How to determine Elastic Stiffness, K0 for ModiMKPinching Material

[ankurjain]

I have done the Moment-Curvature analysis by providing required reinforcement detailing and applied some loads. After getting the result, I got the Elastic Stiffness from the first two rows as

Ko = (4.19E07 - 2.21E07)/(1.00E-06 - 5.00E-7)

Now my doubts are as follows :-
1. As I increase/decrease the load, will the value of Ko changes ?
2. Why only first two rows of this columns are used to find Ko ?
3. How to determine the amount of axial load to be applied to get correct Moment-Curvature relationship ?
curv mom
5.00E-07 2.21E+07
1.00E-06 4.18E+07
1.50E-06 6.14E+07
2.00E-06 8.08E+07
2.50E-06 1.00E+08
3.00E-06 1.19E+08
3.50E-06 1.39E+08
4.00E-06 1.58E+08
4.50E-06 1.77E+08
5.00E-06 1.95E+08
5.50E-06 2.14E+08
6.00E-06 2.33E+08
6.50E-06 2.50E+08
7.00E-06 2.66E+08
7.50E-06 2.79E+08



Thanks in advance guys

[mhscott]

1. If you change the axial load, it will affect the stiffness of your moment-curvature analysis
2. You did the calculation, so not sure what you're asking
3. The amount of axial load is based on the loading condition in your model

[selimgunay]

1. The very initial stiffness before cracking does not depend on the axial force. The effective initial stiffness (e.g. the secant to yield) depends on axial force as the axial force affects the level of cracking
2. If you use the first two rows, that corresponds to the very initial stiffness that I mentioned above and it is not useful as the cracked stiffness is what happens during any reasonable shaking.
3. Several axial forces that you can use are the 1) from gravity analysis of the complete structure, 2) maximum and minimum from time history of the complete structure