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Civil Engineering/response spectrum


QUESTION: hii sir,

in etabs linear analysis like response spectrum analysis, time history can be performed, please say me the pre requisites before performing these analysis.

for example for response spectrum analysis some one says that we need to perform modal analysis 1st and the obtained the graph between frequency and acceleration and feed it to the software, but i can't see any type of modal analysis in etabs, is it there in etabs or not???

and someone says no-need to perform modal analysis, already the graph is preloaded in the form of specific code
in which way i should perform these??

an please point out all prerequisites for performing time history and response spectrum analysis.

thanks in advance

ANSWER: Dear Hemanth

The only tool you need to perform a modal spectral analysis is to have a design response spectrum which is already substituted in etabs or you can extract it from your local code. But you can perform a modal analysis without even a design spectrum. The prerequisites are not for spectral analysis. They are needed for pseudo static analysis which is based on a rough estimate of first mode of vibration of the structure.



QUESTION: thank you sir,

you have said in one discussion that we can model soil as a network of springs and joining them with links, is it feasible to do so good.

and when i am trying to do so, i have few doubts in performing this.

1. why we are modelling soil beneath footings as a network of springs and joining them with link systems. what is the reason for modeling soil as a spring system for performing different types of analysis.

2. and in etabs their are 3 different types of spring systems
point springs, line springs, area springs
which type of springs we should assign and why???

3. etabs is asking to enter the values of simple spring stiffness for global directions, i mean for 3 translation and 3 rotation directions. how we can calculate these 6 values, based on which soil property and how by using which relation.

4. and in the same way for link properties it is asking for the entry of rotational inertia, mass, directional properties what are these properties and how we should calculate these properties

thanks in advance,

ANSWER: Dear Hemanth

1-The soil beneath the foundation is like a matress with springs insude it. Every material can be modelled with a spring system ( linear or non linear, elastic or inelastic).
2- For beginning suppose you need to assign point spring. Later you will follow your own way.

3- Suppose a network of spring with distances of x and y . The Z direction stiffness is equal to x.y.subgrade  modulus of soil for middle points, For sides spring use half of the prementionned calculation and for corners use quarter of it. The other stiffness must be taken to be very big normally.

4- take all other properties equal to zero.



QUESTION: thanks for your prompt response sir,

1. you have explained in a very well manner, but the 3rd answer is not clear to me.

sir, i am asking you how to calculate the spring stiffness for different global directions. by using which soil property and which relations????

2. and sir why we should take all the properties of links to be zero???

thanks in advance

Dear Hemanth

1- I stated that the formula is for z directions and in other directions you need to assign a very big (infinite) stiffness.

2- Because all the properties you need are inherent in your structure . You dont need a link with assigned mass. You need to apply just the stiffness in Z direction.


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Babak Esmailzadeh Hakimi





Sinam Consulting Engineers Co.


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