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# Civil Engineering/response spectrum

Question

spring properties
sir,
you have answered to me in another session that
soil beneath the foundation is like a mattress with springs inside it and can be designed as network of springs and links.
i have asked you about the values (simple spring stiffness for global directions) that should be entered in etabs.
you have answered that for z direction we need to assign form the formulae
1.
The Z direction stiffness is equal to x.y.subgrade  modulus of soil for middle points
in which x and y spring distances.

my question is what is meant by spring distances????

2. and also you have said that for remaining we need to assign a very big (infinite) stiffness.

what is the term infinite means, i mean we cant enter infinite value in etabs ????

3. you have said For sides spring use half of the aforementioned calculation and for corners use quarter of it

aforementioned calculation means the stiffness in z direction it self or something else???

4. and what the terms side springs and corner springs which are mentioned by you????

i have attached a jpeg file with this question, in the picture the different spring properties have been present. so, please say me how to calculate the values of the different spring stiffness in the table

thanks
hemanth

Dear Hemanth

1- every model is an assumption of arrangement of structural members. As I stated the soil is like a matress with a network of spring inside it. You may suppose to have springs spanning 1 meter or 2 . THis is your responsibility to construct your model. You can model a beam as a single member and you can model it assuming several sub-member constructing it. Then the distance between springs is related to your decision haow to deal with the accuracy and cost of the design process including time aand technological restrictions.

2- infinite just means a big number compared to normal situation of the structure. Usually using a 100, 200 or even 1000 times the normal values is ok . But you need to know that sometimes very big values may lead to confusions and mis accuracy in calculations which have to be monitored.

3- I meant the stiffness in Z direction.

4- Please calculate like what I stated before and let me to check it, it is better for yourself. side springs mean springs on the edge of the foundation and corner spring meand the spring located on the corner. The stiffness of corner spring is half the side spring which in turn is half the middle.

REgards
BEH

Civil Engineering

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