SDC into category E or F depending upon its value being equal to or exceeding 0.75.


This section only triggers a condition where the user can make the assumption that, rather than calculated Ta according to equation 12.8-7, he or she can assume Ta equal to 0.8*Ts in both orthogonal directions so long as S1 remains less than 0.75. However, if both methods are used to calculate Ta (the simplified form of 0.8*Ts or 0.8 * SD1/SDS) and the more lengthy equation 12.8-7 then is there a limit? If S1 is less than 0.75 and some condition arises where EQ 12.8-7 exceeds 0.8*Ts then is the greater value for Ta used or is it limited to the approximate value of 0.8*Ts?


The reason for bringing this up is that the value for Ta also plays a role in the calculation of the Seismic Response Coefficient . You really don’t escape Section 11.4.5 since this section is still referenced in Section 11.6. This is where the code becomes somewhat ambiguous. If you disregard the simplified use of Ta = 0.8*Ts as mentioned in Section 11.6 then by the time you calculated Ta for the Seismic Response Coefficient and move on to consideration of the Seismic Design Category, you are back into section 11.6 which then takes you back to 11.4.5 if S1 is less than 0.75.


Therefore, I don’t believe that you can simply ignore the provisions of 11.6 but you can calculate the longer version of Ta as defined in equation 12.8-7 and disregard 0.8*Ts. My only real question is if the code writers intended the simplified form of Ta = 0.8*Ts as an upper limit when calculating the Fundamental Period T defined in the confusing section 12.8.2 where it states;

“The fundamental period, T, shall not exceed the product of the coefficient for upper limit on calculated period (Cu) from table 12.8-1 and the approximate fundamental period, Ta, determined from Eq. 12.8-7. As an alternative to performing an analysis to determine the fundamental period T, it is permitted to use the approximate building period Ta, calculated in accordance with Section, directly [Which is still a lengthier method than the assumption of 0.8*Ts – ed.].”


The trap comes in at the beginning of Section this quotation where you are directed to table 12.8-1 that sets and upper limit on Cu where S1 comes into play as an “upper limit” in the calculation for Cs as brought up in equation 12.8-6 where S1 is equal to or greater than 0.6g which can occur throughout any high seismic region.


It sure what have been easier if they only provided the flow chart showing the logical steps in one diagram rather than having to flip through sections and formula’s J





From: Will Haynes [mailto:gtg740p(--nospam--at)]
Sent: Saturday, March 29, 2008 6:49 PM
To: seaint(--nospam--at)
Subject: Re: Fundamental Period Question ASCE 7-05 12.8.2


Ta is calculated by 12.8-7, 12.8-8, or 12.8.2.

 0.8*Ts is a comparison value in order to apply the SDC determination by the exception of 11.6.

On Sat, Mar 29, 2008 at 9:08 PM, Dennis Wish <dennis.wish(--nospam--at)> wrote:

The designer can calculate the approximate period Ta from the formula 12.8-7 which is;

Ta = Ct (hnx)  (EQ 12.8-7)

Or he or she can use an approximate period from section 11.6(1) equal to 0.8(Ts ) where Ts = SD1/SDS.


My question is if you compare both methods to calculate Ta would you use the maximum or minimum results for Ta when calculating the  Seismic Response Coefficient Cs in formula's 12.8-3 and 12.8-4?


Dennis S. Wish, PE


Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant