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Civil Engineering/fea and matrix methods


people like you give us the feeilng that the world still has love and brotherhood in it.You are doing a wonderful job.

Firstly, can you tell me what is the concept of response spectrum.The seismic frequencies are applied and the maximum displacements of modes are obtained.Then why are the combined using cqc or srss to get the peak displacements.Selecting the modes undergoing max deflection is enough right?

Also, in two bodies with the same geometry and same loading conditions but different youngs modulus,
can we say that the stresses at all points will be the same although the strains are different (because of the difference in E value)as stress is affected only by force and area.

ANSWER: Dear Hani

Thanks about your feelings.

(Design) Response spectrum is the maximum response of a single degree of freedom structure with various stiffness ( period or frequency) under a range of accelerograms (average). It is used in the modal analysis of structures. You may know that each N degree of freedom system has N mode of vibration. For each mode of vibration you calculate a maximum response. To combine the modes you may suppose to add the mode of vibrations together which is extremely conservative . Instead you may  use SRSS or CQC method which suppose no simultanuous maximum in all modes is credible to occur.

For 2 similar structures with different E no difference in stresses occurs but you will have difference in deflections and strains.


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

QUESTION: Thankyou for the quick reply sir.
I dont understand this.after finding the peak responses for each modes,why are they combined using cqc or srss.If we have the peak response ,we can simply design for the peak response itself right.All the other displacements(if we are considering peak displacement for eg) will fall below the peak response of the peak dispacement of this mode

ANSWER: Dear hani

each n degree of freedom system has n natural mode of vibration . each mode has some effect on the behaviour of the structrure under the effect of an external dynamic loading. Then you have to add up the effect of modes to get to the accurate response .


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

QUESTION: Dear sir,
I have this doubt regarding the matrix and fea methods. Are the displacement approach, flexibility method and finite element method all same.If yes, HOw are the related and how do they work in softwares like staad and etabs.

Dear Hani

There are 2 design categuories in structural analysis :1- Stiffness method 2- Displacement method. Almost all software follow the first method. However there are 2 general method in structural design: 1- force method 2- displacement method. Almost all softwares and code based methods are based on force method. But for seismic design or design of structures under specific displacement based loading you can find methods like push over in recent version of softwares which are usually used for rehabilitation structural design.


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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.

Sinam Consulting Engineers Co.

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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