accidental eccentricity

[yasir1989]

hi everyone
i have 10 story building when calculate base shear from reaction and story force they have same value but when use accidental eccentricity(5%) the base shear calculated by summation of the reactions is larger than when calculated by story shear so i cant understand why this difference
thank you

 

[yasir1989]

i use response spectrum method

 

[ayelamayem77]

Dear yasir1989,

use section cut method to calculate base shear it is the only method that gives the same values as base shear

steps as follows:

After running the model

from the menu
select > by story level > story 1

from the menu
assign > group names > add new group

from the menu
define > section cuts , select the group name (used in the previous step)

from the menu
Display > show tables > section cut forces
then compare , i hope it works..

note:
Section cut method is an effective tool to obtain base shear for both columns and walls separately (based on groups defined ) when using a dual frame system to resist seismic forces

 

[yasir1989]

Thank very much ayelamayem77
your idea is very when calculated base shear for over all structure but i have building 10 *10 bay i want to calculate base shear @ each frame and the summation of these base shear @each frame will equal to overall base shear but in my building(shown in attachment) the summation of base shear of frame(a b c d e f g h i j k) will not equal to over all base shear (when use response spectrum and accidental eccentricity 0.05)
thank you

 

[ayelamayem77]

upload the Etabs file

 

[yasir1989]

this is my model
thank you

 

[yasir1989]

please see case of (spec1 ) this case of response spectrum and accidental eccentricity 0.05 and here my problem
thank very much

 

[ayelamayem77]

Dear yasir1989
its better to compress your Etabs file (using winrar) before uploading it because i can't download, any way i guess that i understand your problem so please do the followings:

when selecting one frame to get its amount of base shear due to seismic force, select all frame members and its intersection points at 1st story level only (girders start and end points at first story level)

after that assign your selection to a group.

(selecting both points of the column or selecting the base points of the column instead of the intersection points at fist story will give no results when displaying section cuts results)

i hope it works
best regards..

 

[yasir1989]

Thank you very much Mr ayelamayem77 for helping me wish you all the best in your life
please if you can you give me note about response spectrum method in 3d model
like how to calculate dynamic eccentricity and torque
Sorry for the inconvenience
thank you

 

[yasir1989]

and this is my model

 

[ayelamayem77]

Dear yasir1989:

it seems that you use a response spectrum file under the name c/program files(x86)/computer and structures /sap2000 v15/spectra/yasir4.text
which is not uploaded with the Etabs model, anyway i hope you got my idea in my latest post , if not, please upload the response spectrum text file to check your model.

 

[yasir1989]

Dear ayelamayem77
HERE MY RESPONSE SPECTRUM
thank you

 

[ayelamayem77]

Dear yasir1989

it seems that you are right (your case happened when using response spectrum analysis , but if you did a static one no such problem will occur,anyway i modified section cuts in your model to get values at each frame , also i apply a static seismic force in x-direction to show my idea (data from story shear = total base reaction = section cuts forces)
until you solve this problem (and i hope to show me how) please download the new model:

http://www.mediafire.com/download/oqehmdywikc2dcg/3d+new-modified.rar

 

[yasir1989]

thank you ayelamayem77 for your interest
please the attachment did not work (no EDB file )
best regard

 

[ayelamayem77]

Here it is ...

http://www.mediafire.com/download/6u48pysmbbl79g5/3d-new-modified.rar

don't forget if you find an answer to tell it to me
Best regards..

 

[yasir1989]

dear ayelamayem77
if find this method in link

https://wiki.csiamerica.com/display/etabs/Torsional+amplification

is very good to solve the problem but i cant judge this real value
thanks

 

[ayelamayem77]

Dear yasir
download the following link and you will find all want

http://www.mediafire.com/download/iokmw53hajal7ba/answer.rar

 

[yasir1989]

Dear ayelamayem77
i know this a good way to solve problem and thank u for helping me
but there is small problem with this solution we use SRSS to combine all mode and this method is used only for regular building and 2d ,but for irregular building we should only use CQC .this solution dont have any effect on base shear and moment but have lagre effect on torque
correct me if i am wrong
Best Regards

 

[ayelamayem77]

Dear yasir1989,

in my previous post i showed you how the program calculate total base shear i used SRSS method to show you the procedure in simple way , if you want to use cqc follow the same steps and apply the equation of cqc in my previous excel sheet (which is more complicated)

my conclusion :

if you want the total base shear take its value directly from Etabs tables, do not take the summation of the reactions at the base and compare with the story shear given by etabs especially when you use dynamic response spectrum analysis.

(base shear, displacements,story drifts,atc......)are based on SRSS method or CQC method (THESE TWO METHODS ARE APPROVED BY ASCE-7) and they are totally different than ABS method

example:

if i use SRSS method (for simplicity) for a model and i have 3 mode shapes:

first mode give reaction at the base = X
second mode give reaction at the base = Y
second mode give reaction at the base = Z

Total reaction at base from response spectrum analysis using SRSS method = (X[sup]2[/sup]+Y[sup]2[/sup]+Z[sup]2[/sup])[sup]0.5[/sup] which is not equal to ABS method = X+Y+Z