Civil Engineering/Modeling

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Question
Dear Mr.Hakimi,Hope you are fine.
I want to ask about the following issues:
1-I've modeled a 5 storey (Concrete) bulding which will have a nother 2 stories (STEEL) in the future as an extension ...I've created two models (one with 5 stories and the other with the extension)...comparing the results from the two models I've got some shear walls(that were not failed in the 7 storey model ) failed regarding shear/flexural capacity in the 5 story model ( where the time period used for base shear calculations(0.54sec) were smaller than the one of the 7 sstory model (.98sec)) leading to larger base shear  .... I've modeled the extension stories as equivelant concrete stories (not steel) in ETABS..... also the ratio of column RNFT were different between the two models .... I have no clear information about the time period that the the building will stand without the extention .... should I compare both models and take the worst case between them for each element ?? and should I define another static EQ load case to apply an EQ load on the steel portion with R=8.5 ..this buling locates in zone 3 .
2-Throughout checking columns I found some of them failed as a result of the large " non-sway magnification factor " where ETABS used strange values that do not match the ones calculated by the ACI code formulas and it takes Cm always as 1 ... does ETABS has problems regarding this???
3-To get the non-linear drift the UBC97 says to multiply the elastic drift's value by 0.7R .. what if we used cracked section modifiers in analysis .. doesn't this compensate for 0.7R??
4-Can you provide me with a good refrence on principles of choosing a good structural system (ex. prefered locations of shear walls --- systems for high rise buildings) .

I'm waiting Youre valuabe answers
Thanks and Regards.

Answer
Dear F.D

1- Larger periods is leading  to lower seismic effect in 7 storey structure.You dont need to model a complete 7 storey Rc structure if you dont have to construct it.

2- The non sway assumption depends on ratio of lateral to vertical stiffness of the system. As a check of this matter increase the Elastic modulus of the shear walls by 1 or even 100 times to see wether the results change or not.

3- The cracked section modifiers are to make rational the distribution of moments between colums and girders and are not enough and accurate and assigned to show the effect of nonlinear drift of the structure under seismic action. I suggest you, for drift calculation dont take any modification factor  but the 0.7R, to elastic drift to get inelastic drift. This is much more realistic.

4- I dont know such a jewellery refference but , something that I can propose is that it is better to put shear walls in peripherial locatio of the structure. I suggest you to design the shear walls so that the center of rigidity and mass in each floor get the closest situation to each other. I will take a look on my refferences to find a suiatable reference for you.

Regards
BEH

Civil Engineering

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Babak Esmailzadeh Hakimi

Expertise

I can answer questions in Structural Engineering, Earthquake Engineering, Project Management and Project Control topics!

Experience

I am experienced in the area of earthquake engineering in electrical power industry, i.e. Lifeline Earthquake Engineering. My focus has been on structural seismic design of power installations. Recently I have got involved in project management in the category of constructional engineering.

Organizations
Sinam Consulting Engineers Co.

Education/Credentials
Bs degree in Civil Engineering from Ferdowsi University ,1988, Mashhad Iran, Ms degree in Earthquake Engineering, Sharif University of Technology, 1993, Tehran, Iran

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