Need a book? Engineering books recommendations...
Return to index: [Subject] [Thread] [Date] [Author]
RE: Fundamental Period Question ASCE 7-05 12.8.2
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Fundamental Period Question ASCE 7-05 12.8.2
- From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
- Date: Sun, 30 Mar 2008 14:39:20 -0700
Will, 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 * S_{D1}/S_{DS}) 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 12.8.2.1, 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 Thanks Dennis From: Will Haynes
[mailto:gtg740p(--nospam--at)gmail.com] Ta is calculated by 12.8-7,
12.8-8, or 12.8.2. On Sat, Mar 29, 2008 at 9:08 PM, Dennis Wish <dennis.wish(--nospam--at)verizon.net> wrote: The designer can calculate the approximate period T_{a} from the
formula 12.8-7 which is; Ta = C_{t }(h_{n}^{x}) (EQ 12.8-7) Or he or she can use an approximate period from section 11.6(1) equal to
0.8(T_{s} ) where T_{s} = S_{D1}/S_{DS}. My question is if you compare both methods to calculate T_{a} 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 dennis.wish(--nospam--at)verizon.net http://structuralist.wordpress.com |
- Follow-Ups:
- Re: Fundamental Period Question ASCE 7-05 12.8.2
- From: Jules
- Re: Fundamental Period Question ASCE 7-05 12.8.2
- References:
- Re: Fundamental Period Question ASCE 7-05 12.8.2
- From: Will Haynes
- Re: Fundamental Period Question ASCE 7-05 12.8.2
- Prev by Subject: Re: Fundamental Period Question ASCE 7-05 12.8.2
- Next by Subject: Re: Fundamental Period Question ASCE 7-05 12.8.2
- Previous by thread: Re: Fundamental Period Question ASCE 7-05 12.8.2
- Next by thread: Re: Fundamental Period Question ASCE 7-05 12.8.2
- About this archive
- Messages sorted by: [Subject][Thread][Author][Date]