Civil Engineering/ETABS


QUESTION: Dear sir,

"When you use time history analysis you are using the energy content of earthquake in higher period for non braced system. When you brace it you use higher energy content related to lower periods."

Sir, i verify your answer based on my understanding. Is it that you mean when the structure goes stiffer, the instant shaking is greater that's why increased in the displacement?

Between, I wanted to show that the story displacement and base shear is reduced after i increase the story stiffness. Is it possible? What should i do? Use Modal cases?

Thanks so much. Sorry because i'm very new in this field. I took the initiative to learn it myself for my final Year Thesis. Thanks



your understanding is ok. You know what? in very cases it is better to use a contrast concept. I mean you use stiffer structures on looser soils and lower stiffness structures on stiffer soils. Then to get to your objective you must choose appropriate accelerograms with appropriate frequency content.


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Understand your concept. Between, can tell me more about the difference between Earthquake in "Load Cases" and "Modal Cases"? What are the differences and applications in analysis?



In a modal spectral method you should define a design spectrum which is an average of several earthquakes spectrum. You can find them in seismic codes like IBC or UBC or others. In this method an analysis of significant modes of structure is performed and a good average of structural response is calculated. In time history analysis which is attributed to check and not design of structures you have the response  under of one accelerogram which doesnt provide you with reliable results. But its is ok to check very detailed points.


---------- FOLLOW-UP ----------

QUESTION: Dear sir,

Thanks for the previous answer. There are a few more questions to ask. Sorry if i asked any basic questions. Every answer you gave actually had helped me a lot in getting more understand in using ETABS. I really really really appreciated it very much!

The attachment below showing my questions (for Q1 and Q2).

Remark: I already used the LCase1 to test on a structure in which WITH and WITHOUT shear wall. The result gave is as shown on below:

Maximum Displacement (Without Shear Wall) = 286.5 mm
Maximum Displacement (With 4 additional Shear Wall at corner) = 72.0 mm

Story Shear (Without Shear Wall) = 2800 kN
Story shear (With 4 additional Shear Wall at corner) = 17000 kN (Why increase?)

Overturning (Without Shear Wall) = 54000 kN.m
Overturning (With 4 additional Shear Wall at corner) = 360000 kN.m (Why increase?)

Q3) Does it making any sense? or Should i use "Modal Cases"?
Q4) If use Modal Cases, which "mode" is the best for me to analyze? and will it automatically linked to the response spectrum i defined previously?

Thanks for your patient and really sorry for too many questions.

Best Regard,

Dear JV

Sorry about the delay. I have been out of my office. I guess I have answered all your questions about why unpredicted increase ahs been occured. I stated about the energy and frequency quality of accelerograms. If this is not accepted by you please discuss it here.


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Babak Esmailzadeh Hakimi


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


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.

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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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